Understanding Feline Retroviruses: A Guide to FIV and FeLV

Feline Immunodeficiency Virus (FIV) and Feline Leukaemia Virus (FeLV) are two distinct viral infections that affect cats. While they both belong to the retrovirus family and are frequently tested for simultaneously, they fall into different subcategories and impact feline health in very different ways.

FIV is classified as a lentivirus, which typically results in a lifelong, slowly progressing condition. In contrast, FeLV is a gamma retrovirus, known for its potential to trigger cancerous cellular mutations. Thanks to widespread testing and vaccination efforts, FeLV is now relatively rare in the UK, affecting less than 1% of the feline population. FIV is slightly more common, with an estimated prevalence of around 4%.

Feline Immunodeficiency Virus (FIV)

The Nature of the Virus

FIV operates similarly to the Human Immunodeficiency Virus (HIV) by compromising the immune system. However, these viruses are strictly species-specific; humans cannot contract FIV, and cats cannot catch HIV.

Transmission and Infection

The virus is shed in bodily fluids, particularly blood and saliva. Because FIV is highly fragile and degrades rapidly outside a host, it cannot be carried on human clothing or hands. Transmission almost exclusively requires direct, aggressive contact, such as deep bite wounds sustained during territorial fights. It can also be spread through mating or passed from an infected mother to her litter.

Once a cat contracts FIV, the infection is permanent. The feline immune system will generate antibodies, but they are incapable of neutralizing the virus.

Symptoms and Progression

FIV is characterized by a prolonged dormant phase. An infected cat might live for years without showing a single sign of illness. When symptoms do arise, they are usually the result of a weakened immune system rather than the virus itself. Affected cats become highly susceptible to secondary ailments, which may include:

• Chronic respiratory infections (cat flu)

• Severe dental and gum inflammation

• Gastrointestinal issues and weight loss

• Persistent fevers and general lethargy

• Slow recovery times from minor illnesses

• An increased risk of developing tumors

Diagnosis

Veterinarians typically use a rapid blood test to check for FIV antibodies. Because false positives can happen, it is standard practice to send positive samples to an external laboratory for confirmation, especially if the cat appears perfectly healthy.

Testing kittens under six months old requires special consideration. They may carry harmless antibodies inherited from an infected mother without actually harboring the virus (only about 33% of kittens born to FIV-positive mothers contract the disease). Specialized viral tests or delayed re-testing is necessary for young cats. Additionally, recently infected cats might test falsely negative if their bodies haven't produced antibodies yet.

Management and Care

There is no cure for FIV. Veterinary care focuses on managing symptoms as they appear, which often involves prescribing antibiotics or anti-inflammatories for secondary infections, and occasionally utilizing antiviral medications.

FIV-positive cats can still live fulfilling lives, provided their environment is carefully managed. They should be kept strictly indoors or restricted to fully enclosed, secure outdoor runs to prevent them from spreading the virus or catching illnesses from neighborhood cats. Maintaining a high-quality diet, keeping up with routine vaccinations, and administering regular parasite preventatives are crucial steps in supporting their compromised immune systems.

Boarding and Multi-Cat Environments

Most boarding facilities will accommodate FIV-positive cats as long as they are asymptomatic. Because the virus requires aggressive contact to spread and is easily destroyed by standard cleaning agents, routine cattery hygiene is sufficient to prevent transmission. However, owners should be aware that the stress of boarding and proximity to other animals could expose their immunocompromised cat to secondary infections.

Prevention

Currently, there is no FIV vaccine available in the UK. The most effective preventative measure is neutering or spaying your cat, which drastically reduces the urge to roam and engage in the territorial fighting that spreads the disease.

Feline Leukaemia Virus (FeLV)

The Nature of the Virus

FeLV is a severe, often fatal viral infection that attacks a cat's immune system, leaving them highly vulnerable to severe anemia, secondary infections, and various forms of cancer. Like FIV, it poses zero risk to humans.

Transmission and Infection

Unlike FIV, FeLV is easily spread through casual, friendly contact. The virus is heavily concentrated in saliva, meaning cats can infect one another simply by mutual grooming or sharing food and water dishes. It is also transmitted through mating and from a mother to her kittens via the placenta or nursing.

Interestingly, not all cats exposed to FeLV become permanently infected. Some felines possess strong enough immune responses to eradicate the virus within a few weeks, granting them future immunity. However, kittens, cats with weakened immune systems, or those exposed to high viral loads are much more likely to develop a permanent infection.

Symptoms and Progression

FeLV also features an incubation period, but the prognosis is significantly worse than that of FIV. Approximately 80% of cats with a persistent FeLV infection will pass away within three and a half years of diagnosis.

Clinical signs are diverse and severe, including:

• Swollen, often cancerous lymph nodes (frequently felt around the throat, armpits, or groin)

• Profound anemia

• Chronic, recurring infections (respiratory, oral, or digestive)

• Reproductive failures

• Lethargy and persistent fevers

• The development of various cancers, such as lymphoma

Diagnosis

FeLV is diagnosed via a rapid blood test that detects the presence of the virus itself. As with FIV, laboratory confirmation is highly recommended to rule out false positives. Because some cats successfully fight off the initial infection, vets often advise re-testing a few weeks later. If a cat has had known exposure to an FeLV-positive animal, they should be isolated and tested after an appropriate waiting period to avoid false-negative results.

Management and Care

There is no cure for FeLV, and treatment is entirely palliative. Vets will address secondary infections with medications as needed, but the long-term outlook is generally poor. Due to the severe decline in quality of life, euthanasia is frequently necessary.

Infected cats must be kept strictly indoors or in secure enclosures to protect them from environmental pathogens and to prevent them from infecting the local cat population.

Prevention

Unlike FIV, a highly effective vaccine exists for FeLV. While it cannot help a cat that is already infected, it is a vital tool for protecting healthy cats. Even vaccinated cats should be kept away from known FeLV-positive felines, as no vaccine offers 100% guaranteed protection. Annual booster shots are essential for maintaining immunity and keeping the overall prevalence of the disease low.

Welcoming a New Cat After Loss

If you have recently lost a pet to either FIV or FeLV, you do not need to wait long before bringing a new cat into your home. Both viruses are incredibly fragile and die off rapidly in the environment. It is generally considered safe to introduce a new cat after just 24 hours, provided you thoroughly wash and disinfect all food bowls, water dishes, bedding, and litter boxes. If you have surviving cats in the household, it is highly recommended to have them tested for retroviruses before introducing a new feline companion.