EQUINE | Equine Disease Update
warranted by a specific diagnosis.
Death due to disease or dysfunction of the cardiovas-
cular system may be somewhat unexpected. However,
after delving more deeply into necropsy records, a not-
so-surprising story emerged. Uterine artery rupture was
responsible for approximately half of the cases where
the main cause of death was attributed to the cardiovas-
cular system. This statistic appears to be associated with
the presence of numerous breeding farms in Central
Kentucky. Increased risk of uterine artery rupture is an
important issue for owners and veterinarians worldwide
to consider when breeding mares over fifteen years of
age. Additionally, the potential high risk for uterine ar-
tery rupture in older mares may highlight the option of
embryo transfers to surrogate mares to obtain offspring
from highly valued aged mares. As with any medical
procedure however, embryo transfer and surrogacy are
not without risk and may be restricted by certain breed
registries.
While the main causes of death identified in this pre-
liminary review for horses over the age of fifteen in Ken-
tucky certainly highlight potential problem areas for the
medical management of older horses, a more extensive
review of past cases is needed to fully establish signifi-
cant trends from this study.
Contact: Ashton B. Miller, Graduate Research Assistant,
in collaboration with Alan Loynachan, DVM, PhD and
Amanda Adams, PhD [email protected] (859) 257-
4757 Maxwell H. Gluck Equine Research Center Univer-
sity of Kentucky Lexington, KY
KENTUCKY
Parasite Control: An Update
Internal parasites of horses have been recognized for
centuries. Until the early 1900s, methods for the control
of equine endoparasites lacked a scientific basis. For
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example, in the 1600s one recommendation was to in-
cise the horse’s palate with the intent that the ingested
blood would kill any internal parasites. Beginning in the
1940s and extending to the 1980s, new classes of anti-
parasitic compounds were developed approximately
every 10 years. Currently in the USA, only benzimid-
azoles (fenbendazole and oxibendazole), tetrahydropy-
rimidines (pyrantel pamoate and pyrantel tartrate), and
macrocyclic lactones (ivermectin and moxidectin alone
or combined with praziquantel) are commercially avail-
able for parasite control in horses.
The major endoparasites of horses include bots, large
strongyles, small strongyles or cyathostomes, ascarids,
and tapeworms. Large strongyles (Strongylus spp.) are
one of the most significant equine parasites. The larval
stages can cause disease due to migration in blood ves-
sels and abdominal organs. Drug resistance is not evi-
dent in the case of large strongyles. Cyathostome larvae
do not migrate parenterally like Strongylus spp., but en-
cyst in the mucosa and submucosa of the large intestine
of the horse. Intestinal disease can be induced by cya-
thostomes when large numbers of larvae excyst from
the lining of the large intestine, a condition called “larval
cyathostomiasis.” Resistance to fenbendazole, oxibenda-
zole and pyrantel pamoate is now common among cya-
thostomes. Also, both ivermectin and moxidectin have
become less effective against immature (L4) cyathos-
tomes in the lumen of the large intestine; thus the life
cycle is shortened. Heavy infections with adult ascarids
(Parascaris spp.) can cause intestinal blockage and rup-
ture because of their bulk. These too have become resis-
tant to ivermectin, moxidectin, and pyrantel pamoate.
The final group of equine endoparasites, tapeworms
(Anoplocephala spp.), can also result in intestinal hyper-
trophy, blockage, intussusception, and rupture; they do
not exhibit drug resistance.
Parasite treatment schedules have been based on the
life cycle of the parasites since the early 1900s. In the
• Equine Health Update •