FeLV Case Challenges

Decoding FeLV infection status with serial test panels

The FeLV Lifetime Study was designed to identify the best tests for FeLV and to correlate test results with survival time. A total of 127 FeLV positive cats admitted to the FeLV adoption program at the Austin Pets Alive! animal shelter in Texas were enrolled in the study. Once a month for 6 months blood samples were collected for an extensive panel of FeLV diagnostic tests:

• SNAP ELISA using whole blood, plasma, and serum: FeLV soluble antigen
• Quantitative Petchek plate ELISA with plasma and serum: FeLV soluble antigen
• Immunofluorescent antibody (IFA): cell-associated FeLV antigen
• Quantitative PCR: FeLV proviral DNA

Ocho Rios, Crayola, Monsoon, and T-Rex are 4 of the 127 FeLV+ cats in the study. Their test panel results are displayed in the “cat wheels” below.

Identifying risk for early mortality

Testing for proviral DNA (quantitative PCR) and p27 antigen by microtiter plate ELISA (PetChek) was used to classify each cat’s infection status as either ‘High’ or ’Low’ positive or retroviral status uncertain. In the FeLV Lifetime Study, cats classified as High Positive were at risk for early mortality whereas cats classified as Low Positive had survival rates similar to uninfected cats.

Ocho Rios

Ocho Rios is an approximately 6-year-old neutered male cat. He was transferred from another animal shelter into the FeLV+ adoption program.

Diagnostic considerations

• On entry to the shelter, Ocho Rios was screened for FeLV and FIV using anticoagulated whole blood on SNAP Combo and tested positive for FeLV p27 antigen.  In addition to this, he was positive on SNAP Combo with plasma and serum.  Concordant results between sample types are typically a good indication of higher p27 antigen concentrations.  Follow-up testing using the PetChek FeLV ELISA confirmed the presence of p27 antigen.  PCR testing at the time of enrollment confirmed the presence of proviral DNA in peripheral blood cells at a level greater than the threshold, indicating that Ocho Rios should be classified as a ‘High Positive’ cat.  Over the course of the next 6 months, his test results were consistently positive across all methods of testing, and he remained within the ‘High’ classification.
• Although high positive cats were found to have a shorter survival than Low positive cats in the APA! Study, Ocho Rios went on to live another 6 years. 

Lifestyle considerations

• It is recommended that High positive cats not be placed in a home with other cats that have a compromised immune system. This includes kittens under 6 months of age and senior or geriatric cats with chronic diseases.  This is because High positive cats are more likely to be viremic and potentially infectious to other cats.  Cats with compromised immune systems would be at greater risk for progressive infections.

Crayola

Crayola entered the shelter at two months of age.  He was enrolled in the study at 3.6 months as a neutered male when he reached the minimum enrollment weight of 2 pounds. 

Diagnostic considerations

• On entry to the shelter, Crayola tested SNAP Combo positive for FeLV antigen using anticoagulated whole blood.  He did not enroll in the study until he was over 2 pounds in weight which was at 3.6 months of age.  When retested at this age, he was negative on SNAP Combo using whole blood, plasma and serum and his PetChek ELISA results were also negative.  However, when tested by real-time PCR for FeLV proviral DNA he was found to be positive with copies below the limit for the high threshold.  This indicated that he should be classified as a ‘Low Positive’ cat.  Interestingly, the following month, his results were completely different with SNAP Combo positive for all three sample types, p27 antigen detected at a low level by the PetChek ELISA, and proviral loads present at a high level (number of copies now above the threshold).  By the second month of the study, he again showed a change.  He was now only positive for FeLV p27 antigen on SNAP Combo using whole blood and was back to testing positive for proviral DNA copies at a low level.  By the end of the 6-month study, he was negative on all tests.  Cats mounting an effective immune response against the virus can drive proviral loads and antigen concentrations to undetectable.  Integrated provirus remains within infected cells sequestered in hematopoietic or lymphoid compartments.  Although the infection is not eliminated, these cats appear to control the infection.
• Crayola is alive and doing well more than 7 years after enrolling in the APA! Study.

Lifestyle considerations

• Based on the results of the APA! Study, Low positive cats are expected to have a longer survival time than High positive cats.  Because their proviral DNA loads and antigen concentrations are lower, they are thought to be less likely to transmit the infection.  Nevertheless, it is recommended that Low positive cats not be placed in a home with other cats that have a compromised immune system. This includes kittens under 6 months of age and senior or geriatric cats with chronic diseases.  This is because Low positive cats may revert back to a High positive state under conditions that compromise immune control of the virus.
• It is important for owners and veterinarians to note the Low positive status of an infected cat.  Sometimes cats will develop another disease that may look like a progressive FeLV infection.  Retesting using real-time PCR for proviral loads along with an ELISA for p27 antigen is the best way to determine if a cat’s FeLV classification has changed.  Co-morbidities may arise that are not directly related to the underlying focal or regressive FeLV infection.

Monsoon

Monsoon is a 2-year-old spayed female cat transferred to the shelter from a pet rescue group.

Diagnostic considerations

• Monsoon entered the shelter and tested positive for FeLV p27 antigen on SNAP Combo using anticoagulated whole blood.  Her enrollment results also indicated that she tested positive for FeLV p27 antigen using plasma and serum.  The p27 antigen results were confirmed by testing on the PetChek ELISA with quantitative results indicating that the concentration was below the threshold for a high result.  When tested by PCR for proviral DNA, Monsoon’s results were negative.  This combination of results, confirmed FeLV p27 antigen positive and absence of FeLV proviral DNA, indicated that Monsoon should be classified as a ‘Low Positive’ cat.  Interestingly, Monsoon maintained this pattern of results at each testing event for the next 6 months never demonstrating evidence of FeLV proviral DNA in circulation.  Monsoon represents an example of a focal FeLV infection.
• Monsoon is alive and doing well more than 7 years after enrolling in the APA! Study.

Lifestyle Considerations

• Based on the results of the APA! Study, Low positive cats are expected to have a longer survival time than High positive cats.  For cats without evidence of circulating proviral DNA and probable focal infections, the risk of transmitting the infection is low.  It is thought that cats with focal infections are able to control the spread of the infection within the body and restrict it to lymphoid organs before spread to infect the bone marrow.  A strong immune response against the virus likely helps to control this infection and keeps FeLV p27 antigen levels low.  Like other cats in the Low positive category, these cats have not eliminated the infection as integrated provirus remains within the infected cells sequestered in the lymphoid compartments.
• It is important for owners and veterinarians to note the Low positive status of an infected cat.  Sometimes cats will develop another disease that may look like a progressive FeLV infection.  Retesting using real-time PCR for proviral loads along with an ELISA for p27 antigen is the best way to determine if a cat’s FeLV classification has changed.  Co-morbidities may arise that are not directly related to the underlying focal or regressive FeLV infection.

T-Rex

T-Rex is a 1-year-old intact male cat relinquished to the shelter by his owner.

Diagnostic considerations

• On entry to the shelter, T-Rex was screened for FeLV and FIV using anticoagulated whole blood on SNAP Combo and he tested positive for FeLV p27 antigen.  Testing with plasma and serum also produced positive results.  Concordant results between sample types on the SNAP Combo are usually a good indication of higher p27 antigen concentrations.  However, in the case of T-Rex, follow-up testing using the PetChek FeLV ELISA confirmed the presence of p27 antigen at a level that was below the threshold for a high result. PCR testing at the time of enrollment also detected proviral DNA in peripheral blood at a level that was below the threshold for a high result.  Despite having positive results for both FeLV p27 antigen and proviral DNA, T-Rex would be classified as “Low Positive” cat because the quantitative results of his testing were low. 
• Results for T-Rex were not consistent over the 6 months of the study.  At month 2, the high proviral DNA loads would have suggested that his classification was changing from low to high but at month 3 it was back to low and at month 4, his test results were all negative.  Months 5 and 6 showed a return to his previous low status.  Interestingly, IFA was determined to be positive at each of his visits except month 4.
• T-Rex is alive and doing well more than 6 years after enrolling in the APA! Study

Lifestyle considerations

• A cat’s FeLV classification can change over time.  Because the infection is not eliminated, it must be kept under control by the immune system.  Factors that affect a cat’s immune status may lead to changes in the state of their FeLV infection.  In the case of T-Rex, he was moved between several foster homes during the study and these changes may have served as stressors to his immune system.
• Single point-in-time test results don’t always deliver the full picture but testing monthly for 6 months doesn’t necessarily provide a definitive answer.  Caring for FeLV-infected cats requires being comfortable with some uncertainty.  Knowing that a cat is Low positive does indicate a higher likelihood for longer survival times.  It also elevates the importance of maintaining the cat’s health status and minimizing stressors that could compromise their immune status.