Equine Diseases for Which Vaccinations Exist – Risk-based Vaccines



The AAEP recognizes a second group of risk-based vaccinations that should be administered based on the risk your horse has of contracting a disease and the potential consequences. Horses that live in or travel to high-risk areas (including performance horses) are considered to get the most benefit from risk-based vaccines.


Potomac Horse Fever (PHF)

This is an acute bacterial infection caused by the microbe Neorickettsia risticii. Horses become infected by ingesting infected insects, when horses graze in pastures where these insects are nesting in the grasses. Horses that are grazing at night and are turned out 24/7 are at the highest risk. We also recommend to leave barn lights off at night so that you are not attracting more insects. PHF is seasonal. Most cases occur in July, August, and September, but there is a risk of occurrence from late spring to early fall in temperate areas. PHF only occurs in geographic areas where infected snails and carrier insects live. Once PHF is seen in an area, it usually means that other horses in the same geographic area will be at risk of PHF in future years.


Equine Influenza (EIV)

Equine Influenza (or flu) is a highly contagious acute viral infection of the respiratory tract, similar to human flu. It is most commonly transmitted directly from horse to horse via respiratory secretions (droplets from coughing or nasal discharge), but can also be transmitted indirectly via contaminated equipment and other objects, or by people via contaminated hands or clothing. Equine Influenza tends to spread rapidly through groups of horses in close contact. Most horses recover within a few weeks, although some may develop a more severe illness. Performance horses experience the greatest impact due to loss of condition and competition time. Symptoms can include:

  • fever,

  • behavioural changes,

  • depression,

  • muscle stiffness,

  • lack of appetite,

  • nasal discharge (initially serous, but may become mucoid), and

  • cough.

EIV is considered the most economically detrimental respiratory disease in horses. Therefore, many veterinarians recommend that all horses should be vaccinated against EIV, unless they live in a closed and/or isolated facility.


Equine HerpesVirus 1 and 4 (EHV)

Also called rhinopneumonitis or “rhino,” this is a viral infection that causes respiratory illness that can be easily confused with flu. EHV-1 and EHV-4 are spread by direct contact and through respiratory and nasal secretions. EHV-1 causes respiratory disease, abortion, neonatal death, and neurological disease. EHV-4 is associated mainly with respiratory disease. Symptoms can include:

  • fever,

  • nasal discharge ,

  • depression,

  • cough, and

  • inappetence.

EHV-1 can also cause: Outbreaks of abortion, the birth of weak foals, and Equine Myeloencephalopathy - an infection of the horse’s spinal cord and brain.


Strangles (Streptococcus equi)

Strangles is a highly contagious bacterial upper-respiratory infection caused by the bacterium Streptococcus equi. Foals, weanlings, and yearlings are at the highest risk for strangles. Strangles is spread by direct contact with infected horses, which may continue to shed the bacteria for weeks after they appear to recover. It is also spread through contact with any surface or object that may have been contaminated by an infected horse. Strict biosecurity and hygiene is important in an outbreak situation in order to avoid further transmission.


Botulism (Clostridium botulinum)

This fatal paralytic neurologic disease is caused when horses eat baleage, or ensiled hay (haylage), which contains the toxin from Clostridium botulinum. Feeding these types of wet hay can have benefits for asthmatic horses and horses that experience digestive issues on a dry hay ration. However, of all farm animals, horses are the most s

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