In the past, some veterinarians have relied on commercially available serologic tests to establish, confirm, or rule out a diagnosis of foal pneumonia caused by the soil-borne bacteria Rhodococcus equi. A recent study at Texas A&M University has proven that these tests are not a reliable indicator of the disease. Ronald Martens, DVM, of Texas A&M University, presented "Rhodococcus equi Foal Pneumonia: Failure of Serologic Tests to Accurately Detect Disease" at the 2002 American Association of Equine Practitioner's (AAEP) Convention.

Inhalation of R. equi is thought to be the route of exposure on farms that harbor the bacteria in their soils. Farms that are endemic for the disease have higher concentrations of virulent (disease-causing) forms of R. equi, according to Martens. Although exposure rates to R. equi can be high, not all foals which are exposed will develop the disease. Infection with R. equi usually occurs during the first few days of the foal's life; however, clinical signs of disease are not generally seen for 30-60 days, and might not be apparent for several months.

It has been hoped that the various serologic assays could help detect foals with disease. In the study, serum samples were collected from all foals at a farm on which R. equi infections were persistently present. Samples were collected from foals as they reached two, three, and six to seven weeks of age. These samples were tested using the enzyme-linked immunosorbent assays (ELISA-6939, ELISA-33701, and ELISA-VapA tests), along with agar gel immunodiffusion (AGID), and synergistic hemolysis inhibition (SHI) tests. "Analysis of values for sensitivity and specificity of the various serologic tests indicated that none of them significantly differentiated affected foals from unaffected foals at any of the testing periods," said Martens. However, all test results did increase over time, which merely indicates exposure to R. equi organisms in the environment, whether they are virulent or avirulent (not disease-causing), and not the presence of disease.

"Results of this study indicate that these serologic assays do not reliably detect early stages of disease or identify affected foals by the time a specific diagnosis can be achieved by standard diagnostic methods," he continued. "In addition, because of the high probability of false positive and false negative results, it seems that serologic assessment of R. equi-specific antibodies is not an effective screening tool for the identification of foals that are most likely infected with R. equi