Thursday, April 12, 2007

Mare Reproductive Loss Syndrome - Tent Caterpillars

Spring is here and as the new foals begin to arrive, they are not the only new young creatures showing up on the scene. Also beginning to hatch are the larvae of the Eastern Tent Caterpillars and Forest Tent Caterpillars. What does this have to do with your horse or my Arabian horses?

You may remember in 2001 there were a large number of thoroughbred foals dying in Kentucky and no one knew why. The problem was threatening the racehorse industry and it was imperative to the economy of the state that the reason behind the deaths be identified.

I remember hearing the stories about the dying foals and updates as the numbers of foals increased but don't recall ever hearing what was being done to solve the mystery

I also didn't hear that it wasn't just the thoroughbred farms but 17 other breeds of horses affected. At least one report states was not just limited to the one area of Kentucky. It spread over a number of states. By the time the epidemic ran it's course 4,000 mares had aborted with economic losses reaching $500 million.

The outbreak was catastrophic. The new stories covered the globe as concerns spread about threats to the world horse industry. The state department and FBI were involved wondering about the possibility of contagion associated with agri-terrorism

Mare Reproductive Loss Syndrome was the name given to the phenomenon that included death to fetuses in all stages of development and even foals born alive but dying. What does this have to do with Eastern Tent Caterpillars. Unveiling a Six-Legged Equine Assassin: Mare Reproductive Loss Syndrome describes how the caterpillar became suspect.

Studies were done that proved the caterpillars were the problem but still no one knew specifically why the caterpillars were killing the horse fetuses and foals.

"A series of studies over the next five years has subsequently revealed that horses will inadvertently eat the caterpillars and that the caterpillar hairs embed into the lining of the alimentary tract. Once that protective barrier is breached, normal alimentary tract bacteria may gain access to and reproduce in sites with reduced immunity, such as the fetus and placenta. Fetal death from these alimentary tract bacteria is the hallmark of MRLS. " Quote from MRLS: Eastern and Forest Tent Caterpillars Currently Feeding

The time is right for spraying for caterpillar larvae and keeping these pests out of horse pastures where they might be ingested. While the epidemic was restricted to the Kentucky area where the wild cherry trees this particular species of caterpillars thrives, it would seem to me that making sure that any kind of caterpillar larvae in any part of the country might be wise.

The Kentucky outbreak was what it was because of the large numbers of horses it affected and the economic impact to the horse industry. That's what spawned all the research. The numbers of the larvae were also larger than the norm which contributed to the disaster.

To date, no one's really looking at the effect across the board to horses in general that might ingest caterpillar larvae of any kind. It seems to me that MRLS could easily be the explanation for odd abortions across the country. While it may be a while before science catches up, I think spraying for caterpillars this time of year is a simple enough precautionary step to use to protect my Arabian horses just in case MRLS isn't just restricted to these two forms of caterpillars.


  1. Hi MiKael

    Thanks so much for keeping up with my blog despite my absence recently, I really appreciate it.

    This post is very interesting I need to do a bit more research because we have caterpillars in some of our fruit trees that spin "webs" around the branches and leaves that look like tents I suppose and I wonder if these are what they are talking about? I have taken photos of them but didnt know they were a problem. If these are in fact the same then I will make sure they dont surface this year for sure!!!!

    Hope all is well, I will e-mail soon and give you my telephone number, I also have unlimited long distance so we can have a field day LOL


  2. A few comments, because I also dabble in insects (more specifically, lepidoptera, or moths and butterflies) as well as horses...

    I would STRONGLY suggest contacting your local USDA extension office to determine if a problem really exists on your property before spraying any type of insecticide yourself. The USDA official will be able to suggest a pest management program that best suits your individual issues. This will help ensure that you are only killing the target larvae and not affecting other beneficial insects. For example, the honey bee population in the US has been severely compromised from an unknown cause. Bees pollinate the hay fields. We need bees to have horses. Poorly chosen and poorly applied pesticides have a potential double whammy - not effecting the target insect but then killing other insects unintentionally.

    There are also many biological (not chemical) ways to control pests, such as pesticides made of bacteria which is only harmful to caterpillars, and parasitic wasps that can be matched to a certain species of pest. These have the added bonuses of not contaminating the earth or ground water at your farm.

    Check out for more information on colony collapse disorder of the honeybee.

    Also look at for more information on eastern tent caterpillars.

    Lastly, is the USDA web site.

  3. Thanks for all of the resources. I live on wetlands so I always check things out before I do anything. I am big on natural predators whenever possible. I use fly predators for fly control and have to be careful I don't do anything to jeopordize them either.

  4. Dressagemom,
    I GUESS thanks for the link on the bee die-off - I had no idea how serious things have gotten. :-( It's depressing, but better to know and try to figure some way to help in my little corner of the world rather than not having a clue how bad it is.


  5. It really is drepressing to think about not only the plight of the honey bee but also all the global warming crisis info that's been going around lately. If you get too caught up in it I think it can really be a downer and leave you feeling like there's nothing you can do as one person to fix such large problems.

    I just try and do as much as I can - I recycle, am a chronic light turner-outer to conserve energy, and I pay attention to chemicals going into my body and out into the world. I support causes when I can, but without widespread governmental support I fear that things will get worse before they get better.