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Germs that infect humans


Borrelia burgdorferi

Borrelia burgdorferi

© CDC

Microorganism : the spirochete bacterium Borrelia burgdorferi.

Disease : Lyme disease

Occurrence of the disease

History : Lyme disease was first described in 1975 in Connecticut. The causal agent was identified as Borrelia burgdorferi in 1982.

Current situation : in the United States, where it is endemic to the Atlantic coast, Lyme disease is the most prevalent tick-borne disease, with some 8,000 cases reported annually. In Canada, transmission zones have been identified in Ontario and British Columbia.

Mechanism of action of the microorganism : the bacterium enters the body through a tick bite. It first affects the skin around the bite, then spreads to the nervous system, heart and joints. In the disease's advanced stage, the bacterium strips the protective coating off nerve cells.

Symptoms of the disease : there are three stages to Lyme disease. In the first stage, the tick bite produces a circular skin lesion, often accompanied by cold-like symptoms such as fatigue, malaise, headache, fever, and shivers. During the second stage, which may take several weeks, and even several months, to appear, neurologic symptoms, arthritis, and inflammation of the heart occur. In the final stage, which may take several years to reach, individuals exhibit symptoms similar to those observed in Alzheimer's disease or multiple sclerosis (demyelinization of neurons).

Incubation period : three to 32 days.

Contagious period : no case of transmission between humans has ever been demonstrated.

Hosts : deer and field mice.

Transmission : both Borrelia burgdorferi and two species of ticks (Ixodes dammini and Ixodes pacificus) usually live on deer and field mice. When the ticks bite these animals, they become hosts to the bacterium, which they transmit to humans when they bite them in turn.

Treatment : some antibiotics, such as penicillin or tetracycline, are effective. Arthritis is less likely to develop if treatment is begun early. In cases of established disease, the use of ceftriaxone may reduce the risk of brain damage.

Geographic distribution of the microorganism : the geographic distribution corresponds to that of the ticks: Canada, the United States, Europe, Russia, China, and Japan.

Prevention : information campaigns concerning the mechanism of transmission of the disease may be helpful, as the risk of catching the disease from a tick bite is highest from May to July. Individuals walking in the woods should take precautions to avoid tick bites. Some insecticides can kill ticks. Self-examination for tick bites may help early diagnosis, allowing the condition to be treated rapidly.

Vaccine : a recombinant vaccine against Borrelia burgdorferi has been available in Canada since 2000 and in the United States since 1999.