Lawsonia intracellularis Tests: How Do They Measure Up? (AAEP 2012)

The bacterium Lawsonia intracellularis causes an economically important emerging disease of weanlings called equine proliferative enteropathy (EPE). Thoroughbred foals that recover from EPE reportedly sell for an average of 68% less than nonaffected foals by the same sire, so veterinarians consider detecting the disease agent early a priority. In a recent study researchers recently examined four types of blood serum tests to determine their usefulness in detecting L.intracellularis, and the lead author presented their results at the 2012 American Association of Equine Practitioners' Convention, held Dec. 1-5 in Anaheim, Calif.

Although L. intracellularis has long been considered a common disease agent of swine, veterinarians began detecting its presence in horses about 20 years ago. Equine outbreaks occur sporadically and worldwide, but EPE is considered endemic on some farms that detect cases year after year. Historically, veterinarians based EPE definitive diagnoses on post-mortem test results because clinical signs of the disease could represent any number of diseases. But researchers have become more interested in developing effective serologic EPE diagnostic tests. Successful treatment, after all, depends on early diagnosis.

Connie Gebhart, PhD, associate professor in the University of Minnesota's Department of Veterinary and Biomedical Sciences at the University of Minnesota, and her colleagues collected blood serum samples from 96 weanlings in which they suspected EPE, 117 clinically normal weanlings, and 116 normal horses aged 1-27 years. They tested the samples using three assays: an immunoperoxidase monolayer assay (IPMA, commonly used to test swine for EPE), a slide-based immunoperoxidase assay (SIPA), and an enzyme-linked immunosorbent assay (ELISA).

The IPMA requires a special microscope and individuals trained in and experienced with this type of sample analysis. Personnel can read the SIPA using a standard microscope, and they read an ELISA by comparing color changes in a multi-welled plate.

Investigators also considered a blocking ELISA (b-ELISA), which reports the inverse of the antibody titer. They examined serum total protein, a blood component lost in severe diarrhea, as a screening assay.

The investigators determined that the IPMA, SIPA, and ELISA were both sensitive and specific (they produced few false negative and false positive results, respectively) in weanlings. The results of all three tests were also consistent with each other for weanlings. However, results among the three tests were less consistent for adult horses, suggesting that these tests would require further optimization of the cutoff values for adults.

Because only four horses tested negative on the b-ELISA, researchers concluded that scientists would need to modify the test considerably to optimize it. They found that serum total protein, however, showed promise as a screening test, picking up 85% of positive horses.

Gebhart said to monitor outbreak herds by conducting physical examinations and recording temperatures and body weights daily, along with measuring weekly serology and/or total protein levels. She suggested running monthly serology/total protein analyses post-weaning in endemic herds.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.