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; however, common infectious agents, such as certain bacteria and viruses, are much less expensive to identify using standard laboratory methods that don’t involve genetic testing techniques.
Bacteria are one-celled organisms that contain their own DNA and in some cases can cause serious disease. Even those bacteria that harmlessly live inside our bodies and are involved in beneficial processes can become mutated under unusual conditions and cause us to be very sick. By isolating the DNA from bacteria, and amplifying and analysing specific regions of the DNA, the bacteria can be identified very quickly. Among the many bacteria that can be quickly identified using these genetic testing techniques are: Chlamydia trachomitis, which is an organism that causes a sexually-transmitted disease; Neisseria gonorrhea, which causes gonorrhea, Borrelia burgdorferi which causes Lyme Disease, Legionella pneumophilia which causes Legionnaire’s disease, Mycoplasma pneumoniae which leads to “walking pneumonia”, Mycopbacterium tuberculosis which can cause tuberculosis, and Bordetella pertussis which causes whooping cough. 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 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.A parasite is a complex multi-cellular organism. Parasites usually infect an individual through the saliva of a biting insect, such as a mosquito, or through infected material. 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).