What
is it?
The meningeal worm is an internal parasite (Paralaphostrongylus
tenius) of the white-tailed deer that usually completes its
life cycle in the deer without causing significant problems.
However, when unnatural hosts, such as sheep and goats, become
infested with meningeal worm, the parasite moves into the brain
and/or spinal cord and causes neurological problems that can be
fatal. Llamas and alpacas are even more susceptible to meningeal
worm infection than sheep or goats. Cattle are not known to be
affected. Meningeal worm is not a health concern to humans.
The life
cycle of the meningeal worm requires terrestrial snails or slugs
as intermediate hosts. White-tailed deer become infested with
P. tenius by eating snails or slugs that contain the
infective stage of the larvae. The larvae migrate through the
deer's gut and eventually move into the central nervous system
where they mature into adults, produce eggs, and the life cycle
begins again. However, when P. tenius-infected snails
and slugs are ingested by aberrant hosts, the larvae migrate
into the brain and/or spinal cord. The larvae do not mature into
adults, but rather wander through the central nervous system
causing inflammation and swelling which damages sensitive
nervous tissue producing a variety of neurologic symptoms.
Experimental evidence suggests that it takes 10 to 14 days for
the parasite to reach the brain and/or spinal cord after the
animal eats the infected snail or slug.
Symptoms
The neurologic signs observed in infected sheep and goats depend
upon the number of larvae present in the nervous tissue and the
portion of the brain or spinal cord that has been affected. A
mild infection may produce a slight limp or weakness in one or
more legs, while a more severe infection may cause an animal to
be partially or completely paralyzed. When larvae migrate to the
brain, they may cause blindness, a head tilt, circling,
disinterest in or inability to eat, or other signs that mimic
brain diseases. Affected animals may get progressively worse,
remain static, or in some cases improve without therapeutic
involvement. In most cases, infected animals remain alert and
continue to eat and drink normally.
Click
HERE to see QuickTime movie (1.9 mb) of a goat
displaying symptoms of meningeal worm infection.
Diagnosis
Meningeal worm infection cannot be diagnosed in the live animal.
A fecal examination is not useful since sheep and goats are
“dead end” hosts for the parasite and the larvae do not produce
eggs or pass larvae into the feces. The parasites cannot be
detected by blood testing. The only way to confirm diagnosis is
to find the parasite in the nervous system, which requires a
necropsy examination. Testing the cerebrospinal fluid, which
requires the animal to be tranquilized or anesthetized for
extraction, may help to support suspicions of brain worm
infection.
Thus,
diagnosis of meningeal worm in the live animal is based on
symptoms and clinical history. Usually animals have been grazing
for at least two months and there is a history of deer in the
area. Diseases which look similar to meningeal worm infection
include: listeriosis, CAE, scrapie, rabies, trauma, copper
deficiency, vitamin E/selenium deficiency, spinal cord or brain
abscesses, or polioencephalomalacia.
Treatment
Treatment regimes usually involve high, repetitive doses of
anthelmintics, along with steroids, and other supportive
therapies. Many different anthelmintics (levamisol, ivermectin,
albendazole, fenbendazole, thiabendazole) have been used to
treat meningeal worm infection. It is believed that some
anthelmintics can kill the larvae before it enters the central
nervous system, while others may be able to cross the
blood-brain barrier and kill the larvae regardless of its
location in the body. However, it is important to note that no
controlled studies have confirmed or refuted the efficacy of
different treatment regimes. Nor does treatment repair damaged
nervous tissue. Producers who suspect meningeal worm should
contact their veterinarian for treament recommendations.
Prevention
As with other disease conditions, prevention is usually more
satisfying than treatment. Unfortunately, the meningeal worm is
a hard one to prevent. Reducing deer populations is usually
impractical. A single deer can shed thousands of eggs per gram
of feces, and the larvae are highly resistant to environmental
forces. However if feasible, sheep and goats should not be
pastured in areas which receive high deer utilization or removed
from these pastures before the weather turns wet and cool. It
may be helpful to limit sheep and goat pasturing to fields
without contiguous woodlands and to pastures that are on high
ground and well-drained.
Controlling the intermediate hosts may be a more effective means
of prevention. Sheep and goats can be fenced away from likely
snail and slug habitats: ponds, swamps, wetlands, low-lying,
poorly-drained fields, and woodlands. Some veterinarians
advocate strategic deworming as a means of preventing infection.
However, it is important to realize that regular use of
anthelmintics (e.g. monthly treatments) rapidly leads to
anthelmintic resistance, so while regular treatments may help to
control the meningeal worm, eventually those drugs will lose
their efficacy against ordinary stomach worms, which may be an
even greater problem on most sheep and goat farms.
References: P. tenuis - The White-tailed Deer Parasite, Michigan
State University College of Veterinary Medicine and Goat
Medicine (1994) by Smith and Sherman.
©Copyright 2004.
Maryland Small
Ruminant Page.
