Identifying FeLV or FIV infection is important to optimize individual health care and prevent new infections. Serology is the first line diagnostic test and positive results on serology should be confirmed. Interpretation of retrovirus test results requires integration of the clinical findings, patient risk factors, and performance of the tests being used.

FeLV Overview

FeLV is a gammaretrovirus that infects domestic cats and other felidae worldwide. FeLV is transmitted vertically and horizontally. Infection spreads rapidly within households through oronasal contact with virus-containing secretions, principally saliva. Vaccination and changes in management practices have reduced the global prevalence of FeLV infection. However, testing for FeLV is still relevant because FeLV carries a poor prognosis, there is no effective treatment and transmission of infection to in contacts should be prevented.

FeLV Testing and Outcomes

Screening point-of-care (PoC) tests detect viral antigen, p27, in whole blood, plasma, or serum. These kits usually perform very well; the IDEXX SNAP Combo showed 100% specificity and sensitivity for detecting FeLV antigenaemia recently.¹ Following FeLV exposure, cats initially test antigen positive. A stable outcome, influenced by age, immune status, and virus dose, is reached within weeks. Around 30% of exposed cats develop progressive infection (persistent antigenaemia, high proviral load). Cats with progressive FeLV infection are the source of infection for naïve cats, and they have reduced life-expectancy that is often only days or weeks. FeLV-related diseases include anaemias, immunosuppression, and lympho or myeloproliferative diseases. In the remaining exposed cats, antigenaemia is transient, but residual virus can be detected by quantitative polymerase chain reaction (qPCR, also called real-time PCR). These cats have regressive infection, where integrated provirus (DNA) and, sometimes viral RNA, can be detected by qPCR, but at lower levels and with restricted tropism compared with progressively infected cats. The prevalence of regressive infection reported in the field ranges from less than 1% to 10%. Regressively infected cats pose minimal risk to other cats, although disease outcomes are still under investigation. Loss of immunological containment of regressive infection resulting in progressive infection seems to be rare. Blood donors with regressive infection can transmit FeLV infection with recipients developing fatal progressive infection. Therefore, feline blood donors should test negative on FeLV antigen and qPCR tests.

Abortive infection, only detectable by seroconversion, is rare and of no consequence to the cat or in contacts.

Interpretation of FeLV Test Results

Positive serology must be confirmed with qPCR. Most cats exposed to FeLV will test positive for FeLV antigen initially and retesting is important. p27 positive and qPCR positive cats should be isolated from negative cats and rested around 12 weeks later. If the cat develops progressive infection, it will remain persistently antigen positive. If the cat develops regressive infection the antigen test will become negative. Cats with known exposure to FeLV that test p27 negative should be retested in 30 days. Neither vaccination against FeLV or maternal antibodies affect FeLV p27 serology results.

FIV Overview

FIV is a retrovirus in the lentivirus genus. Worldwide seroprevalence varies from <5% to >30% depending on the region and the population tested. Territorial aggression is the major mode of FIV transmission and free-roaming, entire, sick, male cats are at high risk from infection. FIV-infected cats are at risk from immune dysfunction although many have a normal life expectancy. Immune dysfunction can manifest as atypical and refractory bacterial, viral or protozoal infections and parasitosis. In second stage FIV, persistent cytopenias, high grade lymphoma, unexplained weight loss, and neurological signs can occur. Confinement of uninfected cats is the most effective way to prevent transmission. A field study demonstrated a protective rate of 56% for Fel-O-Vax FIV vaccine (Boehringer Ingelheim).² FIV presents no known zoonotic risk.

FIV Serology

Serology is the first line test for FIV. PoC tests or laboratory-based ELISAs have high sensitivities and specificities for antibody detection. In patients over 6 months of age that have not been vaccinated against FIV, anti-FIV antibodies are a marker for FIV infection. Where vaccination with Fel-O-Vax FIV is available, test selection is important because PoC kits vary in their ability to distinguish vaccine-induced from infection-associated anti-FIV antibodies. Witness FeLV/FIV (Zoetis) and Anigen Rapid FIV/FeLV (BioNote) showed high specificities for identifying FIV-infection in several studies.^3,4 Interference with these tests by vaccination can still occur particularly after a recent booster.^5,6 IDEXX SNAP FIV/FeLV combo is not useful to discriminate vaccinated from infected cats. The reliability of a negative serology result is unchanged by the introduction of the FIV vaccine.

FIV Molecular Testing

Molecular tests target FIV DNA provirus and some also detect viral RNA in plasma by qPCR. Diagnostic sensitivity that is 5–15% lower than serology and specificity comparable with serology are reported for commercial qPCR tests.^7,8 qPCR should not be used as a screening test for FIV. qPCR can be used as an adjunct to serology to determine the true FIV-status of cats in certain circumstances including; seropositive cats that may be vaccinated against FIV, seropositive cats less than 6 months of age, and seronegative cats that may have been recently exposed to FIV. qPCR can detect viral RNA in blood within 1 to 2 weeks and DNA within 3 weeks of experimental infection but differences in virus strain, dose, and assay sensitivities mean that it is hard to predict how long after natural exposure qPCR can reliably detect FIV infection.⁹ A positive FIV qPCR results indicates that the cat is almost certainly infected, whereas negative qPCR results are inconclusive.

Interpretation and Confirmation of FIV Serology

Interpretation of the results of serological screening requires the integration of clinical findings from the individual cat to guide whether repeat serology or qPCR is indicated and what the timing of repeat testing should be. Negative serology results are generally reliable because the sensitivity of serological tests approaches 100%. The time to seroconversion following natural infection is variable and can be prolonged. A seronegative cat that may have been recently exposed should be isolated from FIV-infected cats or cats of unknown FIV-status and retested after 60 days. Rarely, cats in the terminal stage of FIV-infection have impaired antibody production despite high plasma viral loads. If advanced FIV infection is suspected, negative serology results should be followed-up with qPCR testing. Confirmation of positive serology results is recommended. The testing methodology used for confirmation will depend on assessment of the patient’s FIV-infection risk, test availability, whether or not prior FIV vaccination can be ruled out and financial considerations. In cats with a low FIV risk, such as healthy, purebred, young, indoor cats, confirmation of positive serology is essential because the positive predictive value (PPV) of serology is reduced in low-risk groups. The reason for this is that the prevalence of FIV in a low risk population approaches the expected frequency of false positives. So, for each positive test result obtained in cats at low risk, there is a higher chance that the result is a false positive. Maternally-derived FIV antibodies may result in positive serology in uninfected kittens. A positive serology results in a cat under 6 months of age can be confirmed with qPCR. Alternatively, serology can be repeated after 6 months of age. Negative serology in kittens is reliable. Where prior vaccination with Fel-O-Vax FIV is confirmed or cannot be ruled out, Anigen Rapid FIV/FeLV, or Witness FeLV/FIV are useful screening tests. A positive result using one of these kits can be confirmed with the other kit or with qPCR depending on availability and finances.

References

1.  Levy JK, Crawford PC, Tucker SJ. Performance of 4 point-of-care screening tests for feline leukemia virus and feline immunodeficiency virus. J Vet Intern Med. 2017;31(2):521–526.

2.  Westman ME, Malik R, Hall E, Harris M, Norris JM. The protective rate of the feline immunodeficiency virus vaccine: An Australian field study. Vaccine. 2016;34(39):4752–4758.

3.  Westman ME, Malik R, Hall E, Sheehy PA, Norris JM. Determining the feline immunodeficiency virus (FIV) status of FIV-vaccinated cats using point-of-care antibody kits. Comp Immunol Microbiol Infect Dis. 2015;42:43–52.

4.  Crawford C. Does a diva test exist for differentiating FIV infection from FIV vaccination? J Vet Intern Med. 2016;30(4):1475–1476.

5.  Westman ME, Malik R, Hall E, Harris M, Hosie MJ, Norris JM. Duration of antibody response following vaccination against feline immunodeficiency virus. J Feline Med Surg. 2016;0(0):1098612X16673292.

6.  Lappin M. Detection of feline immunodeficiency virus antibodies in serum of vaccinated cats using a commercially available kit. J Vet Intern Med/ACVIM. 2015;29(4):1207.

7.  Morton JM, McCoy RJ, Kann RKC, Gardner IA, Meers J. Validation of real-time polymerase chain reaction tests for diagnosing feline immunodeficiency virus infection in domestic cats using Bayesian latent class models. Prev Vet Med. 2011.

8.  Pinches MDG, Diesel G, Helps CR, Tasker S, Egan K, Gruffydd-Jones TJ. An update on FIV and FeLV test performance using a Bayesian statistical approach. Vet Clin Pathol. 2007;36(2):141–147.

9.  Diehl LJ, Mathiasondubard CK, Onell LL, Hoover EA. Longitudinal assessment of feline immunodeficiency virus kinetics in plasma by use of a quantitative competitive reverse-transcriptase PCR. J Virol. 1995;69(4):2328–2332.