b'FARMCALL TheHorse.com/Farm-CallCan Vaccinated Horses Get Sick?Q This fall I saw reports about horsesrespond as expected to an individual getting infected with diseases likebooster vaccine. In this case you should Eastern equine encephalitis, evenhave your veterinarian administer the though they were up to date on theirpriming dose series (adult horse protocol) vaccinations. Sometimes the horses evenof two vaccines at a three- to four-week died from their infections. Why wouldinterval, followed by booster vaccination vaccinated horses still get sick?at the appropriate times. Without this Carol initial priming series, the vaccination via email likely wont induce a memory immune response, thereby leaving the horse sus-A When questioning vaccine efficacy,ceptible to pathogen challenge. ALEXANDRA BECKSTETT/THE HORSEconsider several factors: Incorrect diagnosis In some instances, Vaccine handling When follow- a horse might develop disease with clini-ing the American Association of Equinecal signs suggestive of a certain condi-Practitioners recommendations for vac- tion but, without confirmatory testing, cine handling, vaccines should maintainthe final diagnosis remains unknown. their ability to protect against pathogens.For example, say a horse properly vac-Specifically, they should be handledcinated against Eastern equine encepha-according to manufacturer recommen- Age, vaccine history, and concurrent health is- litis (EEE)/Western equine encephalitis dations. Storage and handling instruc- sues can affect immune response to vaccination. (WEE)/West Nile virus (WNV) develops tions might be product-specific. For thisacute neurologic disease believed to be a reason, vaccines must be stored at theof vaccination to ensure the vaccine canresult of EEE/WEE or WNV. Diagnosing recommended temperature with little toinduce a protective immune response.the horse based on clinical signs alone no light exposure and be mixed properlyLevel of pathogen challenge In a horsecould be misleading, as the horse could (via shaking) to ensure the suspension isthat was vaccinated previously but devel- have another disease, such as equine uniform prior to administration. A breakoped subsequent disease, one causativeprotozoal myeloencephalitis (EPM), that in the cold chain or any other componentfactor could be the level of pathogenis not a result of vaccine failure. of proper vaccine handling and storagechallenge. For instance, perhaps the dura- Vaccines arent perfect; theyre merely might impact vaccine efficacy, leading totion of time from vaccine administrationone component of a healthy horse pro-failure. to pathogen exposure was at or neargram. Collectively, it is important to have Immune function Factors that can affectthe length of protection provided by thehorses living on the same property well-host immune response might includevaccine. In this scenario, if the pathogenvaccinated to maintain herd immunity horses age, previous vaccine history,exposure was excessive, it could over- against relevant pathogens. Additional and whether he has concurrent issueswhelm immune responsiveness, resultingsteps to maintain optimal health include that could negatively affect immunein disease. This is a good example of whya strong biosecurity program to minimize activation. Possible causes of impairedit is important to maintain accurate medi- the chance of disease transmission from immunity that could negatively impactcal records for all vaccines; this helpsnew arrivals. This is particularly true for a horses vaccination response includeensure you have them administered athorses with unknown vaccine back-excessive stress (e.g., strenuous exercise,the proper intervals to maintain maximalgrounds; dont allow these horses to com-competition, long-distance transport) orhost immunity. mingle with resident horses until they immunosuppressive medications suchVaccination history It is also importanthave been screened and monitored for as high-dose or long-term corticosteroidto be informed about a horses previoushealth status and are vaccinated against therapy. Elderly horses with concurrentvaccine administration. For instance, ifthe same diseases as the resident horses.diseases such as pituitary pars intermediayou purchase a horse with an unknownElizabeth Davis, DVM, PhD, Dipl. ACVIMdysfunction (PPID) might also have re- vaccine history and assume he wasDepartment Head and Professor duced immune responses. Its importantvaccinated previously, when in actual- Equine Internal Medicinethat host immunity is optimal at the timeity he was not, then the horse might notKansas State UniversityThis column features readers questions on specific illnesses, diseases, and conditions, with answers from American Association of Equine Practitioners members or their designates.Send questions to [email protected], or The Horse, 2365 Harrodsburg Rd #A200, Lexington KY 40504-3331. Questions will be edited for publication and must includethe authors name, address, and daytime telephone number. Farm Call is compiled by Managing Editor Alexandra Beckstett.42December 2019The Horse|TheHorse.comFarmCall_Dec.indd 42 11/4/19 2:36 PM'