Lyme disease has recently begun to emerge as a significant threat to human health, both in Europe and the United States. Late sequellae, resembling those of neurosyphilis and multiple sclerosis, may occur many years after initial infection. Spontaneous abortion accompanies arthritis, carditis and neuritis as burdensome short-term sequellae. Thousands of new infections are recognized each year on each side of the Atlantic, although reporting may be incomplete. The disease was described in Europe nearly a century ago and named erythema chronicum migrans, but its etiology has only recently been defined. The name “Lyme disease” was coined to describe a particularly intense American focus of disease, but the term has gained wide acceptance on both continents. The identity of the American and European etiological agents involved has yet to be determined.
In America, a deer-associated, often bird-transported tick transmits this mouse-reservoired spirochete. The European situation seems more complex because the vector tick feeds on a greater variety of vertebrates. The reservoir hosts of the spirochete have yet to be determined. The role ofIxodes ricinus and possible other vectors in perpetuating transmission of the European infection remains to be defined. WhetherI. ricinus as well asI. dammini merely serve as a bridge to the human population or are important for the maintenance of the feral cycle remains to be seen.
The capacity of a tick to maintain transmission of Lyme disease spirochetes depends upon a complex set of properties, including competence as a host for the spirochete, a pattern of feeding that focuses on a particular reservoir favored by a pattern of tick activity, during each transmission season, in which nymphs feed before larvae. Transmission would be favored by an environment, such as that of islands, in which the variety of potential reservoir hosts is restricted. Hosts, for example reptiles, that might fail to support growth of the spirochete would serve to dilute effective transmission in nature.
Similarly, the capacity of a vertebrate to maintain the infection requires long-term support of the spirochete in a tissue site accessible to vector ticks, tolerance of repeated feeding by vector ticks and a pattern of host activity that exposes the host to numerous bites.
The intensity of infection depends upon a continuous pattern of transmission in which each generation is infected anew. The rate event in which the vector inherits infection would serve mainly to transport the spirochete to a new site, most effectively by migrating birds.
Due to the dispersed nature of Lyme disease and its recent emergence as an important hazard to health, measures for prophylaxis have only recently been devised. Lyme disease can be treated with antibiotics. But the effectiveness of such therapy depends upon correct and prompt diagnosis; delayed treatment is less effective, presumably because the spirochete becomes sequestered in immune-privileged sites.