Feline Immunodeficiency Virus (FIV)

In this episode of Current Vet, Dr Lottie talks about feline immunodeficiency virus (FIV)

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Timeline:
00:00 Intro
01:10 Case
02:25 Aetiology & Pathogenesis
09:32 Clinical Signs
11:25 Diagnosis
13:28 Treatment & Prevention
16:14 Key Points
17:31 Outro

Recommended Reading

• Feline Immunodeficiency Virus: Current Knowledge and Future Directions – Bęczkowski, P.M. & Beatty, J.A.
• 2020 AAFP Feline Retrovirus Testing and Management Guidelines – Little, S. et al.
• Feline Immunodeficiency Virus (FIV) – Nelson, R.W. & Couto, C.G. in Small Animal Internal Medicine

References 

• Bęczkowski, P.M. and Beatty, J.A. (2022) ‘Feline Immunodeficiency Virus: Current Knowledge and Future Directions’, Advances in Small Animal Care, 3(1), pp. 145–159. https://doi.org/10.1016/j.yasa.2022.05.007
• Little, S. et al. (2020) ‘2020 AAFP Feline Retrovirus Testing and Management Guidelines’, Journal of Feline Medicine and Surgery, 22(1), pp. 5–30. https://doi.org/10.1177/1098612X19895940
• Feline Immunodeficiency Virus Infection (2021) in Greene’s Infectious Diseases of the Dog and Cat. W.B. Saunders, pp. 414–428. https://doi.org/10.1016/B978-0-323-50934-3.00033-1
• Chhetri, B.K. et al. (2015) ‘Comparison of risk factors for seropositivity to feline immunodeficiency virus and feline leukemia virus among cats’, BMC Veterinary Research, 11(1), p. 30. https://doi.org/10.1186/s12917-015-0339-3
• Gleich, S. and Hartmann, K. (2009) ‘Hematology and Serum Biochemistry of FIV- and FeLV-infected cats’, Journal of Veterinary Internal Medicine, 23(3), pp. 552–558. https://doi.org/10.1111/j.1939-1676.2009.0303.x
• Westman, M.E., Malik, R. and Norris, J.M. (2019) ‘Diagnosing FIV and FeLV infection: an update for clinicians’, Australian Veterinary Journal, 97(3), pp. 47–55. https://doi.org/10.1111/avj.12781
• de Oliveira Medeiros, S. et al. (2016) ‘Follow-up on long-term antiretroviral therapy for cats infected with FIV’, Journal of Feline Medicine and Surgery, 18(4), pp. 264–272. https://doi.org/10.1177/1098612X15580144
• Hartmann, K., Wooding, A. and Bergmann, M. (2015) ‘Efficacy of Antiviral Drugs against FIV’, Veterinary Sciences, 2(4), pp. 456–476. https://doi.org/10.3390/vetsci2040456
• Nelson, R.W. and Couto, C.G. (2019) ‘Feline Immunodeficiency Virus (FIV)’, in Small Animal Internal Medicine. 6th edn. Elsevier, pp. 1491–1494.
• Tilley, L.P. and Smith, F.W.K. (2021) ‘Feline Immunodeficiency Virus (FIV)’, in Blackwell’s Five-Minute Veterinary Consult: Canine and Feline. 7th edn. Wiley, pp. 506–507.

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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.

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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

Hello, hello and welcome back to Current Vet. If you're new here. This is the podcast that makes veterinary medicine simple. I'm your host, Dr. Lottie, and for our topic episodes in March, we're having a month of cat conditions. I wanna say a massive, massive thank you to everyone who's been listening and supporting the podcast since we launched in January.

And a special thank you to those people who have. I got in touch and requested topics to hear about. A lot of those requests have been cat diseases, so today we're kicking off March by talking about feline immunodeficiency virus or FIV. We've then got some episodes on urethral obstruction, feline leukemia virus, and feline hypothyroidism in the next few weeks.

FIV is a condition that causes a lot of confusion, a lot of anxiety for owners. It's also something that comes up repeatedly in exams, usually alongside FELV, and they're very commonly mixed up. So today we're gonna go through FIV properly.

Case

Let's start with our case. You are in small animal consults, and your next patient is Loki, a 7-year-old entire male, domestic short hair.

His owner brings him in because over the last few months he's been losing weight. He started drooling. He seems uncomfortable when he eats and he still goes to the food bowl, but he'll often walk away halfway through his meal. Loki has had outdoor access and according to his owner. He's never been very friendly with other cats. He's had a couple of cases of cat bite abscesses in the past and has a bit of a reputation at your clinic for being feisty. On physical exam, Loki is quiet but alert. His body condition score is around a three out of nine, and when you look in his mouth, you find marked gingivitis and stomatitis with areas of ulceration along the gingival, but the rest of his exam is fairly unremarkable.

This is the third time Loki has been in with a similar complaint, and the owner tells you he's been on antibiotics and pain relief several times before with things improving briefly, but then the same signs always coming back.

Aetiology & Pathogenesis

So now that we've got our case in mind, let's take a step back and talk about what FIV actually is. So FIV is a lifelong viral infection of cats that causes progressive immunosuppression. It belongs to a group of viruses called lentiviruses, which are a subgroup of the retrovirus family. So lentiviruses are characterized by slow disease progression, and the name comes from the Latin word lenis, which literally means slow.

So these viruses take a long time to establish and cause disease, and that is exactly what happens with FIV. It causes immune suppression over months to years. The virus itself is enveloped, which means it's actually quite fragile in the environment. It has a single stranded positive sense, RNA genome, and it is species specific to cats, so it is not zoonotic.

FIV has a particular tropism for immune cells, especially CD four T lymphocytes. But as well as that, it can infect macrophages, dendritic cells, and other immune cells. So these are the cells that are responsible for coordinating immune responses. So damage caused by the virus has wide reaching consequences all through the body.

There are five recognized subtypes of FIV, labeled A through E, and at the moment there is no consistent evidence that these subtypes actually cause meaningful differences in clinical behavior. So from a practical viewpoint, subtype doesn't actually change how we're managing these cases.

FIV is transmitted primarily through bite wounds. The virus is present in high concentrations in saliva, and a bite wound allows the virus in the saliva to be inoculated directly into tissues and the bloodstream. So this transmission, mainly through bites and fighting, explains why the classic FIV patient is male, entire, adult and has outdoor access, especially those cats with a history of fighting.

So when we think back to Loki's case, his signalman, his lifestyle, that is already putting FIV high on your differential list.

Before we go any further, it's really important to separate FIV from FELV because they're often grouped together, but they actually behave differently. Both of the viruses are retroviruses. They both cause immunosuppression and diseases develop secondary to that immunosuppression. They can both present with vague chronic clinical signs, but the transmission routes, the long-term prognosis and the management are not the same.

So a useful way to remember the difference in transmission routes is: FIV is spread through fighting. So I remember FI for fighting. FELV is spread through love. So L for love, cats love through grooming, sharing food bowls, water bowls, litter trays, nursing their kittens. That close contact, that lovey-dovey contact is what spreads. FELV. And then the fighting is what spreads. FIV.

With FIV, the immunosuppression puts cats at risk of secondary infections, but as long as owners minimize the risk of contracting these conditions through good management, many cats can live normal or close to normal lifespans.

With FELV. The virus induces the formation of neoplastic cells and cancer in the infected cats. So disease is typically life limiting and can lead to death within a few years. This distinction is so important to understand when we're talking to owners, discussing prognosis and management and what the next steps are for them.

Now let's talk about how FIV actually causes disease. So FIV infection is classically described as occurring in three stages. Stage one is acute infection. We then have the subclinical infection. Stage two and stage three is the clinical phase. So. The acute infection stage one occurs weeks or months after infection.
After a bite wound, the virus enters the body. It initially infects dendritic cells and macrophages. These cells then carry the virus to lymphoid tissues, such as lymph nodes and the thymus here, FIV infects T lymphocytes, particularly those CD four T cells.

Because FIV is a retrovirus, it uses an enzyme called reverse transcriptase to convert its RNA genome into DNA. That viral DNA is then integrated into the host cells genome. Once that happens, the virus becomes effectively invisible to the host's immune system and cannot be eliminated. In this stage one phase, some cats may show mild signs like a transient fever, lethargy, or lymphadenopathy, but these are often subtle or missed entirely.

We then have a stage two subclinical infection. So after the acute phase, most cats enter a long subclinical stage, which can last months to years. During this time, viral replication continues at low levels. The immune system is partially controlling the virus but not quite clearing it. CD four T cells gradually decline in levels, and as this occurs, the body upregulates CD eight T-cell production.

So that inverts the normal CD four to CD eight ratio, which is a really classic sign of FIV infection,
but overall, despite this inverted ratio, the overall trend is actually reduced numbers of both CD four and CD eight cells, and that is reflective of the overall immune suppression caused by the virus. Clinically, cats appear completely normal during this stage and that is why many cats can actually be infected for years before FIV is ever diagnosed.

Clinical Signs

Stage three of infection is the clinical phase. So eventually immune dysfunction becomes significant enough and we have enough immune suppression that the cat can no longer cope with everyday pathogens or regulate inflammation properly. This is when clinical disease develops.

And really importantly, these clinical disease processes are secondary to the immunosuppression. It is not directly caused by the virus, and this is the stage that our cat Loki is in.

Essentially the immune dysfunction caused by the virus explains the clinical patterns that we see. So because immune responses are impaired, cats develop recurrent or chronic infections, infections that respond really poorly to treatment or infections that relapse once treatment stops. So, for example, oral disease is very common in FIV cats, and this is because the mouth is constantly exposed to bacteria and without proper immune regulation, cats develop severe gingivitis and stomatitis That can be incredibly painful and really difficult to.

This is also why infected cats can develop recurrent skin infections, GI disease, respiratory problems, ocular conditions, literally diseased in any part of the body. The wound healing capacity is really poor, and the risk of lymphoma increases by around five times because immune surveillance against abnormal cells and neoplastic cell development is reduced.

So when we look at Loki's history of repeated treatments with only temporary improvement and then recurrence, it fits perfectly with this classic presentation of FIV.

Diagnosis

So at this point, you're probably wanting to test Loki and confirm your suspicions. And the most commonly used test in practice is a point of care, ELISA antibody test. This detects antibodies to FIV, not the virus itself. In adult cats, it is highly sensitive and specific, and a positive result generally indicates true infection.

There are a few situations where interpretation is trickier, however, so kittens under six months can test positive because they inherit maternal antibodies if they're born to an FIV positive queen. This does not automatically mean that they will be infected, and in fact, they are very likely to not contract the disease.

All we need to do is retest the kitten at around eight months old, once those maternal antibodies have gone, if they're still testing positive at that stage, they are likely to be truly infected. Previously vaccinated cats can also test positive for long periods of time after the vaccination.

And cats in very early infection stages can give false negative results because antibodies can take up to 60 days to develop after infection has occurred. So you run this ELISA test on Loki and it comes back positive. Given his age, his history, and clinical science. This result is likely to be very accurate.

PCR testing can also be used to clarify unusual or conflicting results. So for example, if you have a negative test, but you, you're really sure based on the clinical presentation that it's gonna be FIV, you could perform a PCR test, but it must be done in a lab. It's less widely available and it's also more expensive.

So not all ELISA tests are followed up with a PCR.

Treatment & Prevention

There is no cure for FIV and there is no effective antiviral treatment that reliably improves outcomes. Antivirals like zidovudine have been studied, but they're expensive, they're inconsistent in their effects, and they're associated with significant side effects, particularly something like anemia.

So they're not recommended for routine use. Our treatment process is gonna focus mainly on managing secondary diseases and supporting the cat long term. For Loki, this means aggressive management of his oral disease, appropriate analgesia, treating secondary infections when they're present. But remember that because of their poor response to treatments, antibiotic courses in FIV cats may be longer than required in otherwise healthy cats.

We also need to plan long-term dental care, and we need to be really careful when giving anti-inflammatories or similar medications in FIV cats because their immune systems are already suppressed and we don't want to put them at even further risk.

So judicious use is essential. For supportive management. We're gonna make sure that we are keeping him indoors only both to minimize his exposure to pathogens, but also to prevent him from fighting with other cats and spreading the disease, especially as he's a known fighter. We want to minimize stress so we're not suppressing the immune system even more.

Feed a high quality balanced diet. Avoid raw feeding because this increases the risk of bacterial and parasitic infections. And we're gonna schedule regular health checks and blood work to make sure that we're on top of this condition and to catch any new diseases in really early stages.

With FIV. Communication with owners really matters. We need to be confident in explaining that FIV is a lifelong incurable disease. But it's not a death sentence. The virus itself isn't what's making Loki sick. We have to explain that minimizing exposure to pathogens which may cause disease is really important. He can have flare-ups at any point in his life. Some conditions and diseases can recur, and early intervention in all cases makes a massive difference. Please remember that with good management, many FIV positive cats can live happy, comfortable, normal length lives.

Key Points

So before we finish, these are the key points to remember. Feline immunodeficiency virus is a lifelong viral infection of cats that causes progressive immunosuppression, predisposing cats to secondary infections, inflammatory conditions, and even the development of certain neoplasias like lymphoma.

Transmission is primarily via bite wounds, so adult male cats with outdoor access are at the highest risk of contracting the disease. Many infected cats can remain clinically normal for years. And if clinical disease does develop, these signs reflect conditions that develop secondary to immunosuppression rather than disease caused by the virus itself.

Diagnosis in practice is based on point of care ELISA antibody tests. And remember, there is no cure for the virus. So treatment focuses on good management and minimizing the risk of the cat contracting secondary can live healthy, normal length lives, and the disease should not be considered a death sentence.

Outro

And that's feline immunodeficiency virus. FIV is a condition that sounds a bit scary, but once we understand the underlying mechanisms, it can become quite manageable. Thank you so much for listening to this episode of Current Vet. If you found this helpful, follow the podcast. Subscribe like it so you can find all of our next episodes easily.

And then also share it with a friend or colleague who needs to hear this episode too. You can find us on Instagram and TikTok @veterinaryvista or comment below to let us know which episodes you'd like to hear explained. Come back next week to continue our cat month with an episode on urethral obstruction.

We'll see you next time.