Populations of Ixodes scapularis (Acari: Ixodidae) are modulated by drought at a Lyme disease focus in Illinois

From 1990 through 1997, Ixodes scapularis Say larvae and nymphs were sampled between May and October along a 400-m segment of a nature trail in a Lyme disease endemic site in northern Illinois. Ticks were removed from Peromyscus leucopus mice and collected via tick drags at approximately 3-wk intervals. Mouse population estimates along the trail varied from 2, in the spring of 1996 following a year of drought, to > 200 in 1993, the wettest year on record. During the 8-yr period, there were major droughts during the summers of 1991 and 1995. Cumulative degree-days were positively correlated with the number of ticks collected on drags in the same year and negatively correlated with larval tick populations for the following year (P < 0.05). Cumulative rainfall was positively correlated with larval tick abundance for the following year. This was most readily apparent by examination of the larval density on captured mice. In the year following each of two drought years, larval densities were significantly depressed compared with the 8-yr average at the site.     

Predicting the risk of Lyme disease: habitat suitability for Ixodes scapularis in the north central United States

The distribution and abundance of Ixodes scapularis were studied in Wisconsin, northern Illinois, and portions of the Upper Peninsula of Michigan by inspecting small mammals for ticks and by collecting questing ticks at 138 locations in state parks and natural areas. Environmental data were gathered at a local level (i.e., micro and meso levels), and a geographic information system (GIS) was used with several digitized coverages of environmental data to create a habitat profile for each site and a grid map for Wisconsin and Illinois. Results showed that the presence and abundance of I. scapularis varied, even when the host population was adequate. Tick presence was positively associated with deciduous, dry to mesic forests and alfisol-type soils of sandy or loam-sand textures overlying sedimentary rock. Tick absence was associated with grasslands, conifer forests, wet to wet/mesic forests, acidic soils of low fertility and a clay soil texture, and Precambrian bedrock. We performed a discriminant analysis to determine environmental differences between positive and negative tick sites and derived a regression equation to examine the probability of I. scapularis presence per grid. Both analyses indicated that soil order and land cover were the dominant contributors to tick presence. We then constructed a risk map indicating suitable habitats within areas where I. scapularis is already established. The risk map also shows areas of high probability the tick will become established if introduced. Thus, this risk analysis has both explanatory power and predictive capability.     

Oviposition behavior of Aedes triseriatus (Diptera: Culicidae): prevalence, intensity, and aggregation of eggs in oviposition traps

The oviposition behavior of the treehole mosquito Aedes triseriatus (Say) was studied by counting the eggs laid in 300 oviposition traps weekly for one season (May-October) in Illinois. Eggs were aggregated spatially among traps each week, and temporally among weekly samples and for individual traps over weekly counts. Aggregation could be associated with a nonrandom dispersion pattern of oviposition events and with the oviposition of 20-60 eggs per event. Thus, females were laying clumps of eggs in an overdispersed manner. The greatest number of eggs was found in late July when prevalence (proportion of traps with eggs) and intensity (number of eggs per positive trap) peaked simultaneously. Intensity also peaked in May and prevalence peaked in mid-June and early September. When traps from which eggs were removed each week were compared with traps in which eggs were allowed to accumulate, prevalence was naturally higher in the latter group. However, the intensity of eggs was similar in both groups, indicating that more eggs were laid in traps from which eggs were removed. Thus, the presence of eggs on oviposition surfaces was associated with a decrease in further oviposition.     

Schistosoma haematobium in Upper Egypt: analysis of dispersion patterns

Statistical methods analyzing changes in dispersion patterns of parasites among hosts were applied to Schistosoma haematobium egg excretion data from a five-year study of a cohort of 1,400 boys in 3 villages in Upper Egypt. Despite significant changes in mean density of the parasites in the first 4 years, the degree of aggregation of S. haematobium among the cohort did not change markedly in analysis of egg count data obtained semiannually. Two other related measures of dispersion, mean crowding and patchiness, were also obtained and compared to prevalence and mean intensity, the components determining mean density of schistosomes among hosts. Anti-schistosomal chemotherapy with metrifonate in the cohort did not succeed in appreciably reducing the mean density of S. haematobium over a one-year follow-up. The dispersion pattern of S. haematobium among the study group was not markedly altered. Prevalence and mean intensity were useful indicators of changes in the degree of aggregation of S. haematobium among hosts. The inability of chemotherapy to provide a persistent decline in the mean density parameter is reflected in the rise of mean intensity and prevalence of S. haematobium 1 year post-therapy. It is suggested that analysis of quantifiable changes in dispersion patterns of S. haematobium among their hosts can be a useful aid in planning and assessment of control strategies.     

Active dispersal of natural populations of Triatoma infestans (Hemiptera: Reduviidae) in rural northwestern Argentina

An empirical model of flight initiation coupled with data from a longitudinal study predicted that the flight dispersal of Triatoma infestans from peridomestic sites was more likely to occur in late summer. To partially test this prediction, we operated 11-12 black light traps from 1945 to 2200 hours in March 2003 in two villages in northern Argentina. All peridomestic sites around the light traps were later inspected to assess the relative abundance and nutritional status of T. infestans at each site. Traps were located 19-94 m from the nearest infested site. A total of 2 female, 10 male, and 3 fifth-instar nymphs of T. infestans; 4 adult Triatoma garciabesi; and 1 Triatoma guasayana fifth-instar nymph were collected in 64 trap nights. Nearly two-thirds of the bugs arrived to the traps during the first hour after sunset, when ambient temperatures were 22-28 degrees C; 80% of adults were unfed. The number of T. infestans that flew to the traps was significantly and negatively associated with wind speed, and the number of males positively associated with the abundance of adult T. infestans in peridomestic sites within 200 m around each light trap. This is the first successful application of light traps for collecting dispersing nymphal and adult T. infestans on a village-wide scale. We attribute this success to the placement of traps with consideration to spatial infestation patterns and seasonal variation in nutritional status of peridomestic triatomine populations.     

Invasive group A streptococcal disease: Management and chemoprophylaxis

Given the potentially devastating consequences of severe invasive group A streptococcal disease, attention has been directed toward the role of chemoprophylaxis and the optimization of management strategies. In response to this issue, Canadian guidelines were previously developed. However, the uptake of these recommendations is variable across Canada. The present document summarizes key components of the recommendations for use by Canadian physicians. The importance of penicillin in the treatment of group A streptococcal disease is reaffirmed, and the role of clindamycin is discussed. In addition, in situations in which chemoprophylaxis may be considered, the preferred agents are summarized.