EQUUS - - Medical Front -

A new study sug­gests that spe­cial care may be re­quired when plan­ning the vac­ci­na­tion sched­ule of horses with pi­tu­itary pars in­ter­me­dia dys­func­tion (PPID, also known as Cush­ing’s syn­drome).

A dys­func­tion of the pi­tu­itary gland, PPID is com­mon in horses over the age of 20. “Horses with PPID are more likely to be pre­dis­posed to life-threat­en­ing com­pli­ca­tions such as lamini­tis0 and sec­ondary in­fec­tions,” says Amanda Adams, PhD, as­sis­tant pro­fes­sor at the Gluck Equine Re­search Cen­ter at the Uni­ver­sity of Ken­tucky. “Changes in im­mune func­tion may con­trib­ute to this in­creased sus­cep­ti­bil­ity in PPID horses.”

To de­ter­mine how im­paired im­mune func­tion in PPID horses may af­fect vac­ci­na­tion re­sponse, Gluck re­searchers tested the re­sponse of 33 horses to a com­bi­na­tion vac­cine that protects against in­fluenza0, her­pesvirus0 (EHV) types 1 and 4, West0 Nile virus (WNV), Eastern and West­ern equine en­cephalomye­li­tis0 and tetanus.

The study horses were first tested for PPID us­ing a dex­am­etha­sone0 sup­pres­sion test and a thy­rotropin-re­leas­ing 0 hor­mone stim­u­la­tion test. They were then di­vided into one of the fol­low­ing treat­ment groups: Those with­out PPID given the vac­cine; those that were pos­i­tive for PPID on one test (sin­gle­pos­i­tive) and given the vac­cine; those that were pos­i­tive on both tests (dou­ble pos­i­tive) for PPID and given the vac­cine. Fi­nally, horses that were PPID neg­a­tive or tested pos­i­tive on one or both tests were not vac­ci­nated to serve as con­trols.

Us­ing two di­ag­nos­tic tests gave the re­searchers some in­di­ca­tion of how ad­vanced a par­tic­u­lar horse’s PPID might be, says Adams. “Based on pre­vi­ous re­search, those horses that test pos­i­tive us­ing the dex­am­etha­sone sup­pres­sion test may have later stage dis­ease. Sin­gle­pos­i­tive PPID horses in this study had only sig­nif­i­cantly el­e­vated basal ACTH lev­els and el­e­vated ACTH lev­els fol­low­ing the TRH stim­u­la­tion test. So it is likely that the dou­ble-pos­i­tive PPID horses are in a later stage of dis­ease than sin­gle-pos­i­tive PPID horses.”

Re­searchers drew blood from each study horse prior to the ini­tial vac­ci­na­tion and again two and four weeks later. A vac­cine booster was ad­min­is­tered four weeks into the study and more blood sam­ples were col­lected on weeks six and eight in or­der to mea­sure im­mune re­sponses to the vac­cine.

Blood anal­y­sis re­vealed all the horses had higher titers (in­creased an­ti­bod­ies) against the tar­get dis­eases shortly af­ter vac­ci­na­tion. How­ever, in dou­ble-pos­i­tive PPID horses, an­ti­body titer lev­els to the West Nile virus com­po­nent of the vac­cine dropped off more quickly than in the sin­gle­pos­i­tive horses and those with­out PPID.

“Un­for­tu­nately, we do not have cer­tain ‘cut-off’ titers to as­sume a horse is pro­tected or not pro­tected if faced with West Nile virus,” says Adams. “How­ever, th­ese lev­els sig­nif­i­cantly dropped so that they were not dif­fer­ent com­pared to pre-vac­ci­na­tion titers, which is not a good thing.” The study data showed a sim­i­lar pat­tern with EHV-1 titers, with dou­ble-pos­i­tive horses hav­ing less of a titer re­sponse to vac­ci­na­tion than sin­gle-pos­i­tive horses and horses with­out PPID.

Adams says this means it is pos­si­ble that horses with PPID may need to be vac­ci­nated on a more fre­quent sched­ule than other horses. “We are in the process of fur­ther in­ves­ti­gat­ing this ques­tion. It is im­por­tant to make sure that geri­atric horses are main­tained on a regular vac­ci­na­tion pro­gram, in­stead of just forgotten about,” she says. “Make sure they are al­ways up-to-date on core vac­cines. For horses at higher risk, such as older show jumpers or 4-H horses who travel and co-min­gle, your vet­eri­nar­ian may want to con­sider a booster ev­ery six months for risk-based vac­cines such as EIV/EHV and WNV.”

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