Most cat owners have heard of feline distemper only because the distemper vaccine represents the core recommended regular vaccination for pet cats. Because the vaccine is highly effective, most cat owners fortunately do not have any practical experience with the actual feline distemper infection
Feline distemper is actually nothing like distemper virus that we see in dogs – feline distemper is a slang term for the virus that causes panleukopenia, which is actually a parvovirus. The virus is considered ubiquitous, meaning it is found in virtually every place that is not regularly disinfected, and is highly contagious among unvaccinated cats, most commonly kittens and young adult cats living in groups. Feral colony cats, animal rescue groups, catteries, barn cats, and animal shelters are at high risk for outbreaks.
The feline distemper virus has a single strand of DNA surrounded by a protein coating. It is very stable in the environment, known last up to one full year indoors at room temperature and often even survives freezing. It also survives treatment with such common disinfectants as alcohol and iodine but 10 minute saturation in bleach (diluted 1 part bleach in 32 parts water) is sufficient to kill the virus.
Given its hardiness in the environment, virtually every cat in the course of its life will be exposed to this virus.
Infection occurs when the virus enters the body through the mouth or nose, what we term the oronasal route. Whether illness results or not depends on the immunity in the victim in proportion to the number of individual virus particles entering the body.
As previously stated, feline distemper is caused by a parvovirus. Many people are familiar with this term as parvovirus infection is a generally well known concern for disease in dogs, most notably puppies. Canine parvovirus is actually closely related to the feline panleukopenia virus and thus much of the information regarding canine parvovirus holds true for feline distemper. The feline distemper virus, however, is more difficult to remove from the environment and more lethal than its canine counterpart. Also, cats cannot be infected with feline parvovirus, nor can dogs be infected with feline distemper.
Infected cats shed abundant amounts of virus from all body secretions including feces, vomit, urine, saliva, and mucus. The virus persists long after evidence of the original body secretion has dried or been cleaned by anything other than a disinfectant known to kill the virus. The virus enters the cat’s body with an affinity for rapidly dividing cells in the body, notably the lymph nodes, bone marrow and intestinal cells.
In the bone marrow, the virus suppresses production of red blood cells and white blood cells, leading to anemia (low red blood cells) and compromised immune system (the white blood cells are the immune cells needed to fight the infection). With regard to the immune system, infection with panleukopenia the patient is left vulnerable to more viral invasion, as well as opportunistic secondary bacterial infection.
In the intestines, the virus causes ulceration of the lining, leading to diarrhea and life-threatening dehydration as well as bacterial infection. The patient often dies from either dehydration or secondary bacterial infection.
Because most cats are exposed to this virus to some extent, it is unusual for a cat to have zero immunity to it. Additionally, the vaccine is so effective that even one dose can provide long-lasting protection. As a result, infection is largely limited to unvaccinated younger animals housed in groups, which directly corresponds with exposure to virus numbers large enough to overwhelm their partial immunity. Mortality (death rate) of sick cats is typically considered 90%, making it a deadly viral infection.
A specific syndrome may occur if infection occurs during pregnancy. If infection occurs in early to mid pregnancy, the kittens will likely abort. If the kittens are fairly far along, the cerebellum may become infected, leading to incomplete cerebellar development, known as cerebellar hypoplasia. The cerebellum is the part of the brain that coordinates spatial orientation and movement, enabling the cat to walk and balance. Without a normal cerebellum, a kitten is often born with what are known as intention tremors, that is, whenever he focuses on purposeful movement, he tremors so much that normal movement is impaired, often severely.
Any kitten or unvaccinated adult cat with fever, appetite loss, diarrhea, and/or vomiting is a suspect for feline distemper. Low white blood cells and red blood cells in general bloodwork are hallmark clinical parameters of panleukopenia infection.
The SNAP Fecal ELISA test kit made for parvovirus in dogs is often used for cats as a means to reach a diagnosis of pankeukopenia. This test detects parvovirus in stool and is considered by some veterinarians to be accurate; although the test is not specifically labeled for this use by the manufacturer. Feline distemper vaccination 5 to 12 days prior to running the test can lead to a positive result as the virus from the vaccine will be detected. It is thus important to keep this information in mind when using this test to make a diagnosis; as recent vaccination may cause a false positive. Virus isolation, PCR testing, and antibody level measurement are other commonly used diagnostics to conform panleukopenia.
Infected cats can recover if the cat can be kept alive through aggressive supportive care until the immune system recovers from the panleukopenia and is able to mount an immune response to clear the virus. This is accomplished through aggressive fluid therapy and intravenous antibiotics preventing dehydration and controlling secondary bacterial infections. Given a very guarded prognosis even with aggressive and timely hospital care, there is low probability of survival without hospitalization.
If a cat is fortunate enough to recover from this infection, the feline patient generally suffers no permanent damage is and the cat goes on to live a normal like with lifetime immunity from future infection.
Prevention is very effective with vaccination. The vaccination protocol that I favor begins when a kitten is 6 weeks of age, then bolstered at 9 weeks and 12 weeks. The vaccine should then be bolstered one year later, followed by booster once every 3 years for the rest of the cat’s like.
Vaccination with a modified live vaccine in pregnant cats should be avoided due to risk of the kittens developing cerebellar hypoplasia (see above).
Roger L. Welton, DVM
President Maybeck Animal Hospital