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The Daily Vet by petMD

The Daily Vet is a blog featuring veterinarians from all walks of life. Every week they will tackle entertaining, interesting, and sometimes difficult topics in the world of animal medicine – all in the hopes that their unique insights and personal experiences will help you to understand your pets.

The Life and Times of the Meningeal Worm

Most parasites that I deal with on the farm are your run-of-the-mill roundworms, commonly causing diarrhea and weight loss in cattle and horses, and severe anemia in sheep and goats. However, there’s an insidious threat in the field that goes beyond the usual gastrointestinal upset. This one hits the central nervous system. It’s commonly called the meningeal worm.

Taxonomically speaking, this parasite is called Parelaphostrongylus tenuis (pronounced para-laugh-ah-stron-gilus). The definitive host of this parasite is the white-tailed deer. This means the meningeal worm is supposed to infect the deer; think of the deer as their natural habitat. Adult meningeal worms live in the lining of the brain (called the meninges) and spinal cord of the deer. When this parasite sheds eggs, other animals can become infected through ingestion of the eggs. Sheep, goats, llamas, and alpacas are prone to infection by meningeal worm and are called aberrant hosts.

But let’s back up a second. If the worms are surrounding the brain, how are their eggs making it out to the environment? This is where it gets cool. When the adult female meningeal worm lays eggs, these eggs are washed out of the nervous system via venous circulation. Now in the bloodstream, they get filtered to the lungs where they hatch into larvae. These larvae are then coughed up, swallowed, and then there you go: delivery into the gastrointestinal tract where they get passed in the feces.

OK. The cool stuff isn’t finished just yet. The larvae passed in feces are still too immature; they are not infective to the deer or alpaca or sheep yet. First, snails and slugs, known as intermediate hosts, ingest these tiny larvae. Inside these invertebrates, the larvae continue to develop to a point where they become infective to our farm animals. At this point, if a deer or llama ingests an infected snail or slug, the larvae are ready to migrate from the intermediate host to the definitive (or aberrant) host for completion of the life cycle.

After the snail or slug is ingested — and we’re talking tiny snails and slugs that are accidentally ingested while grazing, not the giant slugs you see on the sidewalk after rain — who’d want to eat those? — the larvae migrate from the digestive system into the spinal canal where they develop into adults and the lifecycle beings again.

When this occurs in a white-tailed deer, there usually aren’t any problems. When this migration into the spinal canal occurs in an aberrant host, the nervous tissue becomes severely inflamed and damaged. This is when we see clinical signs of infection.

The clinical signs of a small ruminant or camelid infected with meningeal worm most often include weakness in the hind limbs that progresses to the front limbs. Affected animals frequently appear uncoordinated or stiff. Since this migration through the nervous system is at the whimsy of the worm, signs and severity of disease vary greatly from one animal to the next. Although the worms commonly destroy spinal tissue, they can also migrate to the brain, potentially causing blindness, change in personality, and seizures.

The course of the disease can vary. Some animals are acutely affected and succumb within days while others are only mildly affected for months.

Frustratingly, there is no test to definitively diagnose meningeal worm infection in a living animal. I say living because the only way to officially diagnose meningeal worm infection is on necropsy, when you observe damage to the spinal cord under the microscope.

Meningeal worm can be a diagnostic challenge because the neurological signs mentioned above can also be indicators of other diseases, such as brain abscesses, bacterial meningitis, certain mineral deficiencies, even rabies. However, usually in the case of a spinal cord infection with meningeal worm, the animal is not running a fever, and still has an appetite. In the field we make what’s called a presumptive diagnosis, begin treatment, and literally hope for the best.

Treatment of meningeal worm infection involves deworming to kill the parasite and supportive treatment to aid in the recovery of the nervous tissue. Here we’re talking about anti-inflammatories and neuro-friendly supplements that help repair oxidative damage like vitamin E and selenium, as well as vitamin B complex and thiamin. Supportive care in the form of physical therapy is also warranted.

The fact of the matter is, though, that nervous tissue, once damaged, does not regenerate. Once damage is done, it’s done. This means that if you’re faced with a badly affected animal, there might not be much you can do and sometimes euthanasia is the most humane option, particularly if the animal cannot walk.

Prevention isn’t a simple option, either. Deer-proofing a pasture sounds good in theory, but difficult in practice. The same with slug- and snail-proofing. Many alpaca owners prophylactically administer dewormer at regular intervals to their herd to kill any potential larvae in the digestive tract that are getting ready to make their break into the central nervous system. However, this gives rise to the concern of the development of antiparasitic resistance, since the same dewormers are used to treat parasites like common roundworms.

So what’s a poor small ruminant or camelid owner to do? Really, education is key. If a farmer knows what signs to look for and can call me out ASAP before severe damage is done, there’s hope.

Dr. Anna O’Brien

Image: Thinkstock

Comments  4

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  • Cost
    08/09/2013 09:00pm

    If suspected early, is it fairly inexpensive to treat this nasty-sounding creature or is a farmer more likely to euthanize an animal that is suspected of hosting this parasite?

    If it's inexpensive, does the farmer usually opt to treat the entire herd?

  • 08/21/2013 01:03am

    Cost of treatment is moderate and therefore is not usually the decision maker. Some farms have their animals on prophylactic monthly injections of a dewormer called ivermectin to help prevent against meningeal worm infection. However, there is concern this will lead to the development of resistance to ivermectin, not by the meningeal worm, but by all the other common gastrointestinal roundworms that this drug is also used for. If an animal is diagnosed with meningeal worm, no, in my experience the farmer does not opt to treat the rest of the herd.

  • 08/09/2013 10:41pm

    Would administering DE (diatomatious earth) help as a de-wormer. Maybe if the pasture is not too large, can DE be dusted to help rid slugs/snails?
    I know it's used in organic farming as pest control.

    And, can this be spread to humans? This is the stuff of nightmares.

  • 08/21/2013 01:06am

    You raise a good question about DE and unfortunately I have never used it, so I cannot answer your question.

    Regarding your other question, thankfully I am not aware of any human cases of meningeal worm but you are right, it IS the stuff of nightmares. However, I will say that some common roundworms found in the feces of cats and dogs can be picked up by humans (usually small children because they put a lot of things in their mouths). These roundworms can undergo aberrant migration in the human and wind up in the eye causing blindness and yes, to the brain as well.

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