More on meningeal worms

On Friday I blogged about a fellow shepherdess who had one of her new ewe lambs stricken by meningeal worms, a terrible parasite that destroys tissues of the central nervous system in ruminants such as sheep, llamas, alpacas, goats, and elk. I think it is important for shepherds to better understand this very dangerous adversary — and other readers may appreciate a bit of insight into some of the complicated decisions that shepherds must make for the good of their flocks.

An adult brain worm (P. tenuis) on the spinal cord of a moose submitted for diagnosis. ~Photo by DEC's Wildlife Pathology Unit~

An adult brain worm (P. tenuis) on the
spinal cord of a moose submitted
for diagnosis.
~Photo by DEC’s Wildlife Pathology Unit~

The life cycle of the meningeal worm begins when an adult worm lays eggs in the brain of a white-tail deer, the typical host. The eggs then pass into the veins of the circulatory system and eventually make their way to the lungs, where they hatch out into larvae. The host coughs the larvae up out of the lungs and they are then swallowed, gaining access to the digestive system, through which they eventually exit the host via the feces. Once in the outside world, the larvae are swallowed by or attach themselves to snails or slugs, which act as intermediate hosts. Over a period of weeks, the larvae develop into infective larvae which can now invade its own ruminant host. The infected snail is eaten by a hungry ruminant (such as a sheep) and releases the larvae into the digestive system where it again migrates into the circulatory system for transport to the brain and/or spinal cord. It takes six to eight weeks from the time of ingestion by an aberrant host like a sheep to the point of visible symptoms, when brain or spinal tissue is being destroyed.

So, how can a shepherd protect their flock from such a destructive threat? Well, it isn’t easy, but there are a number of approaches. First, one can limit the white-tail deer population; but as most people know, this can be a losing battle, since they seem to be everywhere! High fencing can help, but is quite costly. Over the years, we have been able to limit the number of deer simply by keeping sheep and llamas. Our llamas don’t like deer and tend to keep them out; and since the sheep generally keep our acreage well-grazed, the deer don’t see much that appeals to them here! As a result, we thankfully haven’t had to deal with meningeal worms for many years.

Another approach is to try to eliminate the snail and slug population so that the larvae cannot develop into the third (infective) stage. Free-ranging guinea fowl, ducks, and chickens can certainly help with this, reducing the chances that the larvae will be picked up by the grazing sheep or llamas.

Finally, the aberrant host (llamas, sheep, goats, etc.) can be dewormed monthly to kill off any larvae that may be migrating through the digestive or circulatory systems. This type of deworming does not much affect the larvae or worms that are already in the central nervous system, but it can prevent migration to that location. The problem is that the dewormer needed to kill the larvae must be given monthly during grazing to be of use, but it is the same dewormer that shepherds use to control worms specific to the digestive tract. Repeated use of these drugs end up producing drug-resistant parasites in the digestive system, making the drug useless in that particular fight.

The solution to this problem may be to test whether the fenbenzadole dewormer is actually effective for digestive parasites on your particular farm. This involves the collection of fecal samples both before and after about a week of treatment with the dewormer. If the fecal egg count does not decrease by over 70%, then the dewormer is not particularly effective for digestive worms and can easily be used monthly to prevent the migration of meningeal worms. If, however, the drug is effective in killing digestive parasites, then another way to treat for meningeal worms will need to be found. There are already so few families of drugs (only three) available to us for deworming digestive parasites, that losing one so quickly would likely cause more harm than good in the long run. Oh, and don’t worry that the meningeal worms will become resistant to the fenbenzadole — this is not an issue since the typical host (white-tailed deer) have no exposure to this drug.

I’ve been asked whether meningeal worms can be contagious — whether a companion sheep put into quarantine with an affected sheep will eventually show symptoms. The answer is that a sheep with meningeal worms cannot infect others simply by living in close quarters. Yet this doesn’t mean that the second sheep won’t eventually display symptoms. Keep in mind that the entire flock lives in the same fields and buildings, and each of the other flock members has grazed the same areas, so if one is infected with meningeal worms, it’s likely that others in the flock have also been exposed. It is perfectly possible that a week after quarantine, the “friend” in the barn may also begin to exhibit symptoms of a meningeal worm. But that’s because it was already in the brain before quarantine, destroying brain tissue until symptoms became obvious days later.

Meningeal worms are tiny but vicious killers when it comes to our flocks. An adult worm can be 2″ in length, but their eggs and larvae are microscopic. Prompt and effective treatment is critical if there is to be any hope of saving an affected sheep or llama. If you suspect meningeal worms, there is no time to waste.

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