Although pathogens for mammals and birds, Borrelia are the causative agents of tickborne and louseborne relapsing fever and tickborne Lyme disease in humans.
Relapsing Fever
Human relapsing fever is caused by more than 15 species of Borrelia and is transmitted to humans by the bite of a louse or tick. B. recurrentis is responsible for louseborne or epidemic relapsing fever. This spirochete is transmit ted from the louse Pediculus humanus subsp. humanus and disease is found worldwide; humans are the only reservoir for B. recurrentis. All other borreliae that cause disease in the United States are transmitted via tick bites and are named after the species of tick, usually of the genus Ornithodoros (soft tick), from which they are recovered. Common species in the United States include B. hermsii, B. turicatae, B. parkeri, and B. mazzottii. Depending on the organisms and the disease, their reservoir is either humans or rodents in most cases. Although their pathogenic mechanisms are unclear, these spirochetes exhibit antigenic variability that may account for the cyclic fever patterns associated with this disease.
Lyme Disease
Although there are currently at least 10 different Borrelia species within the B. burgdorferi sensu lato complex, only Borrelia burgdorferi sensu stricto (strict sense of B. burgdorferi) as well as B. garinii, B. afzelii, B. spielmanii, B. lusitaniae, and B. valaisiana are agents of Lyme disease and are transmitted by the bite of Ixodes ticks. Lyme disease is the most common vector-borne disease in North America and Europe and is an emerging problem in northern Asia. Hard ticks, belonging primarily to the genus Ixodes, act as vectors in the United States, including Ixodes pacificus in California and I. scapularis in other areas. The ticks’ natural hosts are deer and rodents. However, the adult ticks will feed on a variety of mammals including raccoons, domestic and wild carnivores, and birds. The ticks will attach to pets as well as to humans; all stages of ticks—larva, nymph, and adult—can harbor the spirochete and transmit disease. The nymphal form of the tick is most likely to transmit disease because it is active in the spring and summer when people are dressed lightly and participating in outdoor activities in the woods. At this stage the tick is the size of a pinhead and the initial tick bite may be overlooked. Ticks require a period of attachment of at least 24 hours before they transmit disease. Endemic areas of disease have been identified in many states, including Massachusetts, Connecticut, Maryland, Minnesota, Oregon, and California, as well as in Europe, Russia, Japan, and Australia. Direct invasion of tissues by the organism is responsible for the clinical manifestations. However, IgM antibodies are produced continually months to years after initial infection as the spirochete changes its antigens. B. burgdorferi’s potential ability to induce an autoimmune process in the host because of cross-reactive antigens may contribute to the pathology associated with Lyme disease. Moreover, by virtue of its ability to vary its surface antigens (e.g., outer surface protein [Osp] A to G) as well as avoid complement attack, B. burgdorferi is able to avoid the human host response. The pathologic findings associated with Lyme disease are also believed to be due to the release of host cytokines initiated by the presence of the organism.
