Feline Leukemia Virus (FeLV) has been studied for over 30 years, both for its relevance to the cat population and because it serves as an animal model for certain parallel human diseases. The key characteristics of FeLV are its contagious nature, and the fact that it directly causes both fatal cancerous and non-cancerous diseases. It can lie dormant in the bone marrow for a long time, and vaccination may protect against the diseases. FeLV is not transmissible to humans or animal species other than those of the cat family.

The susceptibility of cats to FeLV infection can differ remarkably and demonstrate varying outcomes such as progressive infection with persistent viremia, and contained infection with transient or undetectable viremia. Recent studies with experimentally FeLV-challenged cats showed them to become provirus-positive, and that vaccination against FeLV did not induce sterilizing immunity. In ostensibly FeLV-recovered cats, the virus may reappear and lead to fatal disease many years after FeLV exposure.

Feline Immunodeficiency Virus (FIV) is a lentivirus first described in the 1986. Five subtypes have been identified based on differences in amino acid sequence coding of the viral envelope. Cats infected with FIV may not die but the infection is persistent throughout life and incurable. Although a vaccine is available, its efficacy remains uncertain.

Prevalence in the United States

• < 2% of healthy cats
  
• Up to 6% in ill cats
  
• 19% of cats with bite wounds or abscesses
  
• 4-8 million cats are infected

Risk factors

• Male sex
  
• Adulthood
  
• Outdoor access

Associated diseases

• Hematologic disorders
  
• Chronic inflammatory conditions
  
• Neoplasia, especially lymphoma
  
• Opportunistic infections

Outcome largely unknown

• Short- vs long-term survival
  
• Elective euthanasia of healthy cats

Retrospective Study of computerized medical records from Banfield veterinary hospitals included:

• > 500 clinics across the US
  
• Fully computerized medical records
  
• All cats tested for FeLV and FIV in 2000 (> 60,000)
  
• FeLV-positive cats: n =823; males, 60%; median age, 2 yrs
  
• FeLV-negative cats: n =7,476; males,= 60%; median age, 2 yr
  
• FIV-positive cats: n = 1,011; males, 75%; median age, 5 yr
  
• FIV-positive cats: n = 1,011; males, 75%; median age, 5 yr

Summary (survival influenced by elective euthanasia)

• Most deaths occurred in first 100 d
  
• FeLV survival : 47% of controls at 3 yr; 29% of controls at 6 yr
  
• 80% of controls at 6 yr

Screening Tests

• High sensitivity
  
• Convenient
  
• Inexpensive
  
• More false positives

Confirmatory Tests

• High specificity
  
• Performed at reference labs
  
• Additional cost
  
• More false negatives

Discordant Tests (status undetermined)