Page 92 - May 2022

EQUINE HEALTH
In a study, it was found that erythromycin disrupts foals’ ability to sweat within 48 hours after starting treatment, essentially making the foals non-sweaters, and persists up to two weeks after stopping treatment.
anyway, because they don’t sweat as efficiently as older horses. Young foals, especially, may be adversely affected by hot weather because they don’t thermoregulate as well as older horses,” he says.
Amy Stieler Stewart, DVM, DACVIM, North Carolina State University, did a lot of research with anhidrosis during her residency at the University of Florida, looking at medication-induced anhidrosis, specifically the effects of macrolide antimicrobials (erythromycin, clarithromycin, azithromycin) that are often used to treat foals with R.
equi pneumonia. There had been reports
of adverse effects during treatment of foals
for R. equi which included overheating, and in severe cases, death of those foals. “Our group at the University of Florida found
that once foals began this medication, their ability to sweat normally and dissipate heat was severely diminished. This was a serious situation, since they are often treated in warm summer months and thus at severe risk for overheating,” she says.
“In our original study we found that erythromycin disrupts foals’ ability to sweat within 48 hours after starting treatment, essentially making the foals non-sweaters. Interestingly this effect persists after the
treatment has stopped; foals can remain at risk of overheating for up to two weeks after stopping the medication,” says Stewart.
“We also tested the more commonly
used macrolides—clarithromycin and azithromycin—and these also decreased normal sweat responses but not as severely
as erythromycin. Our studies showed that
all macrolides commonly used for treating
or preventing R. equi pneumonia in foals suppress normal sweat responses to varying degrees. Foals that are treated with these antimicrobials should be considered at risk for hyperthermia during and after treatment. It is recommended to keep treated foals in a cool environment and out of direct sunlight while being treated with a macrolide antibiotic (erythromycin, clarithromycin, azithromycin) and that these precautions be continued for at least 2 weeks after discontinuing treatment,” she says.
Horsemen should also keep in mind that the thermoregulatory center in newborns is not fully developed. They lack the ability
to stay warm in cold weather or cool in hot weather. “Their normal body temperature tends to run a degree or two warmer than an adult horse,” says Randall.
“Temperature of 102 degrees is not
something we’d worry about in a young foal, but if it gets up to 106 or 107 degrees for very long it affects their brain; some of them are a little dumb for the rest of their life. My family had a foal like that when I was a kid; he had
a bad case of Rhodococcus pneumonia with 106-degree temperature for a couple of days. His registered name was ‘106 In The Shade’ and he never was a very smart horse!”
OTHER SPECIES HAVE DIFFERENT COOLING METHODS
People sweat to cool off, though they don’t have as many sweat glands as horses. Some humans also develop anhidrosis and have a
hard time staying cool. “In our climate it would probably happen more if people weren’t in air- conditioned buildings all the time,” says Randall. Humans must sweat to stay cool, but horses are the most efficient sweaters as a species.
“Dogs and cats don’t sweat; dogs cool themselves by breathing faster and panting (air exchange in the lungs). Cows don’t sweat much (fewer sweat glands); they may get damp and have sweat on their nose, but mainly breathe faster to cool down,” he says.
There are various ways to cool the blood before it gets to the brain. “One of these mechanisms is called the rete mirabile, a Latin name meaning
Horsemen should also keep in mind that the thermoregulatory center in newborns is not fully developed. They lack the ability to stay warm in cold weather or cool in hot weather.
90 SPEEDHORSE May 2022