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, a large assortment of virus that includes the human AIDS virus. The symptoms of the diseases that FIV and FeLV viruses produce in cats are also very similar. Some would say identical. It is just in the frequency and speed with which they produce those illnesses that they differ. The progressive form of FeLV tends to be by far the worst of the two. Those two virus and many others in the retrovirus group have a predilection (preference) for attacking and permanently disabling the cells that comprise the immune system. (read here) With that critical defense system disabled, your cat’s susceptibility to infections and tumors is increased. Should your cat become infected with the FIV virus, most veterinarians believe that the virus will persist in your cat for the remainder of its life. As to whether the FeLV virus persists in every cat in one form or another for the remainder of its life is still debated. However how the presence of the FIV virus will affect your cat is highly variable. (ask me for Westman2019.pdf) Much of that depends on the amount of damage the virus inflicts on your cat’s CD4+ and CD8+ lymphocytes . (read here)   

Although FIV and HIV are distant relatives, humans cannot be infected by the FIV virus, cats cannot be infected by the HIV virus and dogs are not affected by either of them. A fourth retrovirus, feline foamy virus (FFV) is also present in many cats. What health issues, if any, that FFV might cause in cats remains unknown. Some cats are positive for FIV and FFV and a few for FFV, FeLV and FIA. How these mixed viral infections might affect the health of cats is also poorly understood. But none believe that it is a positive event.

How Common Is The Feline immunodeficiency /FIV virus?

The FIV virus is quite common. In prosperous countries it is considerably more so than FeLV. The virus is present worldwide. 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 as likely to be positive for FIV than cats that did not. (ask me for AAFPGuidelines2009) In another study, 0.3% of cats tested at animal shelters and veterinary hospitals were positive for both FeLV and FIV. (read here)

At first glance, FIV+ cats appear to be more common in Australia. 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 were positive using the IDEXX SNAP FIV/FeLV Combo test™. (ask me for Westman2019). However some of those Australian incidence studies went door-to-door sampling cats to be sure that it wasn’t only cats that regularly visited their veterinarians who provided 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 creatures. Few are willing to allow a strange cat to invade their territory without putting up a fuss. That might be confined to some loud warnings, but it often proceeds to a brawl. Over the eons that cats and the FIV virus have evolved together, the virus has “learned” to be sure that it is found in high numbers in the saliva of FIV+ cats. (read here)

No one has studied the percentage of ruling tomcats that are FIV+. I venture that quite a few, if not all of them, are. Ohio State veterinary college estimates that an urban tomcat’s territory includes about 20 city blocks. These cats are king of their territory for a relatively short period of time. I venture that their rule ends when FIV, FeLV or repeated abscesses begin to weaken them. But by then, they have passed FIV virus on to other cat challengers.

In stable colonies of cats, the more likely mode of transfer, when it occurs, is likely to be allogrooming. It is also possible for a mother cat to transmit the FIV virus to its offspring. Since few kittens test positive for FIV, such transfers are likely rare events.

What Does The FIV Virus Do To My Cat?

I mentioned at the beginning of this article that health decline associated with FIV virus is not directly due to the virus. It is due to the virus’ effect on your cat’s immune system. The generators of the defensive cells of that system are in your cat’s bone marrow and lymph nodes. With its immune defenses weakened, your cat’s susceptibility to various sorts of infectious diseases is likely to increase. How much more susceptible depends on the severity of the immunosupression that occurs, the length of time the virus has been present and probably also on the unique genetics of each cat.

You might wonder: “why would a virus that has persisted in cats since the species originated want to kill or weaken the very host that keeps it alive? After all, sick cats don’t move around much to spread the virus. It has to do with virus survival strategy options. By having a very long spreader period before symptoms begin, the FIP virus gets around that problem. During that healthy carrier period, the virus’ RNA co-opts the cat’s own cells into make enormous numbers of FIV virus copies – all the better to spread the disease. To do that successfully, FIV virus must keep your cat’s immune system (primarily its CD4+ T cells in check [disabled]). During that early spreader period, the virus also finds fever undesirable. Most virus reproduce more efficiently at normal body temperatures and less efficiently when body temperature is high. Other virus, such as the Herpes-1 virus associated with feline upper respiratory tract disease follow a different strategy. They thrive in the respiratory tract where your cat’s body temperature is lower and on surfaces where your cat’s immune system has a harder time attacking them. They tend to multiply quickly, get spread to other cats through sneezes and then enter quiescent carrier states where they are never heard from again or only periodically during periods of high stress. Stress for cats often means other cats – a new opportunity for virus to change dwellings. If virus had a brain, they would be judged to be very clever.

Another casualty of disabling the cat’s immune system is the FIV’s effect on CD8+ T cells.   Disabling them also breaks a key element in controlling the spread of cancer. Abnormal cells generate in all of us during our lifetimes. It’s the T cell’s job to find them early and destroy them before they can proliferate into overt cancer.

What Are The Symptoms Of FIV?

I mentioned that early cases of FIP generally show no symptoms. One early study reported that initially these cats were 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 likely to go unnoticed for variable periods 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 reported.

Eventually secondary infections commonly occur due to immune system suppression. For those cats, the term FAIDS (feline acquired immune deficiency syndrome stage) has been coined. That was copied from the term for the later stages of HIV infection being termed AIDS. Those cats often have blood work abnormalities such as low white and/or red blood cells counts, and 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 issues rarely survive more than a few more months.  

Is There More Than One Type Of Feline immunodeficiency virus?

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 is more pathogenic than another is unknown; and because virus continually mutate, subtype designations are 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 United States, the use of two brands of in-office diagnostic test are commonly used to identify cats that carry the FIV virus. Read more about those tests here .

Cats are commonly tested when:

 They show signs of illness that your veterinarian cannot be 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. 

Cats that have had prior vaccinations against the FIV virus present diagnostic challenges. Many believe that these cats are likely to return false-positive test results and require 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.

Should My Cat Be Vaccinated Against FIV?

Through 2021, I believe that Boehringer Ingelheim was still marketing Fel-O-Vax FIV vaccine in Australia. It is composed of an inactivated feline immunodeficiency virus. This vaccine was sold in North America from 2002-2017. It is composed of a strain of inactivated (killed) FIV virus and, I believe, contained a proprietor adjuvant as well to increase its effectiveness. Although Boehringer was not specific about the reasons the vaccine was withdrawn, speculation revolves around several points: Boehringer didn’t sell enough of it in the USA to make production worthwhile. Indoor cats were thought to be at low risk of contracting FIV. The outdoor cats at higher risk rarely if ever received vaccinations. The vaccine offered only limited protection because it only contained one strain of FIV virus. The yearly booster shots this vaccine required (or perhaps the adjuvant included in it) increased the likelihood of cats developing vaccination-related fibrosarcoma cancer at the site of the injection. The vaccine also made it possible for a cat receiving it to test positive on FIV tests for up to four years after a vaccination. That was particularly unfortunate for lost cats that might be mistakenly put down at animal shelters when it was mistakenly assumed that they carried the FIV virus. Later studies in Australia also found the vaccine to be only 56% protective.

Will My Other Cats Catch The FIV Virus?

If you own both FIV-positive and FIV-negative cats and they live in peace with each other (stable, tranquil social structures), the chance that the virus will pass to the FIV-negative cats are said to be low. However I do not know of anyone who has statistically proven that. Cats differ significantly in their FIV virus loads depending on their innate ability to control virus numbers, their age and the stage of FIV or FAIDS that they are currently in. Your group’s individual social habits and home environment probably also come into play as well. 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 When It Still Tests Negative for FIV?

Since the vaccine against FIV is not currently available and was at its best only moderately effective, you need to keep your cat safe from exposure to the feline immunodeficiency virus. That means no exposure to FIV+ cats or cats of unknown FIV virus status. If you are partial to having more than a single feline companion of different virus status, it is possible to maintain negative and positive cats in isolated areas of your home – if you are up to that challenge. In-office tests to detect FIV-carrier cats are not foolproof. They miss some cats that are carrying the virus and falsely accuse other cats of being virus carriers. If you are going to go through the expense and inconvenience of isolating your cats to various locations in your home based on their virus status, the best way to do that is through institutionally-performed PCR tests. I referenced the abstract of an article regarding that in the previous paragraph. At last glance, the full article in PDF was available on ReseachGate.

The FIV virus will not survive for more than a few hours in a normal household environment where common household disinfectants are used to sanitize food and water containers, litter boxes, toys and bedding. After removing grime, I use dilute 1:20 beach in water for utensils and mopping or 1:30 in a mist bottle. When it comes to destroying virus and bacteria, nothing is as effective as common household bleach. 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 will cause a FIV+ cat to become an FIV- cat. No medication, tonic, herbal or complementary brew has been shown to make FIV+ cats live longer. Feline immunodeficiency positive cats that are in apparent good health need no special treatment. The three most common ways to confirm good health in cats are infrequent or lack of infections, close to normal laboratory blood test results and a stable body weight.

Antibiotics:

Most of the illnesses seen in FIV+ cats are due to their decreased resistance to bacterial infections. In those cats, commonly used antibiotics are quite effective. However they might need to be given for a bit longer period than non-carrier cats require.

Bacteria are clever in developing resistance to antibiotics. Some antibiotics that work in the intestine (such as metronidazole/Flagyl®) can be given indefinitely. But over time, antibiotics that perfuse the body generally loose their effectiveness against the bacteria that reside there. So antibiotics are generally dispensed by your veterinarian only when your cat’s condition warrants them.

Nutritional Support:

Cats with advanced FIV symptoms generally loose weight. So everything you do to encourage your cat to eat and maintain a healthy body weight is 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. The same goes for FeLV+ cats with poor appetites.

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

Cats that face health decline as a result of the FIV virus do so because their immune systems have less ability to protect them. A number of medications have been discussed as to their ability to increase the power of your cat’s immune system. In my opinion, none of them have ever been proven to be of benefit to your FIV+ cat. But some cat owners, perhaps you, some veterinarians and many with products they wish to sell you believe they are worth a try on the outside chance that you might gain a little more precious time with your pet. I personally have no faith in any of them; but I see no harm in giving them to your FIV+ cat. Here are a few:

Lymphocyte T-Cell Immunomodulator (LTCI)

In 2006, the USDA issued a condition license for a drug called Lymphocyte T-Cell Immunomodulator (LTCI). The USDA is considerably more lax than the FDA when issuing tentative drug approvals. Supposedly they will “revisit” the drug at some later date to see if it does anything positive. Since the USDA approval, no articles I know of have appeared in the literature documenting the effectiveness of T Cell modulators, enhancers or adjuvants. I do see that the product is still being sold as a treatment for osteoarthritis in dogs. 

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, an 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 obtain 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 to human AIDS patients. The most common of these are AZT (Retrovir®) and PMEA . The results have not been as good as one might hope for. AZT has resulted in toxic complications in cats that include, anemia and liver or bone marrow damage.

At one point veterinarians at the University of California, Davis, explored the possibility that a histone inhibitor, SAHA (vorinostat/Zolina®) 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)  

How Long Might My FLV+ Cat Live?

As many others and I have mentioned, there is often a long asymptomatic period before your FIV+ cat shows signs of illness – and a few cats never do. 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 emanating from cat placement-oriented sources and animal rights advocates tend to be considerably more optimistic than those that do not. No one really knows and few would willingly venture into such an emotionally charged issue.

My Last Cat Died From Feline immunodeficiency/FIV-Related Problems. When Will It Be Safe For Me To Get Another Cat Or Kitten?

I would wait a month before bringing a negative cat into an area that was home to an FIV+ or FeLV+ cat. You never know what other pathogens an immunosuppressed cat might have harbored. It is true that both viruses die very rapidly when they are outside of a cat’s body and that the viruses are killed by ordinary household disinfectants, drying and sunshine. But for your peace of mind and out of respect to your lost feline family member, I think that a month or two is not too long a wait. If you wish, read my thoughts about pet loss here.   

What Special Care Does My FIV+ Cat Require?

As I mentioned earlier, many feline immunodeficiency positive cats do not require any special care. Keep them indoors, feed them a quality or home cooked diet and enjoy each others company.

The general health of all creatures suffer when they are under chronic stress and FIV+ cats are no exception. (read here) What you regard as pleasure or a minor inconvenience your cat may not. The same low-stress lifestyle I suggest for FeLV+ cats are the ones that are likely to benefit your cat the most: avoid stays at boarding kennels, group homes, travel, veterinary hospital inpatient care, optional visits, and grooming shops as best you can. When you travel, hire a cat sitter. Avoid situations that appear to make you cat fearful or unhappy. All of those situations are likely to increase the stress level in its life. Read more about that here. Keep a diary of your cat’s weight. Be particularly attentive to signs of ill health or behavioral changes. Attend to your cat personally. Don’t relegate those chores to others. Regularly check your cats weight, urinary habits and stool consistency. A once a year veterinary visit for blood and urine analysis should be sufficient if there has been no noticeable change in your pet’s behavior, weight or general health. Avoid feeding raw meat or fish. (read here   & here) and practice scrupulous flea control.

You are on the Vetspace animal health website 

Visiting the products that you see displayed on this website help pay the cost of keeping these articles on the Internet.