MRLS in Florida

Editor's Note: By clicking on this link, you will find an image of a cluster of Eastern tent caterpillars on their tent provided by Terry Fitzgerald, PhD, the "tent caterpillar guru" based at The State University of New York who helped researchers during the 2001-2002 outbreak. This image, which might take a few seconds to open, will help you identify the Eastern tent caterpillar.

One confirmed case and two suspect cases of mare reproductive loss syndrome (MRLS) have been diagnosed in Alachua County, Florida, according to Dana Zimmel, DVM, Dipl. ACVIM, Dipl. ABVP, of the University of Florida’s College of Veterinary Medicine. One mare produced a septic foal on March 18, 2006 that was euthanatized after two days of intensive treatment. University of Florida pathologist John Roberts, DVM, Dipl. ACVP, who worked at the University of Kentucky Livestock Disease Diagnostic Center during the MRLS outbreaks in Kentucky in 2001-2002, concluded that the pathology on that foal was consistent with MRLS.

Eastern tent caterpillars


Samples from an earlier abortion on the same farm were re-examined and showed indications of MRLS; Roberts has termed that case suspect. The third case was a foal born on March 26, 2006, that was treated in the neonatal intensive care unit at the University of Florida for 12 hours before being euthanatized. Although complete histopathologic evaluation will not be completed until tomorrow (March 30), Roberts said the pathology was consistent with MRLS.

In the Kentucky outbreaks of 2001-2002, there was a strong association between the presence of unusually large numbers of Eastern tent caterpillars (ETC) and MRLS. Pregnant mares experimentally fed caterpillars typically aborted within several days.  Eastern tent caterpillars were found on the Alachua County farms and have been collected for future study.

Research has been ongoing since the Kentucky outbreak began in 2001. The most recent information on the state of this research was published by The Horse in December 2005. In that article, Bruce Webb, PhD, a University of Kentucky entomologist who has been studying MRLS, said, "We can prevent mare reproductive loss syndrome as we experienced in 2001 and 2002 by keeping horses away from caterpillars."

A tent full of Eastern tent caterpillars.

Kentucky 2001-2002
Mare reproductive loss syndrome (MRLS)--which historically can cause late-term abortions, early-term fetal losses, pericarditis (heart problems), unilateral endophthalmitis (problem in only one eye), hyphema (blood in the eye) in foals, mouth ulcers, and laminitis--was first seen in Kentucky and surrounding states (and as far north as Canada) in 2001, with a reduced incidence in 2002.

MRLS was first recognized as an outbreak of fetal deaths, foals born weak, and late-term abortions in Kentucky on Derby weekend (the first weekend in May) of 2001, but the outbreak was traced back to a start on April 23 of that year. All breeds were affected.

 Because of the high number of pregnant Thoroughbred mares in the area and the records kept on those horses, statistics on that breed reflect what was going on in the general horse industry. In 2001, there were 516 late-term abortions and 2,998 early fetal losses in Kentucky's Thoroughbred industry alone. There also were about 60 cases of pericarditis and 50 cases of unilateral uveitis reported in Central Kentucky horses. 

More than 30% of the anticipated 2002 Thoroughbred foal crop in Kentucky was lost due MRLS. The economic cost to the state from losses suffered by all horse breeds was estimated at nearly $336 million, according to a study commissioned by Kentucky Governor Paul Patton and conducted by the University of Louisville's Department of Equine Business. 

The ETC Link
The cause was never absolutely defined, but most experimental models showed a strong link of MRLS to the Eastern tent caterpillar. There was a huge hatch of caterpillars in 2001, with a large, but reduced number, that hatched in 2002. 

Terry Fitzgerald, PhD, the "tent caterpillar guru" based at The State University of New York who helped Kentucky researchers during the 2001-2002 outbreak, said on March 28, 2006, that where there are apple and cherry trees, there will be Eastern tent caterpillars, although mid-Florida marks the far southern extreme of the ETC's distribution.

"They are common in North Florida and Georgia," he said. It's been reported that the caterpillars have a 10-year cycle of high numbers, then nearly disappear for a time before building back up to high numbers. However, Fitzgerald said there is really no basis for that report. "They have an irregular cycle," he stated. "In one area it was 30 years between, or it could be three or four years. The large number seen in Kentucky (in 2001-2002) might never come back, especially since you cut down all the cherry trees (the ETC's favorite feeding/nesting site). 

The forest tent caterpillar has a general eight- to 10-year cycle, which is where the ETC report might have originated. "They don't come back overnight," said Fitzgerald of the ETC. "It takes a while. You'll see a few nests one year, then a few more. So you get a warning.

"Entomologists at universities ought to keep track of that caterpillar since it has caused problems," he said. "If there were records kept, you could see the trends." 

Webb of the University of Kentucky conducted research to answer why there isn't high tent caterpillar numbers and MRLS abortions every year. In 2001 and 2002, there were unusually large numbers of ETC nests, but that changed dramatically between 2003 and 2005. "Our study indicates that an insect virus that kills tent caterpillars is likely the reason," Webb said. "A tent caterpillar baculovirus was isolated in 2003. This virus is rampant in Central Kentucky. Between 20-80% of the nests we collected in a six-county region were infected (the virus kills the ETC) and 90% of the insects in affected nests had the virus. 

Researchers scientifically showed that the caterpillars cause tiny lesions in the mare's gastrointestinal (GI) tract and suspect that the lesions allow bacteria in the GI tract to enter and circulate in the mare's body to somehow reach the fetus, and ultimately cause abortion. The lesions were found by Neil Williams, DVM, PhD, of the University of Kentucky's Livestock Disease Diagnostic Center, to have tiny setae (caterpillar hairs) in them.

See setae and other caterpillar images here.

Associated Bacteria
Kyle Newman, PhD, of Venture Laboratories, tackled the challenge of identifying levels of "suspect" bacteria for the cause of MRLS in different parts of the mare's digestive tract. The highest levels of the types of Streptococci that are routinely seen in MRLS were found in the mouth, with about 50% of the total isolated bacteria in the mare's mouth being MRLS-associated Streptococcus. Although this same bacterial species was present in the lower GI, it was much less common.  

"In the GI tract, we're looking for a needle in a haystack that is about the size of Chicago," he said. "There are between 500 and 1,000 different bacterial species with a total of between one and 100 trillion total bacteria in the GI tract, and we're looking for one."  

In an unexpected result of this study, the numbers of the MRLS-associated Streptococcus bacteria increased by about 10 times when mares were fed ETC. 

Using a method called DNA fingerprinting, Newman assayed the bacteria from different parts of the mare's GI tract. He determined that different strains of bacteria were found in different gut regions and discovered a 100% match between the suspect bacterial types found in the mouth of a mare and the bacteria that infected its fetus during MRLS. The bacteria causing MRLS "is literally coming straight from the horse's mouth and is part of the normal (gut) flora of that animal," he said.  

The obvious questions raised by this research are: Why these particular bacteria are involved, how and why do they get from the GI tract to the fetus, and why doesn't antibiotic therapy work in treating these infections in mares? 

For more information on the Florida cases contact Zimmel at, or call the University of Florida Large Animal Hospital at 352/392-4700 ext. 5666. 

MRLS Prevention  (recommendations developed for Kentucky)
Primary Preventive Measures
1. Minimize or eliminate exposure of pregnant mares to Eastern tent caterpillars. 
2. Keep pregnant horses away from wild cherry and apple trees. 
3. Frequently mow pastures grazed by pregnant mares. 
4. Offer hay to horses on pasture.

Secondary Preventive Measures
1. Increase the grass-to-clover ratio in pastures. 
2. Restrict time on pasture when a hard freeze is expected following a warm period. 
3. Reduce exposure of pregnant mares to endophyte-infected tall fescue.

Other Measures
1. Mycotoxin binders have been fed by some farms. If mycotoxins are involved, this could help reduce risk. This decision should be made after discussions with a veterinarian and/or nutritionist.
 2. Correct mineral imbalances. While this is always a good idea, there is a theory that mineral imbalances might be associated with MRLS. Again, discuss this with your farm's veterinarian and/or nutritionist.

Source: The University of Kentucky College of Agriculture and Dr. Terry Fitzgerald

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More