The Case of Lyme Disease
As symptoms of many infectious diseases overlap, it is difficult and often not recommended to diagnose a zoonotic disease based strictly on symptoms or clinical signs. An exception possibly is Lyme Disease, a common zoonotic disease in the US caused by Borrelia burgdorferi and transmitted by a tick. If the typical "bull's-eye" rash, also known as erythema migrans, is seen together with a history of a tick bite, it is sometimes diagnosed based on this alone. Even so, a definitive confirmatory test is often recommended. However, the rash occurs only in about 70-80% of cases, making diagnosis of this disease very difficult. Often one is not alerted to the possibility of this disease until later when less specific symptoms occur such as additional rashes, facial palsy, arthritis, heart palpitations, nerve pain, tingling of arms and feet, and many others.
The next step then is laboratory testing, which is initially done on serum specimens. The CDC recommends a two-tiered approach to laboratory testing. For definitive diagnosis, the first tier should be positive or equivocal, and the second one should be positive as well. The first tier involves testing for antibodies using Enzyme-Linked Immunosorbent Assays (ELISA) or similar methods. A pitfall in diagnosis with this testing is that antibodies usually take several weeks to develop, so if tested too soon, false negatives can occur. However, false positives can sometimes occur due to other tickborne diseases.
The second confirmatory test is usually Western Blot. It is always recommended to do the ELISA first, as doing a Western Blot only results in false positives, according to the CDC.
Other tests exist but are not recommended because they have not been FDA-cleared. Research is currently being done on developing new tests such as a cytokine-based immunoassay, rapid point-of-care tests using lateral flow technology, identification of biomarker patterns, and others.
