MRLS outbreak in Washington raises new scientific questions - DVM
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MRLS outbreak in Washington raises new scientific questions


Only the delivery to the mare remained unclear. The working theory was that the horses consumed the caterpillars and their excrement, but how MRLS developed from consumed insects was still a mystery. That began a more thorough look at the role these little pesky creatures play in the disease. During 2002 and 2003 experiments were conducted. From selective mare pastures seeded with caterpillars and their excrement, and mares fed various caterpillar body parts, fresh and preserved, a cause and effect relationship between the tiny bugs and lost foals was determined.

More investigation In February 2002, Bruce Webb, Ph.D., entomologist, and Karen McDowell, Ph.D., Equine Reproduction, University of Kentucky, began to investigate the involvement of Eastern Tent Caterpillars and MRLS.

He began a study to manipulate exposure to ETC and environmental conditions as much as possible to reproduce the syndrome under experimental conditions and take measures that would identify one of more proximate causal agents. To mimic conditions that occurred on the farms in Spring 2001, 29 pregnant mares were placed in each of three adjacent paddocks, 20 feet from each other. One pasture was seeded with caterpillars, the second with caterpillar frass (waste), the third was the control, not contaminated. A physical barrier was placed around the field with the caterpillars to keep the mobile little creatures contained.

In addition to the experiment, sentinel farms were monitored for the emergence of caterpillars. As of mid-March 2002, not only did the Easter bunny begin delivering eggs, the Eastern Tent Caterpillar egg-hatch began. This also began the University of Kentucky's close watch on the effect of ETC's on horses in the central Kentucky, and the farms' diligent program to eradicated the bugs.

Friday, April 26, 2002, similar in time to the previous year, the University of Kentucky Equine Management Advisory noted the beginning of the movement of Eastern Tent Caterpillars. The "worms were beginning to turn," as ETC's were crawling about looking for a sight to pupate. This meant that for two to three weeks the caterpillars would stray as far as 100 feet or more from their nesting trees, depositing waste as they migrated, and accumulating in large numbers on adjacent trees, fence posts and barn walls. University of Kentucky entomologists noted the importance of eradication at this time to limit their potential damage, and recommended several insecticide programs and physical barriers to halt the movement of the insects.

By May 1, 2002, just five days prior to the 128th running of the Derby, results came in from the LDDC-Veterinary Science experimental caterpillar field study. The results were consistent with caterpillars playing a role in the cause of MRLS. Fifteen mares on two treated and one control pasture showed evidence of MRLS. Foals were lost to six of 10 mares on the caterpillar seeded pasture, six of 9 mares on the caterpillar frass seeded pasture, and three of 10 in the control, unseeded pasture. The study was quite conclusive to the relationship of caterpillars to MRLS, though a few mares in the control pasture lost their foals to MRLS. A breach in the physical barrier, which resulted in caterpillar contamination of that field was determined. As a result, the University of Kentucky Department of Agriculture continued to recommend that exposure of pregnant mares to ETC be controlled.

The other bugsAnother piece to the puzzle was noted by Dr. Stuart Brown, DVM, Hagyard-Davidson-McGee Associates, PLLC. "The one thing that I found to be particularly striking in a lot of the mares was these alpha-strep bacteria," Brown explains. Nothing in particular caused him to look for them. It's a routine part of a protocol for working up a mare that has aborted at anytime. They do cell cultures and cytology on them. About 65 percent of the mares Brown examined (of about 300 mares that were affected with MRLS) had grown alpha strep organisms, probably of three to seven different species. Dr. Mike Donahue (University of Kentucky Livestock Disease Diagnostic Center) was growing a large number of alpha streps and also Actinobacilli species. "I find it to be a very striking correlation, that he (Donahue) comes up with 70 percent in late fetal losses from approximately 500 cases he had, and I came up with 65 percent in the 300 or so cases I had," Brown says. He would have initially thought that they were primarily opportunistic organisms. That was very much the feeling, Day 1, when they had the problem, because they found these alpha strep organisms in a lot of fetuses at that time. People thought they were probably just part of the mare's normal flora and they were just secondary invaders.


Source: DVM360 MAGAZINE,
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