The Role of the Deer Tick in 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 one of the most significant threats to public health in the northeastern United States. In 1982, Dr. Burgdorfer isolated a treponema-like spirochete from the midgut of adult Ixodes scapularis and suggested that this organism may be involved in the etiology of Lyme disease. Shortly thereafter, other researchers isolated spirochetes 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 deer tick, I. scapularis, was considered to be the most important vector of the spirochete in the Northeast and upper Midwest.

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 due to 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 most likely transmitted through the saliva, as these ticks salivate excessively during feeding. Apparently, a minimum of 24 hours of tick feeding is required for spirochete transmission. Researchers showed that nymphal I. scapularis transferred B. burgdorferi to one of fourteen rodents after 24 hours, five of fourteen rodents after 48 hours and thirteen of fourteen rodents after 72 hours or more of feeding.

Typically, immature I. scapularis acquires B. burgdorferi during its initial bloodmeal from infected reservoir hosts, primarily from white-footed mice. After molting, the subsequent life stage is transstadially infected. Transstadially infected nymphs and adults can then transmit the spirochete to non-infected hosts. Nymphal deer 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 deer 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 in a given year, 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 of a different population 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. In one study, two of 274 (0.7%) nymphal I. scapularis that were derived from field collected larvae that were fed on non-infected hamsters were infected with B. burgdorferi. Similarly, other scientists found that 44 of 2,297 (1.9%) larval I. scapularis were infected with Lyme disease spirochetes. In another study in Connecticut, none of 148 field collected larvae were infected. However, much higher rates of transovarial transmission have been recorded from two very closley related species of ticks - the European sheep tick, I. ricinus, and from the western blacklegged tick, I. pacificus. Consequently, it appears that transovarial transmission of B. burgdorferi has varied success and importance in the enzootic cycle of B. burgdorferi. However, at least in I. scapularis, transovarial transmission seems to have at best a minimal role in the maintenance of B. burgdorferi in the tick population.

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