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


Brown questioned that, but was not sure.

Thinking about how pathophysiologically that organism would have gotten into those mares, it didn't follow too well for him that this was an ascending infection, from outside the reproductive tract down from the vulva. Brown pondered how they got there. How did they come to be grown in such prominence in these mares? It wasn't that they were super pathogens, but if they didn't know what they were, or where they normally resided in the horse, it drove them to identify the alpha streps and the actinobacilli as a prominent finding of the clinical syndrome. In a paper presented at the 1st Workshop on Mare Reproductive Loss Syndrome (2002), Dr. Mike Donahue concluded that non-beta hemolytic streptococci (alpha-streps) and actinobacilli, two groups of bacteria not normally considered to be important causes of abortion in mares, were recovered from most of the fetuses associated with MRLS.

Two bugs togetherAs of May 2004, "as far as we're concerned caterpillars caused MRLS, from our research and the research of others," Webb states. "The efforts we have taken to control caterpillars have prevented MRLS in the last year or two. We have not seen any this year. We don't know if we are going to see any."

The caterpillars and the bacteria are related parts of the same syndrome. "The results from our last summer's work suggest that the most likely cause of the disease was the physical structure associated with the hairs of the caterpillar," Webb suggests. They dissected thousands of insects into three fractions - the gut, the internal tissues and the cuticle. Only the cuticle causes abortions. They made changes to the structure of the cuticle. They've attempted to extract compounds off of the cuticle. Organic and aqueous extractions have essentially no effect on the abortionagenic activity. If you pulverize the cuticle, you see reduced activity.

Co-investigators McDowell and Dr. Neil Williams, led a study showing that similar abortions can be caused in pigs by feeding them tent caterpillars. They have necropsied pigs that were fed tent caterpillars, and have identified a lesion that occurs throughout the alimentary canal (gut). They call these microgranuloma lesions. At the center of each of those lesions is found a tent caterpillar hair or setae. "We think that the setae are punching holes in the alimentary canal, to allow bacteria to reach the bloodstream," Webb says. The bacteria establish a septicemic infection. They pass through the bloodstream to the reproductive tissue, and you get bacterial replication as the most likely cause of the disease. We think that these alpha streps (as seen by Brown and Donahue) are coming from the mare's gut via this mechanism he adds. "They are normal commensal organisms. We think that the hairs are actively 'opening the door' to let the infection proceed."

"We haven't nailed it down," Webb says. "It still is a hypothesis, but certainly we know that the cuticle is involved. We know that the microgranuloma lesions occur, and we know that the bacteria are in the gut and are somehow passing through the bloodstream to establish infection in the fetal fluids and placenta. We haven't shown formally that those infections cause abortions, and that's where are work is headed over the next six months or so," he says.

The initial cyanide theory has been discounted. It has been discounted both by the equine science department, where they tried to introduce cyanide to try to cause abortions, and in experiments done on the tent caterpillars to show that the cyanide is actually metabolized and rendered harmless.

"It is curious what you are seeing in western Washington, I don't know the Western Tent Caterpillar, but I understand that they are sibling species, so that they would have a lot of similarities," Webb explains. It is possible that in western Washington infestation of WTC's may potentially cause MRLS.


Source: DVM360 MAGAZINE,
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