VMPM 7437: Infectious Diseases and Preventive Medicine of Large Animals - Lecture Notes PDF

VMPM 7437 INFECTIOUS DISEASES and PREVENTIVE MEDICINE of LARGE ANIMALS Lecture Notes – Equine Amanda J. Kreuder, D.V.M., Ph.D., DACVIM (LAIM) R. W. Griffith, D.V.M., M.S., Ph.D., DACVM Copyright 2025 Iowa State University,...

VMPM 7437 INFECTIOUS DISEASES and PREVENTIVE MEDICINE of LARGE ANIMALS Lecture Notes – Equine Amanda J. Kreuder, D.V.M., Ph.D., DACVIM (LAIM) R. W. Griffith, D.V.M., M.S., Ph.D., DACVM Copyright 2025 Iowa State University, Ames, Iowa  EQUINE NOTES EQUINE VACCINATIONS The following information concerning immunization was taken directly from the American Association of Equine Practitioners Vaccination Guidelines (updated 2023). AAEP “Core” Immunization Guidelines The AVMA defines core immunizations as those “that protect from diseases that are endemic to a region, those with potential public health significance, required by law, virulent/highly infectious, and/or those posing a risk of severe disease. Core vaccines have clearly demonstrated efficacy and safety, and thus exhibit a high enough level of patient benefit and low enough level of risk to justify their use in the majority of patients.” The following equine vaccines meet these criteria and are identified as ‘core’ in these guidelines: Tetanus Eastern/Western Equine Encephalomyelitis West Nile Virus Rabies AAEP “Risk-based” Immunization Guidelines These are vaccinations included in a vaccination program after the performance of a risk-benefit analysis. The use of risk-based vaccinations may vary regionally, from population to population within an area, or between individual horses within a given population. Disease risk may not be readily identified by laypersons; it is important to consult a veterinarian when developing a vaccination program. Anthrax Botulism Equine Herpesvirus (Rhinopneumonitis) Equine Viral Arteritis Equine Influenza Leptosporosis Potomac Horse Fever Rotavirus Strangles Venezuelan Equine Encephalomyelitis  EQUINE RESPIRATORY DISEASES In newborn foals, infectious respiratory disease is most commonly associated with either aspiration of contaminated amniotic fluid (particularly in mares with placentitis or in cases of meconium aspiration) or via hematogenous spread from sepsis. The most common bacterial pathogens associated with pulmonary disease in foals is thus identical to those that cause systemic sepsis (E. coli, Klebsiella, Pasteurella, Actinobacillus, and Streptococcus spp.) Older foals are specifically susceptible to Rhodococcus equi infections. Several viruses are also associated with respiratory disease in young foals, including EHV-1 and EHV-4, influenza, EVAV and adenovirus (primarily SCID foals). In adults and older foals, viral infections including influenza, EHV-1/EHV-4, ERAV/ERBV and EVAV are common causes of contagious primary respiratory disease that often lead to secondary bacterial infections; equine adenovirus can cause respiratory disease in healthy foals but it is mostly associated with disease in SCID foals. EHV-5 is an emerging pathogen that has been linked to equine multinodular pulmonary fibrosis (EMPF). Bronchopneumonia in adult horses typically comes from aspiration of normal upper respiratory flora, with Streptococcus equi ss. zooepidemicus being the most commonly isolated organism, and gram-negative organisms such as Pasteurella and Actinobacillus sp. also common. Respiratory infections in horses frequently extend into the pleural space, resulting in pleuropneumonia. In addition to the common bronchopneumonia pathogens, anaerobic bacteria such as Bacteroides, Peptostreptococcus, and Fusobacterium are isolated in 1/3 of cases and are associated with a less favorable prognosis. Fungal lower respiratory infections are uncommon in horses, but are possible, and includes susceptibility to many of the multi-species systemic fungal diseases (Cryptococcus neoformans, Coccidioides immitis, Blastomyces dermatitidis, Histoplasma capsulatum, Aspergillis spp. and Candida spp.) Pneumocystis jiroveci (previously P. carinii) is associated with respiratory disease in SCID foals. Lungworms (Dictyocaulus arnfieldi) can also cause parasitic pneumonia in horses, but the incidence has declined significantly in the past 30 years with the introduction and frequent use of macrocyclic lactones for intestinal parasitism. In addition to the above, Streptococcus equi ss. equi is responsible for strangles, a highly contagious disease, which primary affects the upper respiratory tract and regional lymph nodes. Guttoral pouch infections are also common and can involve both bacterial and fungal infectious causes. EQUINE RHINOPNEUMONITIS and EMPF Etiology Equine herpesviruses 1 and 4 (EHV-1/EHV-4) are the most common herpesvirus involved in respiratory disease in horses. EHV-1 is also an important cause of equine abortion and and will be covered under the reproductive diseases. Occasionally, EHV-4 is involved in equine abortions. EHV-1 occasionally causes severe, fatal disease in neonatal foals and may result from in-utero infection. EHV-1 also causes myeloencephalitis in older horses and this may be associated with respiratory infections and will be covered under the neurologic diseases. EHV-5 has been associated with equine multinodular pulmonary fibrosis (EMPF). Epidemiology EHV-1 and EHV-4: Ubiquitous in horse populations and are transmitted between horses by inhalation (aerosol) and fomites. Young weanling foals often become infected in the fall months. Most animals are infected before 1 yr. of age. Epizootics of the disease will occur on larger horse farms where the  numbers of susceptible foals are high. Some animals may develop disease when they begin training, or when they are stressed in some other fashion. One report indicates that up to a third of the respiratory disease at racetracks is caused by this virus. Because this agent is a herpesvirus, latent carriers may shed it after being stressed. Infection has been shown to periodically recrudesce in some horses leading to sporadic increases in antibody titers. EHV-5 (and possibly EHV-2): Ubiquitous in horse populations but only a small percentage of horses will develop EMPF which is a disease of middle age to older horses. The pathogenesis is not well understood, but host specific factors likely play a role in recrudescence and disease development. Clinical signs EHV-1 and EHV-4: The clinical signs are usually mild and disease may be subclinical; disease is typically more severe with EHV-1 infection than EHV-4 and in younger animals rather than older. Acute disease is characterized by a mild fever, serous nasal discharge, and depression. There may be a cough present. Secondary bacterial infections may develop that are of primary concern. Occasionally, foals are infected at birth, particularly with EHV-1, and may develop severe respiratory disease, diarrhea, and die. These foals present with leukopenia, depletion of myeloid stem cells on bone marrow examination, dilated retinal vessels, and red discolored optic discs. EHV-5 (and possibly EHV-2): Chronic progressive signs of respiratory disease including tachypnea, increased respiratory effort, dyspnea, intermittent fever, cough, exercise intolerance and weight loss. Failure to respond to bronchodilators and/or antimicrobials is a clue that this might be present. Also reports of multisystemic disease in some animals. Diagnosis EHV-1 and EHV-4: PCR on nasal swabs/washes (=shedding virus), blood (=viremic) and tissues (respiratory disease panels) Virus isolation: Nasal swabs (include some unaffected horses) and fetal tissues from abortions FA on tissues Paired sera EHV-5 (and possibly EHV-2): Ultrasound and radiographic lesions with a nodular interstitial pattern PCR from BAL or lung biopsy Treatment EHV-1 and EHV-4: Respiratory disease is typically self-limiting, NSAIDS as needed for fever/malaise; monitor for secondary bacterial infections and treat with antibiotics if warranted for secondary infections. Antivirals are not commonly used for treatment of respiratory disease. EHV-5 (and possibly EHV-2):  Generally poor response to treatment due to pulmonary fibrosis already present at the time of diagnosis; treatment includes corticosteroids, antivirals (valacyclovir/ acyclovir), and immunomodulatory antibiotics (doxycycline). Prevention and control EHV-1 and EHV-4: Prevent introduction of new virus strains using quarantine or separate housing for horses that go to shows, races, etc. Not a lot of variation in this herpesvirus. Spread of virus is through aerosol and fomite transfer. When respiratory cases are identified of EHV-1, it is important to minimize spread to due potential for neurologic disease development in a subset of horses. While not considered a core vaccination, vaccination for EHV1/4 is very common (i.e.- “rhino/flu” vaccines) particularly in young animals and in any equine setting where animals are frequently mixed. Currently used vaccines apparently do not give long-lived immunity nor completely protective immunity. Foals can be immunized starting at 4-6 months with repeat vaccination at 6- 8 and 10-12 months with booster doses every 6 months thereafter. Both inactivated and MLV vaccines are available. EHV-5 (and possibly EHV-2): Unknown at this time. EQUINE RHINITIS VIRUSES Etiology Equine rhinitis A virus (ERAV) is an Aphthovirus (related to Foot and Mouth Disease) and Equine rhinitis B virus (ERBV, divided into 3 subtypes) is an Erbovirus, both of the family Picornaviridae. Both are apparently widespread in equine populations and both are capable of causing acute respiratory disease that can be mistaken for EHV1 infections. Most infections with either virus are mild or subclinical. Epidemiology ERAV infection results in a viremia and may result in long-term fecal and urinary shedding of the virus. It is transmitted via respiratory secretions. ERBV is thought to be transmitted by a similar route. Clinical signs ERAV infection causes systemic disease in horses characterized by fever, anorexia, nasal discharge, coughing, pharyngitis and swelling of the lymph nodes in the head and neck. Some studies have demonstrated the presence of neutralizing antibody in over 50% of horses over 1 year of age indicating that most infections are either mild or subclinical. ERBV is also usually mild and produces pharyngitis, respiratory signs and depressed appetite. Clinical disease caused by both viruses is usually limited to 2-3 days but they can predispose to secondary infections. In one study a significant percentage of horses with influenza virus infections were also positive for rhinitis virus. Diagnosis Infection with both viruses can be detected with a PCR test on nasal (included on respiratory panels) and conjunctival swabs, transtracheal washes and urine. The long shedding period may complicate diagnosis. Prevention and control Prevent introduction of new virus strains using quarantine or separate housing for horses that go to shows, races, etc. A conditionally licensed vaccine is currently available in the US for ERAV only. EQUINE INFLUENZA Etiology The most frequently diagnosed cause of viral respiratory disease in the horse. Equine influenza viruses are primarily from the A subgroup of influenza viruses which have the broadest host range (horses, humans, birds, pigs, cats, dogs). Despite this, horses are typically thought of as dead-end hosts, and only 2 primary subtypes have been described. Orthomyxovirus A/Equi 1 (H7N7) was first isolated in 1956 and until recently had not been isolated anywhere since 1980. A/Equi 1 was subsequently isolated from diseased horses in Wisconsin. Antibody specific for A/Equi 1 is present in the horse population, indicating that the virus may still be circulating. A/Equi 2 (H3N8) was first recognized in 1963 as a cause of widespread epizootics and is currently responsible for equine influenza. A/Equi 2 is generally thought to be much more virulent. Genetic rearrangements typical of influenza viruses do not seem to occur as frequently with this virus, however, there has been considerable antigenic drift and multiple A/Equi 2 (H3N8) virus isolates are included in the vaccine. Interestingly, A/Equi 2 has evolved into two genetically and antigenically distinct lineages (Eurasian and American) and the American lineage has further diverged into distinct South American, Kentucky, and Florida (clade 1 and clade 2) lineages. Convalescent sera had minimal cross-reactivity between the lineages. Epidemiology The virus is endemic in the horse populations in North America and Europe. The epidemiology is similar to influenza viruses of other animals. The virus spreads in an explosive fashion through a group of susceptible horses. Transmission is via the respiratory route (fomite, droplet and aerosol transmission are possible). Latently infected carrier animals may be important in maintenance of the virus in the horse population however, the continuous cycling of the virus through susceptible animals is probably the main factor. Rapid transportation of horses by air travel has presented some problems because the incubation period exceeds the length of time in transport. Where good quarantine procedures were not in place such as in outbreaks that occurred in Australia and Japan in 2007, the virus has escaped into the general equine population. The 2007 Australian outbreak was particularly difficult because the equine population had not previously been exposed to the virus and most of the horses were highly susceptible. Epidemics also occurred in South Africa in 1986 and 2003 and in Hong Kong in 1992. Foals