HRS Article: OLD
RABBIT PARALYSIS PART II: Parasites
This is part II continued from last issue.
PARASITES
The two most common parasites causing neurological disease in the rabbit
are Encephalitozoon cuniculi (also known as Nosema cuniculi) and Baylisascaris
procyonis. In part II of this series we will discuss Encephalitozoon cuniculi.
Encephalitozoon cuniculi
Cause: There is a protosoal (one-celled) organism in the microsporidia
family that can infect a number of species of animals, including humans.
In humans infections are rare except in immunocompromised individuals such
as AIDS patients or those suffering from tropical diseases. Some species,
such as dogs and cats, either die from the disease or survive the infection
and completely clear it from their bodies. In rabbits and mice however,
the infection is persistent throughout their lives and may or may not cause
obvious signs of disease. Even within species it appears that some genetic
strains are more resistant to infection than other. E. cuniculi is passed
from infected animals through the urine. The oral route is the most common
way it is picked up by another rabbit, i.e. when a rabbit eats material
contaminated with urine containing infective spores. The spores also can
be inhaled and enter the body through the lungs. There is still controversy
over whether or not the disease is transmitted through the placenta in
the unborn young. The rate of infection in kits is highest when the dam
is positive for this disease. Whether that is because the spores are in
the environment from previous shedding by the mother or because there is
transmission through the milk or placenta has not been definitively determined.
Once the spores are in the body, they become “active” and spread throughout
various tissues including kidneys, liver, lung, spinal cord and brain.
Microsporidia that infect the kidney go on to actively produce more spores
that are then passed in the urine in large numbers within 30 days. The
spores are produced for up to 90 days post infection, at which time they
stop and the rabbit can no longer infect others. E cuniculi infections
are very common in pet rabbits in the United States. Up to 80% of clinically
normal rabbits tested in some populations were positive for this disease.
It can not only be transmitted from other affected pet rabbits, but also
from wild rabbits and rodents.
Signs: Fortunately most rabbits affected with this parasite remain
completely normal throughout their lives. However, some rabbits develop
clinical disease and others do not, although as mentioned previously it
might depend partially on a genetics. Certainly animals that are immunocompromised
are at a higher risk. The clinical signs in rabbits usually involve disease
of the kidney, brain or spinal cord. If severe kidney disease is present,
a generalized weakness may be observed which could be initially mistaken
for a neurological problem. In addition to weakness and depression, the
rabbit may exhibit a poor appetite, increased water intake, increased urination,
an ammonia odor to the breath and sudden death. Kidney disease is usually
diagnosed by blood tests and occasionally a kidney biopsy. If the brain
or spinal cord is infected, the signs may vary depending on what area is
damaged. A rabbit may experience any one or combination of the following:
unilateral or bilateral facial paralysis, weakness in only one limb, complete
hind limb weakness or paralysis, all four limb weakness or paralysis, head
tilt, loss of appetite, behavior changes, depression, seizures (mild to
severe) and sudden death.
Diagnosis: There is a blood test that can detect the presence
of E cuniculi in the rabbit. It detects antibodies to the parasite. The
test was primarily designed to detect positive animals in a breeding colony
or laboratory setting so they could be removed from the population. The
test is of limited use in pet rabbits because it does not prove that this
parasite is the cause of the clinical signs present. In other words, many
rabbits have been exposed to this parasite and are infected, but will never
show any sign of disease. Therefore, if we get a positive test, it only
means that the rabbit has the parasite in its body, it does not prove that
the parasite is responsible for any of the clinical signs we are currently
seeing. The only way to diagnose if the parasite is actually causing the
signs would be to take a biopsy of the brain or spinal cord, which is dangerous
and not at all practical. Therefore, we can only make a tentative diagnosis
of E cuniculi based on the rule out of other diseases along with the presence
of a positive E. cuniculi test showing that the animal has the potential
for having a problem with this parasite.
Treatment: Unfortunately there is no treatment for an active
infection of E. cuniculi. Drugs such as ivermectin and other parasiticides
have been tried without success. The parasite lives within the cells and
it is very difficult to get medication into this protected area. In Europe
there was a study published in 1994 which suggested that the drugs fumagillin
and albendazole might be useful, but they are not available in the U.S.
and have not been tried on live animals. Even if a drug is discovered that
can clear an infection of E. cuniculi, it will not be able to reverse the
effects of brain, spinal cord or kidney damage once it has occurred. Rabbits
that have clinical signs of the disease should be supported with good nursing
care. Antibiotics, analgesics and anti-inflammatory drugs may be used as
needed for secondary problems that may develop. Most pet rabbits have probably
contracted the infection from the mother. Rabbits that are obtained as
adults and that are not used for breeding are not liely to shed the spores.
Hoever, young rabbits under 4 months of age that may have contracted the
disease from the mother could be actively shedding the spores in their
urine. It would probably be a good idea to isolate rabbits under 5 months
of age from other rabbits in the house. An E. cuniculi test could be run
on these youngsters to determine if they are carrying the parasite. If
they are negative, they could be put with the other rabbits earlier than
5 months of age. We have been unable to find a definitive source that indicates
the best product with which to clean the environment and remove any spores.
Any materials that are contaminated with urine, such as wood or carpet,
that cannot be cleaned thoroughly should be destroyed. Other substances,
such as plastic and metal should be scrubbed with a phenol disinfectant
(Lysol is one example) or a strong bleach solution and then rinsed thoroughly.
In part three of this series on causes of neurological disease we will
discuss another parasite, Baylisascaris procyonis and strokes.