by Dr. Nell Barrett Salter
Feline Leukemia Virus positive. These are some of the most frightening words that a cat breeder or owner will ever hear. In this article I will try to help you to understand what this means, and what steps to take if this should happen to you.
First of all, there are two commonly used types of FeLV tests. One is the in-office ELISA, and the other is the laboratory run IFA test. Both types test for the FeLV protein p27. However, they detect the protein in different forms and detect infection at different stages. 1 If a cat or kitten is ELISA positive, but healthy, an IFA test should be performed. Here is where the confusion begins.
If the cat is IFA negative but ELISA positive there are two possibilities. One is a false positive ELISA test, and the other is a stage of the disease is present at which the IFA test doesn’t detect infection. Therefore, the suspect cat or kitten should be isolated and retested in at least 6 weeks. If, at that time the cat is IFA positive, you know that the cat if infected. The IFA is 99% accurate. An IFA positive cat can be shedding virus and infect other cats and kittens.
But the situation is even more complicated than that. Sixty to eighty percent of cats with a healthy immune system who are exposed to the FeLV virus develop antibodies that prevent progression of the infection. However, a percentage of infected cats will progress to a bone marrow infection and go no further, thereby harboring the virus in the bone marrow. These cats can test negative on both tests, or positive on the ELISA and negative on the IFA. For breeders, this dilemma is significant. If a cat is harboring virus in the bone marrow, it can appear healthy and normal. But a stress, such as pregnancy, can cause a reappearance of the virus, and infection and transmission of the virus. If you think about this, you will understand that even testing new cats and kittens entering your cattery will not completely safeguard you against FeLV. This is the reason that I recommend routine FeLV testing of all breeders at least every 6 months, and testing of all kittens before going to their new home or before being kept as replacement kitties. I personally recommend that a cat who has ever been positive (ELISA or IFA) should NOT be used in the cattery as a replacement. You may lose some prospective breeders this way, but “better safe than sorry”.
As for the other types of tests, saliva and tears, the following has been found. A high incidence of false positives has been reported with the saliva ELISA. Tears correlate better with blood than does saliva (approximately 80 % correlation). 1 Just keep in mind that these are ELISA tests and positive cats should be IFA tested.
I hope that this article has been useful in helping you understand the FeLV tests. I will cover the steps to take if you have a FeLV outbreak in a future article.
By: Dr. Nell Barrett Salter 7/21/97 Reference: 1 Consultations in Feline Internal Medicine, John R. August, Volume 3, pgs. 41-42.
