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COLLEGE OF
AGRICULTURE
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Mare
Reproductive Loss Syndrome (Update)
By Haven Miller
LEXINGTON,
Ky., (June 25, 2003) – Last year several studies produced solid
evidence that the presence of Eastern Tent Caterpillars is strongly
associated with the foal loss problem known as Mare Reproductive Loss
Syndrome. But precisely how the caterpillars were causing mares
to lose their pregnancies was unknown.
Research
this spring has brought the equine industry another step closer to
solving the mystery.
“The
main focus of this year’s research has been to zero in on the caterpillar
and try to figure out what is the chemical or biological nature of
the agent that causes MRLS,” said Nancy Cox, associate dean for research
in the University of Kentucky College of Agriculture.
Results
from UK’s first-reported 2003 research trial, led by Bruce Webb and
Karen McDowell, indicate the causative agent may be associated with
the caterpillar’s exoskeleton, or outside cuticle. This trial
looked at what happens when different parts of the caterpillar are
fed to pregnant mares, and the only treatments that caused abortions
were the ones involving the outside cuticle.
Fetal
losses occurred in five of five mares fed eastern tent caterpillars,
and three of five mares fed exoskeleton (cuticle and related parts).
No losses occurred among mares fed internal parts of caterpillars.
“This
is certainly evidence that we’re moving in a forward direction to
narrow our search for the exact cause,” Cox said.
Another
recent experiment, led by Manu Sebastian and Bill Bernard, demonstrated
that irradiated Eastern Tent Caterpillars can induce fetal loss in
late-term pregnant mares. This suggests MRLS is caused by a
non-infectious agent in caterpillars (irradiation at sufficient levels
has been shown to kill infectious agents such as viruses and bacteria).
The study, conducted jointly by UK and Rood & Riddle Equine Hospital,
was the first experiment in late-term pregnancies which reproduced
a pathological finding similar to naturally-occurring MRLS.
“There’s
been tremendous cooperation by our equine industry in achieving these
results,” Cox said. “We appreciate the support we’ve received
from the Kentucky Association of Equine Practitioners, the Kentucky
Thoroughbred Association, and the Kentucky Thoroughbred Farm Managers
Club in helping design and conduct these experiments, as well as provide
funds.”
UK
researchers also made significant progress this year in terms of pasture
management recommendations and suggestions for caterpillar control.
“After
an intensive program of monitoring farms in 2002 and 2003, we were
able to reduce the importance of certain factors, such as cyanide
and mycotoxins, and at the same time gain a better understanding of
other risk factors such as tall fescue endophyte, which is still under
study,” said Jimmy Henning, assistant Extension director for agricultural
and natural resources in the UK College of Agriculture. “We
learned a lot about what was going on in horse pastures the last two
springs, and were able to pass that information along to farm managers.”
Studies
conducted by Dan Potter of UK’s entomology department in late winter
and spring determined that certain reduced-risk pesticides effectively
eliminated all sizes of ETC larvae in wild cherry trees. Cherry
trees are a preferred host tree for the insect. Micro-injection
of certain insecticides into the base of trees also gave good control
when applied at the right time. This work was supported by the
Grayson-Jockey Club Research Foundation.
Another
study showed that a pyrethroid insecticide containing permethrin provided
an effective “barrier” treatment along fence lines to control large
crawling tent caterpillars once they had left the trees and were wandering
prior to cocoon formation.
“Armed
with the results of UK experiments linking MRLS with exposure to Eastern
Tent Caterpillars and new information on caterpillar behavior and
control, farm managers developed aggressive management plans that
minimized exposure of pregnant mares in 2003,” said Lee Townsend,
UK Extension entomologist. “On top of that, mother nature provided
invaluable help with a caterpillar-specific disease that dramatically
reduced ETC numbers. It certainly proved to be a winning combination.”
Foal
loss numbers from all causes are down dramatically in 2003.
Recent reports from the University of Kentucky’s Livestock Disease
Diagnostic Center show equine fetal submissions (accessions) for January
through May, 2003 are only half the number for the same period in
2002. In May alone accessions dropped by 86 percent.
“Other
than a very few isolated foal losses that we consider at, or below,
normal levels we’ve basically seen nothing that even approaches the
problems of 2001 and 2002, and that’s very good news,” said Lenn Harrison,
LDDC director.
Meanwhile,
researchers remain vigilant. Cox said science is a steady process
that gradually eliminates various possibilities and continually narrows
the focus. The work doesn’t stop until the main question is
answered.
“Given
the discovery that the Eastern Tent Caterpillar is the causative agent
for MRLS, we would be irresponsible not to figure out everything we
can about the mechanisms of the disease,” she said. “Knowing
how the caterpillar affects pregnancy will help our ability to manage
pregnancy in mares in many other scenarios.”
Mare
Reproductive Loss Syndrome
Following is a
statement sent to State Veterinarians the morning of May 8, 2001
TO: State Veterinarians
DATE: May 8, 2001
SUBJECT: Late
term abortions and early fetal deaths among mares in Kentucky
Dr. Don Notter,
Kentucky State Veterinarian, has asked that I provide you the following
information. We would imagine that many of you have heard about the
situation currently occurring in Kentucky. In addition we will be
keeping you apprised of any new information as it becomes available.
After you have had the opportunity to review this report, feel free
to contact either Dr. Notter or myself should you have any additional
questions.
Two clinical problems
have been reported on a number of farms in Kentucky during the week
commencing April 30, 2001.
1. A significant
number of late term aborted fetuses have been submitted to the Livestock
Disease Diagnostic Center (LDDC) in Lexington. Symptoms in mares would
include agalactia, dystocia and stillbirths. A number of foals were
born weak and a few only survived four days. Placentas associated
with a number of these losses have been thickened and edematous, consistent
with red bag syndrome. Several breeds of horses have shown these signs.
Investigations
into all possible causes of these late-term abortions are ongoing.
2. The second
condition relates to an increased number of mares determined to be
in foal at ~40 days but subsequently on ultrasound examination (45-80
days) have undergone early fetal loss (EFL). Affected mares feel normal
on rectal palpation at 60-65 days gestation, but on ultrasound examination,
abnormal eccogenic fluid (cloudy and flocculent) is seen around the
fetus. This is followed by fetal death and expulsion. A number of
mares have exhibited a vaginal discharge and occasional fever. The
incidence of EFL on affected farms has varied considerably with cases
reported on farms in several counties of central Kentucky. The problem
again affects more than one breed of horse.
We have asked
our practicing veterinarians to consider ultrasounding mares at 60-65
days of gestation. If the mare aborts, any fetal tissue found and
a serum sample from the mare should be submitted to the diagnostic
laboratory. Fetal tissue should be kept chilled and delivered as soon
as possible.
We have been meeting
and gathering information in a collaborative effort with local equine
practitioners, faculty at the Livestock Disease Diagnostic Center
and the Maxwell H. Gluck Equine Research Center, farm managers, nutritionists,
and others. The investigations include studies to evaluate whether
these two problems share a common etiology. Discussions have included
the extent and distribution of the cases and what material, including
pasture samples needed to be examined in order to establish an etiologic
diagnosis.
At this time,
though nothing has been ruled out, it would appear that the source
of one or both of these events may be an environmental issue. We have
not identified any agent (viral or bacterial) which would suggest
this to be a contagious situation.
E.S. Rusty Ford
Equine Programs Manager
CR Editors Note
06/2001: The mystery seems half solved. The problem is cyanide poison.
not enough to affect the mare but enough to kill the fetus --- the
culprit-- young Black Cherry Trees in the grazing area. There is some
suspicion that the Eastern Tent Caterpillar may be involved however
the science has not confirmed that hypothesis.
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