Discussion: Epidemiologic Studies

What is the distribution and population density of ticks transmitting the Lyme spirochete? Data on tick distribution are scarce. Ixodes dammini appears to be limited to sea-level ecosystems, whereas L pacificus populations are adequate up to 7,000 feet elevation. Three cases of Lyme disease had their exposure above 5,000 feet in the Sierras of northern California. The nymphal tick is very small and not generally recognized by the public. Qualitative tick surveys in Minnesota done in the late 1950s and early 1960s by the Minnesota Department of Natural Resources found I. dammini in an eight-county area. More recently, qualitative studies showed the tick extending west and somewhat south from that area. Recently a physician detected a case in Mahnomen County, considerably west of the known distribution of I. dammini and actually identified the tick on the patient. Data on exposure in North America are inadequate and can be expected to vary depending on how one looks for ticks, how concentrated ticks are in the woods, and other variables too complicated to analyze as yet. How should we characterize tick density? The terms "high, moderate, and low" tick density were defined. Greater than ten L dammini per animal was considered high, five to nine moderate, and four or less low density. For data to be comparable from one study to another, examination of deer for ticks should be done uniformly, starting at the ears and progressing to the head and neck, ten minutes per animal. Ticks were found to be most concentrated on the ears. Does the adult stage of the tick transmit to people? Sporadic cases of Lyme disease have been detected in virtually every month of the year in the United States north from Maryland. Most of these cases had ECM, implying that the exposure was quite recent in reference to onset of disease and that the vector was the adult tick in cases of late fall and spring ECM. Can ticks other than I. dammini and L pacificus be vectors of the Lyme spirochete in the U.S.? Amblyomma americanum has now been implicated in two cases of ECM. The first of these was in Arkansas where a female tick was found on a worker who developed ECM but failed to seroconvert. A. americanum is an extremely common tick in the South and southeastern U.S. If this tick were to become a significant vector, Lyme disease could develop into a much more serious and widespread health threat. Four cases of Lyme disease from Tennessee were reported to CDC in 1983. A 13-year-old boy camped in south central Tennessee over the Labor Day holiday,