Current Vet

In this episode of Current Vet, Dr Lottie talks about feline leukaemia virus (FeLV)

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Timeline:
00:00 Intro
00:37 Case
01:57 Aetiology & Pathogenesis
06:18 Clinical Signs
07:56 Diagnosis
13:23 Treatment
16:13 Prevention
17:05 Key Points
18:48 Outro

Recommended Reading
  • ABCD’s Guidelines for Feline Leukaemia Virus Infection
  • AAFP’s Feline Retrovirus Testing and Management Guidelines
References

Current Vet is an educational podcast intended for veterinary students, veterinary professionals, and individuals with an interest in veterinary medicine.

All content provided in this podcast and its associated materials is for educational and informational purposes only. It is not intended as, and must not be considered a substitute for, professional veterinary advice, diagnosis, or treatment.

Any clinical cases discussed in this podcast are fictional, and are designed to reflect typical or likely clinical scenarios for educational purposes. They do not represent specific real-life cases, clients, or animals.

While every effort is made to ensure accuracy and alignment with current evidence at the time of publication, veterinary medicine is a rapidly evolving field, and recommendations may change over time.

Creators and Guests

Host
Dr. Lottie Wilkinson
Creator and host of the Current Vet podcast

What is Current Vet?

The podcast that makes veterinary medicine simple.

In each episode, Dr. Lottie breaks down clinical conditions, cases, and concepts across species, focusing on pathophysiology, decision-making, diagnostics, and what actually matters in practice. It’s the kind of context that makes your knowledge finally click.

Every month, we’ll also have honest conversations with guests about the incredible variety of veterinary medicine, what you can do with a vet degree and how to think bigger about your career.

Whether you’re cramming for exams or looking for a soundtrack for your dog walk, Current Vet will make veterinary medicine simple

Intro

Hello, hello and welcome back to Current Vet, the podcast that makes veterinary medicine simple. I am your host, Dr. Lottie, and today we've got a third cat episode for this month, and that is Feline Leukemia Virus or FELV. So today we're gonna talk all about FELV, with our case, of course, for reference. And by the end of this episode, you should feel confident explaining what it is, how it works, how we diagnose it, and what management should look like.

Let's start with our case.

Case

You are in small animal practice and you're seeing marmalade who is a very handsome 2-year-old male neutered, domestic short hair marmalade, was adopted from a rescue about a year ago. He's an indoor outdoor cat. Lives in a household with one other cat, and up until recently has been pretty healthy.

His owner brings him in today because over the past few weeks he's been losing weight, eating less, and just being a bit quieter than normal over the last few days. They've also noticed he seems to get tired more easily and is breathing heavily after activity. On clinical exam, marmalade is quiet but responsive.

His body condition is a three out of nine. His mucus membrane's slightly pale, and when you palpate his peripheral lymph nodes, they feel large. Thoracic occultation also sounds a little bit. Muffled. So at this point, your differential list may not be that serious because he's so young. We don't really assume the worst when it comes to young cats.

And this presentation could be a chronic infection, but FELV should be somewhere on your radar. So let's now talk about what it actually is and how it's going to cause disease.

Aetiology & Pathogenesis

Feline Leukemia virus is an oncogenic retrovirus of cats. So retrovirus means it carries RNA not DNA, and it uses an enzyme called reverse transcriptase to convert this RNA genome into DNA. Once it enters a host cell, that viral, DNA then integrates into the host genome as a provirus, and once it's integrated, it can persist in that cell's genome and that animal's genome for their whole life.

Okay. This ability to integrate into host DNA explains why FELV can cause lifelong infection, evade the host's immune response, disrupt normal cell regulation, and increase the risk of neoplasias developing particularly lymphoma and leukemia. FELV is actually very fragile in the environment and doesn't survive well outside the host, which means transmission requires close contact, as most commonly is through saliva.

But the virus is also shared in nasal secretions, urine, feces, milk, so grooming, shared bowls, licking, and close maternal contact are the main ways that the virus is spread. It can also spread vertically from queens to kittens in utero, or once the kittens start nursing.

This is why FELV is especially relevant in multi cat households, shelters, rescue environments, and for queens and their litters. Basically any environment where there are lots of cats and lots of friendly contact. What happens to a cat after they've been exposed to FELV depends almost entirely on the host immune response to the virus.

So after exposure, the virus enters through the oral pharyngeal or nasal mucosa. It infects local lymphoid tissue and starts replicating inside lymphocytes and monocytes. From there, we get a primary viremia. And the virus spreads to lymphoid organs like the spleen thymus and peripheral lymph nodes. And at this point, one of three things can happen.

We can have an abortive infection, regressive infection develop, or a progressive infection. So the abortive infection occurs if the immune response is rapid and effective, the virus is eliminated before it can establish itself in the bone marrow. So these cats completely clear the infection do not become persistently infected and do not develop disease.

Outcome two is regressive infection. And this is if the immune response is partially effective, the cat can suppress viral replication. The virus integrates into the host genome as pro Proviral DNA, but active replication stops the viremia resolves and the cat becomes antigen negative. Because the virus is not present in the bloodstream, it is only hiding in the cell's genome.

These cats are often clinically normal, but the virus is not truly gone, so reactivation can occur under stress or if there are periods of immunosuppression. The third outcome of infection is progressive infection, and this occurs if the immune response fails to control the virus and FELV infect bone marrow.

Okay. Once bone marrow precursor cells are infected, newly produced blood cells carry this proviral DNA, secondary viremia develops and the cat becomes persistently antigen and infectious, which means that the virus is constantly present in the bloodstream because it's present in these newly produced blood cells. So these cats are at the highest risk of immunosuppression development, of cytopenias, neoplasia, and they have a reduced life expectancy.

Clinical Signs

But what clinical signs are we gonna see in these infected cats? So common patterns include chronic or recurrent infections because of that immunosuppression, weight loss and lethargy, poor coat quality, and pyrexia of an unknown origin. So there's no overt infection that you are aware of because FELV impairs the immune function.

Secondary infections are common and are often severe or really slow to respond to treatment.

Bone marrow infection and infection of precursor cells causes non regenerative anemia, leukopenia, thrombocytopenia, or pancytopenia in advanced cases. Basically it results in a decreased production of all types of blood cells once the bone marrow is affected, so red blood cells and white blood cells and platelets.

Importantly, FELV is strongly associated with development of neoplasia, especially lymphoma. So in cats like marmalade. A mediastinal lymphoma can develop, which is why we are seeing that muffled heart sounds, muffled lung sounds, weight loss, tachypnea, and reduced exercise intolerance. So at this point in Marmalade’s case, you should be thinking we need to take bloods, we need to do imaging, and we want to do FELV testing.

Diagnosis

Diagnosis of FELV is based on detecting the F-E-L-V-P 27 capsid antigen.

But there are two things that we want to find out when we're testing for FELV. First is, is the cat currently viraemic? And actively infectious. And the second is, is the infection progressive or regressive? So our first step in diagnosis is going to be testing for that antigen. The test of choice is a point of care.

P 27 antigen test, commonly known as a SNAP test. These are widely available, are rapid, and they're usually ELISA or immuno chromatography based. They detect the viral antigen, not antibodies, so results are not going to be affected by vaccination or presence of maternal antibodies. This test can be performed with serum, plasma or whole blood, but saliva is not recommended because it lowers the sensitivity of the test.

So our results are gonna be less reliable. Most cats are going to test positive within around 30 days of exposure. But you have to remember that a single positive test does not mean you have diagnosed progressive infection. So if marmalade tests positive today, that tells you he's currently antigen, but it doesn't tell you what the long-term outcome is gonna be.

Positive antigen tests should always be rechecked after 12 to 16 weeks. If the second test is a positive, again, that indicates progressive infection because the cat has remained antigen for all of those weeks. If the second test is negative, it suggests a regressive infection. So they were initially antigen, but that infection has now been suppressed and the virus is no longer present in the blood.

Ideally, these confirmation tests should use a different test brand or a different test method. So for example, it could follow up with a PCR. So PCR is gonna be used as an adjunctive test, not a standalone test. There are two different types of PCR that we can do, and the first is a proviral DNA test, which is gonna detect integrated viral DNA in host cells.

And this is gonna be positive in both regressive and progressive infections. The second PCR choice is a reverse transcriptase PCR testing for viral RNA, which detects active replication and shedding. This can be done on saliva and it's more useful for population screening and identifying cats that are infectious.

PCR test results should always be interpreted alongside antigen testing. In clinics, we can also run blood work and do some imaging for diagnosis. So on blood's, common findings are gonna include that non regenerative anemia, leukopenia, thrombocytopenia, or pan cytopenia.

In more advanced cases you might also see dysplastic or abnormal blood cells on a smear or some evidence of concurrent infections that develops secondary to that immunosuppression. Biochemistry results are typically pretty non-specific, but they can help you assess the overall systemic health, work out if there's any concurrent organ involvement and assess fitness for further diagnostics or treatment.

So this blood work isn't gonna diagnose FELV on its own, but it tells you how much damage the virus has already caused and helps guide prognosis and next steps. Imaging isn't gonna isn't gonna definitively diagnose FALV either, but it's extremely important for identifying neoplasias if they've developed secondary to infection in a young cat like marmalade.

Thoracic imaging is gonna be especially valuable. So common findings may include a mediastinal mass caused by lymphoma, pleural effusion, generalized lymphadenopathy or organomegaly. So particularly in the spleen known as splenomegaly. Abdominal ultrasound can also be useful for assessing peripheral and visceral lymph nodes, the liver and the spleen, or potentially intestinal involvement if there's an intestinal lymphoma

so basically imaging is going to help us narrow down and rule out differentials stage neoplastic disease. And decide whether further diagnostics or treatment is appropriate. If we go back to marmalade, his blood tests show a non regenerative anemia and mild leukopenia. The rustic imaging also reveals a mediastinal mass, and his FELV antigen test is positive.

So now FELV with associated lymphoma is at the top of your list.

Treatment

When it comes to treatment, we need to be really confident in having empathetic but clear conversations with owners about this condition because there is no cure for FELV. We cannot eliminate the virus once persistent infection has been established. So treatment is gonna focus on managing secondary disease, supporting the cat's quality of life, and reducing the transmission risk to other cats.

Opportunistic infections and secondary infections should be treated as they would be in FELV negative cats, so antibiotics, antimicrobials, et cetera, et cetera. But our course may need to be longer and relapses after treatment are more common. Cats with severe anemia may need blood transfusions ,

and it's worth knowing that drugs like erythropoietin or darbepoetin to stimulate more red blood cell production are typically unsuccessful because there's no. Issue with a lack of erythropoietin. The issue is coming from the fact that the bone marrow is diseased and it can't effectively produce more red blood cells.

Chemotherapy for FELV associated lymphoma is an option and can be successful. But know that prognosis is generally poorer than in FeLV negative cats, and the fact that we already have immunosuppression in most cases is gonna increase the risk of complications occurring, especially if we're causing further immune suppression with chemotherapy.

So this is where our quality of life discussions are gonna be really key.

Immuno stimulants like interferons or antiviral medications like zidovudine have been studied, but the benefits are not consistently proven. They're not standard of care, and they should never be framed as a curative medication.

The most important aspect of care for FELV cats is going to be management. So FELV positive cats should be housed indoors only and be isolated from all other cats. To minimize the risk of transmission, you must avoid raw diets as this increases the risk of them contracting infections. You need to provide low stress environments and the cat should be monitored regularly for conditions, even if they seem clinically well.
For marmalade, our treatment options would include chemotherapy for lymphoma, alongside supportive care, palliative care only, or humane euthanasia depending on how severe the disease is and what the owner's goals are gonna be.

Prevention

Our key strategies to prevent FELV are gonna be routine testing for high risk cats. Say for example, those in high density environments

or kittens born to FELV positive queens, we want to segregate EV positive cats and we can vaccinate at risk cats. The FELV vaccines are considered non-core, but they're recommended for kittens born to positive queens for outdoor cats or cats in multi cat environments, the vaccination helps to reduce the risk of persistent viremia and boost the immune response, but it's important to know that it does not prevent infection entirely, so even vaccinated cats can become infected.

Key Points

Before we wrap up, here are the key things that you should know about. FELV. FELV is an oncogenic retrovirus that integrates into the host genome,

the disease outcome is determined by the host's immune response, resulting in either an abortive, regressive, or progressive infection with progressive infections, carrying the highest risk of clinical disease and the poorest prognosis.

Most cats present with recurrent infections, cytopenias, anemia, or neoplasia, so particularly lymphoma. And diagnosis focuses on detection of the FELV p27 antigen. But repeat testing 12 to 16 weeks later is essential to distinguish progressive from regressive infections only antigen positive V Remic cats are actively shedding the virus and are infectious.

There is no cure for FELV, so management focuses on treating secondary disease, supporting the cat's quality of life, and preventing transmission to others. Prevention focuses on routine testing, vaccination of high risk cats and good management and biosecurity, especially in multi cat and shelter environments.
One resource I will recommend if you want to read more about FELV, is the advisory board on CAT Disease, has amazing guidelines on FELV, which gives a good thorough explanation of all aspects of the condition.

Outro

And that is Feline Leukemia Virus. Thank you so much for listening to this episode of Current Vets.

If you found it helpful, follow, subscribe. Like the podcast, wherever you get your podcasts. Give us a rating if you feel like it, and share it with a friend or colleague who needs to listen to this topic. All of the resources used for the episode are gonna be linked in the show notes. And if there's a topic you'd like to hear next, come and find us on Instagram or TikTok at Veterinary Vista and let us know what it is that you wanna hear next.

Join us next week for our final cat episode of the month, which is going to be on feline hypothyroidism. See you next time.