white kitten in couch corner

Photo courtesy Karen James

Feline distemper, also known as feline panleukopenia, is a very contagious, life-threatening infectious disease of cats. It commonly swoops through a group of cats, especially kittens, leaving a trail of death behind it. Fortunately, vaccination is highly effective and the "feline distemper vaccine" (which also covers upper respiratory infectious agents) is considered to be the core immunization for all cats, regardless of their lifestyle. Most people have heard of feline distemper only through discussion of routine cat vaccinations, but let's talk about the actual disease.

The Virus Itself

The feline distemper virus amounts to a single strand of DNA surrounded by a protein coating. It is extremely stable in the environment, which leads to its characterization as ubiquitous (everywhere). It can last a year indoors at room temperature. It survives freezing as well as treatment with such common disinfectants as alcohol and iodine. Fortunately, soaking a surface in bleach (diluted 1 part bleach in 32 parts water) for 10 minutes will kill it.

Infection occurs when the virus enters the body through the mouth or nose. Whether illness results or not depends on the immunity in the victim vs. the number of individual virus particles (i.e. the amount of virus) entering the body.

The feline distemper virus is a parvovirus. Many people are familiar with this term as parvovirus infection is a very real concern for dogs, especially puppies. In fact, canine parvovirus is closely related to the feline distemper virus and much of the information regarding canine parvovirus is true for feline distemper. The feline distemper virus, however, is more difficult to remove from the environment and more lethal in its victims than its canine counterpart.

Infection and Disease

An infected cat sheds large amounts of virus in all body secretions, including feces, vomit, urine, saliva, and mucus. The virus persists long after evidence of the original body secretion has faded away. The virus enters the victim's body and proceeds to seek and infect rapidly dividing cells. The lymph nodes in the throat are first to be affected and from there, over the next two to seven days, the virus rushes to the bone marrow and intestine.

In the bone marrow, the virus suppresses the production of the entire white blood cell line, hence the term panleukopenia (literally, “all-white-shortage”). White blood cells are the immune cells needed to fight the infection, and without them, the victim is completely vulnerable to the advance of the virus. In other words, the first order of the virus's business is to eliminate its host's defenses. From there, it continues to the intestinal tract. 

There, the virus infects the intestinal cells, causing ulceration leading to diarrhea and life-threatening dehydration as well as bacterial infection as the barrier between the body and intestinal bacteria is lost. The patient dies from either dehydration or secondary bacterial infection. The infection can be so rapidly overwhelming that death occurs before the vomiting and diarrhea even begin.

Because most cats are exposed to this virus to some extent, it is unusual for a kitten to have no immunity whatsoever. Furthermore, 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 kept in groups such as a colony of barn cats, feral cats, or even a group of shelter cats. Viral numbers (the amount of virus) must be large enough to overwhelm a cat’s partial immunity, but once virus-laden diarrhea begins to gather in the environment, those large numbers can be reached. Death of sick cats is typically considered to be 90%, though it has been said that a kitten who survives the first 5 days is likely to survive the infection.

Cerebellar Hypoplasia

Graphic courtesy MarVistaVet

A specific syndrome occurs if infection occurs during pregnancy. If infection occurs in mid or early pregnancy, the kittens simply abort. If the kittens are fairly far along, the cerebellum is involved, leading to cerebellar hypoplasia. The cerebellum is the part of the central nervous system that coordinates balance and movement, enabling one to walk or run on an uneven surface without consciously thinking about it. Without a normal cerebellum, the kitten is born with marked intentional tremors: whenever he focuses on purposeful movement, he tremors so much that normal movement is impossible. Such kittens are called wobbly cats and are notably abnormal but are considered to have good life quality and can be kept as pets as long as they are looked after.

Diagnosis of Infection

Any kitten with fever, appetite loss, diarrhea, and/or vomiting is a suspect for feline distemper. Classically, a white blood cell count shows almost no white blood cells; there are few causes of white cell counts this low and the infection can be considered confirmed.

A positive test. Courtesy MarVistaVet.

The SNAP Fecal ELISA test kit made for canine parvovirus is often used in cats as a means to reach a distemper diagnosis. This test detects parvovirus in stool and is felt to be accurate although the test is not specifically labeled for this use by the manufacturer. Vaccination 5-12 days before running the test can lead to a false positive test as the virus from the vaccine will be detected. It is important to keep this information in mind when using this test to make a diagnosis; recent vaccination will cause a false positive.

If a dead kitten is available for necropsy (that’s an autopsy in animals), the infection is readily confirmed by laboratory tissue analysis as there are unique tissue findings in feline distemper.

Virus isolation, PCR testing, and antibody level measurement are also potential tests for feline distemper.

Treatment

An infected cat can recover if the cat can be kept alive until the immune system recovers from the panleukopenia and can throw off the infection. This means that invading intestinal bacteria must be kept at bay with antibiotics and aggressive fluid therapy must control dehydration. Hospitalization is required as fluids and medications must be given intravenously. Blood sugar must be supported as well and control of pain, nausea, and hypothermia is crucial. This is essentially the same therapy as for canine parvovirus infection, though the feline experience seems to be more lethal. There is little chance of survival without hospitalization.

Recovery

If a cat is lucky enough to recover from this infection, generally, no permanent damage is retained, and the cat goes on with lifetime immunity.

The distemper virus is shed for up to six weeks after recovery.

Disinfecting the environment after infection is a huge challenge; the best protection for newly introduced cats will be vaccination. That said, bleach diluted 1:32 in water with a 10-minute contact time will kill the distemper virus, so it would be prudent to bleach any surfaces that can be bleached. 

Prevention

The American Association of Feline Practitioners (AAFP) suggests starting kitten vaccinations at 6-8 weeks old, with boosters every 3-4 weeks until they reach 16 weeks. A follow-up shot is given one year after the initial series, then every three years. Cat’s antibodies from their mothers usually fade by 12 weeks, but the final FPV vaccine is recommended at 16 weeks. In some cats, these antibodies may last up to 20 weeks. Your veterinarian will recommend an appropriate vaccine schedule for your cat depending on their age and other health factors.

• Vaccination, at least with a live vaccine, should be avoided during pregnancy as cerebellar hypoplasia (see above) can result in the kittens.
• Vaccination is generally given every one to three years depending on the protocol of the animal hospital.
• Vaccination can be given as an injection (either modified live or killed virus vaccine) in the right front leg area or can be presented as a nose drop.
• Killed virus vaccine has been associated with vaccination-associated fibrosarcoma (an aggressive cancer) in rare individuals.

Research is continuing in this area; however, the killed virus vaccine has largely been replaced by a modified live virus vaccine, which is not thought to have this problem.