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Diagnosing common infectious diseases – What do all those new tests, titers, PCRs, and blue dots really mean? Part 2: cats (Proceedings)

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Nov 01, 2009

In some ways, diagnosing disease in cats is more difficult than in dogs. Cats often show vague signs of illness no matter the cause. Diagnostic testing is very important but results are subject to interpretation. The following reviews some tips for identifying feline infectious diseases using immunology.

Viral diseases

Panleukopenia

The same fecal antigen (Ag) tests used for canine parvo can be used for diagnosis. As with dogs, false positive results can occur 3-10 days after vaccination with a modified-live product. False negatives occur when antibody (Ab) binds to the fecal antigen. Hemagglutination inhibition (HI) or virus neutralizing (VN) antibody levels may be measured to indicate vaccine response or exposure.

Herpes

Ab titers are not useful for diagnosis, as they can be high or low independently of clinical signs. Virus detection in the early stages of infection can be done with IFA on swabs/scrapings of affected areas such as the conjunctiva, nasal mucosa, or oral cavity. False negatives are common. Virus isolation takes several days to weeks but has a higher sensitivity. PCR, especially real-time PCR, can be useful although cats with latent infections and "carriers" will test positive even without clinical signs.

Calici

As with herpes, Ab detection is not used unless paired titers (3-4 weeks apart) are saved for retrospective diagnosis. IFA and virus isolation are available, with samples taken from multiple sites. Recent vaccination can result in false positives for both herpes and calici. Real-time PCR testing is offered by some reference labs, often as part of a "feline upper respiratory disease" panel.

Leukemia

FeLV virus (antigen) is detectable by ELISA, IFA, and PCR. Ab titers are not clinically useful. ELISA is the most commonly performed test both in-clinic and at reference labs. A positive result indicates the presence of p27, an internal virus protein. Serum, plasma, whole blood, saliva, and tears may be used but the latter two are less accurate. False positives and false negatives are seen with FeLV ELISA testing. Anti-mouse antibody, if present in the cat due to eating mice, may interfere with the test (false positive). False negatives occur in cases of lab error, poor-quality specimens, or if viremia is too low to detect.

IFA testing for FeLV is performed on smears of whole blood, buffy coat cells, or bone marrow aspirates. Fresh samples without anticoagulant are preferred (use blood directly from syringe to make smears, not from a blood tube). IFA detects p27 within cells, instead of simply circulating in the bloodstream, and a positive result indicates a bone marrow infection and worse prognosis. Bone marrow aspirates are also good samples for IFA testing. PCR assays are not standardized but may detect latent infections in cats testing negative on ELISA and IFA. The clinical significance is unknown. Vaccination does not interfere with testing.

If a healthy cat tests positive on ELISA with a screening test, there is a reasonable chance that the result is a false positive or the infection will be temporary. Some clinicians wait 3-4 weeks and then repeat the ELISA test. Others may submit an IFA test right away. If the IFA is positive, then the cat is almost certainly to be infected and should be isolated from other cats. If IFA-negative, there's still a chance the cat is infected so isolation should continue until a followup ELISA test is negative. Discordant results are difficult to interpret, and clients should be informed that no diagnostic test for FeLV is 100% accurate in every situation.

Immunodeficiency

FIV antigen occurs in very low levels in the bloodstream and is therefore difficult to detect. Instead, Ab testing is used to indicate exposure and infection. The detectable Ab response after exposure takes 1-3 months. ELISA testing is used in-clinic and at reference labs, but both false negatives and false positives are possible. If female cats are infected, their kittens will test positive for up to 4-6 months of age. Adult cats may also be falsely positive if cross-reactions occur (especially if whole blood is used instead of serum or plasma), or if any lab errors. A confirmation test should be run on all ELISA positive samples by Western blot. Both disease and maternal Ab are detected with Western blotting, but this assay has a higher specificity. Vaccination for FIV results in positive Ab tests by either method. PCR testing has been advocated to differentiate vaccinated from infected cats but to date this hasn't been proven (both false positive and false negative results occur with PCR). A discriminant ELISA test has been developed to identify vaccinates from exposed cats, but is not commercially available.

Infectious peritonitis

FIP is more difficult to diagnose than the other feline viral diseases. As FIP is caused by a coronavirus, Ab titers can be measured. However, nonpathogenic enteric coronavirus (which many cats have been exposed to), causes positive Ab results. The mutated pathogenic corona does not have any unique features that allow differentiation from nonpathogenic strains. In the "wet" (effusive) from of FIP, Ab titers can be high, low, or even negative as large amounts of viral Ag can bind the Ab. Cats with the "dry" (noneffusive) form typically have high Ab titers, but a low or negative does not absolutely rule out FIP. Cats in crowded, stressful situations (especially catteries) often have high Ab titers with no disease.

Beware of any lab test that offers a "specific" FIP test (Ab or PCR) or one that interprets results for you. Research is currently looking into the 7b protein which is expressed by some infected cats. To date, clinically affected cats are testing positive to 7b antibody but unfortunately some healthy cats also test positive. In cats with neurologic signs, detection of coronaviral Ab in cerebrospinal fluid (CSF) has been recommended as a diagnostic test. A recent study did not support this finding, and there were many false positives and false negatives. A new PCR test detecting mRNA replicating in macrophages outside the GI tract is available from Auburn University. The sensitivity, specificity, and predictive values have not been reported. Despite much effort by diagnostic labs to come up with an "FIP test", we are still left with histopathology as the gold standard for diagnosis.