LYME DISEASE

    Epizootiology of Lyme Disease

    Lyme disease was first recognized as a distinct clinical illness in 1975, when 51 residents from Old Lyme, Lyme and East Haddam, Connecticut were diagnosed as having a unique form of oligoarticular arthritis. Since its description, Lyme disease has emerged as a significant threat to the public's health in the northeastern United States. Nationally, Lyme disease increased from 523 reported cases in 1982 to 4,507 reported cases in 1988, 8,552 cases in 1989, and 13,043 cases in 1994. The Centers for Disease Control reported that Lyme disease accounted for 81% of all reported cases of arthropod-transmitted diseases in the United States between 1986-1991. In 1982, a treponema-like spirochete was isolated from the midgut of adult I. scapularis suggesting that this organism may be involved in the etiology of Lyme disease. Shortly thereafter, spirochetes were isolated from the blood of Lyme disease patients and from adult I. scapularis. The following year the I. scapularis spirochete was recognized as a new species and named Borrelia burgdorferi. Furthermore, the blacklegged tick, I. scapularis, was considered to be the most important vector of the spirochete.

    Acquisition of Spirochetes

    Typically, immature I. scapularis acquires B. burgdorferi during its initial bloodmeal from infected reservoir hosts, primarily from white-footed mice. After molting, the following life stage is transstadially infected. Transstadially infected nymphs and adults can then transmit the spirochete to non-infected hosts. Nymphal blacklegged ticks have had a single previous blood meal (in the larval stage) and subsequently approximately 25% of the nymphs can be found infected. On the other hand, adult blacklegged ticks have had the benefit of two previous blood feedings (as larvae and nymphs) and therefore have a natural infection rate of nearly 50%. Because nymphal feeding precedes larval feeding, the enzootic transmission of B. burgdorferi is highly efficient. Thus, the majority of mice become infected with Lyme disease spirochetes in the spring before serving as hosts to the larvae later that summer. Spirochetes overwinter in the fed larvae, in the unfed nymphs or in the host animal. Ixodes scapularis larvae can also acquire B. burgdorferi transovarially from infected females; however, transovarial transmission seems to play at best a minimal role in the maintenance of B. burgdorferi in the tick population.

    Transmission of Spirochetes

    The Lyme disease spirochete, B. burgdorferi, is transmitted from the tick to the host via salivation, regurgitation, both processes combined or through destruction of the tick during host grooming. Although B. burgdorferi is found primarily in the lumen of the tick's digestive tract, the spirochete also disseminates through the tick's hemolymph and enters the salivary glands. The spirochete is then most likely transmitted through the saliva, as these ticks salivate excessively during feeding. Apparently, a minimum of 24 hours of tick attachment is required for spirochete transmission.

    As stated above, all three life stages of the blacklegged tick can be infected with B. burgdorferi (larvae transovarially, nymphs and adults transstadially). However, the ability of each life stage to transmit the spirochete may vary considerably. Lyme disease transmission is highest during May and June, which coincides with the peak nymphal season. Although adult blacklegged ticks have been shown to transmit B. burgdorferi to laboratory rabbits, disease transmission to humans greatly decreases with the adult season. Several hypotheses may explain the discrepancies between the adult's higher infection rate than the nymphs, yet its lower transmission risk than the nymphal stage. For instance, the adults are much larger than the nymphs and therefore more likely to be removed before spirochetal transmission can occur. Furthermore, adults are most active in the cooler months when people are less active outdoors; and if they are outdoors, they are most likely wearing long pants and long-sleeved shirts. Therefore, fewer adult ticks would be able to attach and begin feeding.

    Peromyscus leucopus is considered to be an extremely efficient and competent reservoir host for the Lyme disease spirochete. Other mammalian and avian species have been implicated as being potential reservoir hosts for B. burgdorferi; however, there is lack of thorough and conclusive research on these species.

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