Mark L. Wilson

Land use change alters malaria transmission parameters by modifying temperature in a highland area of Uganda

Summary As highland regions of Africa historically have been considered free of malaria, recent epidemics in these areas have raised concerns that high elevation malaria transmission may be increasing. Hypotheses about the reasons for this include changes in climate, land use and demographic patterns. We investigated the effect of land use change on malaria transmission in the south‐western highlands of Uganda. From December 1997 to July 1998, we compared mosquito density, biting rates, sporozoite rates and entomological inoculation rates between 8 villages located along natural papyrus swamps and 8 villages located along swamps that have been drained and cultivated. Since vegetation changes affect evapotranspiration patterns and, thus, local climate, we also investigated differences in temperature, humidity and saturation deficit between natural and cultivated swamps. We found that on average all malaria indices were higher near cultivated swamps, although differences between cultivated and natural swamps were not statistically significant. However, maximum and minimum temperature were significantly higher in communities bordering cultivated swamps. In multivariate analysis using a generalized estimating equation approach to Poisson regression, the average minimum temperature of a village was significantly associated with the number of Anopheles gambiae s.l. per house after adjustment for potential confounding variables. It appears that replacement of natural swamp vegetation with agricultural crops has led to increased temperatures, which may be responsible for elevated malaria transmission risk in cultivated areas.

Highland malaria in Uganda: Prospective analysis of an epidemic associated with El Niño

Malaria epidemics in African highlands cause serious morbidity and mortality and are being reported more frequently. Weather is likely to play an important role in initiating epidemics but limited analysis of the association between weather conditions and epidemic transmission parameters has been undertaken. We measured entomological variables before and during an epidemic of malaria (which began in February 1998) in a highland region of south-western Uganda and analysed temporal variation in weather data against malaria incidence (estimated from clinic records), mosquito density and entomological inoculation rates (EIR). Indoor resting density of Anopheles gambiae s.l. was positively correlated with malaria incidence (r = 0.68, P < 0.05) despite extremely low vector densities. EIR totalled only 0.41 infectious bites per person during the entire 8-month study period. Rainfall during and following the El Niño event in 1997 was much higher than normal, and rainfall anomaly (difference from the mean) was positively correlated with vector density 1 month later (r = 0.55, P < 0.05). Heavier than normal rainfall associated with El Niño may have initiated the epidemic; the relationship between temperature and transmission parameters remains to be defined. The results from this study indicate that, in this highland population, epidemic malaria may occur at extremely low inoculation rates.

Epidemiology of hepatitis viruses among hepatocellular carcinoma cases and healthy people in Egypt: A systematic review and meta-analysis

Liver cancers are strongly linked to hepatitis B virus (HBV) and hepatitis C virus (HCV). Egypt has the highest prevalence of HCV worldwide and has rising rates of hepatocellular carcinoma (HCC). Egypts unique nature of liver disease presents questions regarding the distribution of HBV and HCV in the etiology of HCC. Accordingly, a systematic search of MEDLINE, ISI Web of Science, ScienceDirect and World Health Organisation databases was undertaken for relevant articles regarding HBV and HCV prevalence in Egypt among healthy populations and HCC cases. We calculated weighted mean prevalences for HBV and HCV among the populations of interest and examined differences in prevalence by descriptive features, including age, year and geographic region. Prevalences for HBV and HCV were 6.7% and 13.9% among healthy populations, and 25.9% and 78.5% among HCC cases. Adults had higher prevalences of both infections (Adult HBV = 8.0%, Child HBV = 1.6%; Adult HCV = 15.7%, Child HCV = 4.0%). Geographically, HBV was higher in the south, whereas HCV was greater in the north (North HBV = 4.6%, South HBV = 11.7%; North HCV = 15.8%, South HCV = 6.7%). Among HCC cases, HBV significantly decreased over time (p = 0.001) while HCV did not, suggesting a shift in the relative influences of these viruses in HCC etiology in Egypt. Our results highlight large amounts of heterogeneity among the epidemiological factors associated with liver disease in Egypt and underscore the necessity of an integrated strategy for the successful prevention of viral hepatitis infections and chronic liver disease.

Genetic diversity of Cryptosporidium spp. in cattle in Michigan: implications for understanding the transmission dynamics

Epidemiological and molecular data on 248 bovine, 17 human, and 16 water samples of Cryptosporidium spp. collected from the lower peninsula of Michigan between 1997 and 2000 were analysed. Cryptosporidium parvum bovine genotype and Cryptosporidium andersoni were found in 56 and four cattle samples, respectively. A total of six C. parvum subgenotypes were found in 34 bovine samples, and five of the eight farms had two or three subgenotypes in cattle. Six water samples from these farms had C. andersoni, five had the C. parvum bovine genotype, and one had Cryptosporidium muris. In contrast, four PCR-positive human samples produced the C. parvum bovine genotype and two had the C. parvum human genotype. Among the C. parvum bovine genotype samples, two human samples and one water sample had subgenotypes identical to those found on cattle farms. The results of this study demonstrate the potential use of molecular methods in tracking the transmission of Cryptosporidium.

Rapid emergence of a focal epidemic of Lyme disease in coastal Massachusetts

We describe a focal epidemic of Lyme disease, which spread from a nature preserve and affected an adjacent community of permanent residents in coastal Massachusetts. The attack rate from 1980 through 1987 was 35 percent among 190 residents living within 5 km of the nature preserve and was greatest (66 percent) among those living closest to the preserve. The risk of infection bore little relation to sex or age. Late Lyme disease, which clustered near the preserve, occurred mainly in residents infected early in the epidemic who did not have a history of erythema migrans and did not receive antibiotic therapy. All the residents with serologic evidence of infection had early or late clinical manifestations of Lyme disease, or both, during the period of study. The seasonal risk of infection was bimodal--greatest in June, with a secondary peak in October--and corresponded to periods of increased transmission. In the nature preserve, the density of the vector tick, Ixodes dammini, exceeded that in other New England sites. The zoonosis rapidly became endemic, and the severity of its impact correlated with the abundance of deer. This epidemic of Lyme disease demonstrated that outbreaks can be focal and can spread rapidly within a community of permanent residents.

Association of influenza epidemics with global climate variability

The reasons for the seasonality and annual changes in the impact of influenza epidemics remain poorly understood. We investigated the covariations between a major component of climate, namely the El Niño Southern Oscillation (ENSO), and indicators of the impact of influenza, as measured by morbidity, excess mortality and viral subtypes collected in France during the period 1971–2002. We show that both the circulating subtype and the magnitude of ENSO are associated with the impact of influenza epidemics. Recognition of this association could lead to better understanding of the mechanisms of emergence of influenza epidemics.

Demography of a Habitat Generalist, The White-Footed Mouse, in a Heterogeneous Environment

A population of white-footed mice (Peromyscus leucopus) in southeastern Massachusetts was lived-trapped monthly in five habitat types for 5 yr in order to study the demography of a habitat generalist. We identified three demographic groups (two of low density and one of high density), which differed primarily in density, adult survival, proportion of males breeding, and variability in the proportion of males. The low-density segments of this population were at times able to achieve demographic performance equivalent to the high-density segments. Several demographic variables were related linearly to microhabitat gradients derived from a principal components analysis of 24 habitat variables, but similar demographic structure was found in different habitat types. A simple model is presented that relates demography to environmental suitability in a habitat generalist. According to this model, population density, productivity, and survival increase, and numerical and demographic variability decrease, along a gradient of increasing environmental suitability. Intensity of intrinsic regulation also may increase with environmental suitability, which would contribute to population stability. We hypothesize that in habitat generalists such as Peromyscus leucopus, population numbers are regulated along a stable trajectory in areas of high suitability, but may show little or no intrinsic regulation in poorer habitats. Furthermore, demographic structure may shift as environmental conditions improve or decline in quality. This highly flexible demographic structure may enable populations of generalists to persist in poor habitats.

Culex restuans (Diptera: Culicidae) Oviposition Behavior Determined by Larval Habitat Quality and Quantity in Southeastern Michigan

Abstract Oviposition is a critical stage in the mosquito lifecycle, and may determine population levels, distribution, biting behavior, and pathogen transmission. Knowledge of the oviposition behavior of Culex restuans Theobald has become particularly important with the emergence of West Nile virus (WNV) in North America. Laboratory and field studies have examined some factors that contribute to oviposition choice in Culex spp., but few studies have investigated responses to cues of future competition and breeding habitat availability in the field. We hypothesized that female Cx. restuans mosquitoes avoid laying eggs in habitats containing cues of larval competition, and that increased availability of larval habitat decreases egg density. To test these hypotheses, a series of field experiments were conducted in southeastern Michigan during summer 2002. We found that female mosquitoes prefer nutrient-enriched containers and decrease ovipositing in containers with conspecific larvae. In addition, greater habitat abundance decreased egg clutch density per container, although there was considerable aggregation of egg clutches. These results support our hypotheses and have potentially important implications for pathogen transmission by mosquitoes.

Microgeographic distribution of immature Ixodes dammini ticks correlated with that of deer

ABSTRACT. In order to determine whether the small‐scale distribution of immature Ixodes dammini Spielman et al. corresponds closely to the activity patterns of white‐tailed deer, Odocoileus virginianus (Zimmerman), these relationships were examined in a site on Long Island, New York, U.S.A. We first determined the extent and temporal pattern of adult ticks feeding on deer by examining twenty‐three resident deer tranquilized during September‐December 1985.1, dammini adults infested deer throughout this fall period, most abundantly during October and November. With radio‐telemetry collars attached to deer we determined the relative frequency that they occupied 0.25 ha quadrats of the study site. During the following summer, we examined white‐footed mice, Peromyscus leucopus (Rafmesque), that inhabited these quadrats and removed immature ticks from each. 8975 larval and 163 nymphal /. dammini were removed from 208 mice trapped in forty‐three such quadrats. The frequency of deer using these quadrats was positively correlated with both the number of larval and of nymphal ticks per mouse. These results suggest that risk of I.dammini‐borne zoonotic disease may be decreased by locally reducing deer density in sites that experience intense human activity.

Epidemic hepatitis C virus infection in Egypt: estimates of past incidence and future morbidity and mortality

Summary.  Hepatitis C virus (HCV) infection is gaining increasing attention as a global health crisis. Egypt reports the highest prevalence of HCV worldwide, ranging from 6% to more than 40% among regions and demographic groups. Predicting the impact of the epidemic has been difficult because of the long‐latency period and low‐resource setting. Accordingly, we sought to estimate historic incidence and predict the future impact of HCV using Markov simulation modelling techniques. Age‐specific HCV incidence rates (IRs) were estimated using previously acquired age‐specific HCV prevalence data. Data for this analysis were from a highly detailed, community‐based seroprevalence study from 2003. Future HCV‐related morbidity and mortality were estimated using a computer cohort simulation of HCV natural history in the Egyptian population. Population and natural history parameters were defined using results from a meta‐analysis and existing comprehensive literature reviews. Incidence model estimates ranged from 2.01 to 25.47 HCV cases per 1000 person‐years (PYs). The highest IRs were calculated among those over 35 years of age. Our Markov model predicted 127 821 deaths from chronic liver disease and 117 556 deaths from hepatocellular carcinoma in Egypt over the next 20 years. During this period, it was estimated that HCV would yield 750 210 PY of decompensated cirrhosis, 132 894 PY of hepatocellular carcinoma, and a total loss of 32.86 million years of life compared to a non‐infected cohort. Our results support the claim of high HCV incidence in Egypt and suggest that HCV may lead to a substantial health and, consequently, economic burden over the next 10–20 years.