Degree Of Parasite Aggregation Research Articles

The observed relationship between the degree of parasite aggregation and the prevalence of infection within human host populations for soil-transmitted helminth and schistosome infections.

Soil-transmitted helminths (STH) and schistosome parasites are highly aggregated within the human population. The probability distribution of worms per person is described well by the negative binomial probability distribution with aggregation parameter, k, which varies inversely with parasite clustering. The relationship between k and prevalence in defined populations subject to mass drug administration is not well understood. We use statistical methods to estimate k using two large independent datasets for STH and schistosome infections from India and Niger, respectively, both of which demonstrate increased aggregation of parasites in a few hosts, as the prevalence of infections declines across the dataset. A greater attention needs to be given in monitoring and evaluation programmes to find and treat the remaining aggregates of parasites.

Baseline patterns of infection in regions of Benin, Malawi and India seeking to interrupt transmission of soil transmitted helminths (STH) in the DeWorm3 trial.

Global efforts to control morbidity associated with soil-transmitted helminth infections (STH) have focused largely on the targeted treatment of high-risk groups, including children and pregnant women. However, it is not clear when such programs can be discontinued and there are concerns about the sustainability of current STH control programs. The DeWorm3 project is a large multi-country community cluster randomized trial in Benin, India and Malawi designed to determine the feasibility of interrupting the transmission of STH using community-wide delivery of mass drug administration (MDA) with anthelmintics over multiple rounds. Here, we present baseline data and estimate key epidemiological parameters important in determining the likelihood of transmission interruption in the DeWorm3 trial. A baseline census was conducted in October-December 2017 in India, November-December 2017 in Malawi and in January-February 2018 in Benin. The baseline census enumerated all members of each household and collected demographic data and information on occupation, assets, and access to water, sanitation and hygiene (WASH). Each study site was divided into 40 clusters of at least 1,650 individuals per cluster. Clusters were randomized to receive twice yearly community-wide MDA with albendazole (GSK) targeting eligible individuals of all ages (20 clusters), or to receive the standard-of-care deworming program targeting children provided in each country. In each site, a randomly selected group of 150 individuals per cluster (6,000 total per site) was selected from the baseline census using stratified random sampling, and each individual provided a single stool sample for analysis of STH infection using the Kato-Katz technique. Study site, household and individual characteristics were summarized as appropriate. We estimated key epidemiological parameters including the force of infection and the degree of parasite aggregation within the population. The DeWorm3 sites range in population from 94,969 to 140,932. The population age distribution varied significantly by site, with the highest proportion of infants and young children in Malawi and the highest proportion of adults in India. The baseline age- and cluster-weighted prevalence, as measured by Kato-Katz, varied across sites and by species, Baseline hookworm prevalence in India was 21.4% (95% CI: 20.4-22.4%), while prevalence of Ascaris and Trichuris by Kato-Katz was low (0.1% and 0.3% overall). In Malawi, the overall age- and cluster-weighted STH prevalence was 7.7% (95% CI: 7.1-8.4%) predominantly driven by hookworm infections (7.4%) while Ascaris (0.1%) and Trichuris (0.3%) infections were rare. In Benin, the overall age- and cluster-weighted prevalence was significantly lower (5.6%, 95% CI: 5.1-6.2%) and Ascaris (2.0%, 95% CI: 1.6-2.3%) was more common than in other sites. Ascaris infections were more likely to be moderate- or heavy-intensity (43.7%, unweighted) compared to hookworm (5.0%). The force of infection for hookworm was highest in adults in India and Malawi but appeared relatively stable across age groups in Benin. These data demonstrate the significant variability between the sites in terms of demography, socio-economic status and environmental characteristics. In addition, the baseline prevalence and intensity data from DeWorm3 suggest that each site has unique epidemiologic characteristics that will be critical in determining correlates of achieving STH transmission interruption in the DeWorm3 trial. Trial registration: The trial was registered at ClinicalTrials.gov (NCT03014167).

Heterogeneity in transmission parameters of hookworm infection within the baseline data from the TUMIKIA study in Kenya

BackgroundAs many countries with endemic soil-transmitted helminth (STH) burdens achieve high coverage levels of mass drug administration (MDA) to treat school-aged and pre-school-aged children, understanding the detailed effects of MDA on the epidemiology of STH infections is desirable in formulating future policies for morbidity and/or transmission control. Prevalence and mean intensity of infection are characterized by heterogeneity across a region, leading to uncertainty in the impact of MDA strategies. In this paper, we analyze this heterogeneity in terms of factors that govern the transmission dynamics of the parasite in the host population.ResultsUsing data from the TUMIKIA study in Kenya (cluster STH prevalence range at baseline: 0–63%), we estimated these parameters and their variability across 120 population clusters in the study region, using a simple parasite transmission model and Gibbs-sampling Monte Carlo Markov chain techniques. We observed great heterogeneity in R0 values, with estimates ranging from 1.23 to 3.27, while k-values (which vary inversely with the degree of parasite aggregation within the human host population) range from 0.007 to 0.29 in a positive association with increasing prevalence. The main finding of this study is the increasing trend for greater parasite aggregation as prevalence declines to low levels, reflected in the low values of the negative binomial parameter k in clusters with low hookworm prevalence. Localized climatic and socioeconomic factors are investigated as potential drivers of these observed epidemiological patterns.ConclusionsOur results show that lower prevalence is associated with higher degrees of aggregation and hence prevalence alone is not a good indicator of transmission intensity. As a consequence, approaches to MDA and monitoring and evaluation of community infection status may need to be adapted as transmission elimination is aimed for by targeted treatment approaches.

Whether larval amphibians school does not affect the parasite aggregation rule: testing the effects of host spatial heterogeneity in field and experimental studies

Almost all macroparasites show over‐dispersed infections within natural host populations such that most parasites are distributed among a few heavily‐infected individuals. Despite the importance of parasite aggregation for understanding system stability, the potential for population regulation, and super‐spreading events, many questions persist about its underlying drivers. Theoretically, aggregation results from heterogeneity in host exposure, resistance, and tolerance. However, few studies have examined how host spatial arrangement – which likely affects both parasite encounter and density‐dependent interactions – influences infection and dispersion, representing a critical gap in our current knowledge regarding the possible drivers of parasite aggregation. Using field data from over 165 ponds and 8000 hosts, we evaluated how the spatial clustering of amphibian larvae within ponds 1) varied among different amphibian species, and 2), affected the distribution of parasites within the host population using Taylor's power law. A complementary mesocosm experiment used field‐guided manipulations of the spatial arrangement of larval amphibians to create a gradient in host clustering while controlling host density, thereby testing for spatial effects on both infection success and aggregation by three different trematode species. Our field data indicated that larval amphibians exhibited significant spatial clustering that was well captured by Taylor's power law (R2 0.92 to 0.97 for different host species), but the residual variation only weakly correlated with observed patterns of trematode parasite over‐dispersion. Correspondingly, experimental manipulation of host clustering had no effects on parasite infection success or the degree of parasite aggregation among cages or mesocosms. Given the importance of parasite over‐dispersion for host populations and disease dynamics, we advocate for further investigations of host and parasite spatial aggregation, particularly studies that incorporate and/or control for heterogeneity in exposure and susceptibility.

Factors associated with parasite aggregation levels in fishes from Brazil.

The degree of parasite aggregation is determined by a number of factors that are not well understood. In total, 3,746 fish from 73 species and their parasites were analyzed to determine associations between the degree of aggregation by taxon and specific characteristics of the fish. A multiple linear regression (95% confidence interval - CI) was used to show that in most taxa, the degree of parasite aggregation was significantly higher in freshwater fish and schooling fish but varied according to taxon in relation to gender, habitat and feeding habits. Parasite aggregations were also significantly increased in fish with a large number of parasitic larvae and greater body length in all evaluated taxa. Because the coefficients of determination of the models were lower than 60% for all taxa, other factors may be involved in parasite aggregation. Conversely, marked significance was observed for the tested variables in relation to taxon dependence, which indicates that additional studies should be performed for these factors using comprehensive databases with larger samples per parasite species.

Characteristics of non-cerebral coenurosis in tropical goats

The epidemiological, clinical, and biochemical profile of non-cerebral coenurosis in goats and the morphological characteristics of the responsible metacestodes (cysts) were examined in a cross-sectional survey of slaughtered goats in abattoirs of the United Arab Emirates (U.A.E.) originating from Abu Dhabi and various tropical countries. The age, country of origin, and location of each cyst in the body of goats were recorded. Blood samples collected from infected and matching healthy goats were subjected to biochemical analysis. Data on the morphological characteristics of the cysts as well as the clusters, scoleces, and rostellar hooks in one cyst from each affected carcass were collected. The data collected were subjected to statistical analysis. A total of 2,284 slaughtered goats were examined and 40 goats were diagnosed as infected with non-cerebral coenurus cysts. The prevalence of non-cerebral coenurosis was 1.75% and the degree of parasite aggregation (k) was 0.003, which is indicative of overdispersion (k<1). The only abnormalities observed in the infected goats were palpation of large single cysts in thigh muscles and higher serum aspartate aminotransferase (AST) value. A total of 76 non-cerebral coenurus cysts from 14 different body locations were collected. No cysts were found in the brain or spinal cord. Cysts located in psoas muscles had on average significantly bigger volumes and higher numbers of scoleces and clusters compared to cysts located in other body parts (P-value=0.000). Significant differences in the morphometric measurements of the rostellar hooks were observed between cysts found in goats from different countries of origin (P-value<0.05) perhaps due to initial steps of allopatric speciation by geographic isolation. A significant positive correlation was found between number of scoleces and volume of cysts (b=6.37>5; R-Sq=89.4%; P-value=0.000) and between number of clusters and number of scoleces (b=25.13>1; R-Sq=79.8%; P-value=0.000) indicative of following a positive allometric growth as well as between number of clusters and volume of cysts (b=0.25<0.5; R-Sq=69.4%; P-value=0.000) indicative of following a negative allometric growth. The biological significance of the observed allometries is not known, but perhaps for evolutionary reasons the parasite is investing its resources more on the growth of scoleces, less on the growth of cyst volume, and even less on the number of clusters.

Relative host body condition and food availability influence epidemic dynamics: aPoecilia reticulata-Gyrodactylus turnbullihost-parasite model

Understanding disease transmission is important to species management and human health. Host body condition, nutrition and disease susceptibility interact in a complex manner, and while the individual effects of these variables are well known, our understanding of how they interact and translate to population dynamics is limited. Our objective was to determine whether host relative body condition influences epidemic dynamics, and how this relationship is affected by food availability. Poecilia reticulata (guppies) of roughly similar size were selected and assembled randomly into populations of 10 guppies assigned to 3 different food availability treatments, and the relative condition index (Kn) of each fish was calculated. We infected 1 individual per group ('source' fish) with Gyrodactyus turnbulli and counted parasites on each fish every other day for 10 days. Epidemic parameters for each population were analysed using generalized linear models. High host Kn-particularly that of the 'source' fish-exerted a positive effect on incidence, peak parasite burden, and the degree of parasite aggregation. Low food availability increased the strength of the associations with peak burden and aggregation. Our findings suggest that host Kn and food availability interact to influence epidemic dynamics, and that the condition of the individual that brings the parasite into the host population has a profound impact on the spread of infection.

Exposure of sows to Ascaris suum influences worm burden distributions in experimentally infected suckling piglets.

This paper reports on the influence of maternal exposure to Ascaris suum on worm burden distributions in experimentally infected piglets. In the first study, sows were inoculated before and during gestation (6 months, long-term exposure) with 10,000 A. suum eggs twice weekly. In a second study, sows were inoculated during gestation only (3 months, short-term exposure) with increasing doses of eggs (10,000-40,000 eggs twice weekly). Helminth-naive sows served as controls in both studies. The third study used the same design as the short-term exposure study, but piglets from exposed and control sows were cross-suckled within 4 h of birth before colostrum uptake. All piglets were inoculated 2 or 3 times with 50 A. suum eggs on days 4 and 7 (and 14) after birth, and left with the sows. At 10 weeks of age all piglets were necropsied, and liver lesions and worm burdens were recorded. Surprisingly, in piglets born to long-term exposed sows, the prevalence of A. suum infection and the mean worm burden were significantly higher than those in piglets from control sows. In contrast, neither worm burdens nor prevalence were significantly different between piglets from short-term exposed sows compared with their controls. In the cross-suckling experiment, 67% of piglets suckling control sows harboured worms at slaughter, compared with 15% of piglets suckling exposed sows. Maximum likelihood analysis of worm burden distribution and the degree of parasite aggregation showed 3 distinctly different types of overdispersed distributions: worm counts in piglets from control sows, in piglets from short-term exposed sows and in piglets from long-term exposed sows. When the worm burden data were analysed including the cross-suckled piglets by biological mother, it appeared that the control and short-term distributions converged and that only the long-term exposure was significantly different. Overall, the degree of parasite aggregation in piglets infected with A. suum decreased with exposure of the sows. A non-linear relationship was observed between prevalence of infection and mean worm burden, which was different for piglets from exposed and control sows, and similar to relationships of this type that previously have been found in human A. lumbricoides infections. It was concluded that in porcine A. suum infections maternal exposure alters the distribution of worms in their offspring, in which the duration of exposure appeared to be an important influence. The results of the cross-suckling further suggest that maternal factors, e.g. antibodies, are transferred via colostrum.

Frequency distribution of Wuchereria bancrofti infection in the vector host in relation to human host: evidence for density dependence

This paper uses detailed entomological data from Pondicherry to compare the microfilaria distribution in vector with that of human and also to examine the evidence for the operation of density dependence on parasite transmission. Analysis showed that the distribution in vector is similar to that in human. The expected distribution derived from the fit of a zero-truncated negative binomial to the human data, closely reflected the observed microfilaria distribution in the vector. This implies that a relatively large proportion of observed microfilaria negatives in the vector population may be true zeros, as observed in human, arising from the biting on true negatives (as yet uninfected, with unmated female worms or immune) in humans, rather than attributable to the blood sampling process. Further it was found that both the prevalence of infection and the degree of parasite aggregation in the vector population falls significantly with parasite stage, implying the operation of density dependence, perhaps via parasite-induced vector mortality.

Quantification of parasite aggregation: A simulation study

A simulation study is used to examine the statistical behaviour of estimators of parameters of parasite infection in relation to variation in sample size, the degree of parasite aggregation, and mean parasite burden. The most important patterns to emerge are the associations between estimates of parameters and sample size (= number of host individuals). As sample size decreases values of sample mean parasite burden, its associated variance, and the level of parasite aggregation are all systematically underestimated. The geometric mean of parasite burden and the prevalence of infection appear to be independent of associations with other parasite parameters. Estimates of parameter values may also depend on the underlying frequency distribution, but appear insensitive to variation in the population mean parasite burden. Results are discussed in relation to the interpretation of data derived from field-based studies. In particular, establishing the form of the relationship between host age and mean parasite burden and/or the degree of parasite aggregation. It is typical for sample size to decline as a function of host age within cross-sectional field data. This may give rise to artefactual patterns in the shape of age-aggregation curves in which sample sizes are unequal among host age classes.

Life History and Population Biology of Adult Acanthocephalus lucii (Acanthocephala: Echinorhynchidae)

The life history and population biology of adult A. lucii in perch, Perca fluviatilis L., from the Forth and Clyde canal, Scotland, was investigated during May 1979-September 1981. There was an annual cycle in the size of the parasite population; prevalence and abundance (+/- SE) were highest during late spring and summer (70-90% and 14 +/- 4.3 to 16 +/- 5.6 worms/fish, respectively) but declined during late autumn and reached a minimum during winter (50-60% and 2.1 +/- 0.9 to 3.2 +/- 0.6 worms/fish). Parasite maturation was associated with higher water temperatures during spring and summer and most shelled acanthors were probably produced during summer and fall. There was only 1 generation of A. lucii per year, although generations tended to overlap and individuals within each generation did not develop synchronously. The sex ratio of adults was initially near unity but favoured females in the later stages of the infection. The distribution of A. lucii among perch was highly aggregated and stomach content analysis suggested that this was partly due to heterogeneity in perch feeding behaviour. The negative binomial and Poisson lognormal models fitted the data on worm distribution. Seasonal changes in the degree of parasite aggregation were detected, but no conclusive evidence of density-dependent controls on parasite population growth was obtained.

Life History and Population Biology of Larval Acanthocephalus lucii (Acanthocephala: Echinorhynchidae) in the Isopod Asellus aquaticus

Hemocoels of 8,731 Asellus aquaticus collected from the Forth and Clyde canal in Glasgow, Scotland, from January 1980 to March 1981 were examined for larvae of Acanthocephalus lucii. Prevalence and mean intensity were generally low (1.5-8.3% and 1.0-1.6, respectively), but there was a slight seasonal infection pattern with fewer infected isopods during summer, reflecting the appearance of a new isopod generation. Although there were no distinct seasonal trends in the proportions of each larval stage, recruitment of larvae probably occurred mainly during summer and autumn. Some larvae reached the cystacanth stage by late summer or autumn; others overwintered as acanthors or acanthellae and completed development the following spring. The maximum life span of larvae was limited to 1 yr by annual turnover of the isopod population. The distribution of larval A. lucii among isopods was slightly aggregated. There was a peaked pattern in the relationship between isopod length and the prevalence, abundance, and degree of parasite aggregation. The rate of parasite development in laboratory-infected isopods was linearly related to temperature between 9 and 22 C; the temperature threshold was 5.7 C, and the larval parasite required 598 degree-days above threshold to complete development. Among laboratory-infected isopods, 2 mechanisms that could regulate the larval parasite population were detected: intraspecific competition and direct, parasite-induced isopod mortality. However, the intensity of infection in the natural habitat was consistently low and may have remained below the level at which these mechanisms operated.