How Serious is Feline Leukemia (part.3)

A veterinarian stitching a dog, after surgery.Image via Wikipedia
What are the clinical signs of disease?
The clinical signs of disease are going to be variable because so many body systems can be affected. Loss of appetite, fever, weight loss, and weakness are the first signs most commonly seen in infected cats.

How is FeLV infection diagnosed?
Serologic (blood) tests are commonly used to test both asymptomatic and symptomatic cats for FeLV antigen (parts of the virus). The ELISA test can be performed in veterinarians’ offices. If the ELISA test is positive, a confirmatory test should be done.

One confirmatory test is the immunofluorescent antibody (IFA) test, which uses blood smeared on a slide and is performed in special diagnostic laboratories. Fluorescently-tagged antibody made in the laboratory reacts with the FeLV antigen in the blood and fluoresces when viewed through a special microscope.

A poymerase chain reaction (PCR) test is also available which can be helpful if the ELISA test and confirmatory test do not agree.

In 2008, The American Association of Feline Practitioners (AAFP) revised their guidelines for FeLV testing. They recommend that all cats be tested for FeLV. In addition, cats should be (re)tested:
  • During sickness: Regardless of previous negative results. While many signs (such as fever, stomatitis, vomiting, and diarrhea) are obvious indicators of illness, other signs are subtle and may include changes in behavior, grooming, and eating habits.
  • When being adopted/entering new home: Regardless of age and whether or not they will be entering a household with other cats. They should be tested prior to being introduced into the household.
  • When living in multi-cat households in which another cat is infected with FeLV, or are otherwise at high risk (e.g., cats that go outdoors unsupervised).
  • After potential exposures: When cats have had known or possible exposure to other cats who are infected or are of unknown infection status. If a negative test is obtained, the test should be repeated after a minimum of 30 days.
  • Prior to initial FeLV vaccination: FeLV vaccine should not be given to FeLV infected cats.
In addition, it is important to note that:
  • No test is 100% accurate. In populations of cats with low FeLV infection rates, many positive tests may be false positive.
  • Any positive ELISA test should be repeated.
  • In most cases, the ELISA test should be used as the screening test, and IFA as the confirmatory test.
  • ELISA tests using tears or saliva are not recommended.
  • All cats with a negative result but with the potential of having been exposed to FeLV should be retested after a minimum of 30 days.
  • All cats with a suspected or known exposure should be tested.
  • If the ELISA is positive but the IFA test is negative, the cat should be retested in 60 days and then annually until test results agree.
  • The vaccine will not interfere with FeLV testing.
  • The vaccine will not affect the course of disease in already infected cats. A "carrier," though, who is vaccinated and develops FeLV-related disease will appear to be a "vaccine failure".
Finally, a negative FeLV test does not imply the cat is immune to FeLV or was never exposed to it. A negative test could mean the cat:
  • Has never been exposed to FeLV
  • Is infected, but the number of virus particles is too low to detect. The cat may test positive at a later date.
  • Has overcome a previous infection.
To assist your veterinarian in determining the risk of your cat being exposed to or infected with FeLV, the AAFP has developed a history form you can fill out and give to your veterinarian at your cat's next exam.
How is infection with FeLV managed?

FeLV-infected cats can sometimes live for years. Stress and exposure to ill animals should be avoided. FeLV-positive cats should be kept indoors both to protect them from exposure to disease and also to prevent them from spreading FeLV to other cats.

By knowing a cat is FeLV-positive, your veterinarian may select different vaccination protocols, diet, preventive care, and treatments of other diseases than for an uninfected cat. Any sign of disease will require early recognition and often more intensive treatment.

Clinically ill FeLV cats will need to be treated according to the signs of disease they are showing. Supportive care such as fluids, good nutrition, and antibiotics for secondary infections are essential.

Lymphosarcoma is treated using chemotherapy that includes a combination of cytotoxic drugs and prednisolone.

The antiviral drug AZT, which is used in human retrovirus infections (e.g., HIV), has had some success in cats, although it can cause toxic side effects. Certain agents that have used to modify the immune response in FeLV-infected cats include staphylococcal protein (SPA), Propionibacterium acnes (ImmunoRegulin), low doses of oral human alpha interferon, and an aloe derivative called Acemannan. Well-controlled clinic trials of these agents have not been performed.

How is feline leukemia prevented and controlled?


There are a number of FeLV vaccines on the market. None of them produce 100% protection. Almost all of them are derived from inactivated (killed) virus or portions of the virus. Most of these vaccines have one or two additives called adjuvants that enhance the antigenicity (ability of the virus to cause an immune response) of the killed virus. There is also a recombinant vaccine. This means a gene from the virus is placed in bacteria in the laboratory. The bacteria multiply and in doing so produce a large amount of the protein the FeLV gene coded for. This protein is collected and used in the vaccine.
In general, it is recommended that cats who go outdoors, to shows, are boarded, or otherwise have contact with cats other than those in their household should be vaccinated against FeLV. In addition, it is recommended that kittens be vaccinated against FeLV. Remember, all cats should be FeLV-tested before they are vaccinated. Adult cats who are entirely indoors may not need to be vaccinated against FeLV. This is something you should discuss with your veterinarian.

Limiting Exposure: As mentioned above, FeLV vaccines do not produce 100% protection. It is essential that the risk of exposure of a FeLV-negative cat to a potentially FeLV-positive cat be minimized. In single cat households this is best accomplished by keeping the cat indoors.
In catteries and multi-cat households:
  • Test all cats for FeLV.
  • Remove all FeLV-positive cats or totally separate them from the other cats.
  • Disinfect all dishes, litter boxes, and bedding. Remember, FeLV can be transmitted through saliva, urine, and feces.
  • Prevent or minimize the movement of cats in and out of the household or cattery.
  • Test and quarantine all cats before introducing them into the household or cattery.
  • Only FeLV-negative cats should be used for breeding.
What are feline vaccine-associated fibrosarcomas?
A fibrosarcoma is a tumor of the connective tissue. These tumors tend to invade deeply into the underlying tissues. The frequency of these tumors is increasing in cats and has lead researchers to believe some of the tumors may be caused by a local reaction to a vaccine. Although these tumors are being seen more frequently, they are still rare. It is estimated one in every 5,000-10,000 vaccinated cats a year will develop a vaccine-associated tumor. These tumors are most commonly associated with the FeLV vaccine.

A national Vaccine-Associated Feline Sarcoma Task Force composed of veterinarians, researchers, and vaccine manufacturers are working to determine the exact cause of these tumors and how they can be prevented and treated. Possible explanations for the tumors include abnormal reactions to the adjuvant in vaccines, genetic predisposition, and infections with another virus.

The possible risk of vaccine-associated fibrosarcomas has led to a reevaluation of vaccination protocols. This is why it is recommended that cats without potential exposure to FeLV may not need vaccination. In cats with potential exposure, the risk of FeLV infection is greater than the risk of developing a sarcoma so vaccination is advised.

A small, painless swelling sometimes develops at the site of a recent vaccination. This should disappear in several weeks. If it persists, however, it could mean it is developing into a sarcoma and should be checked by your veterinarian.

If a vaccine-associated fibrosarcoma develops, surgical removal is attempted, but generally, this tumor is so invasive it is difficult to remove it all. Radiation or chemotherapy are often recommended in combination with surgery.

Generally, the FeLV vaccine is now given in the left rear leg, in an area distinct from where other vaccinations are given. If a tumor would develop, knowing which vaccine was given where will help us determine with which vaccine the tumor is associated. Also, if a tumor would develop, amputation of the leg would, in many cases, be curative. Cats do incredibly well on three legs, and many owners prefer this to having their cat succumb to a tumor.
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