Specimen: 0.5-1 mL of EDTA anticoagulated blood.
Collection protocol: Venepuncture
Special handling/shipping requirements: Standard
General information about the disease:
Feline immunodeficiency virus (FIV) is a lentivirus in the family Retroviridae. It contains RNA and its life cycle involves the integration of its RNA into the DNA of the genome of the host using the enzyme reverse transcriptase. This proviral DNA is then replicated as the cell divides. The proviral DNA is then translated back into viral RNA, and viruses are released from the host cell, the virus receiving its envelope from the host cell membrane. It shows many similar features to HIV, but is unrelated. A number of different subtypes or clades of FIV have been identified by sequencing the gene involved with the viral envelope. Isolates have been divided into five phylogenetic subtypes designated A, B, C, D and E. New Zealand has been found to have subtype C as the predominant subtype, fewer numbers of subtype A, and a novel, as yet unknown subtype. There is also a putative A/C recombinant strain.
Infected cats carry the virus for life and should be considered infectious at all times. Transmission is predominantly through bite wounds and infected cats are persistently viraemic. Viral replication occurs primarily in CD4+ (T helper) lymphocytes and macrophages, resulting in eventual disruption of cell mediated immunity. Although FIV itself can lead to fatal disease, its main complication is immunodeficiency of the carrier cat making it susceptible to other infections. It is important to know the FIV status of a cat so that these secondary infections, which may be of little consequence in a healthy cat, can be diagnosed and treated before they become serious.
Stages of Infection
1. The acute phase: This stage lasts several weeks. Signs include fever, diarrhoea, gingivitis, jaundice, uveitis, conjunctivitis, generalized lymphadenopathy and neutropenia. The severity depends on age. Young kittens have a more florid lymphadenopathy during the acute phase and there is increased severity in adolescents, while geriatric cats show minimal signs but progress more rapidly to the next stages of disease.
2. Asymptomatic carrier: This stage lasts from months to years with no obvious signs, and the cats appear apparently healthy. This stage may last up to 5 years but cats infected at >10 years of age progress through this stage faster than younger cats.
3. Persistent generalised lymphadenopathy and AIDS related complex: Lasts for 6 months to several years. It is characterized by vague, non-specific signs of illness, weight loss, enlarged lymph nodes, stomatitis, anorexia, anaemia, leucopoenia, neurological signs and apathy. This is the stage at which the majority of cats are presented to veterinarians.
4. Terminal AIDS-like phase: Lasts less than a year. Cats are emaciated. There are opportunistic infections, lymphoid depletion and miscellaneous disorders including neurologic, renal, immunologic and neoplastic disease.
PCR tests are needed to prove infection is present, if the vaccinial status is not known. The demonstration of the FIV proviral DNA sequence in the host genome is consistent with FIV infection. A subclinical phase of several months to years is common in FIV. Infected cats can succumb to various opportunistic infections, however an FIV positive cat may live for several years without any signs of illness.
All kittens born to infected queens will have maternal antibody present, although only one third will be infected with FIV. Maternally derived antibodies may persist for up to 3 months. Then it may be a further two months before infected kittens seroconvert. PCR testing will not be affected by the presence of maternal antibodies, and a positive PCR test in a cat of less than 6 months would therefore indicate FIV infection.
General information about when this test is indicated:
- Persistent unexplained pyrexia.
- Chronic infections (oral, respiratory, ocular, skin, gastrointestinal).
- Neoplastic disease, particularly lymphoma.
- Neurological disease (behavioural change, peripheral lymphadenopathy).
- Introduction of adult cats into multi-cat households.
The real time PCR test detects the presence of the viral genome (antigen), thereby confirming the FIV status of the cat. PCR diagnosis can be made at about 4-5 weeks post infection at which time there should be sufficient virus within the blood stream to make a definite diagnosis (PCR detects integrated virus genome in white cells). Unlike with antibody tests, the presence of vaccinial or maternal antibodies will not affect the PCR result thus a positive PCR test in a cat of less than 6 months would therefore indicate FIV infection.
Comparison with other related tests:
Until recently, diagnosis of FIV has been based on serological tests to identify antibodies to FIV. Infected cats are persistently viraemic and the presence of antibodies in animals over 6 months of age was therefore diagnostic for FIV. The recent release of a FIV vaccine has complicated the diagnosis of FIV. The vaccine elicits a humoral response, thus seroconverting the vaccinated cat. The real time PCR test overcomes this obstacle by detecting the presence of the viral genome (provirus) incorporated into the cat’s lymphocyte genome. This does not occur with vaccination and the detection of FIV genetic material in the lymphocyte genome is therefore specific for FIV infection.