Feline Leukemia Virus (FeLV) is one of the most significant infectious diseases affecting domestic cats worldwide. FeLV is a gammaretrovirus belonging to the family Retroviridae, first discovered in 1964.
Overview and Clinical Importance
Feline Leukemia Virus (FeLV) is one of the most significant infectious diseases affecting domestic cats worldwide. FeLV is a gammaretrovirus belonging to the family Retroviridae, first discovered in 1964. It affects approximately 2-3% of cats in the United States and Canada, with significantly higher prevalence (up to 30%) in high-risk populations.
FeLV is second only to trauma as the leading cause of death in cats, killing approximately 85% of persistently infected cats within three years of diagnosis. The virus causes a wide spectrum of diseases including lymphoma, anemia, and immunosuppression, making it a high-yield topic for the NAVLE examination.
| Subgroup |
Origin |
Disease Association |
Prevalence |
| FeLV-A |
Horizontally transmitted; original virus |
Immunosuppression, T-cell lymphoma |
100% of infected cats |
| FeLV-B |
Recombination with endogenous FeLV |
Increased neoplasia (lymphoma, leukemia) |
50% of infected cats |
| FeLV-C |
Mutation of FeLV-A env gene |
Severe nonregenerative anemia (erythroid aplasia) |
1% of infected cats |
| FeLV-T |
Mutation of FeLV-A |
T-lymphocyte depletion, immunodeficiency |
Uncommon |
Virology and Structure
Virus Classification
FeLV is an enveloped, single-stranded RNA virus that replicates using reverse transcriptase to convert its RNA genome into DNA, which then integrates into the host cell genome as a provirus. This characteristic makes the virus capable of lifelong persistence in infected cells.
FeLV Subgroups
There are four clinically important FeLV subgroups. Only FeLV-A is transmitted between cats; other subgroups arise within infected individuals through recombination or mutation.
High-YieldAll routine FeLV diagnostic tests detect all subgroups but cannot differentiate between them. FeLV-A is the ONLY subgroup transmitted between cats - always present in natural infections. FeLV-C is the subgroup to remember for severe anemia questions!
| High Risk |
Lower Risk |
| Young kittens (less than 16 weeks)
Multi-cat households
Outdoor access cats
Shelter/cattery populations
Cats with bite wounds |
Adult cats (age-related resistance)
Single-cat indoor households
Vaccinated cats
Cats in FeLV-negative environments |
Transmission and Epidemiology
Routes of Transmission
FeLV is transmitted primarily through prolonged close contact between cats. The virus is fragile and survives only a few hours outside the host. Primary shedding occurs in saliva, with lesser amounts in nasal secretions, urine, feces, and milk.
- Horizontal transmission: Mutual grooming, shared food/water bowls, shared litter boxes, bite wounds
- Vertical transmission: In utero, nursing (via milk)
- Blood transfusion: Regressively infected cats with proviral DNA can transmit via transfusion
Risk Factors
NAVLE TipUnvaccinated cats with bite wounds are 7.5 times more likely to be FeLV-infected than vaccinated cats with bite wounds. This statistic highlights vaccine efficacy!
| Outcome |
Prevalence |
Characteristics |
Test Results |
| Abortive |
20-30% |
Complete virus elimination
No provirus integration
Immune to future infection |
Antigen: Negative
Proviral PCR: Negative
Antibodies: Positive |
| Regressive |
30-40% |
Virus contained but provirus persists
No active shedding
Can reactivate with immunosuppression |
Antigen: Negative
Proviral PCR: Positive
Antibodies: Positive |
| Progressive |
30-40% |
Persistent viremia
Bone marrow infected
Active shedding; infectious
Worst prognosis |
Antigen: Positive
Proviral PCR: Positive (high load)
IFA: Positive |
Pathogenesis and Infection Outcomes
Stages of Infection
Following oronasal exposure, FeLV follows a predictable sequence of infection stages:
- Stage 1 (Days 1-2): Virus enters through oropharynx, replicates in tonsilar B-lymphocytes and macrophages
- Stage 2 (Days 2-12): Infected lymphocytes disseminate to regional lymph nodes
- Stage 3 (Days 3-12): Primary viremia - low-level virus in blood mononuclear cells
- Stage 4 (Weeks 2-4): Virus spreads to spleen, lymphoid tissues, intestinal crypts
- Stage 5 (Weeks 4-6): CRITICAL - Bone marrow infection (determines progressive vs regressive outcome)
- Stage 6 (Weeks 4-8): Secondary viremia - epithelial cells infected, massive shedding in saliva
Three Infection Outcomes
High-YieldBONE MARROW INFECTION is the critical turning point! Once bone marrow becomes infected (Stage 5), the cat will develop progressive infection. Young kittens are at highest risk because their immune systems cannot mount an effective response before bone marrow infection occurs.
| Category |
Manifestations |
Clinical Findings |
| Neoplasia (30%) |
Lymphoma (most common): Mediastinal, multicentric, renal, spinal
Lymphoid leukemia
Myeloproliferative disorders
Fibrosarcoma (with FeSV) |
Lymphadenopathy, dyspnea
Pleural effusion (mediastinal)
Renomegaly, azotemia
Paresis/paralysis (spinal) |
| Anemia |
Nonregenerative anemia (most common)
Pure red cell aplasia (FeLV-C)
Immune-mediated hemolytic anemia
Anemia of chronic disease |
Pale mucous membranes
Lethargy, weakness
Tachycardia, hemic murmur
Icterus (if hemolytic) |
| Immunosuppression |
Secondary bacterial infections
Chronic gingivostomatitis
Upper respiratory infections
Mycoplasma haemofelis |
Recurrent infections
Poor wound healing
Oral ulceration, pain
Fever, weight loss |
| Other Syndromes |
Reproductive failure
Fading kitten syndrome
Neuropathy
FeLV-associated enteritis |
Abortion, fetal resorption
Kitten death within weeks
Anisocoria, urinary incontinence
Vomiting, diarrhea, weight loss |
Clinical Signs and Associated Diseases
Clinical manifestations of FeLV are highly variable. Cats may remain asymptomatic for months to years before developing disease. The four major categories of FeLV-associated diseases are: neoplasia, anemia, immunosuppression, and other syndromes.
Major Clinical Syndromes
NAVLE Tip80% of cats with MEDIASTINAL lymphoma are FeLV-positive! Young cat + dyspnea + anterior thoracic mass + pleural effusion = think FeLV-associated mediastinal lymphoma. Also remember: 50% of cats with RENAL lymphoma are FeLV-positive.
| Test |
What It Detects |
When Positive |
Key Points |
| ELISA (POC) |
Free p27 antigen in blood |
Early infection (before bone marrow); Primary screening test |
Highly sensitive
In-clinic test
All positives need confirmation |
| IFA |
p27 antigen in WBCs/platelets |
After bone marrow infection (6-8 weeks post-exposure) |
IFA+ = progressive infection
Laboratory test
May miss early/regressive infections |
| PCR (Proviral) |
FeLV proviral DNA integrated in cells |
Earliest (1-3 weeks post-exposure); detects regressive infections |
Most sensitive
Required for blood donors
Quantitative PCR helps stage |
Diagnosis
AAFP Testing Recommendations
According to the 2020 AAFP Feline Retrovirus Guidelines, the FeLV status of every cat at risk of infection should be known. Testing is recommended:
- When cats are first acquired
- Prior to vaccination against FeLV
- Following potential exposure to infected cats or bite wounds
- Whenever clinical illness occurs
- Before blood donation (ELISA and PCR required)
Diagnostic Tests Comparison
High-YieldELISA+ / IFA- suggests early or regressive infection. ELISA+ / IFA+ confirms PROGRESSIVE infection with bone marrow involvement and poor prognosis. PCR turns positive EARLIEST (as early as 2 weeks post-infection) and is the test of choice when ELISA+ needs confirmation or for blood donor screening.
Diagnostic Algorithm
Level 1 (Screening): Point-of-care ELISA or rapid immunomigration test
Level 2 (Confirmatory): IFA or PCR for positive screening results
Interpretation:
- ELISA negative: Cat is not viremic (retest in 30 days if recent exposure suspected)
- ELISA positive: Confirm with IFA or PCR
- ELISA+/IFA+ or high proviral load: Progressive infection confirmed
- ELISA+/IFA- or low proviral load: Early or regressive infection; retest in 30-60 days
| Condition |
Treatment |
Notes |
| Secondary infections |
Appropriate antimicrobials (often prolonged courses) |
May require longer treatment duration; consider lifelong therapy for some infections |
| Severe anemia |
Blood transfusions, erythropoietin |
Screen donors with ELISA AND PCR; use FeLV-negative blood only |
| Lymphoma |
COP protocol: Cyclophosphamide, Vincristine (Oncovin), Prednisolone |
Response rate approximately 80-97%; median survival 338 days with treatment |
| Chronic gingivostomatitis |
Full mouth extraction, anti-inflammatories |
Medical management rarely resolves condition; dental radiography essential |
| Immune modulation |
Feline interferon omega (rFeIFN-omega); LTCI |
Limited evidence; may improve survival in some cats |
Treatment and Management
There is currently no cure for FeLV infection. Treatment is supportive and aimed at managing secondary conditions and maintaining quality of life.
General Management Recommendations
- Keep FeLV-positive cats indoors to prevent transmission and protect from secondary infections
- Feed high-quality, nutritionally complete diet (avoid raw food due to infection risk)
- Biannual veterinary exams with CBC, chemistry panel, urinalysis
- Spay/neuter to prevent stress of breeding and transmission to offspring
- Continue routine vaccinations (killed vaccines preferred) as immune response may be reduced
NAVLE TipPrognosis: 80-85% of progressively infected cats die within 3-4 years of diagnosis. Median survival time is approximately 2.5 years. However, many cats can live good quality lives for extended periods with proper management. A decision for euthanasia should NEVER be made solely on the basis of FeLV-positive status!
| Vaccine Type |
Examples |
Characteristics |
| Inactivated whole virus (adjuvanted) |
Nobivac Feline 2-FeLV, Fel-O-Vax |
Contains adjuvant to enhance immune response
Some studies show higher efficacy
Theoretical risk of injection-site sarcoma |
| Recombinant canarypox-vectored (non-adjuvanted) |
PureVax Recombinant FeLV |
No adjuvant - reduced injection-site reaction risk
Stimulates cell-mediated and humoral immunity
No risk of reversion to virulence |
Prevention and Vaccination
Vaccine Types
AAFP Vaccination Protocol
- Core vaccine for cats less than 1 year old
- Non-core for adult cats; administer based on risk assessment
- Initial series: 2 doses, 3-4 weeks apart, starting at 8 weeks of age
- Booster at 1 year, then annually or every 2 years based on risk
- Test before vaccination: Only FeLV-negative cats should be vaccinated
High-YieldFeLV vaccination does NOT affect FeLV test results (unlike FIV vaccination which causes positive antibody tests). No FeLV vaccine provides 100% protection, and vaccination does not eliminate proviral DNA if cat is already infected. Efficacy ranges from 80-100% depending on study and vaccine type.