Lyme disease (Lyme borreliosis) is an infectious disease caused by at least three species of bacteria belonging to the genus Borrelia. Borrelia burgdorferi sensu stricto is the main cause of Lyme disease in North America, whereas Borrelia afzelii and Borrelia garinii cause most Europeancases. The disease is named after the towns of Lyme and Old Lyme, Connecticut, US, where a number of cases were identified in 1975. Although it was known that Lyme disease was a tick-borne disease as far back as 1978, the cause of the disease remained a mystery until 1981, when B. burgdorferi was identified by Willy Burgdorfer.
Lyme disease is the most common tick-borne disease in the Northern Hemisphere. Borrelia is transmitted to humans by the bite of infected ticks belonging to a few species of the genus Ixodes (“hard ticks”). Early symptoms may include fever, headache, fatigue, depression, and a characteristic circular skin rash called erythema migrans (EM). Left untreated, later symptoms may involve the joints, heart, and central nervous system. In most cases, the infection and its symptoms are eliminated by antibiotics, especially if the illness is treated early. Delayed or inadequate treatment can lead to more serious symptoms, which can be disabling and difficult to treat.
Signs and symptoms
Lyme disease can affect multiple body systems and produce a range of symptoms. Not all patients with Lyme disease will have all symptoms, and many of the symptoms are not specific to Lyme disease, but can occur with other diseases as well. The incubation period from infection to the onset of symptoms is usually one to two weeks, but can be much shorter (days), or much longer (months to years).
Symptoms most often occur from May through September, because the nymphal stage of the tick is responsible for most cases. Asymptomaticinfection exists, but occurs in less than 7% of infected individuals in the United States. Asymptomatic infection may be much more common among those infected in Europe.
Lyme disease is caused by spirochetal bacteria from the genus Borrelia. Spirochetes are surrounded by peptidoglycan and flagella and are composed of about 40% of DNA. Because of their double-membrane envelope, Borrelia are often mistakenly described as Gram negative despite the considerable differences in their envelope components with Gram-negative bacteria. The Borrelia species that are Lyme-related are collectively known as Borrelia burgdorferi sensu lato, and show a great deal of genetic diversity.
Borrelia burgdorferi sensu lato is made up of 18 closely related species, but only three clearly cause Lyme disease: B. burgdorferi sensu stricto(predominant in North America, but also present in Europe), B. afzelii, and B. garinii (both predominant in Eurasia). Some studies have also proposed B. bissettii and B. valaisiana may sometimes infect humans, but these species do not seem to be important causes of disease.
Exposure to the Borrelia bacterium during Lyme disease possibly causes a long-lived and damaging inflammatory response, a form of pathogen-induced autoimmune disease. The production of this reaction might be due to a form of molecular mimicry, where Borrelia avoid being killed by the immune system by resembling normal parts of the body’s tissues.
It is therefore possible that if some chronic symptoms come from an autoimmune reaction, this could explain why some symptoms persist even after the spirochetes have been eliminated from the body. This hypothesis may explain chronic arthritis that persists after antibiotic therapy, similar to rheumatic fever, but its wider application is controversial.
Folk remedies for tick removal tend to be ineffective, offer no advantages in preventing the transfer of disease, and may increase the risks of transmission or infection. The best method is simply to pull the tick out with tweezers as close to the skin as possible, without twisting, and avoiding crushing the body of the tick or removing the head from the tick’s body. The risk of infection increases with the time the tick is attached, and if a tick is attached for less than 24 hours, infection is unlikely. However, since these ticks are very small, especially in the nymph stage, prompt detection is quite difficult.