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


Farm managementAfter the devastation of spring 2001, several drastic measures were taken by Kentucky farm managers to control the caterpillars and mares exposure to them in 2002. These measures included spraying trees with pesticides, eliminating cherry and crab-apple trees, manual removal of ETC eggs and tents, tree injection to make the leaves toxic to the ETC, keeping mares off pastures adjacent to cherry trees (the primary ETC host tree), keeping mares stalled at night, and limiting mares pasture exposure during the daytime.

According to Wolbert, for western Washington, the best control method is a bacterial insecticide, Bacillus thuringiensis , a non-toxic spray to be used around the immediate area of small or juvenile tents.

In Kitsap County, especially in 2003, though there was a tremendous population of caterpillars inhabiting several trees (alders, birch, apple and ornamental cherry), there was a significant reduction in numbers somewhere between the caterpillar and metamorphosis to the moth. Certain wasps are the natural predator of the WTC. The eggs by wasps of the Eulophidae family and the mature larvae are parasitized by wasps in the Braconidae family. These have seemed to help in the control of the WTC.

Wolbert advises farm managers to prune alders and other trees that overhang the pastures, have trees sprayed when they initially see the tents, remove and burn tents from trees, and monitor areas within their pastures to remove accumulating numbers of caterpillars, especially from water troughs and buckets. Another caterpillar control method is to paint the trunks of trees that border pastures with Tree Tanglefoot® Pest Barrier (The Tanglefoot Co., Grand Rapids MI), a sticky substance that entraps migrating caterpillars (once caterpillars strip foliage from a tree they migrate significant distances searching for alternative food sources).

According to Wolbert, most area veterinarians believed the contamination of caterpillars to horses occurred from those floating at the surface of water troughs. Though not the natural choice of grazing horses to eat caterpillars, though where they were extremely plentiful, consumption was almost inevitable. Similar explanation was offered in Kentucky. It would be prudent of farm managers to remove accumulations of caterpillars that would be in the areas where horses would be forced to consume them with food or water.

According to Wolbert, infestation in 2003 and 2004 in parts of Pierce County and Kitsap County was extreme. Elaine Anderson, Washington State University, Master Gardner Program Coordinator for King County, confirmed that 2003 was an exceptionally bad year for caterpillars, and predicted that 2004 would be a similar occurrence. By June, the movement and infestation of the caterpillars is finished.

EpilogueIt is really amazing how quickly this MRLS puzzle has been solved. Looking into the future the scientists are hoping that this is going to tell them something more about equine immunology. It may be relative in other contexts. The question is certainly whether the other insects, other similar insults to the digestive tract may cause similar problems. These are questions worth thinking about.

Whether this occurrence in western Washington is a duplicate of what occurred in Central Kentucky is open to speculation, but with a significant infestation of Western Tent Caterpillars I would suggest that the possibility is a relatively good one.

In Kentucky they are also looking at gypsy moths, which are basically on the borders of Kentucky at this point. If it is the physical structure of the hairs that is causing the abortions, then you have to suspect other hairy caterpillars to have a similar effect. They have done a few things with gypsy moths, but up to this time do not have any evidence that there is an exactly parallel problem.

It is certainly curious that horses eat things that have greater texture and coarseness than Tent Caterpillar hairs, but they do not cause such problems. Is there something unique about those hairs or cuticle that creates the problem? As far as anything that would be an irritant to the gut wall, it is curious that the fiber of hay and grain does not cause a similar problem. It is obvious that the horse's diet is of such material but it begs the question of something peculiar about the caterpillar setae that causes the lesions.


Source: DVM360 MAGAZINE,
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