Human Diseases Caused by Viruses and Prions

Airborne Diseases

• Airborne viruses are propelled from the respiratory tract by coughing, sneezing, or vocalizing
• Examples of airborne diseases include Chickenpox (Varicella) and Shingles (Herpes Zoster), Measles (Rubeola), Rubella (German Measles), and Smallpox (Variola)

Chickenpox and Shingles

• Caused by a DNA virus, a member of Herpesviridae
• Humans serve as the reservoir and source
• Acquired by droplet inhalation into the respiratory system
• Chickenpox results from initial infection, and vaccine prevents or shortens illness
• Shingles (herpes zoster; postherpetic neuralgia) is a reactivated form of chickenpox virus that resides in cranial and sensory neurons
• Treatment is supportive, and acyclovir and others are used

Measles

• Caused by an RNA virus that enters the body through the respiratory tract
• Subacute sclerosing panencephalitis is a rare progressive degeneration of the central nervous system caused by the measles virus
• Treatment, prevention, and control involve symptomatic/supportive therapy and attenuated MMR vaccine (measles, mumps, rubella)

Rubella

• Caused by an enveloped RNA virus
• Mild brief rash is acquired from respiratory droplets
• Congenital rubella syndrome is a disastrous disease in the first trimester of pregnancy that can lead to fetal death, premature delivery, or congenital defects
• Attenuated vaccine has reduced cases to 1000 and 10 congenital rubella cases per year

Smallpox

• Caused by the variola virus, a large, brick-shaped complex virus with linear dsDNA
• Transmitted by aerosol or contact, and humans are the only natural host
• Clinical forms include Variola major, the most common and severe form, and Variola minor, a less common and less severe form
• Eradication was possible due to the disease's obvious clinical features, humans being the only hosts and reservoirs, no asymptomatic carriers, and a short infectivity period (3–4 weeks)

Smallpox Eradication and Vaccination

• Last case occurred in Somalia in 1977
• Protection is obtained from vaccination using vaccinia virus in a live virus vaccine
• Routine immunization is no longer done in the U.S.

West Nile Fever

• Transmitted by Culex spp. mosquitoes that feed on sparrows and crows
• Human-to-human spread has occurred through blood and organ donation
• Only one antigen type exists, and immunity is presumed to be permanent
• Clinical manifestations include fever, lymphadenopathy, rash, and serious complications such as encephalitis in >1/100 patients
• Treatment, prevention, and control involve serology tests, supportive treatment, and mosquito control measures

Direct Contact Diseases

• Transmission occurs through direct personal contact (touching, kissing, sexual contact, contact with body fluids, contact with open wounds)
• Many diseases, including AIDS, are transmitted through direct contact

Genital Herpes

• Usually caused by herpes simplex type 2, a linear dsDNA enveloped virus
• A very common sexually transmitted disease and can be transmitted to infants during vaginal delivery (congenital or neonatal herpes)
• Active and latent disease with reactivations occur
• Clinical manifestations include fever, burning sensation, genital soreness, and blisters in the infected area
• Blisters heal spontaneously, but the virus remains latent and is periodically reactivated
• Can be treated with antiviral drugs (e.g., acyclovir)

Congenital Herpes

• Congenital (neonatal) herpes is usually HSV-2 and is one of the most life-threatening infections in newborns (~1,500–2,200 babies/year in the U.S.)

Arthropod-Borne Diseases

• Arboviruses are transmitted by bloodsucking arthropods from one vertebrate host to another
• Multiply in the tissues of the vector without producing disease
• Vector acquires a lifelong infection

West Nile Fever (Encephalitis)

• Caused by a flavivirus, an RNA virus
• Appeared in New York in 1999, causing human and animal deaths
• By 2006, it was found in all continental U.S.
• Can result in neurologic involvement and blindness
• Caesarian section is recommended

Human Herpesvirus 6 Infections

• HHV-6 is an enveloped virus with an icosahedral capsid and dsDNA
• Tropism is wide, including CD8+, CD4+, natural killer, and epithelial cells
• Probably transmitted in saliva
• Clinical manifestations include exanthem subitum, a short-lived disease of infants with high fever for 3 to 4 days, followed by a macular rash
• Pneumonitis in immunocompromised individuals and implicated in chronic fatigue syndrome, lymphadenitis, and multiple sclerosis in immunocompetent adults
• No treatment or prevention measures are available

Warts

• Verrucae are horny projections on the skin associated with human papillomaviruses, a DNA virus
• More than 100 different strains, infect epithelium and mucus membranes, and cause benign epithelial tumors and cancers
• Four major types include plantar warts, verrucae vulgaris, flat or plane warts, and anogenital condylomata (genital warts)
• Direct contact and autoinoculation occur
• Treatment involves physical destruction, topical application of drug podophyllum, or injection of IFN-α

Genital Warts (Anogenital Condylomata)

• A very common sexually transmitted HPV with a 1–6 month incubation period
• Warts, usually multiple, are found on external genitalia, in the vagina, or in the rectum
• Most infections clear spontaneously
• Specific types of HPV cause at least 90% of cervical cancers
• Vaccine is available against HPV 16 and 18 (responsible for causing ~80% of cervical cancers)

Gastroenteritis (Viral)

• Acute viral gastroenteritis is an inflammation of the stomach or intestines and is an important disease in infants and children
• Leading cause of childhood death in developing countries
• Probably spread by the fecal-oral route
• Caused by four major groups of viruses
• Rotavirus, adenovirus, and astrovirus cause ~5–10 million deaths/year worldwide
• Viral diarrhea is transmitted by the fecal-oral route
• Rotavirus live oral vaccines are used
• Norovirus causes ~23 million cases of acute gastroenteritis per year
• Usually self-limited disease, and symptomatic/supportive therapy is used

Zoonotic Diseases

• Human viral infections in animal reservoirs before transmission to and between humans
• RNA viruses, many are on the Select Agents list as potential bioweapons
• Examples include Ebola and Marburg viruses, hantaviruses, Lassa fever virus, and Nipah virus

Ebola and Marburg Hemorrhagic Fevers

• Viral hemorrhagic fever (VHF) is a severe multisystem syndrome caused by many distinct viruses
• Overall host vascular system is damaged, leading to vascular leaking (hemorrhage) and dysfunction (coagulopathy)
• Ebola hemorrhagic fever is a member of the single-stranded, negative sense RNA family called the Filoviridae
• Infection is severe and ~80% fatal
• No known carrier state; fruit bat may be reservoir
• Transmission occurs from direct contact with Ebola victim, body fluids, or clinical samples
• Internal hemorrhaging occurs
• Supportive therapy is used, and no treatment is available
• Experimental vaccines are being evaluated

Prion Diseases

• Transmissible spongiform encephalopathies are fatal neurogenerative disorders caused by prions
• Remains clinically silent for months or years, ending in profound disability or death
• Diseases include Kuru, Creutzfeldt-Jacob (CJD) disease, and variant CJD, bovine spongiform encephalopathy (BSE)
• Transmission occurs through medical procedures, genetic, or food-borne routes
• Prions (protein infectious particles) consist of abnormally folded proteins (PrPsc) that can induce normal forms of protein PrPc to abnormally fold
• BSE agent survives gastrointestinal tract passage and is neurotropic, both serving as a source of agent
• Dementia is the primary symptom
• Usually accompanied by motor dysfunction
• Symptoms appear after prolonged incubation and last from months to years prior to death
• Produce characteristic spongiform degeneration of brain and deposition of amyloid plaques
• Share many characteristics with Alzheimer's disease