Treatment of placentitis includes combating the infection, decreasing the inflammatory response and supporting and maintaining
At the University of Florida, Macpherson and her colleagues have tested several treatment methodologies. Mares have been treated
with combinations of antimicrobials (penicillin, gentamicin, trimethoprim sulfamethoxazole), anti-inflammatory drugs (pentoxifylline,
flunixin meglumine) and altrenogest (synthetic progestin), designed to control the infection, delay premature delivery and
improve the foal's health.
In her studies, Macpherson looked at various combinations of medications. "Long-term administration of trimethoprim sulfamethoxazole
(TMS) and pentoxifylline (PTX) extended gestational length in mares with placentitis when compared with infected, untreated
mares," she says. "However, foal survival was not improved in treated animals (TMS and PTX were present in fetal and placental
tissues). When progestins (altrenogest) were added to the TMS and PTX treatment regimen, 10 of 12 mares (83 percent) delivered
A more recent study explored the use of TMS alone or combined with anti-inflammatory agents (dexamethasone, aspirin) and progestins
(altrenogest plus aspirin). In this study, Macpherson says, experimentally infected mares receiving only TMS were as likely
to deliver viable foals (63 percent) as mares receiving TMS in combination with dexamethasone, aspirin and altrenogest (72
percent). "A caveat to the administration of a TMS-based therapy for mares with placentitis is that foal viability is not
consistently improved with this therapy in clinical practice, as it is not consistently shown effective in vivo as it is in
vitro," she says.
"We have used TMS a lot in one of the projects done at the University of Florida, where we've had a very high live-foal rate.
About 83 percent of our foals survived after infection and treatment with TMS, PTX and altrenogest," Macpherson says. "TMS
is a drug that has some limitations, particularly for streptococcal organisms. The No. 1 cause of placentitis is Streptococcus equi, so in order to combat it, we need a better armament of drugs."
New drug holds promise
Macpherson recently evaluated the use of a long-acting ceftiofur, a third-generation cephalosporin,7 which has excellent bacterial activity against streptococcal organisms as well as many gram-negative aerobes and some anaerobes.
Ceftiofur sodium is a commonly used antimicrobial in equine practice, and its injectable formulation provides a practical
method of administration.
The newer drug—ceftiofur crystalline free acid—has a broad appeal for equine practitioners not only because it is a potent
antimicrobial against Streptococcus zooepidemicus but also because it is administered intramuscularly at four-day intervals and provides therapeutic drug concentrations in
horses for 10 days after the second dose. In addition, studies showed that it did not cross the placenta at all, which was
a big eye-opener.
"In many ways, this exciting new drug would appear to be the perfect antimicrobial treatment for mares with placentitis,"
Macpherson says. "But we have to continue to explore that drugs are passing the placenta and that we can use effectively."
Ed Kane, PhD, is a researcher and consultant in animal nutrition. He is an author and editor on nutrition, physiology and
veterinary medicine with a background in horses, pets and livestock. Kane is based in Seattle.
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