FeLV Infection

• Both exposure and vaccination result in FeLV antibodies
  
• Generally high FeLV antigens (proteins) found in blood of infected cats
  
• Detection of p27 antigens is primary diagnostic tool
  
• Some age resistance to infection
  
• Outcome of infection: 1/3 transient; 1/3 latent; 1/3 persistent viremia

New FeLV Response Categories

Response Category	Antigenemia	Proviral Loads	Virus from Bone Marrow
Abortive	Latent	Regressive*	Progressive
Undetecable	Transient	Undetectable/Transient	Persistent
Undetectable	Low/ Moderate	Low/ moderate	High
Not tested	Positive	Negative

* Clinical significance unknown: no viremia, shedding, or disease (Hofmann-Lehmann et al, 2007)

Diagnosis of FeLV
Antigen tests: detect viral proteins (p27 antigen); screen with ELISA, lateral flow; require adequate amount of antigen in sample; detected at 28+ days following exposure

FeLV Confirmation
Immunofluorescent antibody: detects antigen in WBC, platelets; must be sent to reference lab; good confirmatory test; correlates with persistent infection; more false negatives
Virus culture: gold standard; identifies infectious virus; susceptible to degradation during transport
PCR: identifies FeLV RNA or DNA (provirus) genetic sequences; little information about test performance

Testing Kittens for FeLV

• Queen to kitten transmission is common
  
• Virus shed in placenta, saliva, milk
  
• Infected queen usually tests positive
  
• Not all kittens in litter may be infected
  
• Infected kittens may seroconvert over weeks to months

• Persistent infection
  
• Low antigens (proteins) found in blood of infected cats
  
• Generally high FIV antibody titers form the cornerstone of diagnosis

Diagnosis of FIV
Antibody tests: detect FIV antibodies; screen with ELISA, lateral flow; require adequate immune response; detected at 60 + days following exposure

FIV Confirmation
Western Blot: detects antibodies against multiple FIV proteins; must be sent to reference lab; good confirmatory test; more false negatives
Virus culture:
gold standard; identifies infectious virus; susceptible to degradation during transport PCR: identifies FIV RNA or DNA (provirus) genetic sequences; high rate of false-negatives due to marked strain sequence variation; little information about test performance

Testing Kittens for FIV

• Queen to kitten transmission is uncommon
  
• Kittens absorb queen’s FIV antibodies in colostrum
  
• Kittens test FIV+ for maternal antibodies
  
• Kittens are usually not infected!
  
• Kittens should not be diagnosed FIV + until 6 mo or older

Sample Selection and Handling

• Serum or plasma (non-hemolyzed) preferred to whole blood
  
• Saliva and tear samples may be less reliable than blood
  
• Refrigeration and rapid processing of samples for culture
  
• Guard against contaminating PCR samples

Testing Mistakes to Avoid

• Pooling samples: combining several cat samples in one test (litters of kittens, feral cat colonies); decreases sensitivity of test
  
• Sampling “representative” cats: infections within litters are frequently sporadic, prevalence is too low (4%) for meaningful results in feral cat colonies, queens are not always representative of their kittens
  
• Relying on a single test: kittens infected neonatally may seroconvert over weeks to months; recently exposed cats require a minimum of 1-2 mo to seroconvert; cats may become infected throughout life

Fel-O-Vax FIV vaccine induces antibodies that result in false + [i.e. not infected] with current antibody tests. Vaccinated cats still antibody positive at one year, and > 3 years in some vaccinates.

Vaccine contains dual subtype whole virus plus cells inactivated with formalin.

Vaccinated queens passively transferred FIV antibodies to their kittens for up to 11 weeks; 100% of kittens antibody positive through 3 weeks of age with gradual fall off of antibody prevalence until only ~ 10% of kittens remained positive at 11 weeks old.

FIV Testing Conclusions

• Current testing technologies are unable to distinguish cats that are vaccinated against FIV, infected with FIV, or both vaccinated and infected
  
• Resolution awaits new test development or introduction of subunit vaccines
  
• Fel-O-Vax vaccination interferes with all currently available antibody-based diagnostic tests
  
• FIV antibodies appear in the first few weeks following vaccination and persist at least a year
  
• Kittens absorb FIV antibodies from vaccinated queens and more than half test FIV+ past the age of weaning
  
• Commercial FIV PCR remains problematic [coded testing at 3 commercial labs revealed 41-93 % sensitivity and 44-95% specificity]
  
• New dual ELISA FIV antibody test appears promising

References: Julie K. Levy, DVM, PhD, ACVIM, Coll Vet Med, Univ Florida, Dec 2007 presentation to Antech Consultants; Hofmann-Lehmann et al, Vaccine, 10:1016, 2007.