Brain Worm

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Though it might sound like something I should have saved for the Halloween issue, llamas and alpacas are susceptible to a very serious condition commonly referred to as brain worm or meningeal worm.  It is caused by the parasite Parelaphostrongylus tenuis (how’s that for a mouthful?!), which is spread by white-tailed deer via slugs and snails.  The parasite does not harm the deer, but in llamas and alpacas the larvae migrate aimlessly through the central nervous system, causing symptoms that include hind end weakness, incoordination, and even paralysis.  Oral deworming products are ineffective for prevention and treatment is not always successful.

The meningeal worm is a nematode parasite that uses the white-tailed deer as its definitive host.  Adult worms lay eggs in the meninges of the deer’s brain, which pass into the bloodstream and circulate to the lungs where they develop into larvae.  The larvae are then coughed up, swallowed, and passed in the feces of infected deer.  Slugs and snails feeding on leaf litter and organic debris contaminated by feces serve as an intermediate host, and are ingested accidentally while animals are browsing or grazing. 

When the infected snail or slug is ingested by another deer, the tiny larvae penetrate the wall of the small intestine, enter the body cavity, migrate along nerves to the spinal cord and back to the meninges to begin the cycle again.  However, in susceptible aberrant hosts such as llamas, alpacas, and moose (and occasionally sheep and goats) the larvae migrate abnormally within the central nervous system causing neurologic disease.

Clinical signs vary with the location of migration and the tissue being damaged.  Signs in affected animals begin in the hind end with weakness, wobbly gait, knuckling, or paralysis most often reported.  Most animals will continue to be bright and alert with a normal appetite, although brain involvement can result in head tilt, circling, depression, blindness, seizures, and even death.  Other diseases with similar signs include listeria, polioencephalomalacia (thiamine deficiency), eastern encephalitis, West Nile virus, and Rabies.  Diagnosis is made based on clinical signs and response to therapy. 

Prompt aggressive treatment is required to halt progression of the disease.  Oral fenbendazole (Safeguard, Panacur), though ineffective at preventing infection, is administered once symptoms are noted because unlike ivermectin it can penetrate the blood brain barrier to reach and kill migrating larvae.  Very high doses (much higher than on the product label) of the drug are given daily for several days, in addition to anti-inflammatories such as flunixin (Banamine).  Supportive care including physical therapy to prevent muscle contractions, pressure sores, and urine scald are also important, though some cases may not recover completely, or may appear to improve and then relapse after several weeks or months.

As previously stated, oral deworming products are ineffective for prevention or this disease, so monthly injections of an ivermectin type product must be given.  While experts historically recommended administration of ivermectin only during the months of May through October, year-round prophylaxis during milder winters is likely prudent.  Measures should also be taken to reduce access to the pasture by deer, and to avoid grazing animals near leaf piles or in poorly drained areas where snails and slugs are most abundant.

Researchers are currently working on a vaccine for meningeal worm infection, but until then monthly injections are critical.  If you own a llama or alpaca in the northeastern part of the country, contact your veterinarian to learn the appropriate dose and injection technique to provide protection from this devastating disease.  Also call at the first sign of hind end weakness: early, aggressive treatment gives the best chance for recovery.

Dr. Yoanna Maître is the owner of Berkshire Ambulatory Veterinary Services, a mobile clinic that provides on-farm medical and surgical care to horses and farm animals in Berkshire County and neighboring areas of New York State.

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