Feline Immunodeficiency Virus (FIV) – NAVLE Study Guide

Overview and Clinical Importance

Feline Immunodeficiency Virus (FIV) is a lentivirus in the Retroviridae family that causes progressive immune dysfunction in domestic and wild cats worldwide. First isolated in 1986 by Pedersen and Yamamoto at UC Davis, FIV is analogous to Human Immunodeficiency Virus (HIV) in its pathogenesis and clinical manifestations, though it is strictly species-specific and poses no zoonotic risk to humans.

FIV affects approximately 2.5-4.4% of cats in the United States, with higher prevalence in outdoor, intact male cats due to the primary transmission route of bite wounds. Understanding FIV is essential for the NAVLE as it frequently appears in questions regarding retroviral infections, diagnostic testing interpretation, and management of immunocompromised feline patients.

Etiology and Virology

Viral Classification and Structure

FIV is a lentivirus (family Retroviridae), characterized by slow disease progression and lifelong persistent infection. The virion consists of a diploid single-stranded RNA genome (~9400 nucleotides) enclosed within a protein capsid (p24), surrounded by a matrix protein (p17) and lipid envelope derived from the host cell membrane.

Key Viral Components

FIV Subtypes (Clades)

Five major FIV subtypes (A-E) have been identified based on env gene sequence diversity. Geographic distribution varies: Subtype A predominates in California and Northern Europe; Subtype B is common in central/eastern USA and Southern Europe; Subtype C occurs in California and British Columbia; Subtypes D and E are found in Japan and Argentina, respectively.

Transmission and Epidemiology

Routes of Transmission

Risk Factors

• Intact males: 2-3 times higher risk due to territorial fighting
• Outdoor access: Free-roaming cats have significantly higher exposure
• Age: Average age at diagnosis is 5 years; prevalence increases with age
• Shelter/feral populations: Higher prevalence due to fighting

Pathogenesis and Immunopathology

Cellular Tropism and Entry

FIV primarily targets CD4+ T lymphocytes through a two-step receptor-mediated entry process. The envelope glycoprotein gp120 first binds to CD134 (OX40), a primary receptor expressed on activated T cells. This binding induces conformational changes exposing the V3 loop, enabling interaction with the chemokine co-receptor CXCR4, triggering membrane fusion and viral entry.

Additional target cells include: CD8+ T cells, B lymphocytes, macrophages, dendritic cells, microglia, and astrocytes. This broad tropism contributes to diverse clinical manifestations including neurological disease.

Three Phases of FIV Infection

Phase 1: Acute (Primary) Phase (2-16 weeks)

Following infection, FIV replicates rapidly in lymphoid tissues. Peak viremia occurs 8-12 weeks post-infection, accompanied by transient lymphopenia affecting both CD4+ and CD8+ populations. Clinical signs are often mild and frequently unrecognized by owners.

Clinical findings: Transient fever, mild lethargy, anorexia, generalized lymphadenopathy (lymphoid hyperplasia), diarrhea, stomatitis. Neutropenia may occur due to neutrophil apoptosis.

Immunologic events: Sharp decline in CD4+ T cells; CD4+CD25+ regulatory T cells (Tregs) become infected and activated, contributing to immunosuppression through inhibition of effector T cell proliferation.

Phase 2: Subclinical (Asymptomatic) Phase (months to years)

Following development of humoral and cell-mediated immune responses, plasma viral load decreases to low or undetectable levels. This phase can last many years, and some cats remain in this phase for their entire lives without progressing to FAIDS.

Key immunologic changes: Inversion of CD4:CD8 ratio (normal approximately 3.0, infected approximately 1.0 or less) due to CD4+ decline and CD8+ expansion. Antibody titers remain high. Progressive but gradual decline in both CD4+ and CD8+ populations over time.

Phase 3: Clinical/Terminal Phase (FAIDS)

When CD4+ depletion reaches critical levels, Feline Acquired Immunodeficiency Syndrome (FAIDS) develops, characterized by opportunistic infections, neoplasia, and wasting. Not all FIV-infected cats reach this phase.

Clinical Manifestations

Common Clinical Signs by System

Secondary Infections

FIV-associated immunosuppression predisposes to infections that would typically be controlled in immunocompetent cats:

• Toxoplasma gondii: Reactivation of latent infection
• Cryptococcus: Disseminated fungal infection
• Mycobacteria: Atypical mycobacteriosis
• Hemoplasmas: Mycoplasma haemofelis
• Demodex: Generalized demodicosis

Diagnostic Approach

AAFP Testing Recommendations

Per the 2020 AAFP Feline Retrovirus Testing Guidelines, FIV testing is recommended:

• When first acquired (all cats should be tested at least once)
• Prior to initial FIV vaccination
• Following potential exposure (bite wound, fight) - retest 60 days post-exposure
• When clinical signs of illness are present
• Annually for high-risk cats (outdoor, multi-cat households)

Diagnostic Testing Methods

Diagnostic Algorithm

Initial Screening (POC antibody test):

• Negative result: Generally reliable. If recent exposure possible, retest at 60 days.
• Positive result: Confirm with different test (different POC kit, Western blot, or PCR).

Special Situations:

• Kittens less than 6 months: May have maternal antibodies. Retest every 60 days until 6 months of age, or use PCR immediately.
• Previously vaccinated cats: Will test antibody-positive. Use PCR to distinguish vaccine response from true infection.
• Terminal-phase cats: May be antibody-negative (antigen-antibody complexes). PCR more reliable.

Treatment and Management

General Principles

There is no cure for FIV. Management focuses on maintaining quality of life, preventing secondary infections, and treating complications as they arise. Healthy FIV-positive cats may not require specific treatment.

Treatment Options

Supportive Care and Husbandry

• Indoor-only lifestyle: Prevents transmission to other cats and reduces exposure to pathogens
• High-quality diet: Avoid raw food (bacterial/parasitic risk); balanced commercial diet
• Biannual examinations: Every 6 months with CBC, chemistry, urinalysis
• Dental care: Regular dental examinations and treatment
• Parasite control: Strict flea, tick, and intestinal parasite prevention
• Spay/neuter: Prevents transmission to offspring and reduces roaming/fighting
• Avoid immunosuppressives: Use corticosteroids cautiously

Vaccination Considerations

FIV Vaccine: An inactivated whole-virus vaccine (Fel-O-Vax FIV) was previously available but is no longer marketed in the US/Canada. It provided variable protection (62-84%) and caused antibody-positive test results, complicating diagnosis. Considered non-core where available.

Other vaccines in FIV-positive cats: Core vaccines (FVRCP, rabies) are generally recommended based on risk assessment. Killed vaccines preferred over modified-live. Consult with veterinarian regarding individual risk-benefit.

Prognosis

Prognosis for FIV-positive cats is guarded but often favorable. Many cats live normal or near-normal lifespans with appropriate management. Key factors affecting prognosis:

• Viral subtype and strain pathogenicity: Some strains more virulent
• Age at infection: Earlier infection may have worse outcomes
• Co-infections: FeLV co-infection significantly worsens prognosis
• Quality of care: Regular veterinary care extends survival

Survival data: One study found FIV-positive cats lived an average of 4.9 years after diagnosis compared to 6 years for uninfected cats. However, many cats remain healthy for 10+ years.

FIV = F.I.V. Mnemonic

• Fighters get infected (bite wounds = primary transmission)
• Immune system attacked (CD4+ T cell depletion)
• Virus stays for life (no cure, lifelong infection)

Three Classic FIV Findings: "GUL"

• Gingivostomatitis
• Uveitis (anterior)
• Lymphoma (B-cell)

60-Day Testing Rule

"6-0" = Retest at 60 days post-exposure if initial test negative (seroconversion window)

Risk Factor Memory: "MAO"

• Male (intact)
• Adult (older cats)
• Outdoor access