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42 C H A P T E R Rabies, Slow Virus I...

42 C H A P T E R Rabies, Slow Virus Infections, and Prion Diseases Many different viruses can invade the central nervous sys- C. Reactions to Physical and Chemical Agents tem and cause disease. This chapter discusses rabies, a viral Rabies virus survives storage at 4°C for weeks and at −70°C encephalitis feared since antiquity that is still an incurable for years. It is inactivated by CO2, so on dry ice it must be disease; slow virus infections; and transmissible spongiform stored in glass-sealed vials. Rabies virus is killed rapidly by encephalopathies—rare neurodegenerative disorders that are exposure to ultraviolet radiation or sunlight, by heat (1 hour caused by unconventional agents called “prions.” at 50°C), by lipid solvents (ether, 0.1% sodium deoxycholate), by trypsin, by detergents, and by extremes of pH. RABIES D. Virus Replication Rabies is an acute infection of the central nervous system The rhabdovirus replication cycle is shown in Figure 42-2. that is almost always fatal. The virus is usually transmit- Rabies virus attaches to cells via its glycoprotein spikes; the ted to humans from the bite of a rabid animal. Although nicotinic acetylcholine receptor may serve as a cellular recep- the number of human cases is small, rabies is a major pub- tor for rabies virus. The single-stranded RNA genome is lic health problem because it is widespread among animal transcribed by the virion-associated RNA polymerase to five reservoirs. mRNA species. The template for transcription is the genome RNA in the form of ribonucleoprotein (RNP) (encased in N protein and containing the viral transcriptase). The monocis- Properties of the Virus tronic mRNAs code for the five virion proteins: nucleocapsid A. Structure (N), polymerase proteins (L, P), matrix (M), and glycopro- Rabies virus is a rhabdovirus with morphologic and bio- tein (G). The genome RNP is a template for complementary chemical properties in common with vesicular stomatitis positive-sense RNA, which is responsible for the generation virus of cattle and several animal, plant, and insect viruses of negative-sense progeny RNA. The same viral proteins (Table 42-1). The rhabdoviruses are rod- or bullet-shaped serve as polymerase for viral RNA replication as well as for particles measuring 75 × 180 nm (Figure 42-1). The particles transcription. Ongoing translation is required for replica- are surrounded by a membranous envelope with protruding tion, particularly of viral N and P proteins. The newly rep- spikes, 10 nm long. The peplomers (spikes) are composed of licated genomic RNA associates with the viral transcriptase trimers of the viral glycoprotein. Inside the envelope is a ribo- and nucleoprotein to form RNP cores in the cytoplasm. The nucleocapsid. The genome is single-stranded, negative-sense particles acquire an envelope by budding through the plasma RNA (12 kb; molecular weight 4.6 × 106). Virions contain membrane. The viral matrix protein forms a layer on the an RNA-dependent RNA polymerase. The particles have a inner side of the envelope, whereas the viral glycoprotein is buoyant density in CsCl of about 1.19 g/cm3 and a molecular on the outer layer and forms the spikes. weight of 300–1000 × 106. E. Animal Susceptibility and Growth of Virus B. Classification Rabies virus has a wide host range. All warm-blooded ani- The viruses are classified in the family Rhabdoviridae. Rabies mals, including humans, can be infected. Susceptibility viruses belong to the genus Lyssavirus, whereas the vesicular varies among mammalian species, ranging from very high stomatitis-like viruses are members of the genus Vesiculovirus. (foxes, coyotes, wolves) to low (opossums); those with inter- The rhabdoviruses are very widely distributed in nature, mediate susceptibility include skunks, raccoons, and bats infecting vertebrates, invertebrates, and plants. Rabies is the (Table 42-2). The virus is widely distributed in infected ani- major medically important rhabdovirus. Many of the animal mals, especially in the nervous system, saliva, urine, lymph, rhabdoviruses infect insects, but rabies virus does not. milk, and blood. Recovery from infection is rare except in 623 Riedel_CH42_p623-p634.indd 623 05/04/19 4:54 PM  624   SECTION IV  Virology TABLE 42-1 Important Properties of Rhabdoviruses brain-to-brain passage in rabbits yields a “fixed” virus that no longer multiplies in extraneural tissues. This fixed (or mutant) Virion: Bullet-shaped, 75 nm in diameter × 180 nm in length virus multiplies rapidly, and the incubation period is shortened Composition: RNA (4%), protein (67%), lipid (26%), carbohydrate (3%) to 4–6 days. Inclusion bodies are found only with difficulty. Genome: Single-stranded RNA, linear, nonsegmented, negative- sense, molecular weight 4.6 million, 12 kb F. Antigenic Properties Proteins: Five major proteins; one is the envelope glycoprotein There is a single serotype of rabies virus. However, there are strain differences among viruses isolated from different Envelope: Present species (raccoons, foxes, skunks, canines, bats) in different Replication: Cytoplasm; virions bud from plasma membrane geographic areas. These viral strains can be distinguished by Outstanding characteristics: epitopes in the nucleoprotein and glycoprotein recognized by monoclonal antibodies as well as by specific nucleotide Wide array of viruses with broad host range sequences. There are at least seven antigenic variants found Group includes the deadly rabies virus in terrestrial animals and bats. The G glycoprotein is a major factor in rabies virus neuroin- vasiveness and pathogenicity. Avirulent mutants of rabies virus certain bats, where the virus has become peculiarly adapted have been selected using certain monoclonal antibodies against to the salivary glands. Hematophagous vampire bats may the viral glycoprotein. A substitution at amino acid position 333 transmit the virus for months without themselves ever show- of the glycoprotein results in loss of virulence, indicating some ing any signs of disease. essential role for that site of the protein in disease pathogenesis. When freshly isolated in the laboratory, the strains are Purified spikes containing the viral glycoprotein elicit referred to as street virus. Such strains show long and vari- neutralizing antibody in animals. Antiserum prepared able incubation periods (usually 21–60 days in dogs) and against the purified nucleocapsid is used in diagnostic immu- regularly produce intracytoplasmic inclusion bodies. Serial nofluorescence for rabies. Glycoprotein spikes Matrix protein Nucleocapsid A B FIGURE 42-1 Structure of rhabdoviruses. A: Electron micrograph of bullet-shaped particle typical of the rhabdovirus family (100,000×). Shown here is vesicular stomatitis virus negatively stained with potassium phosphotungstate. (Courtesy of RM McCombs, M Benyesh-Melnick, and JP Brunschwig.) B: Schematic model of rabies virus showing the surface glycoprotein spikes extending from the lipid envelope that surrounds the internal nucleocapsid and the matrix protein lining the envelope. The nucleocapsid comprises the single RNA genome plus nucleoprotein and the polymerase proteins. (Reproduced with permission from Cowan MK, Talaro KP: Microbiology. A Systems Approach, 2nd ed. McGraw-Hill, 2009. © McGraw-Hill Education.) Riedel_CH42_p623-p634.indd 624 05/04/19 4:54 PM  CHAPTER 42 Rabies, Slow Virus Infections, and Prion Diseases    625 Phosphatidylserine APICAL SURFACE Pathogenesis and Pathology receptor Virus Rabies virus multiplies in muscle or connective tissue at the 1 Adsorption site of inoculation and then enters peripheral nerves at neu- G protein romuscular junctions and spreads up the nerves to the central nervous system. However, it is also possible for rabies virus 2 Penetration to enter the nervous system directly without local replica- tion. It multiplies in the central nervous system and progres- sive encephalitis develops. The virus then spreads through Uncoating Fuse with 3 peripheral nerves to the salivary glands and other tissues. 4 endosome (– genome) The organ with the highest titers of virus is the submaxil- lary salivary gland. Other organs where rabies virus has been 5 N P An Transcription found include pancreas, kidney, heart, retina, and cornea. An M An mRNA Rabies virus has not been isolated from the blood of infected G LATERAL SURFACE An persons. 6 L An Replication (+) Susceptibility to infection and the incubation period may (+) Viral proteins depend on the host’s age, genetic background, and immune Genome status, the viral strain involved, the amount of inoculum, the severity of lacerations, and the distance the virus has to travel Progeny (–) from its point of entry to the central nervous system. There 7 is a higher attack rate and shorter incubation period in per- N,P,L sons bitten on the face or head; the lowest mortality occurs in M protein 8 those bitten on the legs. Rabies virus produces a specific eosinophilic cytoplasmic inclusion, the Negri body, in infected nerve cells. Negri bodies are filled with viral nucleocapsids. The presence of such inclu- G protein sions is pathognomonic of rabies but is not observed in at least 9 20% of cases. Therefore, the absence of Negri bodies does not rule out rabies as a diagnosis. The importance of Negri bodies BASAL SURFACE in rabies diagnosis has been lessened by the development of the FIGURE 42-2 Steps in the replication of a rhabdovirus: (1) virus more sensitive fluorescent antibody and reverse transcription- attachment; (2) penetration within an endosome; (3) fusion of polymerase chain reaction diagnostic tests. virus with endosomal membrane, releasing core into cytoplasm; (4) uncoating of nucleocapsid; (5) viral negative-sense genomic RNA transcribed into positive-sense RNA; (6) positive-sense RNA serves as template for synthesis of viral genome, plus mRNA that gives rise Clinical Findings to viral proteins; (7) negative-sense RNA becomes incorporated into Rabies is primarily a disease of lower animals and is spread nucleocapsids (N); (8) nucleocapsids join matrix protein (M) at cell to humans by bites of rabid animals or by contact with saliva surface; (9) budding of virus from cell surface. (Reproduced with from rabid animals. The disease is an acute, fulminant, fatal permission from Levy JA, Fraenkel-Conrat H, Owens RA: Virology, encephalitis. The incubation period in humans is typically 3rd ed. Prentice Hall, 1994.) 1–3 months but may be as short as 1 week or more than a year. It is usually shorter in children than in adults. The clinical spectrum can be divided into three phases: a short prodromal phase, an acute neurologic phase, and coma. The prodrome, TABLE 42-2 Animal Susceptibility to Rabies lasting 2–10 days, may show any of the following nonspecific symptoms: malaise, anorexia, headache, photophobia, nau- Very High High Moderate Low sea and vomiting, sore throat, and fever. Usually, there is an Foxes Hamsters Dogs Opossums abnormal sensation around the wound site. Coyotes Skunks Sheep During the acute neurologic phase, which lasts 2–7 days, Jackals Raccoons Goats patients show signs of nervous system dysfunction, such Wolves Cats Horses as nervousness, apprehension, hallucinations, and bizarre Cotton rats Bats Nonhuman Rabbits primates behavior. General sympathetic overactivity is observed, Cattle including lacrimation, pupillary dilatation, and increased salivation and perspiration. A large fraction of patients will Modified with permission from Baer GM, Bellini WJ, Fishbein DB: Rhabdoviruses. In Fields BN, Knipe DM (editors-in-chief). Fields Virology, 2nd ed. Raven Press, exhibit hydrophobia (fear of water) or aerophobia (fear when 1990. feeling a breeze). The act of swallowing precipitates a painful Riedel_CH42_p623-p634.indd 625 05/04/19 4:54 PM  626   SECTION IV  Virology Negri bodies Photo/CDC A B FIGURE 42-3 Histopathologic examination of central nervous system tissue from autopsy of a decedent with suspected rabies infection, showing neuronal cytoplasmic inclusions (Negri bodies) after hematoxylin and eosin staining (A) and rabies virus antigen (red) after immunohistochemical staining (B). (Reproduced with permission from Centers for Disease Control and Prevention: Human rabies—Kentucky/ Indiana, 2009. MMWR Morb Mortal Wkly Rep 2010;59:393.) spasm of the throat muscles. This phase is followed by con- structure with basophilic granules in an eosinophilic matrix. vulsive seizures or coma and death. The major cause of death Negri bodies contain rabies virus antigens (Figure 42-3). is cardiorespiratory arrest. Paralytic rabies occurs in about Both Negri bodies and rabies antigen can usually be found in 30% of patients, most frequently in those infected with bat animals or humans infected with rabies, but they are rarely rabies virus. The disease course is slower, with some patients found in bats. surviving 30 days. Recovery and survival are extremely rare. Reverse transcription-polymerase chain reaction testing Rabies should be considered in any case of encephali- can be used to amplify parts of a rabies virus genome from tis or myelitis of unknown cause even in the absence of an fixed or unfixed brain tissue or saliva. Sequencing of ampli- exposure history, and particularly in a person who has lived fied products can allow identification of the infecting virus or traveled outside the United States. Most cases of rabies in strain. the United States are in individuals with no known exposure. Because of the long incubation period, people may forget a B. Serology possible exposure incident. People who contract bat rabies Serum antibodies to rabies can be detected by immunoflu- often have no recollection of being bitten by a bat. orescence or neutralization tests. Such antibodies develop The usual incubation period in dogs ranges from 3 to slowly in infected persons or animals during progression of 8 weeks, but it may be as short as 10 days. Clinically, the disease the disease but promptly after vaccination with cell-derived in dogs is divided into the same three phases as human rabies. vaccines. Antibodies in cerebrospinal fluid are produced in rabies-infected individuals but not in response to vaccination. Laboratory Diagnosis There are no tests to diagnose rabies infections in humans C. Viral Isolation before the onset of clinical symptoms. Rabies can be diag- Available tissue is inoculated intracerebrally into suckling nosed from euthanized animals by direct fluorescent anti- mice. Infection in mice results in encephalitis and death. The body testing of brain tissue. central nervous system of the inoculated animal is examined for Negri bodies and rabies antigen. In specialized laborato- A. Rabies Antigens or Nucleic Acids ries, hamster and mouse cell lines can be inoculated for rapid Tissues infected with rabies virus are currently identified (2- to 4-day) growth of rabies virus; this is much faster than most rapidly and accurately by means of immunofluorescence virus isolation in mice. An isolated virus is identified by fluo- or immunoperoxidase staining using anti-rabies monoclonal rescent antibody tests with specific antiserum. Virus isola- antibodies. A biopsy specimen is usually taken from the skin tion takes too long to be useful in making a decision about of the neck at the hairline. Impression preparations of brain whether to give vaccine. or cornea tissue may be used. A definitive pathologic diagnosis of rabies can be based D. Animal Observation on the finding of Negri bodies in the brain or the spinal cord. All animals considered “rabid or suspected rabid” They are sharply demarcated, more or less spherical, and (Table 42-3) should be sacrificed immediately for laboratory 2–10 µm in diameter, and they have a distinctive internal examination of neural tissues. Other animals should be held Riedel_CH42_p623-p634.indd 626 05/04/19 4:54 PM  CHAPTER 42 Rabies, Slow Virus Infections, and Prion Diseases    627 TABLE 42-3 Rabies Postexposure Prophylaxis Guide—United States, 2008 The following recommendations are only a guide. In applying them, take into account the animal species involved, the circumstances of the bite or other exposure, the vaccination status of the animal, and the presence of rabies in the region. Note: Local or state public health officials should be consulted if questions arise about the need for rabies prophylaxis. Animal Type Evaluation of Animal Treatment of Exposed Persona Domestic Dogs, cats, and ferrets Healthy and available for 10 days of None, unless animal develops symptoms of rabies observation Immediately begin prophylaxis Rabid or suspected rabid Consult public health officials Unknown (escaped) Wild Skunks, raccoons, bats, foxes, coyotes, Regard as rabid unless animal is proved Consider immediate prophylaxis and other carnivores negative by laboratory tests Other Livestock, rodents, and lagomorphs Consider individually. Local and state public health officials should be consulted about the need for (rabbits and hares) rabies prophylaxis. Bites of squirrels, hamsters, guinea pigs, gerbils, chipmunks, rats, mice, other rodents, rabbits, and hares almost never require rabies prophylaxis. a Prophylaxis consists of immediate and thorough cleansing of bites and wounds with soap and water, administration of rabies immune globulin, and vaccination. Reproduced with permission from The Centers for Disease Control and Prevention: MMWR Morb Mortal Wkly Rep 2008;57(RR-3):1. for observation for 10 days. If they show any signs of enceph- is to neutralize some of the inoculated virus and lower the alitis, rabies, or unusual behavior, they should be killed concentration of virus in the body, providing additional humanely and the tissues examined in the laboratory. If they time for a vaccine to stimulate active antibody production appear normal after 10 days, decisions must be made on an to prevent entry into the central nervous system. Successful individual basis in consultation with public health officials. postexposure prophylaxis will therefore prevent the devel- opment of clinical rabies. Immunity and Prevention B. Types of Vaccines Only one antigenic type of rabies virus is known. More All vaccines for human use contain only inactivated rabies than 99% of infections in humans and other mammals virus. Two vaccines are available in the United States, that develop symptoms end fatally. Survival after the although a number of others are in use in other countries. onset of rabies symptoms is extremely rare. It is therefore Both rabies vaccines available in the United States are equally essential that individuals at high risk receive preventive safe and efficacious. immunization, that the nature and risk of any exposure be evaluated, and that individuals be given postexposure pro- 1. Human diploid cell vaccine (HDCV)—To obtain a phylaxis if their exposure is believed to have been danger- rabies virus suspension free from nervous system and foreign ous (Table 42-3). Because treatment is of no benefit after the proteins, rabies virus is grown in the MRC-5 human diploid onset of clinical disease, it is essential that postexposure cell line. The rabies virus preparation is concentrated by ultra- treatment be initiated promptly. Postexposure rabies pro- filtration and inactivated with β-propiolactone. No serious phylaxis consists of the immediate and thorough cleansing anaphylactic or encephalitic reactions have been reported. of all wounds with soap and water, administration of rabies This vaccine has been used in the United States since 1980. immune globulin, and a vaccination regimen. 2. Purified chick embryo cell vaccine (PCEC)—This A. Pathophysiology of Rabies Prevention by vaccine is prepared from the fixed rabies virus strain Flury Vaccine LEP grown in chicken fibroblasts. It is inactivated with Presumably the virus must be amplified in muscle near the β-propiolactone and further purified by zonal centrifugation. site of inoculation until the concentration of virus is suf- It became available in the United States in 1997. ficient to accomplish infection of the central nervous sys- A recombinant viral vaccine consisting of vaccinia virus tem. If immunogenic vaccine or specific antibody can be carrying the rabies surface glycoprotein gene has successfully administered promptly, virus replication can be depressed immunized animals following oral administration. This vac- and virus can be prevented from invading the central ner- cine can be used for the immunization of both wildlife reser- vous system. The action of passively administered antibody voir species and domestic animals. Riedel_CH42_p623-p634.indd 627 05/04/19 4:54 PM  628   SECTION IV  Virology C. Types of Rabies Antibody of wild animals (especially bats, raccoons, skunks, and foxes) 1. Rabies immune globulin, human (HRIG)—HRIG is or occurs in travelers bitten by dogs elsewhere in the world. a γ-globulin prepared by cold ethanol fractionation from the The most serious problem in livestock appears to be vam- plasma of hyperimmunized humans. There are fewer adverse pire bat-transmitted rabies in Latin America. The increase in reactions to human rabies immune globulin than to equine wildlife rabies in the United States and some other developed anti-rabies serum. countries presents a far greater risk to humans than do dogs or cats. 2. Anti-rabies serum, equine—This is concentrated Primarily as a result of the successful control of rabies serum from horses hyperimmunized with rabies virus. It has in domestic dogs, the incidence of human rabies in the been used in countries where HRIG is not available. United States declined to fewer than three persons per year during the last two decades. D. Pre-exposure Prophylaxis Antigenic analysis with monoclonal antibodies and genotyping by nucleotide sequence analysis can distinguish This is indicated for persons at high risk of contact with rabies rabies virus isolates from different animal reservoirs. From virus (research and diagnostic laboratory workers, spelunkers) 2000 to 2011, there were 32 diagnosed human rabies cases in or with rabid animals (veterinarians, animal control, and the United States, of which more than 95% of domestically wildlife workers). The goal is to attain an antibody level pre- acquired cases were proved to be due to bat-associated virus. sumed to be protective by means of vaccine administration Eight of nine patients with imported rabies had dog-associated prior to any exposure. It is recommended that antibody titers strains. of vaccinated individuals be monitored periodically and that Raccoons are an important reservoir for rabies in boosters be given when required. the United States and account for over half of all reported cases of animal rabies. It is believed that raccoon rabies was E. Postexposure Prophylaxis introduced into the mid-Atlantic region in the 1970s, when Although few (0–5) cases of human rabies occur in the infected raccoons were transported there from the south- United States per year, more than 20,000 persons receive eastern United States to replenish hunting stocks. The rac- some treatment every year for possible bite wound expo- coon rabies epizootic has spread and now covers the eastern sure. The decision to administer rabies antibody or rabies United States into Canada. vaccine—or both—depends on several factors: (1) the nature Bats present a special problem because they may carry of the biting animal (species, state of health, domestic, or rabies virus while they appear to be healthy, excrete it in wild) and its vaccination status, (2) the availability of the saliva, and transmit it to other animals and to humans. animal for laboratory examination (all bites by wild animals Among human rabies cases in the United States attributed and bats require rabies immune globulin and vaccine), to bat-associated variants, the majority were caused by the (3) the existence of rabies in the area, (4) the manner of attack silver-haired bat and eastern pipistrelle bat variants. How- (provoked or unprovoked), (5) the severity of the bite and ever, only two cases were associated with a history of bat bite, contamination by saliva of the animal, and (6) advice from as most bat exposures go undetected. Bat caves may contain local public health officials (Table 42-3). Schedules for aerosols of rabies virus and present a risk to spelunkers. postexposure prophylaxis involving the administration of Migrating fruit-eating bats exist in many countries and are a rabies immune globulin and vaccine are available from the source of infection for many animals and humans. Bat rabies Centers for Disease Control and Prevention and state public may be important in the initiation of terrestrial enzootics in health offices. new regions. Australia, long considered to be a rabies-free continent, was found in 1996 to harbor rabies virus in fruit bats. All persons bitten by bats must receive postexposure Epidemiology rabies prophylaxis. Rabies is enzootic in both wild and domestic animals. World- Human-to-human rabies infection is very rare. The wide, at least 50,000 deaths due to human rabies occur each only documented cases involve rabies transmitted by cor- year; however, rabies is grossly underreported in many coun- neal and organ transplants. One example involves cor- tries. Almost all rabies deaths (>99%) occur in developing neal transplants—the corneas came from donors who died countries, with Asia accounting for over 90% of all rabies with undiagnosed central nervous system diseases, and the fatalities. In these countries, where canine rabies is still recipients died from rabies 50–80 days later. The first docu- endemic, most human cases develop from bites of rabid dogs. mented case involving solid organ transplants occurred in Children aged 5–15 years are at particular risk. An estimated the United States in 2004. The liver and kidneys from a single 15 million persons are given postexposure prophylaxis annu- donor were transplanted into three recipients, all of whom ally, the majority in China and India. died of confirmed rabies 5–7 weeks later. Transmission likely In the United States, Canada, and Western Europe, occurred via neuronal tissue in the transplanted organs, as where canine rabies has been controlled, dogs are responsible rabies virus is not spread in the blood. Theoretically, rabies for very few cases. Rather, human rabies develops from bites could originate from the saliva of a patient who has rabies and Riedel_CH42_p623-p634.indd 628 05/04/19 4:54 PM  CHAPTER 42 Rabies, Slow Virus Infections, and Prion Diseases    629 exposes attending personnel, but such transmission has never TABLE 42-4 Important Properties of Bornaviruses been documented. Virion: Spherical, 90 nm in diameter Genome: Single-stranded RNA, linear, nonsegmented, negative- Treatment and Control sense, 8.9 kb, molecular weight 3 million There is no successful treatment for clinical rabies. Inter- Proteins: Six structural proteins ferons, ribavirin, and other drugs have shown no beneficial Envelope: Present effects. Symptomatic treatment may prolong life, but the out- come is almost always fatal. Replication: Nucleus; site of maturation not identified Historically, several key events have contributed to the Outstanding characteristics: control of human rabies: the development of a human rabies Broad host range vaccine (1885), the discovery of the diagnostic Negri body (1903), the use of rabies vaccines for dogs (1940s), the addition Neurotropic of rabies immune globulin to human postexposure vaccina- Cause neurobehavioral abnormalities tion treatments (1954), the growth of rabies virus in cultured cells (1958), and the development of diagnostic fluorescent Borna disease virus (BDV) is an enveloped, non- antibody tests (1959). segmented, negative-stranded RNA virus in the family Pre-exposure vaccination is desirable for all persons who Bornaviridae (Table 42-4). BDV is novel among nonseg- are at high risk of contact with rabid animals, such as veteri- mented, negative-sense RNA viruses in that it transcribes narians, animal care personnel, certain laboratory workers, and replicates its genome in the nucleus and uses RNA and spelunkers. Persons traveling to developing countries splicing for regulation of gene expression. BDV is noncyto- where rabies control programs for domestic animals are not lytic and highly neurotropic; it establishes persistent infec- optimal should be offered pre-exposure prophylaxis if they tions. There is a single recognized serotype of BDV. Titers of plan to stay for more than 30 days. However, pre-exposure neutralizing antibodies produced in host species are usually prophylaxis does not eliminate the need for prompt postex- very low. posure prophylaxis if an exposure to rabies occurs. Many species can be infected by bornaviruses, including Isolated countries (eg, Great Britain) that have no indig- humans. Serologic or reverse transcription-polymerase chain enous rabies in wild animals can establish quarantine pro- reaction data suggest that BDV may be associated with neu- cedures for dogs and other pets to be imported. In countries ropsychiatric disorders in humans, although those findings where dog rabies exists, stray animals should be destroyed are controversial and it remains to be established whether and vaccination of pet dogs and cats should be mandatory. BDV is etiologically involved in the pathophysiology of cer- In countries where wildlife rabies exists and where contact tain human mental disorders. between domestic animals, pets, and wildlife is inevitable, all domestic animals and pets should be vaccinated. An oral vaccinia–rabies glycoprotein recombinant virus SLOW VIRUS INFECTIONS AND PRION vaccine (V-RG) proved effective at controlling rabies in foxes DISEASES in Europe. Added to baits, the oral vaccine is being used to curtail rabies epizootics in wildlife in the United States. Some chronic degenerative diseases of the central nervous system in humans are caused by “slow” or chronic, persis- tent infections by classic viruses. Among these are subacute Emerging Rhabdovirus Infections sclerosing panencephalitis and progressive multifocal leu- A small outbreak of viral hemorrhagic fever in central Africa koencephalopathy. Other diseases known as transmissible in 2009 was associated with a novel rhabdovirus named spongiform encephalopathies—such as Creutzfeldt-Jakob Bas-Congo virus. Two patients died and two health care disease (CJD)—are caused by unconventional transmissible workers survived, indicating potential person-to-person agents termed “prions” (Table 42-5). The progressive neuro- transmission. The probable animal reservoir is unknown, logic diseases produced by these agents may have incubation and no additional cases have since been identified. periods of years before clinical manifestations of the infec- tions become evident (Table 42-5). BORNA DISEASE Slow Virus Infections Borna disease, a central nervous system disease primarily of A. Visna horses and sheep in certain areas of Germany, is manifested Visna and progressive pneumonia (maedi) viruses are by behavioral abnormalities usually ending in death. Inflam- closely related agents that cause slowly developing infections matory cell infiltrates are present in the brain. The disorder in sheep. These viruses are classified as retroviruses (genus is immune mediated. Lentivirus; see Chapter 44). Riedel_CH42_p623-p634.indd 629 05/04/19 4:54 PM  630   SECTION IV  Virology TABLE 42-5 Slow Virus and Prion Diseases Disease Agent Hosts Incubation Period Nature of Disease Diseases of humans Subacute sclerosing Measles virus Humans 2–20 years Chronic sclerosing panencephalitis panencephalitis variant Progressive multifocal Polyomavirus JC Humans Years Central nervous system leukoencephalopathy virus demyelination Creutzfeldt-Jakob disease Prion Humans, chimpanzees, Months to years Spongiform encephalopathy (CJD) monkeys Variant CJDa Prion Humans, cattle Months to years Spongiform encephalopathy Kuru Prion Humans, chimpanzees, Months to years Spongiform encephalopathy monkeys Diseases of animals Visna Retrovirus Sheep Months to years Central nervous system demyelination Scrapie Prion Sheep, goats, mice, Months to years Spongiform encephalopathy hamsters Bovine spongiform Prion Cattle Months to years Spongiform encephalopathy encephalopathy Transmissible mink Prion Mink, other animals Months Spongiform encephalopathy encephalopathy Chronic wasting disease Prion Mule deer, elk Months to years Spongiform encephalopathy CJD, Creutzfeldt-Jakob disease. a Associated with exposure to bovine spongiform encephalopathy-contaminated material. Visna virus infects all the organs of the body of the efficiency of measles virus transcription in differentiated infected sheep; however, pathologic changes are confined brain cells is important in maintaining the persistent infec- primarily to the brain, lungs, and reticuloendothelial system. tion that leads to subacute sclerosing panencephalitis. Inflammatory lesions develop in the central nervous system soon after infection, but there is usually a long incubation C. Progressive Multifocal Leukoencephalopathy period (months to years) before observable neurologic symp- JC virus, a member of the family Polyomaviridae (see toms appear. Disease progression can be either rapid (weeks) Chapter 43), is the etiologic agent of progressive multifocal or slow (years). leukoencephalopathy, a central nervous system complication Virus can be recovered for the life of the animal, but viral that occurs in some immunosuppressed individuals. Once expression is restricted in vivo so that only minimal amounts exceedingly rare, the disease can occur in a significant pro- of infectious virus are present. Antigenic variation occurs portion (about 5%) of patients with AIDS; however, as antivi- during the long-term persistent infections. Many mutations ral drugs slow the progression of human immunodeficiency occur in the structural gene that codes for viral envelope gly- virus infections, fewer patients develop this disease. Progres- coproteins. Infected animals develop antibodies to the virus. sive multifocal leukoencephalopathy is also a rare complica- tion of some therapeutic monoclonal antibodies for diseases B. Subacute Sclerosing Panencephalitis such as multiple sclerosis. Demyelination in the central ner- This is a rare disease of young adults caused by measles virus, vous system of patients with progressive multifocal leukoen- with slowly progressive demyelination in the central nervous cephalopathy results from reactivation and replication of JC system ending in death (see Chapter 40). Large numbers of virus when an immune system is compromised. viral nucleocapsid structures are produced in neurons and glial cells. There is restricted expression of the viral genes that encode envelope proteins, so the virus in persistently Transmissible Spongiform infected neural cells lacks proteins needed for the production Encephalopathies (Prion Diseases) of infectious particles. Patients with subacute sclerosing pan- Degenerative central nervous system diseases—kuru, CJD, encephalitis have high titers of anti-measles antibody except Gerstmann-Sträussler-Scheinker syndrome, fatal famil- that antibody to the M protein is frequently lacking. Reduced ial insomnia of humans, scrapie of sheep, transmissible Riedel_CH42_p623-p634.indd 630 05/04/19 4:54 PM  CHAPTER 42 Rabies, Slow Virus Infections, and Prion Diseases    631 encephalopathy of mink, bovine spongiform encephalopathy circulating B cells in scrapie-infected sheep. Infectivity has (BSE) of cattle, and chronic wasting disease of deer—have also been detected in milk from sheep incubating natural similar pathologic features. These diseases are described as scrapie. Maximum titers of infectivity are reached in the transmissible spongiform encephalopathies. The causative brain long before neurologic symptoms appear. The disease agents are not conventional viruses; infectivity is associated is characterized by the development of amyloid plaques in the with proteinaceous material devoid of detectable amounts of central nervous system of infected animals. These areas rep- nucleic acid. The term “prion” is used to designate this novel resent extracellular accumulations of protein; they stain with class of agents. Congo red. The different types of prions appear to have common A protease-resistant protein of molecular mass 27–30 kDa mechanisms of pathogenesis. Species barriers exist for all can be purified from scrapie-infected brain and is designated transmissible spongiform encephalopathies, but some pri- prion protein PrP. Preparations containing only PrP and no ons have crossed such barriers. These diseases are associated detectable nucleic acid are infectious. PrP is derived from a with acquisition of misfolded prion proteins that can cause larger host-encoded protein, PrPSc, that is an altered version misfolding and aggregation of normal cellular prion protein of a normal cellular protein (PrPC). The protein is a glycolipid- expressed in brain tissue. anchored membrane protein. The level of PrPSc is increased These agents are unusually resistant to standard means in infected brains because the protein becomes resistant to of inactivation. They are resistant to treatment with formal- degradation. Genetic susceptibility to scrapie infection is dehyde (3.7%), urea (8 M), dry heat, boiling, ethanol (50%), associated with point mutations in the PrPC gene, and mice proteases, deoxycholate (5%), and ionizing radiation. How- genetically altered to be devoid of PrPC are resistant to scra- ever, they are sensitive to phenol (90%), household bleach, pie. A conformational model for prion replication proposes ether, NaOH (2 N), strong detergents (10% sodium dodecyl that PrPSc forms a heterodimer with PrPC and refolds it so that sulfate), and autoclaving (1 hour, 121°C). Guanidine thiocya- it becomes like PrPSc. “Strains” of prions are speculated to nate is highly effective in decontaminating medical supplies reflect different conformations of PrPSc. In the last few years, and instruments. several studies have generated synthetic prions in vitro that There are several distinguishing hallmarks of these prion caused disease when inoculated in vivo, further suggesting diseases. Although the etiologic agent may be recoverable that prions are infectious proteins. from other organs, the diseases are confined to the nervous system. The basic features are neurodegeneration and spongi- form changes. Amyloid plaques may be present. Long incuba- B. Kuru and Classic Creutzfeldt-Jakob tion periods (months to decades) precede the onset of clinical Disease (CJD) illness and are followed by chronic progressive disease (weeks Two human spongiform encephalopathies are kuru and the to years). The diseases are always fatal, with no known cases classic form of CJD. Brain homogenates from patients have of remission or recovery. The host shows no inflammatory transmitted both diseases to nonhuman primates. Kuru response and no immune response (the agents do not appear occurred only in the eastern highlands of New Guinea and to be antigenic); no production of interferon is elicited; and was spread by customs surrounding ritual cannibalism there is no effect on host B-cell or T-cell function. Immuno- of dead relatives. Since the practice has ceased, the disease suppression of the host has no effect on pathogenesis; how- has disappeared. CJD in humans develops gradually, with ever, chronic inflammation induced by other factors (viruses, progressive dementia, ataxia, and myoclonus, and leads to bacteria, autoimmunity) may affect prion pathogenesis. It has death in 5–12 months. Sporadic CJD is believed to be caused been observed that prions accumulate in organs with chronic by the spontaneous transformation of normal prion pro- lymphocytic inflammation. When coincident with nephritis, tein into abnormal prions. This occurs with a frequency of prions are excreted in urine. approximately one case per million population per year in the United States and Europe and involves patients over 50 years of age. The estimated incidence is less than one case per A. Scrapie 200 million for persons under 30 years of age. Sporadic CJD Scrapie shows marked differences in susceptibility of differ- is believed to be caused by the spontaneous transformation ent breeds of animal. Susceptibility to experimentally trans- of normal prion protein into abnormal prions. However, the mitted scrapie ranges from 0 to over 80% in various breeds new variant form of CJD linked to BSE (see below) has mainly of sheep, whereas goats are almost 100% susceptible. The affected people under the age of 30. transmission of scrapie to mice and hamsters, in which the Two familial forms of CJD are Gerstmann-Sträussler- incubation period is greatly reduced, has facilitated study of Scheinker syndrome and fatal familial insomnia. These the disease. diseases are rare (10–15% of CJD cases) and are due to inheri- Infectivity can be recovered from lymphoid tissues tance of mutations in the PrP gene. early in infection, and high titers of the agent are found in Iatrogenic CJD has been transmitted accidentally the brain, spinal cord, and eye (the only places where patho- by contaminated growth hormone preparations from logic changes are observed). Prion protein is associated with human cadaver pituitary glands, by corneal transplant, by Riedel_CH42_p623-p634.indd 631 05/04/19 4:54 PM  632   SECTION IV  Virology contaminated surgical instruments, and by cadaveric human Most rabies deaths worldwide occur in Asia and result dura mater grafts used for surgical repair of head injury. It from bites of rabid dogs. In the United States, most human appears that recipients of contaminated dura mater grafts cases are acquired from wild animals. remain at risk of developing CJD for more than 20 years fol- Killed rabies vaccines exist for use in humans; live attenu- lowing receipt of grafts. There is currently no suggestion of ated virus vaccines are available for animal immunization. CJD transmission by blood or blood products, although the There are no tests to diagnose rabies infections in humans potential is there. before disease develops. There is no successful treatment for clinical rabies. C. Bovine Spongiform Encephalopathy (BSE) and Postexposure prophylaxis consists of administration of New Variant CJD rabies antibody, rabies vaccine, or both, following a pos- A disease similar to scrapie, BSE, or “mad cow disease,” sible exposure. emerged in cattle in Great Britain in 1986. This outbreak was Subacute sclerosing panencephalitis is a rare and fatal cen- traced to the use of cattle feed that contained contaminated tral nervous system disease caused by measles virus. bone meal from scrapie-infected sheep and BSE-infected Progressive multifocal leukoencephalopathy is a rare, usu- cattle carcasses. The use of such cattle feed was prohibited ally fatal, central nervous system disease caused by poly- in 1988. The epidemic of “mad cow disease” peaked in Great omavirus JC virus in immunosuppressed individuals. Britain in 1993. It is estimated that over 1 million cattle were The prion diseases (transmissible spongiform encephalop- infected. BSE has also been found in other European countries. athies) are caused by unconventional agents with proper- In 1996, a new variant form of CJD was recognized in the ties of infectious proteins. United Kingdom that occurred in younger people and had Human prion diseases include kuru, CJD, and variant CJD. distinctive pathologic characteristics similar to those of BSE. Prion agents are very resistant to inactivation, including It is now accepted that the new variant forms of CJD and formaldehyde, boiling, and radiation; they can be inacti- BSE are caused by a common agent, indicating that the BSE vated by bleach and autoclaving. agent had infected humans. Through 2006, over 150 people The progressive neurologic diseases may have very long had been diagnosed with new variant CJD in England, and incubation periods, ranging from months to years. most had died. A particular polymorphism in the amino acid sequence of the human prion protein seems to influence sus- ceptibility to disease. REVIEW QUESTIONS D. Chronic Wasting Disease 1. Rabies virus is rapidly destroyed by A scrapie-like disease, designated chronic wasting disease, is (A) Ultraviolet radiation found in mule deer and elk in the United States and Canada. (B) Heating at 56°C for 1 hour It is laterally transmitted with high efficiency, but there is no (C) Ether treatment evidence that it has been transmitted to humans. Infectivity (D) Trypsin treatment (E) All of the above has been detected in feces of deer before they become ill; the agent is retained in the soil, where it can then be ingested by 2. Prions are readily destroyed by other deer and elk. (A) Ionizing radiation (B) Formaldehyde (C) Boiling E. Alzheimer’s Disease (D) Proteases There are some neuropathologic similarities between CJD (E) None of the above and Alzheimer’s disease, including the appearance of amy- 3. The presence in neurons of eosinophilic cytoplasmic inclusion loid plaques. However, the disease has not been transmitted bodies, called Negri bodies, is characteristic of which of the fol- experimentally to primates or rodents, and the amyloid mate- lowing central nervous system infections? rial in the brains of Alzheimer’s patients does not contain (A) Borna disease PrPSc protein. (B) Rabies (C) Subacute sclerosing panencephalitis (D) New variant Creutzfeldt-Jakob disease CHAPTER SUMMARY (E) Postvaccinal encephalitis 4. Which of the following statements about rabies vaccines for Rabies is a viral encephalitis that is almost always fatal once human use is true? symptoms appear. It is caused by an RNA virus classified (A) Contain live, attenuated rabies virus as a rhabdovirus. (B) Contain multiple antigenic types of rabies virus Humans get infected with rabies by a bite from a rabid ani- (C) Can treat clinical cases of rabies mal. The incubation period can range from 1 week to more (D) Can be used for postexposure prophylaxis than a year. (E) They are associated with Guillain-Barré syndrome. Riedel_CH42_p623-p634.indd 632 05/04/19 4:54 PM  CHAPTER 42 Rabies, Slow Virus Infections, and Prion Diseases    633 5. A 22-year-old man is a resident of a small town near London. (C) The vaccine was directed against dog rabies and the patient He likes to eat beefsteak. He develops a severe progressive was infected with bat rabies. neurologic disease characterized by psychiatric symptoms, (D) The rabies immune globulin should not have been admin- cerebellar signs, and dementia. Probable bovine spongiform istered as it interfered with the vaccine. encephalopathy (BSE) is diagnosed. New variant Creutzfeldt- (E) Interferons—and not the treatment regimen administered— Jakob disease in humans and BSE appear to be caused by the are the treatment of choice once rabies symptoms same agent. Which of the following statements is true of both develop. diseases? 9. Which of the following animals is most commonly reported (A) Immunosuppression of the host is a predisposing factor. rabid in the United States? (B) It is an immune-mediated degenerative neurologic (A) Squirrels disorder. (B) Raccoons (C) There is a long incubation period (months to years) from (C) Rabbits time of exposure to appearance of symptoms. (D) Swine (D) The agent is recoverable only from the central nervous sys- (E) Rats tem of an infected host. 10. A runner reports an “unprovoked bite” from a neighborhood (E) The interferon response persists throughout the incuba- dog. The dog was captured by local animal control authorities, tion period. and it appears healthy. What is the appropriate action? (F) There is a high-titer antibody response toward PrPSc pro- (A) Confine and observe the dog for 10 days for signs sugges- tein of the agent. tive of rabies. 6. Rabies virus has a wide host range and the ability to infect all (B) Begin postexposure prophylaxis of the bitten person. warm-blooded animals, including humans. Which statement (C) Immediately euthanize the dog. about the epidemiology of human rabies is true? (D) Because canine rabies has been eliminated in the United (A) Africa accounts for the majority of rabies fatalities. States, dog bites are no longer an indication for postexpo- (B) Dog bites cause most cases of human rabies in England. sure prophylaxis, and no further action is needed. (C) Domestic animals are the source of most human rabies in (E) Test the dog for rabies antibody. the United States. 11. The slow virus disease that most clearly has immunosuppres- (D) Human-to-human rabies transmission places medical per- sion as an important factor in its pathogenesis is sonnel at serious risk. (A) Progressive multifocal leukoencephalopathy (E) Bat rabies has caused most human rabies cases in the (B) Subacute sclerosing panencephalitis United States since the 1990s. (C) Creutzfeldt-Jakob disease 7. Infectious scrapie agent can be detected in amyloid plaques in (D) Scrapie infected brains of sheep and hamsters. The genome of the infec- 12. Scrapie and kuru possess all of the following characteristics tious agent is characterized by which of the following nucleic except acid types? (A) A histologic picture of spongiform encephalopathy (A) Negative-sense, single-stranded RNA (B) Transmissibility to animals associated with a long incuba- (B) Small interfering RNA, smallest known infectious RNA tion period (C) DNA copy of RNA genome, integrated in mitochondrial (C) Slowly progressive deterioration of brain function DNA (D) Prominent intranuclear inclusions in oligodendrocytes (D) Single-stranded, circular DNA (E) No detectable nucleic acid 13. A 5-year-old boy in San Francisco reaches into a car to pet another family’s dog and is bitten on the finger. Six weeks after 8. A 49-year-old man visited a neurologist after 2 days of increas- the bite, the child develops fever, headache, and a seizure. He ing right arm pain and paresthesias. The neurologist diagnosed becomes combative and hallucinates. What is the best diagnos- an atypical neuropathy. The symptoms increased and were tic test to perform on the patient to rule in rabies as a cause of accompanied by hand spasms and sweating on the right side his illness? of the face and trunk. The patient was admitted to the hospital the day after developing dysphagia, hypersalivation, agitation, (A) Detection of serum anti-rabies antibody and generalized muscle twitching. Vital signs and blood tests (B) Culture of cerebrospinal fluid for virus were normal, but within hours the patient became confused. (C) Direct fluorescent antibody stain of a biopsy from the nape The consulting neurologist suspected rabies. Rabies immune of the neck globulin, vaccine, and acyclovir were administered. The patient (D) Brain biopsy was placed on mechanical ventilation the following day. Renal (E) Cerebrospinal fluid anti-rabies antibody failure developed, and the patient died 3 days later. Rabies test 14. Each of the following statements concerning rabies and rabies results were positive. The patient’s wife reported the patient virus is correct except had suffered no bites by dogs or wild animals. The most likely (A) The virus has a lipoprotein envelope and single-stranded explanation for treatment failure is RNA as its genome. (A) The rabies test results were falsely positive and the patient (B) The virus has a single antigenic type (serotype). did not have rabies. (C) In the United States, dogs are the most common reservoir. (B) Treatment was initiated after the onset of clinical symp- (D) The incubation period is usually long (several weeks) toms of rabies. rather than short (several days). Riedel_CH42_p623-p634.indd 633 05/04/19 4:54 PM  634   SECTION IV  Virology 15. A 20-year-old man, who for many years had received daily Colby DW, Prusiner SB: De novo generation of prion strains. Nat injections of growth hormone prepared from human pitu- Rev Microbiol 2011;9:771. itary glands, develops ataxia, slurred speech, and dementia. At Compendium of animal rabies prevention and control, 2011. autopsy the brain shows widespread neuronal degeneration, National Association of State Public Health Veterinarians, Inc. a spongy appearance due to many vacuoles between the cells, MMWR Morb Mortal Wkly Rep 2011;60(RR-6):1. no inflammation, and no evidence of virus particles. The most De Serres G, Dallaire F, Côte M, et al: Bat rabies in the likely diagnosis is United States and Canada from 1950 through 2007: (A) Herpes encephalitis Human cases with and without bat contact. Clin Infect Dis (B) Creutzfeldt-Jakob disease 2008;46:1329. (C) Subacute sclerosing panencephalitis Grard JG, Fair JN, Lee D, et al: A novel rhabdovirus associated (D) Progressive multifocal leukoencephalopathy with acute hemorrhagic fever in central Africa. PLoS Med (E) Rabies 2012;8:e1002924. Human rabies prevention—United States, 2008. Recommenda- tions of the Advisory Committee on Immunization Practices. Answers MMWR Morb Mortal Wkly Rep 2008;57(RR-3):1. 1. E 6. E 11. A Lipkin WI, Briese T, Hornig M: Borna disease virus—fact and fan- 2. E 7. E 12. D tasy. Virus Res 2011;162:162. 3. B 8. B 13. C Lyles DS, Rupprecht CE: Rhabdoviridae. In Knipe DM, Howley PM 4. D 9. B 14. C (editors-in-chief). Fields Virology, 5th ed. Lippincott Williams 5. C 10. A 15. B and Wilkins, 2007. 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