Feline Leukemia...
about this deadly disease
... and a little about the
new vaccine available

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Feline Leukemia

Introduction: Despite the widespread use of vaccines, feline leukemia virus (FeLV) remains one of the most important causes of disease and death in cats.

This disease causes a variety of problems:

1.  Cats that don't act sick at all, but carry and spread the virus around to other cats.  It's estimated that about 2% of all healthy looking stray cats in the USA are carriers and spreaders of leukemia virus.

2.  Cats that are deathly ill with multiple symptoms and end up dying soon after getting sick.  Common problems caused by the leukemia virus include fevers, weight loss, abortions, severe diarrhea, severe respiratory diseases, and anemia.

3.  Cats that develop cancer and die slowly over a period of months.

4.  And most common of all ... patients that are NOT deathly ill but because of a weakened immune system, they are susceptible to colds, respiratory infections, pneumonia, bowel infections, eye infections, and wounds that won't heal normally.  A miserable life.

The virus is present worldwide.

It is mostly a disease of young cats.

It is more common in outdoor cats, in households with multiple cats, and in catteries and wherever cats live in groups such as dairy barns, horse barns and so forth.

The disease is spread by the saliva, the urine, the feces, the tears, and through bite wounds.

Kittens can get the virus from their mother while in the uterus or while nursing.

Diagnosis:  We now have reliable, test kits in our clinics that test for leukemia as well as Feline AIDS.

These test kits are easy, reliable, inexpensive, and NOT affected by vaccinations.  But they aren't perfect.  Here are some rules of thumb:

If a patient is very ill and the test comes up positive, we assume the test kit is correct and depending on how ill the patient is, we are likely to give a very poor prognosis and frequently recommend euthanasia.

But if the patient isn't ill and the test comes up positive ... a frequent occurrence when someone brings in a newly found stray cat for a check up ...  we either confirm the result by sending blood into a commercial lab (which will use a different type of test) OR we'll retest again in about 1 month because many cats that have had recent exposure to the virus will test positive but will successfully fight off the disease and have a negative test result a month or two later.

There is also a small chance of a false negative result in those situations where a cat was tested in the first few weeks after exposure before the virus had time to replicate enough to make the test positive.  This is why a negative test is unreliable in kittens under 9 weeks of age.

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Prevention: Vaccination has been very successful.  As a young vet, I helplessly watched or humanely euthanized hundreds of cats die from this disease each year.  Now, I still see several cases a month, but almost exclusively in non-vaccinated cats and strays.

You may already know that there has been quite some controversy over the safety of leukemia vaccine and another controversy about whether or not we are  vaccinating middle aged and older cats unnecessarily.

As to the first issue of vaccine safety, evidence is pretty conclusive that a small percentage of cats (about 1 in 10,000 is the most common estimate) developed cancer as a result of the vaccine.
That problem has recently been fixed ... the new "sub unit" or adjuvant free vaccines now available for leukemia and rabies are safer, more effective, and don't cause cancer.  At present, they are a little more expensive.

As to the second issue of when to vaccinate?  We vets are in near unanimous agreement that it's important to vaccinate kittens twice about 3-4 weeks apart .  I like to recommend the vaccine at 9-11 weeks of age and then again a month later.  Most of us agree that young cats should be revaccinated yearly at 1 and 2 years of age.  After age 2, the need for repeated annual boosters is probably not justified ... UNLESS... exposure risk is high.  You should consider giving annual boosters past age 2 if you have lots of cats, if you have a cat that roams the neighborhood, or if there is a known positive cat living in the same household.


We can treat the secondary diseases that leukemia positive patients are so prone to, such as diarrhea, respiratory infections, eye infections, and so forth.

We can give supportive care and supplemental care such as IV fluids, immune stimulants, anti-oxidants, vitamin cocktails, and so forth.

But, as of yet, there isn't a cure for the disease ... a treatment that eliminates the virus from the body.

Not that veterinary scientists and pharmacists haven't tried various antiviral drugs and treatments.  A lot of time and money has been devoted to conquering this disease.  And perhaps a cure is just around the corner.

The evolution of a vaccine for leukemia:

In the 1970s, there was an epidemic of feline leukemia that killed thousands of pet cats

There was great excitement in our profession when it was discovered that this disease was caused by a virus and a successful vaccine was produced ... not only because we would now be able to prevent this deadly disease... but because we were hopeful that we would soon be successful in producing a preventive vaccine for the similar disease in humans.

The initial vaccine contained modified live virus and were very effectice but a percentage of cats actually got the disease from the vaccine... not acceptable.

There soon followed a killed virus vaccine that was much safer... but not very effective because killed virus' don't elicit a strong immunologic response, which is the whole purpose of a vaccine.

So, adjuvants were added to the vaccine.  Adjuvants are chemicals that stimulate a strong response from the immune system and when combined with the killed virus made the vaccine very effective.
This is still the most common vaccine used today in veterinary practice.  But it's not the best vaccine available.

In the 1990s it was noticed that more and more cats were getting deadly sarcoma cancers at places where vets tended to give vaccinations.  It took another decade, but it was discovered that the cause was the chemical adjuvant used in killed virus vaccines like rabies and leukemia vaccines !

The percentage of cats that get cancer because of vaccination is not great, but it's a significant number...several hundred each year in the United States... especially if it's your cat.

The risk of getting leukemia is much, much higher than the risk of having a vaccine related cancer, so I certainly recommended vaccinating your cat for rabies as required by law and also for leukemia
(leukemia vaccination is probably NOT needed in middle aged and older cats)

But now, because of DNA recinbubabt Sub-unit technology there are both rabies and leukemia vaccines available from MERIAL that contain no chemical adjuvants and are very effective and safe.  No injection site cancers.  The only reason not to use these newer, safer, and better vaccines is because they are about twice as expensive.

All the major veterinary cat associations and vaccination boards recommend using adjuvant free vaccines when available.
I won't pretend to know much about this new technology but there is a lot of successful research going on in vaccination laboratories in our quest to develop effective protection for AIDs, Rabies, Flu, Marek's Disease, and other major causes of death.  The first of these new technology vaccines were produced by Merial ... now a huge, global health care conglomerate... in 2005 for avian flu.  Then Marek's disease ( a devasting neoplastic viral disease of poultry)  Now there are "Purevax" vaccines available for feline leukemia and rabies.