James Rothschild Environmental Factors Affecting Abundance May 12 2008 p. 1 Environmental Factors Affecting Nymphal Ixodes pacificus Abundance Under Field and Laboratory Conditions James Rothschild Abstract The prevalence of human cases of Lyme disease in the United States has been steadily increasing since 1982, and in northern California, the seasonal occurrence of human Lyme disease cases coincides with the peak activity period of nymphal Ixodes pacificus Cooley and Kohls 1943 (Eisen and Lane 2000). This study examined nymphal I. pacificus in Marin County in attempt to elucidate what biotic and abiotic factors influence their abundance. Field and laboratory studies were conducted to determine both the correlations and effects of soil pH, air and soil temperature on nymph abundance. Vegetation association among nymphs was also determined in the field. In the field, it was observed that nymphs were significantly associated with areas consisting predominately of oak trees and showed highest abundance in areas with air temperatures of 14oC, soil temperatures of 9oC and a soil pH of 6.7. Nymph abundance was significantly correlated with soil pH in the field but no significant correlation was observed between soil and air temperature. Under experimental conditions, it was found that nymphs had a significant association to a maintained soil temperature of 9oC and a significant association to maintained air temperatures of 14oC and 11oC. No significant association of nymphs was observed to maintained soil pH. The significant association of nymphs with conditions in the field and laboratory may provide an explanation as to which natural conditions determine nymphal abundance in certain locations.James Rothschild Environmental Factors Affecting Abundance May 12 2008 p. 2 Introduction The western black-legged tick, Ixodes pacificus Cooley and Kohls 1943, is the primary vector of Lyme disease in the western United States, transmitting the disease causing spirochete, Borrelia burgdorferi Johnson, Hyde, Steigerwalt and Brenner 1984, in two of its three life stages (Padgett and Lane 2001). Larval I. pacificus hatch in the end of spring to midsummer and are typically uninfected with B. burgdorferi as transovarial transmission between adult female and larvae is rare (Schoeler and Lane 1993). Larvae seek western fence lizards, Sceloporus occidentalis Baird & Girard 1852, western scrub jay’s, Aphelocoma californica Grinnell 1901 and dusky-footed woodrats, Neotoma fuscipes Baird 1858, for a blood meal and attach to these hosts for several days (Brown 1992). Larvae drop off the hosts and proceed to the nymphal life stage which may remain inactive for several months. Nymphs will seek the same host species as larvae for a blood meal and molt into adults who seek Columbia black tailed deer, Odocoileus hemionus columbianus Richardson 1829, dusky-footed woodrats, and western gray squirrels, Sciurus griseus Ord 1818 as hosts (Brown 1992). Both nymphs and adults are competent vectors for Lyme disease but a discrepancy exists between the infection and transmission rates of nymphs and adults. Adults have been shown to have lower infection rates because of borreliacidal factors in the blood of western fence lizards that kill B. burgdorferi during the feeding of I. pacificus nymphs (Lane and Quistad 1998). In addition, the adults larger size and activity during winter months when people wear more protective clothing causes their transmission of Lyme disease to be low (Xiahong et al. 2000). Due to the more inconspicuous size of nymphs, they are less likely to be noticed and removed promptly and are thus primarily responsible for transmitting Lyme disease causing spirochetes to humans (Vredevoe 1998). Lyme disease is the most frequently reported vector-borne disease in the U.S. (Bacon et al. 2005). Systemic symptoms of Lyme disease include headache, neck stiffness and heart palpitations while chronic symptoms in untreated cases include arthritis, joint swelling and neurological problems (Zhang et al. 2006). In 2005, 23,305 cases of Lyme disease were reported, yielding a national average of 7.9 cases for every 100,000 individuals (Bacon et al. 2005). The cost to treat Lyme disease has been decreasing since 1997, but the loss of productivity to an individual may still be very high (Zhang et al. 2006). In 2000, the expected mean total cost attributable to Lyme disease was $1,965 per patient due to the cost of hospitalization,James Rothschild Environmental Factors Affecting Abundance May 12 2008 p. 3 consultations, office visits and prescription of antibiotics. However, the loss of productivity and suffering accrued by an individual with late-stage Lyme disease could be high as $9,108 due to days missed from work (Zhang et al. 2006). Control measures to decrease the transmission of Lyme disease to humans may result in the alleviation of financial costs associated with disease treatment. The implementation of such measures requires a strong understanding of the factors that promote nymphal abundance. Prior studies regarding the biotic and abiotic factors that influence the prevalence of ticks have focused mainly on the distribution of vertebrate hosts and climatic fluctuations (Brownstein et al. 2005). These findings indicated that landscape features such as the presence of logs, topographic exposure and the amount of vegetation affected population sizes and tick physiological processes, such as their potential to move to find either hosts or mates (Eisen 2000). I. pacificus nymphs predominantly occur in areas with oak trees as opposed to grassland areas. This association with oak trees may be because oak trees and surrounding leaf litter create areas with high humidity and low air temperature which has been shown to be necessary for tick survival in the lab (Xiahong et al. 2000). In addition, oak habitats may also be suitable habitats for vertebrate hosts which would in turn cause the presence of nymphs. In laboratory experiments, tick mortality has been greater under high temperatures and low humidity suggesting a physiological
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