THE USE OF DNA PROBES DETECTING INFECTIOUS AGENTS

By David Hannon, DVM

A DNA probe is a test used to identify a portion of nucleic acid from a virus or other organism. The recent technology was first developed to test for psittacine beak and feather disease (PBFD) virus and polyomavirus, but now new research has created one that can accurately detect the Chlamydia organism. These tests are much more sensitive than previously developed virus-identification tests, and can detect smaller concentrations of virus particles, as well as detecting their nucleic acid before significant pathological changes have occurred.

These tests work by identifying portions of an organisms nucleic acid, or more specifically, DNA. Since all living organisms have nucleic acid, including viruses, bacteria, and other infectious agents, it is just a matter of identifying a certain sequence that is specific to that organism imparticular. Once a sequence has been identified, then a test is made to react when it comes into contact with that specific DNA. Of course, the entire process is a bit more complicated than this, but I'll leave the details up to the scientists developing them.

DNA probes can be used to detect PBFD virus in a bird's blood, feather pulp, and body tissue samples. Samples collected from a toenail clip are acceptable, but since the virus can be present in a bird's feather dander before the bird is actually infected, it is recommended that birds that test positive be rechecked on a blood sample drawn from a vein. DNA probes can also be used to detect polyomavirus and Chlamydia psittaci in swabs from a bird's cloaca, choana, feces, liver, spleen, and kidney. All three can be used to detect the presence of the organisms in the bird's environment, and it is recommended that swabs of air ducts, cages, floors, walls, and any other dust-collecting surface be submitted if environmental contamination is suspected.

DNA probes are far superior to previous test methods in that they actually detect the presence of the organisms, not just the body's response to them, such as antibody tests and titres do. Most older virus detection tests also relied on a large volume of virus particles, whereas minute amounts can be detected with the probes. They are highly sensitive and very specific (i.e. they won't cross react with other similar viruses), therefore the instances of false positives and false negatives are minimal. The only drawback is if cloacal swabs are used to test for polyoma or chlamydia, a false negative may occur if the bird is infected but not shedding the organisms into the environment.

Currently these tests are being performed at Veterinary Molecular Diagnostics in Milford, Ohio. Group samples or selective sampling is acceptable. Since the PBFD and polyomavirus probes were developed by Branson Ritchie's group at the University of Georgia, $5 from each test goes to the University of Georgia Research Foundation to support further research in avian viruses.