MED­I­CAL FRONT

• How Cush­ing’s dis­ease af­fects im­mu­nity

EQUUS - - Contents - By Chris­tine Barakat and Mick McCluskey, BVSc, MACVSc

New re­search from Ok­la­homa State Univer­sity helps ex­plain why horses with the hor­monal dis­or­der pi­tu­itary pars intermedia dys­func­tion (PPID, also known as Cush­ing’s dis­ease) can be more sus­cep­ti­ble to in­fec­tion.

In a two-phase study, the re­searchers an­a­lyzed blood sam­ples from 23 horses with PPID and 39 who were healthy. They specif­i­cally in­ves­ti­gated the pro­cesses that im­mune cells called neu­trophils use to neu­tral­ize in­fec­tious or in­flam­ma­tory agents and help the body heal.

“Neu­trophils mi­grate to ar­eas of in­fec­tion or in­flam­ma­tion in a process called chemo­taxis. To exit the blood ves­sels, neu­trophils first stick onto the ves­sel walls, a process known as ad­he­sion. With­out ad­he­sion and chemo­taxis the neu­trophils can’t get to the site of in­fec­tion or in­flam­ma­tion,” ex­plains Dianne McFar­lane, DVM, PhD. “Once they ar­rive [at an area of in­flam­ma­tion] neu­trophils can en­gulf for­eign or necrotic ma­te­rial in­clud­ing bac­te­ria---a process called phago­cy­to­sis. They are able to kill bac­te­ria by re­leas­ing en­zymes and chem­i­cals, a process known as ox­ida­tive burst. To­gether, these four ac­tions of neu­trophils are nec­es­sary for a ro­bust im­mune sys­tem.”

McFar­lane and her team dis­cov­ered that horses with PPID had sig­nif­i­cantly re­duced ox­ida­tive burst ac­tiv­ity and ad­he­sion than did healthy horses. None of the study horses had ac­tive in­fec­tions, and the re­duc­tion in these func­tions was not cor­re­lated to sever­ity of PPID.

McFar­lane says this study un­der­scores the im­por­tance of man­ag­ing PPID horses with med­i­ca­tion to bal­ance their hor­mone con­cen­tra­tions and avoid sec­ondary in­fec­tions. “In older horses with in­fec­tions that are not re­spond­ing well to treat­ment, it is im­por­tant to de­ter­mine their PPID sta­tus,” she says. “Even when giv­ing the cor­rect an­tibi­otics, it can be very dif­fi­cult to cure in­fec­tions in horses with high con­cen­tra­tions of im­muno­sup­pres­sive hor­mones such as oc­curs in horses with PPID.”

Ref­er­ence: “Neu­trophil func­tion in healthy aged horses and horses with pi­tu­itary dys­func­tion,” Vet­eri­nary Im­munol­ogy and Im­munopathol­ogy, June 2015

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