DNA Testing

By Malcolm Weir, DVM, MSc, MPH; Kristiina Ruotsalo, DVM, DVSc, Dip ACVP & Margo S. Tant BSc, DVM, DVSc

What is DNA?

DNA (deoxyribonucleic acid) is a large complex molecule that carries the genetic information or genetic "code" of an organism. It is made of two parallel strands that are wrapped around each other forming a double helix shape. Each strand is made up of repeating subunits called nucleotides (See handout "Genetics Basics - Understanding DNA" for more information).

DNA is often called the "blueprint of life" because it carries the "instructions" on how to build every part of a specific organism. All common forms of life, such as viruses, bacteria, plants, and animals carry a copy of their own genetic code in each of their cells. Organisms that are closely related genetically have very similar DNA patterns, while organisms that are quite different share little DNA in common. For example, the DNA of a cat is very similar to that of a lion, but it is quite different from the DNA of the dog or horse. However, regardless of how similar two organisms might be, there are differences in their DNA that makes each organism unique.

What is the DNA-PCR test and how does it work to find infectious disease?

The letters PCR stand for polymerase chain reaction. The word 'polymerase' refers to DNA-polymerase, which is an enzyme that plays an essential role in building DNA.

"The principle behind DNA-PCR is that each organism has a unique section
of DNA that is just like a fingerprint."

The principle behind DNA-PCR is that each organism has a unique section of DNA that is just like a fingerprint. When the unique DNA sequence of a particular organism is known, a diagnostic probe can be created. Then, using DNA-PCR, the probe can search the sample to determine if the organism of concern is present. If the probe finds the fingerprint DNA sequence, it means the organism is present.

How is DNA-PCR used in veterinary medicine?

DNA-PCR is most commonly used in veterinary medicine to detect the presence of infectious organisms, such as viruses, bacteria, and some types of parasites. Less commonly, DNA testing is used in breeding animals to determine parentage, to detect genes responsible for heritable disease, and to look for genes that control specific characteristics such as coat color.

Is DNA-PCR a good test?

The DNA-PCR is an excellent test for many reasons. It is especially useful for detecting extremely small numbers of infectious organisms, and for detecting infectious organisms such as viruses and some bacteria that are difficult to diagnose by other methods. It is often more economical than traditional methods of diagnosing infectious disease, and results are usually available in just a few days.

Can the DNA-PCR test be wrong?

Any diagnostic test can be wrong, including DNA-PCR. Success with DNA-PCR depends on submitting the proper sample and collecting the sample before treatment is started. The DNA-PCR is a technically demanding procedure, and laboratory error may occur due to contamination of the sample or poor technique. However, the error rate is extremely small, especially if the sample is sent to a qualified laboratory where the potential for error is minimized through the use of strict safeguards and highly trained professionals. The enormous diagnostic power of DNA-PCR greatly outweighs its weakness and provides veterinarians with a valuable tool in the battle against infectious disease.

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