Lyme Disease

Causes

Lyme disease is caused by Borrelia burgdorferi, Borrelia afzelii, and Borrelia garinii, all belonging to the bacterial phylum of corkscrew-shaped spirochaete. In the United States, only cases of B. burgdorferi have been reported; in Asia, only B. afzelii and B. garinii; in Europe, all three types of the infection are present, although the latter two are more common. There are slight clinical differences between the symptoms of the United States version and the infection typical of Europe.

In all regions, the bacterium is transmitted through Ixodes ticks. This type of tick is commonly known as a deer tick in the United States, where the increased incidence of Lyme disease in the last few decades has coincided with an increase in the deer population. Deer represent a medium to transport and maintain the tick population, as they are not infected by the disease.

The Ixodes tick has three active feeding stages as a larvae, nymph, and adult. Typically, an uninfected larval Ixodes tick acquires the spirochaete along with its blood meal while feeding on an infected vertebrate, and then transmits it during its nymphal feeding stage to an uninfected human or other vertebrate. Thus, the majority of human cases occur between late spring and early summer, when nymphs are most active. Recent molecular and genetic analysis of the causative bacterium aims to reveal additional information behind the mechanisms of infection and transmission.

Symptoms

The early stage of the infection by the spirochaete begins at the site of the tick bite as an expanding round or oval skin lesion, known as erythema migrans. Erythema migrans in the United States is characterized by more severe inflammation and dissemination than in Europe and Asia, where the lesions are milder but persist for a longer period of time. This first stage is usually accompanied by an array of flu-like symptoms including malaise, fatigue, headache, fever, chills, and swollen lymph nodes.

The second stage of the illness involves the spread of the infection to the heart, nervous system, and joints. Most patients are treated before progressing to this stage and experiencing such symptoms.

In 5 percent of those untreated patients, however, heart complications occur within several weeks of infection. The most common problem includes various degrees of atrioventricular block. Occasionally, patients also experience dysfunction in the left ventricle and inflammation of the heart wall. Additional cardiac symptoms in certain Asian and European cases include chronic, persistent problems with cardiac muscle.

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