When we hear the term “infectious disease”, we usually think of something that can infect us and cause a disease process to begin. That “something” can be a bacteria, virus, parasite, or fungus obtained from many different sources (other infected individuals, poor hygiene, transfusion with infected blood, shared needles between drug users, etc.). Disease-causing bacteria and viruses are known as infectious agents and some of them can be quickly identified by using genetic testing techniques. This has led to a revolution in microbiology laboratories where many of the most common infectious organisms are now identified in clinical specimens using genetic testing techniques to identify them by their DNA or RNA.
Specimens that might contain these bacteria include urine, blood, sputum, cerebrospinal fluid and others.
Viruses are unusual organisms that sometimes insert their DNA into a host’s genome. The viral RNA or DNA utilises the host’s cells to produce proteins and make more viruses. Viruses such as hepatitis C virus (HCV) and human immunodeficiency virus (HIV) are examples of RNA viruses.
Other disease-causing viruses that contain DNA instead of RNA include Herpes simplex virus, cytomegalovirus, Epstein-Barr virus, parvovirus and varicella-zoster (chicken pox) viruses. All of these viruses can be identified by first removing the suspected viral DNA or RNA from a patient specimen and then using it to provide a “fingerprint” of the suspected virus. Specimens usually include blood, cerebrospinal fluid, sputum, other body fluids, amniotic fluid, tissue, or bone marrow. Much of the testing on donor blood that will be used in a blood transfusion utilises genetic testing to inspect the blood for viral contamination.
Determining how many copies of a virus’ RNA or DNA are present in an individual’s blood is another use of infectious disease genetic testing techniques. The number of copies present is typically referred to as the “viral load” or “viral burden”. This testing is usually done after a drug therapy is initiated to assess whether it is working or not to remove or decrease the viral RNA load. The most common viral load tests are for HCV or HIV, and the tests require a sample of blood.
Parasites may be single-cell organisms such as Giardia lamblia or complex multi-cellular organisms such as intestinal worms. Parasites may infect a person through the saliva of a biting insect, such as a mosquito, through contact with infected material such as dirty water or they may even burrow through the skin. An example of a parasite that can be identified using genetic tests is Toxoplasma gondii which can cause encephalitis or congenital infections that lead to severe damage of a fetus (fetal toxoplasmosis).