Feline Immunodeficiency Virus Infection

Feline immunodeficiency virus (FIV) infection is a chronic, progressive, and ultimately immunosuppressive retroviral disease of domestic and wild felids, caused by a lentivirus of the family Retroviridae. The disease is characterized by a long asymptomatic carrier state followed by a gradual decline in immune function, leading to increased susceptibility to opportunistic infections, certain neoplasms, and neurologic and hematologic disorders. FIV infection is classified into five stages: acute (Stage I), asymptomatic carrier (Stage II), persistent generalized lymphadenopathy (Stage III), AIDS-like complex (Stage IV), and terminal (Stage V). The virus targets CD4+ T lymphocytes, macrophages, and dendritic cells, causing profound immune dysregulation. FIV is transmitted primarily through bite wounds, making outdoor, intact male cats the highest risk population. Diagnosis relies on serologic detection of antibodies (ELISA, Western blot) or molecular detection of proviral DNA (PCR). There is no cure, but management focuses on supportive care, treatment of secondary infections, and in some regions, the use of immunomodulatory or antiretroviral drugs. Prognosis is variable, with many infected cats living years of good quality life if managed appropriately.

The causative agent is the feline immunodeficiency virus (FIV), a lentivirus belonging to the family Retroviridae, subfamily Orthoretrovirinae. FIV is an enveloped, single-stranded positive-sense RNA virus with a diameter of approximately 100-125 nm. The viral genome encodes three major structural genes (gag, pol, env) and accessory genes (vif, rev, tat, and in some strains, dUTPase). The envelope glycoprotein gp120 mediates binding to the primary cellular receptor CD134 (OX40) and the chemokine coreceptor CXCR4, facilitating entry into CD4+ T lymphocytes, macrophages, and dendritic cells. After entry, the viral RNA is reverse-transcribed into proviral DNA, which integrates into the host genome, establishing lifelong infection. FIV is species-specific and does not infect humans. There are five subtypes (clades A-E) based on env gene sequences, with subtype B being most common in North America and Europe, and subtype A in the UK and South Africa. Transmission occurs primarily via inoculation of virus-laden saliva through bite wounds during aggressive encounters. Vertical transmission (in utero, intrapartum, or via milk) is possible but less common. Sexual transmission is rare. The virus is fragile in the environment and is inactivated by common disinfectants, heat, and desiccation.

FIV infection is distributed worldwide, with prevalence varying by geographic region, cat population, and risk factors. In North America, seroprevalence in healthy cats is approximately 2-5%, but in high-risk populations (outdoor, aggressive, intact male cats) it can reach 15-20%. In Europe, prevalence ranges from 1-10%, with higher rates in southern Europe and in cats with clinical signs. In Asia and Australia, prevalence can be higher, up to 20-30% in some feral colonies. The infection is more common in male cats, particularly those with outdoor access and a history of fighting. Age distribution is bimodal, with peaks in young adults (2-4 years) and older cats (8-10 years). No breed predisposition is recognized, but purebred cats may have lower risk due to indoor confinement. FIV is endemic in many wild felid species, including lions, pumas, and cheetahs, but these infections are often nonpathogenic. The disease is not zoonotic. Risk factors include intact male status, outdoor access, and multi-cat households with aggressive interactions. Indoor-only cats have a very low risk of infection.

The pathophysiology of FIV infection is complex and involves progressive immune dysregulation. After primary inoculation, the virus replicates in local dendritic cells and macrophages, then spreads to regional lymph nodes, where it infects CD4+ T lymphocytes. Acute viremia occurs within 1-2 weeks, leading to a transient febrile illness and lymphadenopathy. The virus establishes a chronic, lifelong infection with high viral turnover, particularly in lymphoid tissues. The hallmark of FIV infection is the progressive depletion of CD4+ T cells, leading to a reversed CD4:CD8 ratio. The mechanisms of CD4+ T-cell loss include direct viral cytopathic effects, apoptosis of uninfected bystander cells, cytotoxic T-lymphocyte (CTL) responses, and antibody-dependent cell-mediated cytotoxicity. Chronic immune activation leads to exhaustion of T-cell responses and increased expression of programmed cell death protein 1 (PD-1). Additionally, FIV infects macrophages and microglia, contributing to neurologic dysfunction and serving as a viral reservoir. The virus also causes B-cell dysregulation, leading to hypergammaglobulinemia and increased risk of B-cell lymphomas. The progressive immunosuppression results in susceptibility to opportunistic infections (e.g., feline herpesvirus, calicivirus, Toxoplasma gondii, Mycoplasma haemofelis, Candida albicans) and certain neoplasms (lymphoma, leukemia). The clinical course is variable, with some cats remaining asymptomatic for years, while others progress to AIDS-like syndrome within 2-5 years.

Predisposing factors for FIV infection include: (1) Outdoor access, which increases exposure to infected cats and fighting behavior; (2) Intact male status, due to territorial aggression and roaming; (3) Age: young adult cats (2-4 years) are more likely to be infected due to high-risk behaviors; (4) Multi-cat households with aggressive interactions; (5) Lack of neutering, which increases roaming and fighting; (6) Co-infections with other pathogens (e.g., FeLV, Mycoplasma haemofelis, Toxoplasma gondii) which may accelerate disease progression; (7) Immunosuppressive therapy (e.g., corticosteroids) which may exacerbate viral replication; (8) Poor nutrition and stress, which can impair immune function; (9) Genetic factors: certain MHC haplotypes may influence susceptibility and progression; (10) Environmental factors: overcrowding and poor sanitation may increase transmission risk.

Clinical signs of FIV infection vary depending on the stage of disease. Acute stage (Stage I): Occurs 4-6 weeks post-infection, characterized by transient fever, lethargy, lymphadenopathy, and sometimes neutropenia. These signs are often mild and may go unnoticed. Asymptomatic carrier stage (Stage II): This is the longest phase, lasting months to years, during which the cat appears clinically normal despite persistent infection. Persistent generalized lymphadenopathy (Stage III): Some cats develop generalized lymph node enlargement without other signs. AIDS-like complex (Stage IV): This stage is characterized by chronic, recurrent, or opportunistic infections. Common signs include: (1) Stomatitis and gingivitis (severe, refractory); (2) Chronic rhinitis and sinusitis; (3) Upper respiratory infections (herpesvirus, calicivirus); (4) Dermatitis and skin abscesses; (5) Chronic diarrhea (due to bacterial or protozoal infections); (6) Weight loss and cachexia; (7) Fever of unknown origin; (8) Neurologic signs: behavioral changes, seizures, peripheral neuropathies, and cognitive dysfunction; (9) Hematologic abnormalities: anemia, leukopenia, thrombocytopenia; (10) Ocular signs: uveitis, glaucoma, chorioretinitis; (11) Neoplasia: lymphoma (especially B-cell), leukemia, squamous cell carcinoma. Terminal stage (Stage V): Severe immunosuppression leading to overwhelming infections, severe cachexia, and multi-organ failure. The clinical course is highly variable; some cats remain in Stage II for life.

Differential diagnoses for FIV infection include: (1) Feline leukemia virus (FeLV) infection: Both cause immunosuppression and similar clinical signs, but FeLV is more commonly associated with anemia, lymphoma, and myelosuppression. Diagnosis via ELISA for p27 antigen or PCR. (2) Feline infectious peritonitis (FIP): Causes fever, weight loss, and effusions; diagnosis via coronavirus serology, Rivalta test, and histopathology. (3) Toxoplasmosis: Can cause uveitis, neurologic signs, and fever; diagnosis via serology (IgM/IgG) and response to clindamycin. (4) Hemoplasmosis (Mycoplasma haemofelis): Causes hemolytic anemia; diagnosis via blood smear or PCR. (5) Chronic stomatitis/gingivitis: Can be due to dental disease, calicivirus, or autoimmune conditions; FIV is a common underlying cause. (6) Lymphoma: Can cause lymphadenopathy, weight loss, and organ infiltration; diagnosis via cytology/histopathology. (7) Systemic fungal infections (e.g., cryptococcosis, histoplasmosis): Cause respiratory, cutaneous, and systemic signs; diagnosis via cytology, histopathology, and antigen testing. (8) Chronic bacterial infections (e.g., abscesses, pyothorax): Can cause fever and lethargy; diagnosis via culture and imaging. (9) Autoimmune diseases (e.g., immune-mediated hemolytic anemia, thrombocytopenia): Can cause similar hematologic abnormalities; diagnosis via Coombs test, flow cytometry. (10) Nutritional deficiencies (e.g., thiamine deficiency): Can cause neurologic signs; diagnosis via response to supplementation.

The diagnostic algorithm for FIV infection begins with a thorough history and physical examination, with particular attention to risk factors (outdoor access, fighting, male sex) and clinical signs suggestive of immunosuppression. Initial screening is typically performed using a point-of-care ELISA test that detects antibodies against FIV p24 and gp40 antigens. A positive result should be confirmed with a second, different method, such as Western blot or immunofluorescence assay (IFA), especially in low-risk populations or if the result is unexpected. In kittens under 6 months of age, maternal antibodies may cause false positives; therefore, a positive test in kittens should be repeated after 6 months of age. PCR testing for proviral DNA is available and can be used to confirm infection, particularly in cats with indeterminate serology or in early infection before seroconversion (which may take up to 8 weeks). However, PCR is not recommended as a sole screening test due to variability in sensitivity. If the cat is clinically ill, additional diagnostic workup is warranted to identify secondary infections or neoplasia: complete blood count, serum biochemistry, urinalysis, FeLV antigen testing, and imaging (thoracic radiographs, abdominal ultrasound). In cases with neurologic signs, cerebrospinal fluid analysis and MRI may be indicated. The diagnostic algorithm should also include assessment of CD4:CD8 ratio via flow cytometry, which can help stage the disease and predict progression.

Laboratory findings in FIV infection are variable and often reflect secondary complications. Hematology: Common abnormalities include neutropenia (especially in acute stage), lymphopenia, anemia (normocytic, normochromic, or hemolytic), and thrombocytopenia. Leukocytosis may occur with secondary infections. Serum biochemistry: Hyperglobulinemia (polyclonal gammopathy) is common due to chronic immune stimulation. Hypoalbuminemia may occur with chronic disease. Liver enzymes (ALT, ALP) may be elevated due to hepatic lipidosis or lymphoma. Renal parameters (BUN, creatinine) may be elevated if chronic kidney disease develops. Electrolyte imbalances may occur with vomiting or diarrhea. Urinalysis: Proteinuria may be present with glomerulonephritis. Specific biomarkers: FIV antibody testing (ELISA, Western blot) is the mainstay. PCR for proviral DNA is confirmatory. CD4:CD8 ratio is often decreased (<1.0). Flow cytometry can quantify T-cell subsets. Other biomarkers: Alpha-1 acid glycoprotein (AGP) may be elevated in chronic inflammation. Feline pancreatic lipase immunoreactivity (fPLI) may be elevated if pancreatitis is present. NT-proBNP may be elevated with cardiac disease. Serology for co-infections (FeLV, Toxoplasma, Mycoplasma) is recommended.

Imaging findings in FIV infection are nonspecific but may reveal secondary conditions. Thoracic radiographs: May show evidence of pneumonia (interstitial or alveolar patterns), pleural effusion (with pyothorax or lymphoma), or mediastinal masses (lymphoma). Abdominal ultrasound: May reveal lymphadenopathy (enlarged mesenteric or iliac lymph nodes), hepatosplenomegaly, renal changes (if chronic kidney disease), or intestinal thickening (with inflammatory bowel disease or lymphoma). Echocardiography: May be indicated if cardiac disease is suspected. Computed tomography (CT) and magnetic resonance imaging (MRI): Useful for evaluating neurologic signs (e.g., cerebral atrophy, meningoencephalitis) or for staging lymphoma. Endoscopy: May be used to evaluate chronic vomiting or diarrhea and to obtain biopsies. Fluoroscopy: Rarely needed. Imaging is not diagnostic for FIV itself but is essential for identifying complications.

Cytology and histopathology are valuable for diagnosing secondary infections and neoplasia. Fine needle aspirates (FNA) of enlarged lymph nodes may reveal reactive hyperplasia or lymphoma (large, atypical lymphocytes). FNA of cutaneous lesions may show inflammatory cells or infectious agents (e.g., fungi, bacteria). Fluid analysis (e.g., pleural or peritoneal effusion) can differentiate transudate from exudate and may reveal neoplastic cells. Histopathology of lymph nodes, liver, spleen, or intestinal biopsies can confirm lymphoma or inflammatory bowel disease. In FIV-infected cats, lymph node histopathology often shows follicular hyperplasia in early stages and lymphoid depletion in later stages. Immunohistochemistry can identify viral antigens in tissues. Bone marrow biopsy may be indicated for persistent cytopenias, revealing myelodysplasia or aplasia. Special stains (e.g., Gram, Giemsa, acid-fast) can identify infectious agents. Histopathology is also useful for diagnosing toxoplasmosis (tachyzoites or cysts) or fungal infections.

There is no cure for FIV infection, but treatment aims to manage clinical signs, treat secondary infections, and support immune function. The cornerstone is supportive care: (1) Nutritional support: High-quality, palatable diet; consider appetite stimulants (e.g., mirtazapine 1.88 mg/cat PO q48h) or feeding tubes if anorexic. (2) Fluid therapy: For dehydration or shock, use balanced crystalloids (e.g., Lactated Ringer's solution) at maintenance rates (60 ml/kg/day) or shock doses (90 ml/kg IV bolus). (3) Treatment of secondary infections: Antibiotics should be chosen based on culture and sensitivity. Common choices include amoxicillin-clavulanate (12.5-25 mg/kg PO q12h), doxycycline (5-10 mg/kg PO q12h), or azithromycin (5-10 mg/kg PO q24h). For stomatitis, clindamycin (11 mg/kg PO q12h) or metronidazole (10 mg/kg PO q12h) may be used. Antiviral therapy: Zidovudine (AZT) at 5-10 mg/kg PO q12h has shown some efficacy in reducing viral load and improving clinical signs, but its use is limited by bone marrow suppression. Interferon-omega (1 MU/kg SC q24h for 5 days, repeated) may be used as an immunomodulator. Immunomodulatory drugs: Recombinant feline interferon-omega, or human interferon-alpha (30 IU PO q24h) may be used. Corticosteroids (e.g., prednisolone 1-2 mg/kg PO q24h) may be used for immune-mediated conditions but should be used cautiously. For lymphoma, chemotherapy protocols (e.g., CHOP) may be considered. Supportive care for neurologic signs: Anticonvulsants (e.g., phenobarbital 2.5 mg/kg PO q12h) if seizures. Management of chronic kidney disease: If present, follow IRIS guidelines (e.g., renal diet, ACE inhibitors). Dental care: Professional cleaning and extractions for stomatitis. Environmental management: Keep cats indoors to reduce exposure to secondary infections and prevent transmission.

The prognosis for FIV-infected cats is highly variable. Many cats remain asymptomatic for years and have a normal lifespan. Median survival time from diagnosis is approximately 5 years, but some cats live 10 years or more. Factors associated with a poorer prognosis include: (1) Advanced age at diagnosis; (2) Presence of clinical signs at diagnosis (especially stomatitis, chronic infections, or neurologic signs); (3) Low CD4:CD8 ratio (<0.5); (4) High viral load; (5) Co-infection with FeLV; (6) Development of lymphoma or other neoplasia; (7) Poor body condition. Cats that are asymptomatic at diagnosis and receive good preventive care have a good prognosis. Regular veterinary check-ups, prompt treatment of infections, and a stress-free indoor environment can improve outcomes. The terminal stage is characterized by severe immunosuppression and is ultimately fatal.

FIV-infected cats should be monitored regularly, at least every 6 months, or more frequently if clinical signs develop. At each visit, a thorough physical examination should be performed, including oral cavity examination, lymph node palpation, and body condition scoring. Baseline laboratory tests (CBC, serum biochemistry, urinalysis) should be repeated every 6-12 months to monitor for anemia, leukopenia, hyperglobulinemia, and renal or hepatic changes. CD4:CD8 ratio and viral load (if available) can be measured periodically to assess disease progression. Dental prophylaxis should be performed every 6-12 months to prevent stomatitis. Owners should be educated to monitor for signs of secondary infections (e.g., sneezing, diarrhea, weight loss) and seek veterinary care promptly. Vaccination: FIV vaccines are available in some countries but are not universally recommended; if used, they can interfere with serologic testing. Cats should be kept indoors to prevent transmission and reduce exposure to pathogens. Spaying/neutering is recommended to reduce fighting behavior. Nutritional support and supplementation with omega-3 fatty acids may be beneficial. In cats with chronic kidney disease, follow IRIS guidelines for monitoring and treatment.

Pearls: (1) A positive FIV antibody test in a healthy cat should not be considered a death sentence; many cats live long, healthy lives. (2) Always confirm a positive ELISA with a second test (Western blot or IFA) to avoid false positives, especially in low-risk cats. (3) In kittens under 6 months, a positive test may be due to maternal antibodies; retest after 6 months. (4) FIV-infected cats are at higher risk for stomatitis, so proactive dental care is essential. (5) Use antibiotics judiciously; culture and sensitivity are important due to increased risk of resistant infections. (6) Consider FIV testing in any cat with chronic, recurrent infections, especially if they have outdoor access. (7) Provide a stress-free environment to minimize immune suppression. Pitfalls: (1) Do not euthanize a cat solely based on a positive FIV test; many cats have good quality of life. (2) Avoid using corticosteroids indiscriminately, as they can exacerbate immunosuppression. (3) Do not rely solely on PCR for diagnosis; false negatives can occur. (4) Do not vaccinate FIV-infected cats with modified-live vaccines, as they may cause disease; use killed vaccines only. (5) Do not overlook co-infections such as FeLV, hemoplasma, or Toxoplasma, which can worsen prognosis. (6) Do not ignore dental disease; it is a common source of pain and infection.

Drug protocols for FIV infection are primarily supportive and aimed at treating secondary infections and complications. Antibiotics: Amoxicillin-clavulanate (12.5-25 mg/kg PO q12h) for skin, respiratory, and urinary infections. Doxycycline (5-10 mg/kg PO q12h) for Mycoplasma, Chlamydia, and Bartonella. Clindamycin (11 mg/kg PO q12h) for toxoplasmosis, dental infections, and anaerobic infections. Azithromycin (5-10 mg/kg PO q24h) for respiratory and skin infections. Metronidazole (10 mg/kg PO q12h) for diarrhea and anaerobic infections. Antiviral: Zidovudine (AZT) (5-10 mg/kg PO q12h) may reduce viral load and improve clinical signs, but monitor for anemia and neutropenia. Interferon-omega (1 MU/kg SC q24h for 5 days, then every 48h for 3 doses) as immunomodulator. Immunomodulators: Recombinant feline interferon-omega (Virbagen Omega) at 1 MU/kg SC q24h for 5 consecutive days, repeated every 6 months if needed. Human interferon-alpha (30 IU PO q24h) may be used. Corticosteroids: Prednisolone (1-2 mg/kg PO q24h) for immune-mediated conditions, but use with caution. Appetite stimulants: Mirtazapine (1.88 mg/cat PO q48h) or cyproheptadine (2 mg/cat PO q12h). Antiemetics: Maropitant (1 mg/kg SC q24h) or ondansetron (0.5-1 mg/kg IV q12h). For chronic kidney disease: ACE inhibitors (benazepril 0.25-0.5 mg/kg PO q24h) and renal diet. For lymphoma: Chemotherapy protocols (e.g., CHOP: cyclophosphamide, doxorubicin, vincristine, prednisolone) should be managed by a veterinary oncologist. Always adjust dosages for renal or hepatic impairment and monitor for drug interactions.

Key studies and consensus guidelines: (1) The 2020 AAFP Feline Retrovirus Testing and Management Guidelines provide evidence-based recommendations for diagnosis and management of FIV and FeLV. They recommend testing all cats at risk, using confirmatory tests, and managing infected cats as individuals with regular monitoring. (2) A study by Levy et al. (2008) found that FIV-infected cats in a long-term study had a median survival of 5 years, with many living longer, and that clinical stage at diagnosis was a significant predictor of survival. (3) A study by Hartmann et al. (2015) evaluated the efficacy of zidovudine (AZT) in FIV-infected cats and found modest improvements in clinical signs and CD4 counts, but no significant effect on survival. (4) A study by Gil et al. (2013) showed that recombinant feline interferon-omega improved clinical signs and quality of life in FIV-infected cats with stomatitis. (5) A meta-analysis by Hosie et al. (2009) on FIV vaccination concluded that current vaccines have variable efficacy and can interfere with serologic testing, so they are not universally recommended. (6) The European Advisory Board on Cat Diseases (ABCD) guidelines recommend that FIV-infected cats be kept indoors, neutered, and receive regular veterinary care to prevent secondary infections. (7) A study by Diehl et al. (2014) found that CD4:CD8 ratio is a useful prognostic indicator, with ratios <0.5 associated with shorter survival. (8) A study by Litster et al. (2014) on co-infections found that FIV-infected cats are more likely to be co-infected with Mycoplasma haemofelis and Bartonella henselae, which can exacerbate clinical signs. Overall, the evidence supports a proactive, individualized approach to managing FIV-infected cats, with emphasis on preventive care and early treatment of secondary conditions.