Measurement of Ascaris lumbricoides infection intensity and the dynamics of expulsion following treatment with mebendazole

Revisiting density-dependent fecundity in schistosomes using sibship reconstruction

The stability of parasite populations is regulated by density-dependent processes occurring at different stages of their life cycle. In dioecious helminth infections, density-dependent fecundity is one such regulatory process that describes the reduction in egg production by female worms in high worm burden within-host environments. In human schistosomiasis, the operation of density-dependent fecundity is equivocal and investigation is hampered by the inaccessibility of adult worms that are located intravascularly. Current understanding is almost exclusively limited to data collected from two human autopsy studies conducted over 40 years ago, with subsequent analyses having reached conflicting conclusions. Whether egg production is regulated in a density-dependent manner is key to predicting the effectiveness of interventions targeting the elimination of schistosomiasis and to the interpretation of parasitological data collected during monitoring and evaluation activities. Here, we revisit density-dependent fecundity in the two most globally important human Schistosoma spp. using a statistical modelling approach that combines molecular inference on the number of parents/adult worms in individual human hosts with parasitological egg count data from mainland Tanzania and Zanzibar. We find a non-proportional relationship between S. haematobium egg counts and inferred numbers of female worms, providing the first clear evidence of density-dependent fecundity in this schistosome species. We do not find robust evidence for density-dependent fecundity in S. mansoni because of high sensitivity to some modelling assumptions and the lower statistical power of the available data. We discuss the strengths and limitations of our model-based analytical approach and its potential for improving our understanding of density dependence in schistosomiasis and other human helminthiases earmarked for elimination.

Schistosomiasis is a devastating disease of poverty currently estimated to infect over 220 million people. It is caused by parasitic worms (blood flukes) that live for, on average, 5–7 years inside the blood vessels of infected hosts and produce hundreds of eggs daily. Whether egg production is regulated in a density-dependent manner, and if so under what conditions, has been controversial for schistosomiasis, and investigation is hampered due to the inaccessible location of adult worms. Resolving this fundamental question is important because density dependencies determine the resilience of helminthiases to interventions. Here, we have revisited this longstanding and unresolved question of density-dependent fecundity in human schistosomes using a novel statistical modelling approach that combines information from molecular and parasitological data. We report the first clear evidence of density-dependent fecundity in S. haematobium, the causative agent of millions of cases of urogenital schistosomiasis. Our findings are of critical importance both to mathematical modellers predicting the impact of interventions and to public health policy makers striving to meet the 2030 elimination targets for schistosomiasis. This study also serves to illustrate a new biostatistical approach that could be applied to investigate density dependencies in other helminthiases where adult parasites are inaccessible.

The impact of community-wide, mass drug administration on aggregation of soil-transmitted helminth infection in human host populations

Background

Soil-transmitted helminths (STH) are intestinal parasites estimated to infect over 1.5 billion people. Current treatment programmes are aimed at morbidity control through school-based deworming programmes (targeting school-aged children, SAC) and treating women of reproductive age (WRA), as these two groups are believed to record the highest morbidity. More recently, however, the potential for interrupting transmission by treating entire communities has been receiving greater emphasis and the feasibility of such programmes are now under investigation in randomised clinical trials through the Bill & Melinda Gates Foundation funded DeWorm3 studies. Helminth parasites are known to be highly aggregated within human populations, with a small minority of individuals harbouring most worms. Empirical evidence from the TUMIKIA project in Kenya suggests that aggregation may increase significantly after anthelminthic treatment.

Methods

A stochastic, age-structured, individual-based simulation model of parasite transmission is employed to better understand the factors that might induce this pattern. A simple probabilistic model based on compounded negative binomial distributions caused by age-dependencies in both treatment coverage and exposure to infection is also employed to further this understanding.

Results

Both approaches confirm helminth aggregation is likely to increase post-mass drug administration as measured by a decrease in the value of the negative binomial aggregation parameter, k. Simple analytical models of distribution compounding describe the observed patterns well.

Conclusions

The helminth aggregation that was observed in the field was replicated with our stochastic individual-based model. Further work is required to generalise the probabilistic model to take account of the respective sensitivities of different diagnostics on the presence or absence of infection.

Estimating helminth burdens using sibship reconstruction

BACKGROUND

Sibship reconstruction is a form of parentage analysis that can be used to identify the number of helminth parental genotypes infecting individual hosts using genetic data on only their offspring. This has the potential to be used for estimating individual worm burdens when adult parasites are otherwise inaccessible, the case for many of the most globally important human helminthiases and neglected tropical diseases. Yet methods of inferring worm burdens from sibship reconstruction data on numbers of unique parental genotypes are lacking, limiting the method's scope of application.

RESULTS

We developed a novel statistical method for estimating female worm burdens from data on the number of unique female parental genotypes derived from sibship reconstruction. We illustrate the approach using genotypic data on Schistosoma mansoni (miracidial) offspring collected from schoolchildren in Tanzania. We show how the bias and precision of worm burden estimates critically depends on the number of sampled offspring and we discuss strategies for obtaining sufficient sample sizes and for incorporating judiciously formulated prior information to improve the accuracy of estimates.

CONCLUSIONS

This work provides a novel approach for estimating individual-level worm burdens using genetic data on helminth offspring. This represents a step towards a wider scope of application of parentage analysis techniques. We discuss how the method could be used to assist in the interpretation of monitoring and evaluation data collected during mass drug administration programmes targeting human helminthiases and to help resolve outstanding questions on key population biological processes that govern the transmission dynamics of these neglected tropical diseases.

Testing for soil-transmitted helminth transmission elimination: Analysing the impact of the sensitivity of different diagnostic tools

In recent years, an increased focus has been placed upon the possibility of the elimination of soil-transmitted helminth (STH) transmission using various interventions including mass drug administration. The primary diagnostic tool recommended by the WHO is the detection of STH eggs in stool using the Kato-Katz (KK) method. However, detecting infected individuals using this method becomes increasingly difficult as the intensity of infection decreases. Newer techniques, such as qPCR, have been shown to have greater sensitivity than KK, especially at low prevalence. However, the impact of using qPCR on elimination thresholds is yet to be investigated. In this paper, we aim to quantify how the sensitivity of these two diagnostic tools affects the optimal prevalence threshold at which to declare the interruption of transmission with a defined level of confidence. A stochastic, individual-based STH transmission model was used in this study to simulate the transmission dynamics of Ascaris and hookworm. Data from a Kenyan deworming study were used to parameterize the diagnostic model which was based on egg detection probabilities. The positive and negative predictive values (PPV and NPV) were calculated to assess the quality of any given threshold, with the optimal threshold value taken to be that at which both were maximised. The threshold prevalence of infection values for declaring elimination of Ascaris transmission were 6% and 12% for KK and qPCR respectively. For hookworm, these threshold values are lower at 0.5% and 2% respectively. Diagnostic tests with greater sensitivity are becoming increasingly important as we approach the elimination of STH transmission in some regions of the world. For declaring the elimination of transmission, using qPCR to diagnose STH infection results in the definition of a higher prevalence, than when KK is used.

Systematic review of studies generating individual participant data on the efficacy of drugs for treating soil-transmitted helminthiases and the case for data-sharing

Background

Preventive chemotherapy and transmission control (PCT) by mass drug administration is the cornerstone of the World Health Organization (WHO)’s policy to control soil-transmitted helminthiases (STHs) caused by Ascaris lumbricoides (roundworm), Trichuris trichiura (whipworm) and hookworm species (Necator americanus and Ancylostama duodenale) which affect over 1 billion people globally. Despite consensus that drug efficacies should be monitored for signs of decline that could jeopardise the effectiveness of PCT, systematic monitoring and evaluation is seldom implemented. Drug trials mostly report aggregate efficacies in groups of participants, but heterogeneities in design complicate classical meta-analyses of these data. Individual participant data (IPD) permit more detailed analysis of drug efficacies, offering increased sensitivity to identify atypical responses potentially caused by emerging drug resistance.

Methodology

We performed a systematic literature review to identify studies concluding after 2000 that collected IPD suitable for estimating drug efficacy against STH. We included studies that administered a variety of anthelmintics with follow ups less than 60 days after treatment. We estimated the number of IPD and extracted cohort- and study-level meta-data.

Principal findings

We estimate that there exist individual data on approximately 35,000 participants from 129 studies conducted in 39 countries, including 34 out of 103 countries where PCT is recommended. We find significant heterogeneity in diagnostic methods, times of outcome assessment, and the reported measure of efficacy. We also quantify cohorts comprising pre-school age children, pregnant women, and co-infected participants, including with HIV.

Conclusions

We argue that establishing a global IPD repository would improve the capacity to monitor and evaluate the efficacy of anthelmintic drugs, respond to changes and safeguard the ongoing effectiveness of PCT. Establishing a fair, transparent data governance policy will be key for the engagement of the global STH community.

Soil-transmitted helminthiases (STHs) caused by roundworm, whipworm or hookworm affect over one billion of the world’s poorest people mostly living in low and middle income countries, exerting a major health and economic toll. These infections are controlled by regular mass drug distribution to affected populations. But with very few alternative medicines, the effectiveness of treatment programmes is vulnerable to the potential emergence of drug resistance. Despite a recent scale-up of mass drug distribution, systematic monitoring and evaluation of the efficacy of treatment is too rarely undertaken and our knowledge of how the drugs are performing is largely based on information from clinical trials. However, the design and reporting of information from these trials is very variable which makes it difficult to form a comprehensive picture of the status and trends in drug efficacy. Here, we present a systematic review of published studies completed since 2000, characterise variation in their design, implementation and reporting and estimate the abundance of individual participant data. We argue that the co-ordinated sharing of these individual data would greatly increase the capacity of the global health community to monitor effectively drug efficacy, to respond accordingly to changes, and thereby to safeguard the effectiveness of STH control.

Estimating the prevalence and intensity of Schistosoma mansoni infection among rural communities in Western Tanzania: The influence of sampling strategy and statistical approach

BACKGROUND

Schistosoma mansoni is a parasite of major public health importance in developing countries, where it causes a neglected tropical disease known as intestinal schistosomiasis. However, the distribution of the parasite within many endemic regions is currently unknown, which hinders effective control. The purpose of this study was to characterize the prevalence and intensity of infection of S. mansoni in a remote area of western Tanzania.

METHODOLOGY/PRINCIPAL FINDINGS

Stool samples were collected from 192 children and 147 adults residing in Gombe National Park and four nearby villages. Children were actively sampled in local schools, and adults were sampled passively by voluntary presentation at the local health clinics. The two datasets were therefore analysed separately. Faecal worm egg count (FWEC) data were analysed using negative binomial and zero-inflated negative binomial (ZINB) models with explanatory variables of site, sex, and age. The ZINB models indicated that a substantial proportion of the observed zero FWEC reflected a failure to detect eggs in truly infected individuals, meaning that the estimated true prevalence was much higher than the apparent prevalence as calculated based on the simple proportion of non-zero FWEC. For the passively sampled data from adults, the data were consistent with close to 100% true prevalence of infection. Both the prevalence and intensity of infection differed significantly between sites, but there were no significant associations with sex or age.

CONCLUSIONS/SIGNIFICANCE

Overall, our data suggest a more widespread distribution of S. mansoni in this part of Tanzania than was previously thought. The apparent prevalence estimates substantially under-estimated the true prevalence as determined by the ZINB models, and the two types of sampling strategies also resulted in differing conclusions regarding prevalence of infection. We therefore recommend that future surveillance programmes designed to assess risk factors should use active sampling whenever possible, in order to avoid the self-selection bias associated with passive sampling.

Sources of variability in the measurement of Ascaris lumbricoides infection intensity by Kato-Katz and qPCR

Background

Understanding and quantifying the sources and implications of error in the measurement of helminth egg intensity using Kato-Katz (KK) and the newly emerging “gold standard” quantitative polymerase chain reaction (qPCR) technique is necessary for the appropriate design of epidemiological studies, including impact assessments for deworming programs.

Methods

Repeated measurements of Ascaris lumbricoides infection intensity were made from samples collected in western Kenya using the qPCR and KK techniques. These data were combined with data on post-treatment worm expulsions. Random effects regression models were used to quantify the variability associated with different technical and biological factors for qPCR and KK diagnosis. The relative precision of these methods was compared, as was the precision of multiple qPCR replicates.

Results

For both KK and qPCR, intensity measurements were largely determined by the identity of the stool donor. Stool donor explained 92.4% of variability in qPCR measurements and 54.5% of observed measurement variance for KK. An additional 39.1% of variance in KK measurements was attributable to having expelled adult A. lumbricoides worms following anthelmintic treatment. For qPCR, the remaining 7.6% of variability was explained by the efficiency of the DNA extraction (2.4%), plate-to-plate variability (0.2%) and other residual factors (5%). Differences in replicate measurements by qPCR were comparatively small. In addition to KK variability based on stool donor infection levels, the slide reader was highly statistically significant, although it only explained 1.4% of the total variation. In a comparison of qPCR and KK variance to mean ratios under ideal conditions, the coefficient of variation was on average 3.6 times larger for KK highlighting increased precision of qPCR.

Conclusions

Person-to-person differences explain the majority of variability in egg intensity measurements by qPCR and KK, with very little additional variability explained by the technical factors associated with the practical implementation of these techniques. qPCR provides approximately 3.6 times more precision in estimating A. lumbricoides egg intensity than KK, and could potentially be made more cost-effective by testing each sample only once without diminishing the power of a study to assess population-level intensity and prevalence.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2164-y) contains supplementary material, which is available to authorized users.

Soil-Transmitted Helminths: Mathematical Models of Transmission, the Impact of Mass Drug Administration and Transmission Elimination Criteria

Infections caused by soil-transmitted helminthias (STHs) affect over a billion people worldwide, causing anaemia and having a large social and economic impact through poor educational outcomes. They are identified in the World Health Organization (WHO) 2020 goals for neglected tropical diseases as a target for renewed effort to ameliorate their global public health burden through mass drug administration (MDA) and water and hygiene improvement. In this chapter, we review the underlying biology and epidemiology of the three causative intestinal nematode species that are mostly considered under the STH umbrella term. We review efforts to model the transmission cycle of these helminths in populations and the effects of preventative chemotherapy on their control and elimination. Recent modelling shows that the different epidemiological characteristics of the parasitic nematode species that make up the STH group can lead to quite distinct responses to any given form of MDA. When connected with models of treatment cost-effectiveness, these models are potentially a powerful tool for informing public policy. A number of shortcomings are identified; lack of critical types of data and poor understanding of diagnostic sensitivities hamper efforts to test and hence improve models.

Multi-parallel qPCR provides increased sensitivity and diagnostic breadth for gastrointestinal parasites of humans: field-based inferences on the impact of mass deworming

BACKGROUND

Although chronic morbidity in humans from soil transmitted helminth (STH) infections can be reduced by anthelmintic treatment, inconsistent diagnostic tools make it difficult to reliably measure the impact of deworming programs and often miss light helminth infections.

METHODS

Cryopreserved stool samples from 796 people (aged 2-81 years) in four villages in Bungoma County, western Kenya, were assessed using multi-parallel qPCR for 8 parasites and compared to point-of-contact assessments of the same stools by the 2-stool 2-slide Kato-Katz (KK) method. All subjects were treated with albendazole and all Ascaris lumbricoides expelled post-treatment were collected. Three months later, samples from 633 of these people were re-assessed by both qPCR and KK, re-treated with albendazole and the expelled worms collected.

RESULTS

Baseline prevalence by qPCR (n = 796) was 17 % for A. lumbricoides, 18 % for Necator americanus, 41 % for Giardia lamblia and 15% for Entamoeba histolytica. The prevalence was <1% for Trichuris trichiura, Ancylostoma duodenale, Strongyloides stercoralis and Cryptosporidium parvum. The sensitivity of qPCR was 98% for A. lumbricoides and N. americanus, whereas KK sensitivity was 70% and 32%, respectively. Furthermore, qPCR detected infections with T. trichiura and S. stercoralis that were missed by KK, and infections with G. lamblia and E. histolytica that cannot be detected by KK. Infection intensities measured by qPCR and by KK were correlated for A. lumbricoides (r = 0.83, p < 0.0001) and N. americanus (r = 0.55, p < 0.0001). The number of A. lumbricoides worms expelled was correlated (p < 0.0001) with both the KK (r = 0.63) and qPCR intensity measurements (r = 0.60).

CONCLUSIONS

KK may be an inadequate tool for stool-based surveillance in areas where hookworm or Strongyloides are common or where intensity of helminth infection is low after repeated rounds of chemotherapy. Because deworming programs need to distinguish between populations where parasitic infection is controlled and those where further treatment is required, multi-parallel qPCR (or similar high throughput molecular diagnostics) may provide new and important diagnostic information.

The Role of More Sensitive Helminth Diagnostics in Mass Drug Administration Campaigns: Elimination and Health Impacts

Diagnostics play a crucial role in determining treatment protocols and evaluating success of mass drug administration (MDA) programmes used to control soil-transmitted helminths (STHs). The current diagnostic, Kato-Katz, relies on inexpensive, reusable materials and can be used in the field, but only trained microscopists can read slides. This diagnostic always underestimates the true prevalence of infection, and the accuracy worsens as the true prevalence falls. We investigate how more sensitive diagnostics would impact on the management and life cycle of MDA programmes, including number of mass treatment rounds, health impact, number of unnecessary treatments and probability of elimination. We use an individual-based model of STH transmission within the current World Health Organization (WHO) treatment guidelines which records individual disability-adjusted life years (DALY) lost. We focus on Ascaris lumbricoides due to the availability of high-quality data on existing diagnostics. We show that the effect of improving the sensitivity of diagnostics is principally determined by the precontrol prevalence in the community. Communities at low true prevalence (<30%) and high true prevalence (>70%) do not benefit greatly from improved diagnostics. Communities with intermediate prevalence benefit greatly from increased chemotherapy application, both in terms of reduced DALY loss and increased probability of elimination. Our results suggest that programmes should be extended beyond school-age children, especially in high prevalence communities. Finally, we argue against using apparent or measured prevalence as an uncorrected proxy for true prevalence.

Current status of soil-transmitted helminthiases among pre-school and school-aged children from Ile-Ife, Osun State, Nigeria

A cross-sectional survey was conducted to determine the prevalence and intensity of soil-transmitted helminths among pre-school and school-aged children attending nursery and primary schools in Ile-Ife. Single stool samples were collected between January and March, 2009 from 352 children randomly selected from a total of 456 children attending both private and government schools. The stool samples were processed using the modified Kato-Katz technique, and then examined for the eggs of soil-transmitted helminths (STHs). One hundred and twenty-one (34.4%) samples were positive for STH eggs. The overall prevalences of Ascaris lumbricoides, Trichuris trichiura and hookworm were 33.2%, 3.7% and 0.9%, respectively. The prevalence of STH infection in government schools (47.8%) was significantly higher than in private schools (16.1%) (P < 0.001). The most common type of mixed infection was the combination of A. lumbricoides and T. trichiura (6.8%). The prevalence and intensity of A. lumbricoides rose with age. The lowest prevalence and intensity (7.7%; 0.240 ± 0.136 eggs per gram (epg)) were recorded in the 2- to 3-year-old age group, while the highest prevalence and intensity (58.7%; 1.820 ± 0.237 epg) were recorded in children aged 10 years and above. A questionnaire survey indicated that 73% of the children attending private school had been treated with anthelminthics less than 2 months prior to the collection of stool specimens, while 43% of the children attending government school received anthelminthic treatment during the same period. The findings indicate that STH infections are endemic among schoolchildren in Ile-Ife and that the burden of parasitic infections is greater in government schools than in private schools.

The prevalence and intensity of gastrointestinal parasites of dogs in Ile-Ife, Nigeria

A study of gastrointestinal parasites in 269 faecal samples from dogs (Canis familiaris) collected from Ile-Ife, Nigeria between January and December 2004, revealed seven helminth species:Toxocara canis33.8%,Ancylostomasp. 34.6%,Toxascaris leonina3.3%,Trichuris vulpis3.7%,Dipylidium caninum4.1%,Uncinaria stenocephala0.7% andTaeniasp. 1.1%. The faecal egg intensities, determined as mean eggs per gram of faeces ( ± SEM) were:T. canis393.8 ± 83.4,Ancylostomasp. 101.5 ± 32.8,T. leonina14.3 ± 7.9,T. vulpis3.4 ± 1.5,D. caninum2.2 ± 0.8,U. stenocephala0.2 ± 0.2. The prevalence of intestinal parasites was significantly higher (P < 0.05) in dogs of age 0–6 months than in older age groups. There was no significance difference in overall prevalence of intestinal helminth parasites between male (58.3%) and female (50.0%) dogs (P>0.05). The prevalence of helminth parasites was significantly higher (P < 0.05) in free-ranging than in kennelled dogs. The prevalence of helminth parasites was also significantly higher (P < 0.05) in African shepherds than in Alsatians and other exotic breeds. Each helminth parasite had similar prevalences and intensities among both genders (P>0.05) except inT. vulpis. The overall prevalence of intestinal parasites may continue to rise due to lack of functional veterinary clinics for dog care in Ile-Ife. Therefore, there is the need to establish a veterinary facility in Ile-Ife.

Parasites of importance for human health in Nigerian dogs: high prevalence and limited knowledge of pet owners

Background

Dogs are the most common pet animals worldwide. They may harbour a wide range of parasites with zoonotic potential, thus causing a health risk to humans. In Nigeria, epidemiological knowledge on these parasites is limited.

Methods

In a community-based study, we examined 396 dogs in urban and rural areas of Ilorin (Kwara State, Central Nigeria) for ectoparasites and intestinal helminths. In addition, a questionnaire regarding knowledge and practices was applied to pet owners.

Results

Nine ectoparasite species belonging to four taxa and six intestinal helminth species were identified: fleas (Ctenocephalides canis, Pulex irritans, Tunga penetrans), mites (Demodex canis, Otodectes sp., Sarcoptes scabiei var. canis), ticks (Rhipicephalus sanguineus, Ixodes sp.), and lice (Trichodectes canis); and Toxocara canis, Ancylostoma sp., Trichuris vulpis, Dipylidium caninum, Taenidae and Strongyloides sp. Overall prevalence of ectoparasites was 60.4% and of intestinal helminths 68.4%. The occurrence of C. canis, R. sanguineus, T. canis, Ancylostoma sp. and T. vulpis was most common (prevalence 14.4% to 41.7%). Prevalence patterns in helminths were age-dependent, with T. canis showing a decreasing prevalence with age of host, and a reverse trend in other parasite species. Knowledge regarding zoonoses was very limited and the diseases not considered a major health problem. Treatment with antiparasitic drugs was more frequent in urban areas.

Conclusion

Parasites of importance for human health were highly prevalent in Nigerian dogs. Interventions should include health education provided to dog owners and the establishment of a program focusing on zoonotic diseases.

Parasites of the endangered Cantabrian capercaillie (Tetrao urogallus cantabricus): correlates with host abundance and lek site characteristics

In April 2000, we collected and analyzed for parasites 174 feces belonging to 48 different and identified Cantabrian capercaillies (Tetrao urogallus cantabricus) from 39 lek sites in the Cantabrian Mountains (North Spain) and determined the factors related with prevalence and abundance of propagule excretion: (1) interrelations with other parasite species, (2) characteristics of the capercaillie population (number of capercaillies in the lek site, number of occupied lek sites in a 2-km area, and abundance trend), and (3) characteristics of the lek site (orientation and altitude). The higher prevalence was observed for Eimeria sp. oocysts (29% of feces). Capillaria-like eggs were the most frequently detected among nematodes (9%). Excretion of Heterakis sp., Ascaridia sp., and Trichostrongylus sp. eggs was low. The presence of Capillaria propagules in feces was negatively related with those of Eimeria and Heterakis. This could be explained by differences in parasite transmission and capercaillie exposure because Eimeria and Heterakis have direct life cycle, whereas subfamily Capillariinae includes many heteroxenous species. Capillaria eggs were more prevalent in lek sites at lower altitude, which could be explained by altitude-related factors such as humidity, temperature, or soil characteristics that may affect intermediate host abundance. Although no relation was found with host abundance, the low parasite species diversity, prevalence, and abundance may indirectly reflect the decline of the Cantabrian capercaillie populations.

Epidemiology of intestinal helminth parasites of dogs in Ibadan, Nigeria

An epidemiological study of gastrointestinal helminths of dogs (Canis familiaris) in two veterinary clinics in Ibadan, Nigeria, was conducted between January 2001 and December 2002. Faecal samples collected from 959 dogs were processed by modified Kato–Katz technique and then examined for helminth eggs. The results of the study showed that 237 (24.7%) of the dogs examined were infected with different types of helminths. The prevalences for the various helminth eggs observed were: Toxocara canis 9.0%, Ancylostoma spp. 17.9%, Toxascaris leonina 0.6%, Trichuris vulpis 0.5%, Uncinaria stenocephala 0.4% and Dipylidium caninum 0.2%. The faecal egg intensities, determined as mean egg count/gram of faeces ( ± SEM), were: T. canis 462.0 ± 100.5, Ancylostoma spp. 54.1 ± 8.6, T. leonina 0.8 ± 0.4, T. vulpis 0.1 ± 0.0, U. stenocephala 1.0 ± 0.7 and D. caninum 0.2 ± 0.1. Host age was found to be a significant factor with respect to the prevalence and intensity of T. canis and Ancylostoma spp. There was no significant difference in the prevalence of intestinal helminth parasites between male (27.0%) and female (22.5%) dogs (P>0.05). The prevalence of helminth parasites was significantly higher (P < 0.05) in the local breed (African shepherd) (41.2%) than in Alsatian dogs (16.2%) or in other exotic breeds (21.0%). Single parasite infections (85.7%) were more common than mixed infections (3.5%).

Prevalence and intensity ofToxocara canis(Werner, 1782) in dogs and its potential public health significance in Ile-Ife, Nigeria

A study on the prevalence and intensity ofToxocara canis(Werner, 1782) in dogs was carried out in Ile-Ife, Nigeria. Faecal samples were collected from 269 dogs between January and December 2004, processed by the Kato–Katz technique and then examined forT. caniseggs. The prevalence ofT. canisobtained was 33.8%. The intensity of infection, measured as mean egg count per gram of faeces ( ± SEM) was 393.8 ± 83.4. The prevalence and intensity ofT. canisin dogs aged 0–6 months were significantly higher (P < 0.05) than older age groups. The prevalence and intensity ofT. canisinfection were significantly higher in males than in female dogs (P < 0.05). SinceT. canisis known to cause visceral larva migrans (VLM) in young children, there is the possibility that the high prevalence ofT. canisinfection obtained in this study might constitute an important risk factor for transmission to humans. Therefore, there is the need to educate the residents of Ile-Ife on the danger of close association of their children with household pets.

An asthma-associated genetic variant of STAT6 predicts low burden of ascaris worm infestation

Th-2 immune mechanisms are involved in the pathology of asthma and in the protective immune response to parasitic worms. Common upregulating genetic variants of Th-2 immune signalling are risk factors for asthma, and we tested whether they may confer a counteradvantage in protecting against parasitic worms. We examined the intensity of infection by the parasitic worm, Ascaris lumbricoides, by microsopic counting of ascaris eggs in the stool of 614 schoolchildren from an area of endemic ascaris infection in China. We investigated the relationship between the intensity of ascaris infection and common, asthma-associated genetic variants of Th-2 and Th-1 immune signalling. Ascaris egg counts per gram of stool (epg), mean 1068 epg, ranged from barely detectable (<240 epg) to heavy (approximately 9600 epg) in a skewed distribution. Logistic regression, after exploratory discriminant analysis, showed a major association between a common genetic variant of the 3'-UTR regulatory elements of the signal transducer and transactivating factor (STAT6) (P=0.0002) and egg counts, at the 77 th centile. Linear regression after log transformation of egg counts confirmed a highly significant association with this STAT6 variant (P=0.001). Thus, a common, asthma-associated, genetic variant of the pivotal transduction and transactivating factor for Th-2 immune signalling, STAT6, predicts increased resistance to ascaris worm infection. The evolution of enhanced resistance to parasitic worm infection, through human genetic variation in Th-2 immune signalling, may represent one origin for asthma.

Ascaris egg profiles in human faeces: biological and epidemiological implications

Since 2 morphological forms (fertilized and unfertilized) of egg can be produced by Ascaris, infected humans can release in their faeces fertilized eggs only (FEO), unfertilized eggs only (UEO) or both fertilized and unfertilized eggs (FUE) (designated herein as the 3 different egg profiles). Epidemiologically, fertilized eggs are of significance as they enable effective transmission of the parasite. This study, for the first time, characterizes the Ascaris egg profiles in human faeces in an endemic region of China, explores possible host- and parasite-factors related to these profiles, and discusses the biological and epidemiological implications of the findings. The 3 egg profiles were recorded throughout the study period of 2 years, and the overall percentages of people with FEO, FUE and UEO profiles were approximately 41-47%, 32-42% and 17-21%, respectively. The overall number of unfertilized eggs for the entire population accounted for approximately 6-9% of all eggs excreted. The different Ascaris egg profiles showed no correlation to host gender, but they did relate to age and worm burden of the host and to the sex ratio and developmental status of the parasite. While an annual universal anthelmintic treatment resulted in some fluctuation in the values of individual egg profiles, the general features of these profiles remained similar throughout the study period. The findings of this study should have significant implications for understanding transmission patterns of Ascaris and for the implementation of control measures against ascariasis in endemic regions.

Effect of water supply and sanitation on the prevalence and intensity of Ascaris lumbricoides among pre-school-age children in Ajebandele and Ifewara, Osun State, Nigeria

The relationship between Ascaris lumbricoides infection and provisions for safe water supply and excreta disposal was investigated in Ajebandele and Ifewara, 2 peri-urban communities near Ile-Ife, Osun State, Nigeria, from September 1998 to December 1999. Although there were more facilities for safe water supply (pipe-borne and shallow wells) and sanitation (flush toilets and pit latrines) in Ajebandele than in Ifewara, they were not evenly distributed and only a few families, mainly in Ajebandele, were well provided for. Our results indicated that the prevalence of A. lumbricoides was significantly higher in Ifewara (21.2%) than in Ajebandele (13.1%), and within villages was much lower in houses with tap water and flush toilets. However, the mean intensity of infection was unexpectedly higher in Ajebandele (166.5 +/- 692.0) than in Ifewara (153.1 +/- 485.8) suggesting that the present level of investment in Ajebandele is probably not sufficient to reduce morbidity. These findings confirm suggestions that there is a threshold level of investment below which interventions on water supply and sanitation may not significantly improve human health. Sustainable ascariasis control in the study communities will require significant improvements to existing water supplies and sanitation facilities and any future interventions should incorporate a programme of health education to ensure that the facilities provided are properly used and hygienically maintained.

Geographical variation in Ascaris lumbricoides fecundity and its implications for helminth control

The observation by microscopy of nematode eggs in human faeces is used to diagnose a helminthic infection, while the concentration of those eggs is used to estimate the number of worms in the host. Within a community, the prevalence of infection and the mean egg count provide useful information about the extent of a public health problem, and are being used to guide the growing efforts to control disease caused by helminths. Here, Andrew Hall and Celia Holland examine data on the relationship between the worm burdens of Ascaris lumbricoides and the concentration of eggs in faeces, and discuss the implications of the variation found for using such data to plan helminth control programmes.

Multiple infection with Plasmodium and helminths in communities of low and relatively high socio-economic status

A study was conducted in the city of Lubumbashi, Zaire: (1) to survey parasitic infections and clinical conditions in the local children and their mothers; (2) to identify combinations of parasites and clinical conditions that commonly occurred together in individuals; and (3) to determine whether single- and/or multiple-species infections were risk determinants of the observed clinical conditions. Overall, 1100 children and mothers from three subdivisions, two of low socio-economic status (LSES) and one of relatively high socio-economic status (HSES), provided stool and blood samples and were clinically examined. Plasmodium prevalence was higher in the two LSES subdivisions than in the HSES subdivision. Prevalence and intensity of Ascaris lumbricoides infection were low in the HSES subdivision and one of the two LSES subdivisions. In contrast, prevalence and intensity of Trichuris trichiura and of hookworms were similar in all subdivisions. Plasmodium and A. lumbricoides were the most frequently found single-species infections. The combination of A. lumbricoides and Plasmodium was the most frequent double-species infection and that of A. lumbricoides, Plasmodium and T. trichiura was the most frequent triple-species infection. Significant positive associations between parasite species were detected in the HSES subdivision, and in one of the two LSES subdivisions. Because the relationships were not consistently detected, it is hypothesized that the associations are determined by environmental conditions rather than synergy between the parasites in the host. The most commonly observed clinical conditions were abdominal pain, diarrhoea, fever, and low packed-cell volume (PCV). The occurrence of each was significantly lower in the HSES subdivision than in at least one of the two LSES subdivisions. Abdominal pain and low PCV were most common in individuals presenting with only a single clinical condition, and the combination of this symptom and sign was the most commonly observed pair of conditions. Abdominal pain, low PCV and diarrhoea was the most common combination in individuals with three clinical conditions. Logistic regression revealed that hookworm infection, T. trichiura infection, young age and residence in an LSES subdivision were determinants of diarrhoea. Trichuris trichiura infection, young age and living in an LSES subdivision were risk factors for abdominal pain. Plasmodium infection and young age were risk factors for fever. LSES was the only predictor of low PCV. Infection with A. lumbricoides did not enter any of the models. No significant interactions were detected among parasites, indicating that there was no synergism or antagonism among parasites in the induced disease.

The effects of deworming on indicators of school performance in Guatemala

Intestinal helminths are among the most common infections in school-age children. Of 246 children, aged 7-12 years, attending school in rural Guatemala, 91% carried Ascaris lumbricoides and 82% carried Trichuris trichiura. These children were randomly assigned to receive either albendazole or placebo at 0 and 12 weeks in a 'double-blind' study of the effects of deworming on indicators of school performance. Albendazole successfully rid the children of Ascaris but it was less effective against Trichuris. The children's performance in tests of reading and vocabulary were measured at 0 and 24 weeks, the Peabody picture vocabulary test was given at 24 weeks, and attendance was measured throughout the school year. Comparison of the treated and placebo groups showed no positive effect of deworming. The treated children were largely free of Ascaris for at least 6 months, but during that period we could not detect any improvement in reading, vocabulary, or attendance. The effects of being Trichuris-free were not examined because of the limited effectiveness of albendazole against this worm at the dosage used.

Ascaris lumbricoides infection and environmental risk factors in an urban African setting

Identification of appropriate strategies for controlling gastro-intestinal nematodes in communities depends, in part, on an understanding of the conditions that increase risk of exposure to infective stages. The present study was conducted in Lubumbashi, Zaire. The objectives were to identify features of the environment and living conditions that were significant predictors of Ascaris lumbricoides infection, and to determine whether the same predictors were important in populations living in subdivisions of lower (LSES) and higher (HSES) socio-economic status. Forty-two households from each of three subdivisions (two LSES and one HSES) were selected at random. Mothers were interviewed, observations on the environment around the home were recorded, and single stool samples, collected from all children and mothers, were examined for nematode eggs. Maternal education was a significant predictor of A. lumbricoides intensity in both LSES and HSES subdivisions. Factors related to poor sanitation (open defecation and high number of people using the same toilet) were important in the LSES subdivisions but not in the HSES subdivision. In contrast, the ratio of relatives to direct family members per household was a significant predictor of high intensity of infection in the HSES subdivision, but not in the LSES subdivisions, indicating that relatives and live-in visitors contribute to Ascaris transmission in the HSES population.

Comparison of age-dependent antigen recognition in two communities with high and low Trichuris trichiura transmission

A previous ELISA-based study using whole worm extract, compared age-antibody profiles in two communities with high and low levels of Trichuris trichiura transmission (Needham et al., 1992). This showed that specific IgG1 levels mirrored infection intensity at the population level, while IgA levels exhibited a weak trend to remain elevated in the adult age classes in the area of highest transmission. This was interpreted as preliminary evidence for IgA-mediated resistance in the population with greatest prior experience of infection. The present study extends this work to compare IgG1 and IgA isotype recognition of separated antigens by Western blot between the two communities. Comparison of age-dependent antigen recognition in the two communities shows that both qualitative and quantitative recognition by IgG1 antibodies is related to the current intensity of infection (as assessed by eggs per gram of faeces, epg). The magnitude of the IgA response to separated antigens of 16-17 kDa and 90 kDa exhibits a stronger trend to remain elevated in adults and to reflect the past experience of infection: IgA antibodies are present at significantly higher levels in adults from the high transmission area compared with those from the community with low levels of T. trichiura endemicity, despite infection levels in these age groups being of similar magnitude. This comparative study therefore, provides further evidence to support a role for IgA in acquired immunity to T. trichiura in areas of intense transmission.

The prevalence and intensity of soil-transmitted helminthiases in rural communities in southern Nigeria

An investigation of the prevalence and intensity of soil-transmitted helminthiases was undertaken in four villages in Oyo state, Nigeria. Diagnosis was based on examination of stool samples for the presence of helminth ova in all age classes. The prevalence of Ascaris lumbriocoides ranged from 61.5-72.2%, of Trichuris trichiura from 65-74%, and of hookworm from 52.4-63% depending on the village concerned. Intensity, assessed indirectly as the mean eggs per gram of faeces (epg), varied with regard to the four villages. The mean egg count of A. lumbricoides varied from 6815-10,823 per gram, of T. trichiura from 127-246, and of hookworm from 122-267. Host age, sex and household size were found to be significant factors influencing faecal egg output, and indirect measure of the intensity of the helminth infections.

Predisposition of individuals and families in Mexico to heavy infection with Ascaris lumbricoides and Trichuris trichiura

Stool specimens from 90% of the population of 2 marginal communities in Coatzacoalcos, Mexico, were examined by the Kato-Katz technique for the estimation of eggs per gram of faeces (epg). Average epg values for Ascaris lumbricoides and Trichuris trichiura were calculated for each family in the community, based on the arithmetic mean epg of family members for each parasite. 39 families were classified as either lightly or heavily infected with A. lumbricoides and infected members were treated with mebendazole. A. lumbricoides expelled following treatment were collected from the children 2-10 years of age, and reinfection was monitored monthly for 6 months. At the end of the reinfection interval, a final stool specimen was collected from all family members and those infected were treated. Again, A. lumbricoides expelled by the children 2-10 years of age were collected. There was a significant correlation between the number of A. lumbricoides expelled by the children at the first and second treatments as well as in epg values before the first and second treatments for both A. lumbricoides and T. trichiura in children (2-10 years) and adults (greater than 19 years). At the family level, there were significant correlations between the values of family mean epg before each treatment for both A. lumbricoides and T. trichiura.