What Is The Difference Between Feline Immunodeficiency Virus/FIV and Feline Leukemia Virus/FeLV?

Both the Feline immunodeficiency virus (FIV) and the feline leukemia virus (FeLV) are members of the retrovirus family. So is the human AIDS virus. All three attack important cells of the immune system, decreasing the ability to fight infections and tumor. The symptoms of the diseases that FIV and FeLV viruses produce in cats can be very similar. However, with good care and a stress-free life, FIV positive cats tend to live considerably longer than FeLV positive cats. When a cat harbors the FeLV or the FIV virus, it is the viruses effects on CD4+ T-lymphocytes that are most damaging.  (read here) Many of your cat’s lymphocytes originate from stem cells in its bone marrow before they move through the blood or congregate in your cat’s lymph nodes. But T-lymphocytes originate in its thymus gland, hence the T. CD4+ lymphocytes are born “naïve” but with the help of another immune system cell instruct your cat’s other immune system cells to produce antibodies. Another small group of them are responsible for remembering pathogens encountered in your cat’s past. So, when CD4+ cells are disabled by the FeLV or FIV virus, your cat’s entire immune system is crippled.  

The FIV virus is quite common. In prosperous countries, it is considerably more common than FeLV. The FIV virus is present worldwide. It survives in a variety of wild feline species, not just house cats. Cornell University believes that in the United States, 2.5-5% of cats that exhibit no symptoms carry the virus. When they sampled the blood of cats that were already showing signs of various illnesses, the percentage increased to 15%. Similar increases were found for cats that were allowed to range out of doors. In another study, 1.7% of cats at animal shelters and 3.1% of cats presented to animal hospitals, and 3.9% of feral and colony cats tested positive for FIV. Male cats had four times the number of positives as female cats, and adult cats four time the number of positives as juvenile cats. In that sampling, cats showing evidence of illness were about three times more likely to be positive for FIV than cats that did not. In another study, 0.3% of cats tested at animal shelters and veterinary hospitals were positive for both FeLV and FIV. (read here) As with most infectious feline diseases, statistics depend on the lifestyle of the cat’s being sampled. 

At first glance, FIV+ cats appear to be more common in Australia than in North America.  In house cats two years old and older, that had no access to the outdoors, 13% in New South Wales, 15% in Victoria, 16% in Queensland and 20% in Western Australia tested positive using the IDEXX SNAP FIV/FeLV Combo test™. (ask me for Westman2019). Some of those Australian incidence studies went door-to-door collecting blood samples from the cats to be sure that it wasn’t only cats that regularly visited a veterinarian who were providing blood samples. (read here)  So the North American, UK, and EU studies may have actually missed a lot of FIV+ cats.

Where Did My Cat Catch The Feline immunodeficiency Virus?

Cats are by nature very territorial. Few are willing to allow strange cats to invade their territory without putting up a fuss. That might be confined to some loud warnings, but it often proceeds to a brawl and bites. Over the eons that cats and the FIV virus have evolved together, the virus has “learned” to be sure that it is found in large numbers in the saliva of FIV+ cats. (read here) No one has studied the percentage of king-of-the-hill roaming tomcats that are FIV+. I venture that quite a few, if not all of them, are. That is why a ruling tomcat’s reign is so short. But by then, they have passed the FIV virus on to all the other cat challengers. Ohio State veterinary college estimates that an urban tomcat’s territory includes about 20 city blocks. 

What Are The Symptoms Of FIV?

I mentioned that early in infection, the FIP virus rarely causes symptoms. One early study did report that soon after infection, cats became lethargic, ran fevers and had enlarged superficial lymph nodes (lymphadenopathy). However, that was a study that artificially created an enhanced (more powerful) FIV virus variant. (read here). Most early cases are more likely to go unnoticed for a variable period of time – months to years. When health issues finally do occur, unexplained low-grade fevers, weight loss, lack of normal interest, unkempt hair coat and diarrhea are the most common symptoms that cat owners report.

Eventually, secondary infections commonly begin due to the immune system suppression I mentioned earlier. For those cats, the term FAIDS (feline acquired immune deficiency syndrome stage) has been coined. That was copied from the term used for the later stages of HIV infection, AIDS. Those late-stage cats often develop blood report abnormalities such as a low white and/or a low red blood cell count, and abnormally low blood protein levels. Eye, respiratory tract, and mouth infections (gingivostomatitis) in the advanced stages of FIV are common. Various cancers also increase in frequency. Neurological issues brought on by the virus sometimes result in seizures or behavioral changes. (read here) FIV+ cats with these advanced issues rarely survive more than a few additional months.  

Is There More Than One Type Of Feline immunodeficiency virus?

Yes,

FIV isolates have been assigned five different subtypes, designated A, B, C, D and E. (read here) In Melbourne, Australia the Subtype A and A/B were at one point the most common. (read here) If one of these subtypes of FIV is more pathogenic than another is unknown. Because retrovirus continually mutate, knowing which subtype infected your cat is of little practical value (subtypes appear, subtypes disappear, subtypes combine). 

How Will My Veterinarian Decide If My Cat Carries The Feline immunodeficiency /FIV Virus?

In the North America, two major brands of in-office diagnostic test are commonly used by your veterinarian to identify cats that carry the FIV virus. Read more about those tests here. When the results of those tests are borderline, or when they are negative, but your veterinarian is still concerned that your cat may be FIV positive, it is always best if a blood sample is sent to a central veterinary laboratory for conformation. 

Cats are commonly tested when:

  They show signs of illness that your veterinarian cannot readily attribute to a specific disease

  They relapse despite what should have been effective treatment

  A new cat is being considered as a family member

   Sixty days after a pet was lost, and substantial time passed before it was found or returned or after a potential FIV exposure

  Another FIV+ cat was identified in your household

  Prior to release for adoption from an animal shelter or purchase

  Kittens born to FIV+ queens, initially and at 4 months of age 

Kittens, born to FLV-positive mothers, present your veterinarian with a diagnostic challenge. Many believe that these cats are likely to return false-positive test results, and to require later repeat testing or confirmational PCR tests. (read here) Others believe that is not required. (read here)  or ask me for Westman2019. It is always wise to confirm a positive FIV test using a different test brand or a different testing method. Negative in-office (point-of-care) tests results are usually accurate. However, all tests occasionally yield false-positives. When FIV test results do not agree, they are called “discordant”. Sixty days is a commonly chosen period between repeating tests when they are discordant.

When kittens are tested, the tests need to be repeated sixty days later. That is because early false positives are possible due to persistence of a FIV+ or FIV-vaccinated mother’s anti-FIV antibodies passed on to the kittens through her first milk (colostrum). Early false negative are also possible when FIV virus numbers in the kitten are still low. Some suggest waiting up to six months before making a final decision on a kitten’s FIV status. Reference laboratory-performed tests tend to be the most reliable.

Cats that received a prior vaccination against the FIV virus with a vaccine that is no longer marketed will test false positive, both in in-office tests and those performed at central laboratories. This vaccine, Fel-O-Vax FIV®, ceased production in 2017 due to the issue of false positive tests subsequent to vaccination as well as the vaccine’s apparent lack of effectiveness. 

Will My Other Cats Catch The FIV Virus?

If you own both FIV-positive and FIV-negative cats, and they live in harmony with each other (a stable, tranquil social structures), the chance that the virus will pass to your FIV-negative cat(s) are thought to be low. However, I do not know of anyone who has scientifically proven that. Cats differ significantly in their FIV virus loads, which affects their ability to pass on the virus to other cats. The amount of FIV virus within them, depends on their innate ability to control virus numbers, their stress level, their age, and the stage of FIV or FAIDS that they are currently in. Unlike FeLV, it is thought that a saliva-contaminated bite wound, not cat-to-cat contact or mutual grooming, is the primary (or perhaps the only) way the FIV virus moves to a new cat. In making decisions as to an individual cat’s virus load and likelihood of shedding the virus, PCR testing would probably yield the most useful information. (read here)   

What Can I Do To Protect My Cat While It Still Tests Negative for FIV?

Since effective vaccines against FIV do not currently exist, the most important thing you can do to protect your cat from FIV is to keep it indoors. Cats are perfectly content living entirely indoors, as long as you provide them with a stimulating environment. The main reason cat owners allow their cats to roam is because they are too lazy to provide their cat(s) clean litter boxes in sufficient numbers. 

The FIV virus will not survive for more than a few hours in a typical household environment. In a group home, rescue or even your home, when it comes to destroying virus and bacteria, nothing is as effective as common 6% hypochlorite household bleach (e.g., Clorox™) diluted with 19 parts of water. Diluted bleach is only full strength on the day you make it. Check Dollar Store bleaches to be sure that they are not approaching their expiration dates. Do not inhale the fumes. Do not add bleach to your pet’s drinking water.

What Treatments Can My Veterinarian Offer My FIV+ Cat?

No medication at your veterinarian’s disposal or obtained through other channels will cause a FIV+ cat to become an FIV- cat. No medication, tonic, herbal or complementary brew or procedure has been shown to make FIV+ cats live longer. Feline immunodeficiency positive cats that are in apparent good health need no special treatment, other than a low stress environment and good nutrition. The three most common ways to confirm good health in your cat are infrequent or lack of infections, a stable healthy body weight and close to normal laboratory blood test results.

Antibiotics Have Their Place:

Most of the illnesses seen in FIV+ cats are due to their decreased resistance to bacterial infections. With their immune system crippled in the later (FAIDS) stages of FIV, bacteria, fungi, and other viruses that a healthy cat would resist, become secondary health issues. (read here) When required, the commonly used antibiotics that veterinarians rely on are quite effective – for a time. Those antibiotics, however, might need to be given to your cat for a bit longer period than a non-FIV cat would require. With long-term or repeated use, bacteria are clever in developing resistance to all antibiotics.

Medications That Might Give A Boost To Your Cat’s Immune System – Immunomodulators:

Lymphocyte T-Cell Immunomodulator (LTCI)

In 2006, the USDA issued a conditional license for an immune cell stimulant called Lymphocyte T-Cell Immunomodulator or LTCI. The product is produced and patented by T-Cyte Therapeutics. I believe that this product is identical, or close to identical, to GM-CSF aka sargramostim. The Company’s website and a review of their patent furnishes little or no data that would confirm or refute that. It is sold as a subcutaneous injection. If it is, indeed, Granulocyte-macrophage colony-stimulating factor (GM-CSF), it is a cytokine that has been explored as a treatment for melanoma, to stimulate wound healing, to combat anemia, as an HIV treatment, as beneficial in the treatment of fugal infections, and as a treatment for cancer, as a treatment for Crohn’s disease, among others. None of these studies reported or confirmed dramatic improvements. However, in 2005, a group of veterinarians published a study that administered Amgen’s Neupogen^® Filgrastim (r-metHuG-CSF) human GM-CSF product to a group of six FIV-positive kittens. (read here) Fifty percent of the cats that received Neupogen injections showed an increase in their neutrophil, eosinophil, lymphocyte, monocyte, and/or red blood cell numbers – for 11–21 days. After that, the cats apparently developed antibodies against the product because it was designed for human use and the cats recognized it as a foreign protein. This is similar to what occurs when anemic cats are given another compound designed for anemic humans. (read here) After extensive online searching, I still do not know if the T-Cyte product is compatible with the feline body (ie, rFeG-CSF), or if it is compatible only with the human body (ie, rHuG-CSF) or is something entirely different. The study in the six cats would have been more meaningful if the authors had followed the experimental cats for a longer period. Their results would also have been more meaningful if another group of control (placebo) cats had received a common adjuvant, instead of just an injection of sterile salt water. Adjuvants, too, are foreign proteins that boost white blood cell numbers in the way that the authors described. So, the Neupogen may have functioned as no more than an adjuvant. When a product is presented so opaquely, I hesitate to recommend it. But if it was found helpful in your FIV-positive cat, please let me know. 

Many Other products besides Lymphocyte T-Cell Immunomodulator have been given to FIV or FeLV- positive cats out of desperation. I, personally, have no faith in any of them, but I see no harm in your giving them to your FIV+ cat:

Interferon Alpha

In 2017, an article appeared that claimed that interferons (IFN-α and IFN-ω) might be beneficial to FIV+ cats. They base that on what I believe to be a rather low-power Italian study that you can read here. In 2019, another article appeared that claimed that oral Human Interferon Alpha was beneficial to FIV+ cats. (read here)   Virbac markets an interferon product, Virbagen Omega, in the UK and the EU, but not as of this writing, in North America. Several of my clients have successfully obtained the drug here in the USA through a trans-shipper of veterinary pharmaceuticals based in the UK. They had it administered to their cats that were suffering from FeLV  (not FIV). For that use, it appeared to provide no benefit.

Medicines Developed To Treat Human AIDS

A few veterinarians have attempted to treat fading FIV+ cats with drugs that are helpful in treating human AIDS patients. The most common of these are zidovudine=(Retrovir®)  and PMEA. The results have not been as good as one might wish for. AZT has resulted in toxic complications in cats that include, anemia and liver or bone marrow damage. At one time, veterinarians at the U.C., Davis, explored the possibility that a histone inhibitor, SAHA might be beneficial to FIV+ cats, based on its possible ability to control HIV virus numbers in humans. (read here) But in 2020, the drug failed its clinical trials in treatment studies on HIV patients. (read here)  

Nutritional Support And A Low Stress Lifestyle

Cats with advanced FIV symptoms generally lose weight. So, anything you do to encourage your cat to eat and maintain a healthy body weight is very important. I mentioned in my article on feline leukemia that a more tempting diet prepared at home for your cat can be very helpful in combating poor appetite and weight loss. It will also give your cat considerably more pleasure than the industrial stuff sold in cans and bags. When that is no longer sufficient, appetite stimulants such as mirtazapine,   Entyce/Elura® or cyproheptadine might be helpful. 

How Long Might My FLV+ Cat Live?

As I mentioned, there is often a long asymptomatic period before your FIV+ cat’s health declines – and a few cats appear to never become ill. In one report from Germany, it was estimated that the average life expectancy forward from the time of FIV virus exposure was about 5 years. (read here) But another study, that included one of the same authors, concluded that the median survival time of FIV-infected cats was not significantly different from non-infected cats. (read here) Estimates on the lifespan emanating from cat placement sources and animal rights advocates tend to be considerably more optimistic than those that do not. No one really has hard data, and few veterinarians would be willingly to delve into such an emotionally charged issue. 

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