Bill Barton, DVM
Administrator/State Veterinarian in the Division of Animal Industries
Idaho State Department of Agriculture
Is "pooling" samples viable for trichomoniasis testing?
For many years, culture of trichomoniasis samples followed by microscopic evaluation was the gold standard for diagnosing trich. As PCR, or Polymerase Chain Reaction, has become more readily available in laboratories across the country, it is rapidly replacing culture as the preferred method for diagnosing trichomoniasis.
PCR offers several advantages over culture/microscopy. Microscopy relies on the trained eye of the examiner to identify the actual T.foetus organism under the microscope. If the sample is contaminated with too much debris, blood or other matter, the trich organism may not be visible on microscopic evaluation and a positive bull could be called negative. If the sample is not shipped or incubated at the proper temperature the trich organisms may die and the sample will be called negative when testing via microscopy. Culture samples must be read over a period of 7 days which lengthens the result reporting time.
PCR analysis of a sample identifies the presence of T.foetus DNA within the sample. If the sample is collected and handled appropriately, PCR analysis is highly sensitive, meaning; if the trich DNA is present the test will likely detect it. PCR results can usually be reported within 1 to 3 days depending on the laboratory.
PCR does not require a live organism to detect. Even if a positive sample is shipped to a lab and the T.foetus organism dies, the T.foetus DNA can still be detected via PCR. Testing this sample through microscopy would lead to a false negative.
PCR can also distinguish between T.foetus and fecal trichomonads, whereas microscopy cannot.
Recent peer-reviewed studies have shown that, because of the high sensitivity gained by using PCR, “pooling” of samples is a reasonable method of screening for trich in a herd. To pool samples, each individual bull is collected and the sample is placed in the appropriate media. The samples are submitted to a laboratory approved in your state to pool samples and the pooling is done at the lab. The laboratory will typically pool 5 individual samples in to 1 group sample. The individual samples will be maintained pending the testing of the group sample. If the group or “pooled” sample is negative for trich, all 5 bulls are considered negative. If the pooled sample is positive, the 5 individual samples that made up the pool are then tested individually to identify which of the bulls is/are infected.
Pooling of trich samples provides an economic advantage to the producer by reducing diagnostic testing costs. However, because not all states, countries or sales currently accept pooled samples, it is recommended that the state or country of destination or the sale be contacted for their requirements prior to submitting samples to the laboratory.