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Equine Vaccinations

• The information was directly influenced by the American Association of Equine Practitioners Vaccination Guidelines (updated 2023).

AAEP "Core" Immunization Guidelines

• The AVMA describes core immunizations, they protect from diseases that are endemic to a region, or of public health significance, are required by law, are virulent/highly infectious, or pose a risk of severe disease.
• Core vaccines have demonstrated efficacy and safety, exhibiting high patient benefit and low risk, justifying their use in most patients.
• Equine core vaccines:
• Tetanus
• Eastern/Western Equine Encephalomyelitis
• West Nile Virus
• Rabies

AAEP "Risk-based" Immunization Guidelines

• Risk-based vaccinations are included in a program after risk-benefit analysis.
• Use of risk-based vaccinations may vary by region, population, area, or individual horse.
• Consult a veterinarian when developing a vaccination program, as disease risk is not easily identifiable.
• Examples of risk-based vaccines:
• Anthrax
• Botulism
• Equine Herpesvirus (Rhinopneumonitis)
• Equine Viral Arteritis
• Equine Influenza
• Leptospirosis
• Potomac Horse Fever
• Rotavirus
• Strangles
• Venezuelan Equine Encephalomyelitis

Equine Respiratory Diseases

• Infectious respiratory disease in newborn foals is commonly from aspiration of contaminated amniotic fluid (especially with placentitis or meconium aspiration) or hematogenous spread from sepsis.
• Common bacterial pathogens in foals with pulmonary disease are similar to those causing systemic sepsis (E. coli, Klebsiella, Pasteurella, Actinobacillus, Streptococcus spp.).
• Older foals are susceptible to Rhodococcus equi infections.
• Viruses linked to respiratory disease in young foals include EHV-1 and EHV-4, influenza, EVAV, and adenovirus (mainly SCID foals).
• In adults and older foals, viral infections like influenza, EHV-1/EHV-4, ERAV/ERBV, and EVAV cause contagious primary respiratory disease, often leading to secondary bacterial infections.
• Equine adenovirus can cause respiratory issues in healthy foals but is mainly linked to disease in SCID foals.
• EHV-5 is an emerging pathogen linked to equine multinodular pulmonary fibrosis (EMPF).
• Bronchopneumonia in adult horses usually results from aspiration of normal upper respiratory flora, with Streptococcus equi ss. zooepidemicus being the most isolated organism.
• Gram-negative organisms like Pasteurella and Actinobacillus sp. are also typical.
• Respiratory infections in horses often extend into the pleural space, causing pleuropneumonia.
• Anaerobic bacteria like Bacteroides, Peptostreptococcus, and Fusobacterium are isolated in 1/3 of cases alongside typical bronchopneumonia pathogens and are linked to a less positive outlook.
• Fungal lower respiratory infections are rare but possible in horses, with susceptibility to many multi-species systemic fungal diseases (Cryptococcus neoformans, Coccidioides immitis, Blastomyces dermatitidis, Histoplasma capsulatum, Aspergillis spp., and Candida spp.).
• Pneumocystis jiroveci is linked to respiratory disease in foals with SCID.
• Lungworms can cause parasitic pneumonia in horses, but incidence has decreased in the past 30 years with the use of macrocyclic lactones for intestinal parasitism.
• Streptococcus equi ss. equi causes strangles, affecting the upper respiratory tract and regional lymph nodes.
• Guttoral pouch infections are common and can involve both bacterial and fungal infectious causes.

Equine Rhinopneumonitis and EMPF

• Equine herpesviruses 1 and 4 (EHV-1/EHV-4) are the most common herpesviruses linked to respiratory disease in horses.
• EHV-1 causes equine abortion (covered as reproductive disease).
• EHV-4 is occasionally linked to equine abortions.
• Occasionally, EHV-1 causes severe, fatal disease in neonatal foals and may result from in-utero infection.
• EHV-1 also causes myeloencephalitis in older horses and may be associated with respiratory infections (covered as neurologic diseases).
• EHV-5 has been associated with equine multinodular pulmonary fibrosis (EMPF).

EHV-1 and EHV-4 Epidemiology

• Ubiquitous in horse populations and are transmitted between horses by inhalation (aerosol) and fomites
• Young weanling foals often become infected in the fall months, and most animals are infected before 1 year old.
• Epizootics will occur on larger horse farms where susceptible foal numbers are high. Certain animals may develop disease when they begin training or are stressed.
• Up to a third of respiratory disease at racetracks is caused by this virus (EHV-1 or EHV-4).
• Latent carriers may shed it after stress, leading to increased antibody titers in some horses.

EHV-5 (and possibly EHV-2) Epidemiology

• Ubiquitous in horse populations, however only a small percentage develop EMPF (a disease of older horses).
• The pathogenesis is not well understood, but host specific factors likely play a role in recrudescence and disease development.

EHV-1 and EHV-4: Clinical Signs

• Usually mild/subclinical; EHV-1 infections are typically more severe than EHV-4, and more severe in younger animals than older.
• Acute disease: mild fever, serous nasal discharge, depression; may have cough. Secondary bacterial infections are a primary concern
• Occasionally, foals infected at birth (especially with EHV-1) 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): Clinical Signs

• Chronic progressive respiratory signs such as tachypnea, increased respiratory effort, dyspnea, intermittent fever, cough, exercise intolerance and weight loss.
• Failure to respond to bronchodilators and/or antimicrobials is a clue.
• Also reports of multisystemic disease in some animals.

EHV-1 and EHV-4: Diagnosis

• 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): Diagnosis

• Ultrasound and radiographic lesions with a nodular interstitial pattern.
• PCR from BAL or lung biopsy.

EHV-1 and EHV-4: Treatment

• Respiratory disease is typically self-limiting.
• NSAIDs as needed for fever/malaise and monitor for secondary bacterial infections and treat with antibiotics if warranted and antivirals are not commonly utilized.

EHV-5 (and possibly EHV-2): Treatment

• Response is generally poor due to pulmonary fibrosis.
• Treatment: corticosteroids, antivirals (valacyclovir/acyclovir), and immunomodulatory antibiotics (doxycycline).

EHV-1 and EHV-4: Prevention and Control

• Prevent new virus strains with quarantine or separate housing for horses at shows, races, etc. Not a lot of variation in this herpesvirus.
• Virus spreads through aerosol and fomite.
• When respiratory cases of EHV-1 are identified minimizing spread is important due to potential neurologic disease development in a subset of horses.
• Vaccination for EHV1/4 is very common (i.e., “rhino/flu" vaccines), particularly in young animals and any setting where animals are mixed.
• Currently used vaccines apparently don't give long-lived nor completely protective immunity.
• Foals can be immunized starting at 4-6 months, repeat vaccination at 6-8 and 10-12 months with booster doses every 6 months. Both inactivated and MLV vaccines are available.

EHV-5 (and possibly EHV-2): Prevention and Control

• Unknown at this time.

Equine Rhinitis Viruses

Etiology

• Equine rhinitis A virus (ERAV) is an Aphthovirus (related to Foot and Mouth Disease). Equine rhinitis B virus (ERBV, divided into 3 subtypes) is an Erbovirus. Both are of the family Picornaviridae.
• Both are widespread in equine populations and can cause acute respiratory disease that can be mistaken for EHV1 infections.
• Most infections are mild or subclinical.

Epidemiology

• ERAV infection results in viremia and can cause long-term shedding of the virus in feces and urine.
• It's transmitted via secretions from the respiratory system.
• ERBV is thought to be transmitted by a similar route.

Clinical Signs

• ERAV infection causes systemic disease characterized by fever, anorexia, nasal discharge, coughing, pharyngitis, and swelling of lymph nodes in head and neck. Neutralizing antibody presence in over 50% of horses over 1 year old indicates most infections are mild/subclinical. ERBV also usually mild and causes pharyngitis, respiratory signs, and depressed appetite.
• Clinical disease caused by both viruses usually limited to 2-3 days, but can predispose to secondary infections. In one study, a significant percentage of horses with influenza virus infections were also PCR-positive for rhinitis virus.

Diagnosis

• Infection can be detected with a PCR test on nasal (included on respiratory panels) and conjunctival swabs, transtracheal washes and urine.
• a prolonged shedding period may complicate diagnosis

Prevention and control

• Prevent introduction of new virus strains using quarantine or separate housing at shows, races, etc.
• A conditionally licensed vaccine is available for ERAV in the US.

Equine Influenza

• The most frequently diagnosed cause of viral respiratory disease in the horse.

Etiology

• Equine influenza viruses are from the A subgroup of influenza viruses with the broadest host range, though horses are usually dead-end hosts.
  • There are two primary subtypes:
  • Orthomyxovirus A/Equi 1 (H7N7): First isolated in 1956; has been isolated from diseased horses in Wisconsin since 1980. Antibody specific for A/Equi 1 in the horse population indicates circulation.
  • A/Equi 2 (H3N8): First recognized in 1963 as a cause of widespread epizootics and is currently responsible for equine influenza and generally thought to be much more virulent.
• Genetic rearrangements typical of influenza viruses do not seem as frequent with this virus. There has been considerable antigenic drift, and multiple A/Equi 2 (H3N8) virus strain isolates are included in the vaccine.
• A/Equi 2 has evolved into two lineages (Eurasian and American) with the American lineage diverging into distinct clades (South American, Kentucky, and Florida). Convalescent sera show minimal cross-reactivity between lineages.

Epidemiology

• Virus is endemic in horse populations in North America and Europe. Epidemiology is similar to influenza viruses in other animals and spreads explosively through susceptible horses. Transmission: respiratory route (fomite, droplet and aerosol possible)
• Latently infected carrier animals may be maintence in the horse population.
• Rapid transportation of horses by air travel has presented some issues. The virus has escaped into the equine population where good quarantine procedures were lacking (e.g., outbreaks in Australia and Japan in 2007).
• The 2007 Australian outbreak was difficult because the equine population had not previously been exposed.
• Foals <6 months of age are least susceptible due to maternal antibody protection. Most outbreaks are associated with large congregations of horses at shows, racetracks and sale barns.

Clinical signs

Virtually every young horse and many older will become acutely ill on a farm where the disease appears. Incubation time: short (2 days).

• Typically there are: Pyrexia (103 to 107.5 F) Anorexia Depression
• Dry, harsh, nonproductive cough (appears early/persists weeks) Conjunctivitis (cloudy corneas) Nasal discharge Marked lymph node swelling
• Recovery occurs after 2-3 weeks Virus infection itself is rarely fatal (exceptions: naïve neonatal foals) but secondary bacterial infection may lead to pneumonia and death.

Diagnosis Often based on clinical signs once diagnosed

• tests:
• PCR on nasal swabs/washes (shedding virus)
• virus isolation
• Rapid ELISA
• test kits, available/good specificity but moderate sensitivity (70%). Paired sera indicates RECENT infection

Treatment

Respiratory disease is usually self-limiting

• NSAIDs as needed for fever/malaise
• Monitor for secondary bacterial infections and treat with antibiotics if warranted
• Antivirals not commonly used

Prevention and control

Quarentine animals when they return from shows, farms, racetracks. EIV may be reportable in some states Vaccination is very common- (“rhino/flu” vaccines)

• several vaccine types exist Killed adjuvanted product- limited immunity duration which may result in vaccine failures so administer three inital doases and annual/semi annual revaccidention. Intranasal modified live vaccine appears to give good results.
• Give first dose at 11 months
• Booster recommended every 6 months Not labeled for use in pregnant animals but indicated if the face of a non immunized outbreak exists

Note

• Horses that are undergoing strenuous training do respond well to influenza vaccination.