The prevalence and intensity of Ascaris lumbricoides infections in Moslem children from northern Bangladesh

Post-ERCP pancreatitis: Frequency and risk stratification from four tertiary care referral hospitals in South East Asia

Post-endoscopic retrograde cholangiopancreatography (ERCP) pancreatitis (PEP) is a dreaded complication. But the frequency of this complication varies widely (1%-40%) among the literature and across geographical regions. With uncertainty in the estimation of occurrence and having no accepted, safe, effective method to prevent it, identification of its frequency and predictable risk factors is very crucial in a local or regional setting. The aim of this study was to identify the frequency of PEP and its risk factors from 4 tertiary care referral centers from a country in the South East Asia Region. This cross-sectional study was done on 1042 consecutive ERCP in adult patients (age >18 years) with baseline serum amylase levels less than 3 times the upper level of standard value. On the basis of clinical features and the amylase level at 24 hours (the next morning), the procedure was evaluated to confirm PEP. The frequency of PEP was calculated and procedure, patient, or operator-related independent risk factors for occurring PEP were analyzed using univariate and multivariate analyses. The study found 204 (19.6%) cases of PEP. In multivariate regression analysis, young age, suspected Sphincter of Oddi Dysfunction, recurrent pancreatitis, needle papillotomy, and pancreatic sphincterotomy were significant risk factors. The univariate analysis also found difficult cannulation and pancreatic duct injection with contrast as significant risk factors. Both patient-related and procedure-related risk factors acted independently for the development of PEP, which occurred in 19.6% of cases. This multicenter assessment of the frequency and risk factors for PEP in our patients would be extremely beneficial for future prevention of this complication.

Soil transmitted helminth infection in primary school children varies with ecozone in the Ngorongoro Conservation Area, Tanzania

BACKGROUND

Soil-transmitted helminthiasis is a neglected tropical disease, thriving in environments of poverty and disadvantage. Our objective was to determine the prevalence and intensity of four soil transmitted helminths (STH) among primary school children in the Ngorongoro Conservation Area (NCA), Tanzania.

METHODS

A cross-sectional study was performed between May 15^th and June 2^nd, 2014. Six of 20 primary schools were chosen, based on elevation, designated low elevation ecozone (LEE) or high elevation ecozone (HEE). A total of 340 children from standards one to four were recruited. Height and weight of each child was determined and body mass index (BMI) was calculated. Stool samples were analyzed using the Kato-Katz, Wisconsin, and Baermann techniques to detect STH eggs and larvae. An interviewer-administered questionnaire on socio-demographic variables was used to capture information from the school's headmaster and a checklist was used to assess sanitation facilities.

RESULTS

STHs identified included Ascaris (presumably lumbricoides), Trichuris (presumably trichiura), hookworms (presumably Ancylostoma duodenale and/or Necator americanus), and Strongyloides (presumably stercoralis). The overall prevalence of STH infection was 29.0% in LEE and 34.0% in HEE. Prevalence was 34.3% and 28.2% in males versus females, respectively. Ascaris sp. were only identified in HEE, prevalence of Trichuris sp. and hookworms were significantly higher in HEE compared to LEE, and Strongyloides spp. prevalence was lower in HEE. Intensity of Trichuris sp. was significantly higher in HEE and lower for hookworms. We did not detect a significant relationship between BMI and helminth intensity; however, BMI was lower in lower elevations and in males vs. females. Sanitation practices are taught at the schools, but challenges were identified when implementing. Latrine facilities were available and latrine-cleaning routines were practiced; however, hand washing practices were challenging due to restricted water availability.

CONCLUSIONS

Significant differences in prevalence in HEE and LEE exist, and STH infections are still very common among school children suggesting that anthelminthic intervention and education may be necessary in this region. Based on this outcome, the study area in the NCA would be classified as a medium risk area, where periodic treatment recommendations should be based on prevalence estimations in the different ecozones.

TRIAL REGISTRATION

Ethics approval was obtained from the Catholic University of Health and Allied Sciences (CUHAS; Lake Zone Institutional Review Board MR/53/100/307)); the Conjoint Health Research Ethics Board (CHREB) at the University of Calgary in Canada (Study ID REB14-0127); the National Institute of Medical Research (NIMR) of Tanzania; and the Tanzania Commission for Science and Technology (COSTEC).

Identifying optimal threshold statistics for elimination of hookworm using a stochastic simulation model

BACKGROUND

There is an increased focus on whether mass drug administration (MDA) programmes alone can interrupt the transmission of soil-transmitted helminths (STH). Mathematical models can be used to model these interventions and are increasingly being implemented to inform investigators about expected trial outcome and the choice of optimum study design. One key factor is the choice of threshold for detecting elimination. However, there are currently no thresholds defined for STH regarding breaking transmission.

METHODS

We develop a simulation of an elimination study, based on the DeWorm3 project, using an individual-based stochastic disease transmission model in conjunction with models of MDA, sampling, diagnostics and the construction of study clusters. The simulation is then used to analyse the relationship between the study end-point elimination threshold and whether elimination is achieved in the long term within the model. We analyse the quality of a range of statistics in terms of the positive predictive values (PPV) and how they depend on a range of covariates, including threshold values, baseline prevalence, measurement time point and how clusters are constructed.

RESULTS

End-point infection prevalence performs well in discriminating between villages that achieve interruption of transmission and those that do not, although the quality of the threshold is sensitive to baseline prevalence and threshold value. Optimal post-treatment prevalence threshold value for determining elimination is in the range 2% or less when the baseline prevalence range is broad. For multiple clusters of communities, both the probability of elimination and the ability of thresholds to detect it are strongly dependent on the size of the cluster and the size distribution of the constituent communities. Number of communities in a cluster is a key indicator of probability of elimination and PPV. Extending the time, post-study endpoint, at which the threshold statistic is measured improves PPV value in discriminating between eliminating clusters and those that bounce back.

CONCLUSIONS

The probability of elimination and PPV are very sensitive to baseline prevalence for individual communities. However, most studies and programmes are constructed on the basis of clusters. Since elimination occurs within smaller population sub-units, the construction of clusters introduces new sensitivities for elimination threshold values to cluster size and the underlying population structure. Study simulation offers an opportunity to investigate key sources of sensitivity for elimination studies and programme designs in advance and to tailor interventions to prevailing local or national conditions.

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.

Unprogrammed deworming in the Kibera slum, Nairobi: implications for control of soil-transmitted helminthiases

BACKGROUND

Programs for control of soil-transmitted helminth (STH) infections are increasingly evaluating national mass drug administration (MDA) interventions. However, "unprogrammed deworming" (receipt of deworming drugs outside of nationally-run STH control programs) occurs frequently. Failure to account for these activities may compromise evaluations of MDA effectiveness.

METHODS

We used a cross-sectional study design to evaluate STH infection and unprogrammed deworming among infants (aged 6-11 months), preschool-aged children (PSAC, aged 1-4 years), and school-aged children (SAC, aged 5-14 years) in Kibera, Kenya, an informal settlement not currently receiving nationally-run MDA for STH. STH infection was assessed by triplicate Kato-Katz. We asked heads of households with randomly-selected children about past-year receipt and source(s) of deworming drugs. Local non-governmental organizations (NGOs) and school staff participating in school-based deworming were interviewed to collect information on drug coverage.

RESULTS

Of 679 children (18 infants, 184 PSAC, and 477 SAC) evaluated, 377 (55%) reported receiving at least one unprogrammed deworming treatment during the past year. PSAC primarily received treatments from chemists (48.3%) or healthcare centers (37.7%); SAC most commonly received treatments at school (55.0%). Four NGOs reported past-year deworming activities at 47 of >150 schools attended by children in our study area. Past-year deworming was negatively associated with any-STH infection (34.8% vs 45.4%, p = 0.005). SAC whose most recent deworming medication was sourced from a chemist were more often infected with Trichuris (38.0%) than those who received their most recent treatment from a health center (17.3%) or school (23.1%) (p = 0.05).

CONCLUSION

Unprogrammed deworming was received by more than half of children in our study area, from multiple sources. Both individual-level treatment and unprogrammed preventive chemotherapy may serve an important public health function, particularly in the absence of programmed deworming; however, they may also lead to an overestimation of programmed MDA effectiveness. A standardized, validated tool is needed to assess unprogrammed deworming.

The coverage and frequency of mass drug administration required to eliminate persistent transmission of soil-transmitted helminths

A combination of methods, including mathematical model construction, demographic plus epidemiological data analysis and parameter estimation, are used to examine whether mass drug administration (MDA) alone can eliminate the transmission of soil-transmitted helminths (STHs). Numerical analyses suggest that in all but low transmission settings (as defined by the magnitude of the basic reproductive number, R₀), the treatment of pre-school-aged children (pre-SAC) and school-aged children (SAC) is unlikely to drive transmission to a level where the parasites cannot persist. High levels of coverage (defined as the fraction of an age group effectively treated) are required in pre-SAC, SAC and adults, if MDA is to drive the parasite below the breakpoint under which transmission is eliminated. Long-term solutions to controlling helminth infections lie in concomitantly improving the quality of the water supply, sanitation and hygiene (WASH). MDA, however, is a very cost-effective tool in long-term control given that most drugs are donated free by the pharmaceutical industry for poor regions of the world. WASH interventions, by lowering the basic reproductive number, can facilitate the ability of MDA to interrupt transmission.

How effective is school-based deworming for the community-wide control of soil-transmitted helminths?

Background

The London Declaration on neglected tropical diseases was based in part on a new World Health Organization roadmap to “sustain, expand and extend drug access programmes to ensure the necessary supply of drugs and other interventions to help control by 2020”. Large drug donations from the pharmaceutical industry form the backbone to this aim, especially for soil-transmitted helminths (STHs) raising the question of how best to use these resources. Deworming for STHs is often targeted at school children because they are at greatest risk of morbidity and because it is remarkably cost-effective. However, the impact of school-based deworming on transmission in the wider community remains unclear.

Methods

We first estimate the proportion of parasites targeted by school-based deworming using demography, school enrolment, and data from a small number of example settings where age-specific intensity of infection (either worms or eggs) has been measured for all ages. We also use transmission models to investigate the potential impact of this coverage on transmission for different mixing scenarios.

Principal Findings

In the example settings <30% of the population are 5 to <15 years old. Combining this demography with the infection age-intensity profile we estimate that in one setting school children output as little as 15% of hookworm eggs, whereas in another setting they harbour up to 50% of Ascaris lumbricoides worms (the highest proportion of parasites for our examples). In addition, it is estimated that from 40–70% of these children are enrolled at school.

Conclusions

These estimates suggest that, whilst school-based programmes have many important benefits, the proportion of infective stages targeted by school-based deworming may be limited, particularly where hookworm predominates. We discuss the consequences for transmission for a range of scenarios, including when infective stages deposited by children are more likely to contribute to transmission than those from adults.

Large donations of drugs to treat soil-transmitted helminths (STHs, intestinal worms) means that many more school-aged children will be treated, improving their well-being and development. These children will have to be repeatedly treated since reinfection will occur due to contaminated environments in the absence of improvements in hygiene and sanitation. Repeated treatment of school-aged children may have the added benefit of reductions in levels of infection for the whole community. This will in part be determined by the proportion of the total worms harboured or eggs output by school-aged children, a product of how heavily infected school-aged children are and how many school-aged children there are in the community. In one setting school-aged children output as little as 15% of hookworm eggs whereas in another setting they harbour up to 50% of roundworms. Thus, whilst school-based programmes may have important health benefits, the community-level impact on transmission could be limited unless school-aged children over-contribute to infection. We use mathematical models to show that if children contribute more infective stages to the environment which adults are exposed to than adults do, the reductions in transmission resulting from treating children will be larger, but may still be limited.

Investigating over-dispersion; Moniliformis (Acanthocephala) and rats

An analysis of the frequency distribution of numbers of Moniliformis dubius in rats of an outbred strain of Wistar origin (CFHB) and feeding ad libitum on Oxoid 41B diet, showed that over-dispersion occurred regardless of the age and sex of the rats and the infective dose given (12, 20 or 40 cystacanths/rat). Over-dispersion was also shown to be independent of variability in the age and sex of the cystacanths given. The analysis demonstrated that the over-dispersion declined as the course of the infection proceeded. As expected, parasite survival was found to be age-dependent with female worms living longer, on average, than males, and both male and female worms living longer in rats given 12 as opposed to 20 cystacanths. Possible mechanisms for generating the over-dispersion observed during this work are discussed and a tentative hypothesis, invoking host heterogeneity with regard to carbohydrate availability in the small intestine, is proposed for further experimental investigation.

Density-dependent effects on the weight of female Ascaris lumbricoides infections of humans and its impact on patterns of egg production

BACKGROUND

Ascaris lumbricoides exhibits density-dependent egg production, a process which has a marked impact on both the transmission dynamics and the stability of the parasite population. Evidence suggests that the egg production of female Ascaris is also associated with the size of the worm. If worm size is mediated by density-dependent processes then the size of female worms may have a causal impact upon patterns of Ascaris egg production.

RESULTS

We analyse data collected from a cohort of human hosts, and demonstrate that the per host mean weight (a proxy for size) of female Ascaris is dependent on the number of infecting females (worm burden) following a pattern of initial facilitation followed by limitation. Applying a negative binomial (NB) generalized linear model (GLM) and a zero-inflated negative binomial (ZINB) model we confirm that the per host female mean weight is significantly associated with per host egg production. Despite these associations, the mean weight of female Ascaris has little causal impact on patterns of density-dependent egg output. The ZINB model is able to account for the disproportionately large number of zero egg counts within the data and is shown to be a consistently better fit than the NB model. The probability of observing a zero egg count is demonstrated as being negatively associated with both female worm burden and female mean weight.

CONCLUSION

The mean weight of female Ascaris is statistically significantly associated with egg output, and follows a consistent pattern of facilitation preceding limitation with increasing female worm burden. Despite these relationships, incorporation of female Ascaris mean weight into models of egg output has little effect on patterns of density dependence. The ZINB model is a superior fit to the data than the NB model and provides additional information regarding the mechanisms that result in a zero egg count. The ZINB model is shown to be a useful tool for the analysis of individual-based egg output data.

Susceptibility to intestinal infection and diarrhoea in Zambian adults in relation to HIV status and CD4 count

BACKGROUND

The HIV epidemic in sub-Saharan Africa has had a major impact on infectious disease, and there is currently great interest in the impact of HIV on intestinal barrier function. A three year longitudinal cohort study in a shanty compound in Lusaka, Zambia, carried out before anti-retroviral therapy was widely available, was used to assess the impact of HIV on susceptibility to intestinal infectious disease. We measured the incidence and seasonality of intestinal infection and diarrhoea, aggregation of disease in susceptible individuals, clustering by co-habitation and genetic relatedness, and the disease-to-infection ratio.

METHODS

Adults living in a small section of Misisi, Lusaka, were interviewed every two weeks to ascertain the incidence of diarrhoea. Monthly stool samples were analysed for selected pathogens. HIV status and CD4 count were determined annually.

RESULTS

HIV seroprevalence was 31% and the prevalence of immunosuppression (CD4 count 200 cells/microL or less) was 10%. Diarrhoea incidence was 1.1 episodes per year and the Incidence Rate Ratio for HIV infection was 2.4 (95%CI 1.7-3.3; p < 0.001). The disease-to-infection ratio was increased at all stages of HIV infection. Aggregation of diarrhoea in susceptible individuals was observed irrespective of immunosuppression, but there was little evidence of clustering by co-habitation or genetic relatedness. There was no evidence of aggregation of asymptomatic infections.

CONCLUSION

HIV has an impact on intestinal infection at all stages, with an increased disease-to-infection ratio. The aggregation of disease in susceptible individuals irrespective of CD4 count suggests that this phenomenon is not a function of cell mediated immunity.

The potential impact of density dependent fecundity on the use of the faecal egg count reduction test for detecting drug resistance in human hookworms

Current efforts to control human soil-transmitted helminth (STH) infections involve the periodic mass treatment of people, particularly children, in all endemic areas, using benzimidazole and imidothiazole drugs. Given the fact that high levels of resistance have developed to these same drugs in roundworms of livestock, there is a need to monitor drug efficacy in human STHs. The faecal egg count reduction test (FECRT), in which faecal egg output is measured pre- and post-drug treatment, is presently under examination by WHO as a means of detecting the emergence of resistance. We have examined the potential impact of density dependent fecundity on FECRT data. Recent evidence with the canine hookworm indicates that the density dependent egg production phenomenon shows dynamic properties in response to drug treatment. This will impact on measurements of drug efficacy, and hence drug resistance. It is likely that the female worms that survive a FECRT drug treatment in some human cases will respond to the relaxation of density dependent constraints on egg production by increasing their egg output significantly compared to their pre-treatment levels. These cases will therefore underestimate drug efficacy in the FECRT. The degree of underestimation will depend on the ability of the worms within particular hosts to increase their egg output, which will in turn depend on the extent to which their egg output is constrained prior to the drug treatment. As worms within different human cases will likely be present at quite different densities prior to a proposed FECRT, there is potential for the effects of this phenomenon on drug efficacy measurements to vary considerably within any group of potential FECRT candidates. Measurement of relative drug efficacy may be improved by attempting to ensure a consistent degree of underestimation in groups of people involved in separate FECRTs. This may be partly achieved by omission of cases with the heaviest infections from a FECRT, as these cases may have the greatest potential to increase their egg output upon removal of density dependent constraints. The potential impact of worm reproductive biology on the utility of the FECRT as a resistance detection tool highlights the need to develop new drug resistance monitoring methods which examine either direct drug effects on isolated worms with in vitro phenotypic assays, or changes in worm genotypes.

Current efforts to control soil-transmitted helminth (STH) infections in humans in endemic countries involve the mass administration of drugs. The use of these same drugs for many years to control livestock nematodes has resulted in the emergence of significant levels of resistance. Hence, there is a need to manage the use of drugs against human STHs in order to reduce the likelihood of resistance developing. An important component of managing drug use will be an ability to detect drug resistance should it emerge. WHO and the World Bank are presently supporting initiatives to develop tools for detecting drug resistance in human STHs. The tool to be assessed in the short term is the faecal egg count reduction test (FECRT). We have examined literature on an aspect of worm reproductive biology with potential to impact significantly on the FECRT. We describe the potential effects that density dependent egg production by female hookworms may have on interpretation of FECRT data. This study highlights a potential weakness in reliance on the FECRT for assessment of drug resistance in human hookworms, hence emphasising the need to develop more advanced worm bioassay and molecular methods.

Soil-transmitted helminthiasis among mothers and their pre-school children on Unguja Island, Zanzibar with emphasis upon ascariasis

Soil-transmitted helminthiasis (STH) is a scourge to the health and well-being of infants and pre-schoolchildren throughout many parts of sub-Saharan Africa. To improve maternal and child health, regular de-worming is recommended and often delivered from mother and child health (MCH) clinics, yet there have been few studies monitoring the progress and impact of interventions on local levels of disease. A cross-sectional parasitological survey, supplemented with questionnaires, was therefore conducted across 10 Ungujan villages examining mothers (n=322) and their pre-school children (n=359). Within children, mean prevalence of ascariasis, trichuriasis and hookworm was 8.6% (95% CI 5.5-11.8), 18.9% (95% CI 14.5-23.4) and 1.7% (95% CI 0.2-3.5) while in mothers mean prevalence was 6.7% (95% CI 3.7-9.7), 11.9% (95% CI 8.0-15.8) and 1.9% (95% CI 0.2-3.5), respectively. There was, however, significant spatial heterogeneity of STH by village, 2 villages having much elevated levels of infection, although general access to anthelminthics and utilization of village MCH clinics was good. Levels of parasite aggregation (k) were determined and a multilevel logistic regression model identified access to a household latrine [OR=0.56 (95% CI 0.32-0.99)] and having an infected household member [OR=3.72 (95% CI 2.22-6.26)] as observed risk factors. To further investigate worm burdens of Ascaris lumbricoides, adult worms were expelled using Combantrin and measured. A negative relationship between mean worm burden and mean worm mass was found. Villages in the north of Unguja represent locations where there is elevated prevalence of both ascariasis and trichuriasis and it appears that local factors are particularly favourable for transmission of these helminths. From a perspective of control, in such locations, intervention efforts should be stepped up and greater efforts placed upon improving household sanitation.

Intestinal helminths in a population of children from the Kashmir valley, India

In any geographical area, surveys of the prevalence of intestinal helminths are necessary to suggest appropriate control measures. The aim of this study was to determine the prevalence of intestinal helminth infections in children of the Kashmir valley and to identify the risk factors. Stool samples were collected from 2256 children from rural as well as urban areas of the Kashmir valley. The samples were examined by simple smear and zinc sulphate concentration methods. Intensity of the infection was quantified by Stoll's egg-counting technique. Infection by at least one intestinal helminth was found in 71.18% of the sampled population. The prevalence of Ascaris lumbricoides was highest (68.30%), followed by Trichuris trichiura (27.92%), Enterobius vermicularis (12.67%) and Taenia saginata (4.60%). Light (57.1%) to moderate (42.8%) intensity of infection was observed for A. lumbricoides, while the majority of the infected children (92.3%) harboured a light intensity of infection for T. trichiura. The age group, rural or urban residence, type of water source, boiled or unboiled water, type of defecation site, level of personal hygiene and maternal education were associated with helminth infection. Adequate control measures are urgently needed to combat the high prevalence of intestinal helminths and risk factors in the children of Kashmir valley.

Prevalence of biliary ascariasis and its relation to biliary lithiasis

Hepatobiliary ultrasound was carried out on 2224 consecutive patients at the Center for Nuclear Medicine and Ultrasound and at one private diagnostic center in Mymensingh for ultrasound examination of different systems. The purpose of this study was to investigate the prevalence of biliary ascariasis and its association with other biliary diseases, specially biliary lithiasis. Data regarding the presence of stone, worm or other diseases were recorded accordingly in this prospective study. There were 952 male and 1272 female patients with an age range of 5-90 years. Biliary diseases were detected in 305 patients (13.71%), of whom 97 were male (10.19%) and 208 were female (16.35%). The most common biliary disease in both sexes was cholelithiasis (11.87%), which was found in 84 male patients (8.82%) and in 180 female patients (14.15%). Other diseases found were choledocholithiasis in 14 patients (3 males, 11 females), gall bladder mass in 9 patients (3 males, 6 females), common bile duct mass in 7 patients (4 male, 3 female) and biliary ascariasis in 10 patients (3 male, 7 female). Overall prevalence of biliary ascariasis was 0.45% (0.31% in male patients and 0.55% in female patients), and age range of patients with the condition was 6-50 years. No case of biliary ascariasis was associated with cholelithiasis or choledocholithiasis. Acute cholecystitis was associated with 8 cases (80%) of biliary ascariasis. Common sonographic findings in patients with biliary ascariasis were a single long, linear or curved echogenic structure within the bile duct, without acoustic shadowing. Other findings were gall bladder distention with sludge inside, an edematous wall and mildly dilated biliary tree. Prevalence of biliary ascariasis in the study was 0.45%, with incidence being higher in female subjects (0.55%). No correlation was found between biliary ascariasis and biliary lithiasis. Most of the cases of biliary ascariasis were associated with acute cholecystitis. We concluded that a careful search for biliary ascariasis should be considered for patients with acute acalculus cholecystitis, especially in areas in which ascariasis is endemic, such as Bangladesh.

Impact of sanitation and health education on intestinal parasite infection among primary school aged children of Sherpur, Bangladesh

This study was carried out in 1999-2000 in the northern part of Bangladesh to determine the impact of sanitary latrine use and of health education on intestinal parasites in school-aged children. The children were between 5 and 13 years of age and stool samples revealed that more than half (53%) of the study sample was still infected with one or more intestinal parasites even after 4 years of intervention. Ascariasis was found to have the highest prevalence rate (36.2%) and hookworm the lowest (10.7%). Intestinal parasite infection was significantly lower (P < 0.05) among those who used a sanitary latrine and received health education. This result is consistent with observations that the effect of sanitation and health education is slow to develop. Concerted primary healthcare activities with community development efforts should be undertaken to improve the overall living condition of the people of this area to control this problem.

Immunity, antigenic heterogeneity, and aggregation of helminth parasites

Empirical studies of helminth parasites reveal that the distribution of parasite burdens in their host populations is highly aggregated. This aggregation is fundamental to the ecology and epidemiology of helminth parasites. Results from a stochastic model predict that aggregation of helminth parasites is inversely related to the intensity of host immunity. Aggregation also decreases with antigenic heterogeneity and increases with heterogeneity in transmissibility among parasite strains. It is also found that the degree of aggregation is greater when immunity affects parasite fecundity than when immunity acts on host susceptibility. Potential relevance of this result for assessing the influence of vaccines that target either host susceptibility or parasite fecundity on the level of aggregation and consequent effects on drug resistance and disease prevalence are discussed.

Evaluation of the Integrated Management of Childhood Illness guidelines for treatment of intestinal helminth infections among sick children aged 2-4 years in western Kenya

Anthelmintic treatment of sick preschool-age children at health facilities is a potentially effective strategy for intestinal helminth control in this age-group. We conducted a study from July 1998 to February 1999 in western Kenya to determine whether the Integrated Management of Childhood Illness (IMCI) guidelines' clinical assessment can be used to identify helminth-infected children, and to evaluate the nutritional benefit of treating sick children without pallor with an anthelmintic (mebendazole is already part of IMCI treatment for sick children aged 2-4 years with palmar pallor in areas where hookworm and Trichuris trichiura infections are endemic). Sick children aged 2-4 years seen at 3 rural health facilities were clinically evaluated and tested for haemoglobin concentration, malaria parasites, and intestinal helminths. Children without pallor were randomly assigned to receive a single dose of 500 mg of mebendazole or a placebo and re-examined 6 months later. Among the 574 children enrolled, 11% had one or more intestinal helminths. Most infections were of light intensity. Selected clinical signs and symptoms available from the IMCI assessment, including palmar pallor and low weight-for-age, were not associated with helminth infection. Six months after enrollment, no differences in growth of children without pallor were observed between the mebendazole (n = 166) and placebo (n = 181) groups. However, there was a significantly greater mean increase in weight, height, and weight-for-age Z score among the helminth-infected children in the mebendazole group (n = 22) as compared with helminth-infected children in the placebo group (n = 20). We conclude that even lightly infected preschool-age children without palmar pallor benefit from anthelmintic treatment; however, in this study setting of low helminth prevalence and intensity, helminth-infected children could not be identified using the IMCI guidelines. Cost-effectiveness studies are needed to help define helminth prevalence thresholds for routine anthelmintic treatment of sick preschool-age children seen at first-level health facilities.

Anthelmintic treatment of rural Bangladeshi children: effect on host physiology, growth, and biochemical status

BACKGROUND

The effect of helminth infestation on the nutrition, growth, and physiology of the host is still poorly understood. Anthelmintic treatment of children in developing countries has had varying success in terms of growth improvements.

OBJECTIVE

The objective of this study was to assess the effect of regular deworming on child growth, physiology, and biochemical status.

DESIGN

The study was a 12-mo longitudinal intervention in 123 Bangladeshi children aged 2-5 y. Treatment (mebendazole) or placebo tablets were administered every 2 mo for 8 mo and again at 12 mo. Weight, height, midupper arm circumference, intestinal permeability, plasma albumin, alpha(1)-antichymotrypsin, and total protein concentration were assessed every 2 mo.

RESULTS

Treatment with mebendazole reduced the prevalence of Ascaris lumbricoides from 78% to 8%, of Trichuris trichiura from 65% to 9%, and of hookworm from 4% to 0%. There was no significant difference in the growth of treated children compared with those given placebo tablets. No changes in intestinal permeability or plasma albumin were observed after deworming. Significant decreases in total protein (P<0.001) and alpha(1)-antichymotrypsin (P<0.001) were observed in the treatment group, indicating possible reductions in inflammation and immunoglobulin concentration after deworming. A significant increase in the prevalence of Giardia intestinalis (from 4% to 49%) in the treatment group was associated with a short-term reduction in weight (P = 0.02) and higher intestinal permeability (P <0.001) in infected subjects. No long-term effects of G. intestinalis on growth were observed.

CONCLUSION

Low-intensity helminth infections, predominantly of A. lumbricoides and T. trichiura, do not contribute significantly to the poor growth and biochemical status of rural Bangladeshi children.

Anaemia in Bangladesh: a review of prevalence and aetiology

OBJECTIVE

: This paper provides a comprehensive review of the changes in the prevalence and the extent of anaemia among different population groups in Bangladesh up to the present time. The report also focuses on various factors in the aetiology of anaemia in the country.

DESIGN AND SETTING

: All the available data have been examined in detail, including data from national nutrition surveys, as well as small studies in different population groups.

RESULTS

: Over the past three decades a number of studies including four national nutrition surveys (1962/64; 1975/76; 1981/82 and 1995/96) have been carried out to investigate the prevalence of anaemia among different population groups in Bangladesh, and have demonstrated a significant public health problem. Since the 1975/76 survey the average national prevalence of anaemia has not fallen; in 1995/96, 74% were anaemic (64% in urban areas and 77% in rural areas). However, age-specific comparisons suggest that the rates have fallen in most groups except adult men: in preschool children in rural areas it has decreased by about 30%, but the current level (53%) still falls within internationally agreed high risk levels. Among the rural population, the prevalence of anaemia is 43% in adolescent girls, 45% in non-pregnant women and 49% in pregnant women. The rates in the urban population are slightly lower compared with rural areas, but are high enough to pose a considerable problem. It appears that severe anaemia in the Bangladeshi population is less frequent, possibly present among only 2-3% of the population. The data on the aetiology of anaemia reveal that iron deficiency may be a substantial cause of anaemia in the Bangladeshi population. Other dietary factors in addition to parasitic infestations may also precipitate the high prevalence of anaemia.

CONCLUSIONS

: While the overall prevalence of anaemia among the Bangladeshi population is still very high, the rates of severe anaemia are almost non-existent. A large proportion of anaemia can be attributed to iron deficiency. There is a need for a comprehensive strategy for the prevention and control of anaemia in Bangladesh.

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.

Relationships between vitamin A, iron status and helminthiasis in Bangladeshi school children

OBJECTIVE

To explore the relationships between biochemical indicators of vitamin A and iron status and the intestinal helminths Ascaris lumbricoides and hookworm in primary school children.

SETTING

Two rural governmental schools in northwestern Bangladesh.

DESIGN

Cross-sectional study.

SUBJECTS

The sample consisted of 164 children in grades 3-5.

METHODS

Serum retinol and beta-carotene (by high-performance liquid chromatography, HPLC), haemoglobin (HemoCue), ferritin (enzyme-linked immunoadsorbent assay, ELIZA) and height and weight were measured. Dietary intake of vitamin A was assessed using a food frequency questionnaire and faecal analyses were done using Stoll's egg-count technique.

RESULTS

The mean serum retinol was 26.8 microg dl(-1) and 20% had a level of < 20 microg dl(-1), the cut-off value for low vitamin A status. There was a strong positive association between serum beta-carotene and serum retinol (r = 0.44, P < 0.001), suggesting those with higher retinol levels had a higher carotene intake. Thirty-one per cent were anaemic (Hb < 11.5 g dl(-1)), 30% had iron deficiency (serum ferritin < 12.0 microg l(-1)) and 14% were suffering from iron deficiency anaemia. Children with a serum retinol level of 20 microg dl(-1) had significantly lower ferritin (14.0 compared to 26.0 microg l(-1), P = 0.005) and Hb levels (11.7 compared to 12.4 g dl(-1), P = 0.005) than those with higher levels. The proportion of iron deficiency anaemia was significantly greater among children with hookworm. Our data suggest that hookworm exerts its impact on iron status independently of the vitamin A status of the host.

CONCLUSIONS

Programmes to improve iron status should consider including both vitamin A prevention programmes and deworming.

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.

Perceptions and treatment of intestinal worms in rural Bangladesh: local differences in knowledge and behaviour

A survey was conducted on 131 mothers in rural Bangladesh to examine knowledge and perceptions of helminth infection in relation to use of health facilities and treatment-seeking behaviour. Almost all respondents considered worms to be a cause of bad health and a high percentage of mothers had obtained deworming treatment for their children. However, marked differences were found in mothers' descriptions of the causes and prevention of helminth infection in two adjacent areas; Pullakandi and Shekpara. The discrepancies in biomedical knowledge corresponded with differences in treatment-seeking behaviour in the two areas. All households in the area had access to free deworming treatment provided by a health clinic, but this facility was predominantly used by women living nearby in Pullakandi. Because of the cultural and social constraints on female activities, women living further from the clinic, in Shekpara, preferred to send their husbands to a pharmacy in the nearby town to buy deworming treatment. As a consequence, these households were at a relative disadvantage in respect of the low exposure of women to health education and the greater financial cost of deworming treatment. The study highlights the influences of social and cultural factors on treatment-seeking behaviour, which in turn affect women's exposure to health education and biomedical knowledge of helminths. Further questions are raised, however, on the ability of women to implement preventive measures and the impact of health education on rates of parasitic infection.

An 18-month study of the effect of periodic anthelminthic treatment on the growth and nutritional status of pre-school children in Bangladesh

An 18-month study was conducted from February 1989 to August 1990 to examine the effect of regular deworming on child growth and nutritional status. A sample of 1402 children, from 2 to 6 years old, were divided into a treatment group and a control group. The 688 children in the treatment group received a 500 mg single dose of mebendazole, while the 714 children in the control group were given a placebo. Height, weight and mid-upper arm circumference (MUAC) were measured on monthly household visits. Growth was measured in terms of the change in height-for-age, weight-for-age, weight-for-height and MUAC over 18 months. The initial prevalence of infection was estimated from a random sample of 96 children (49 treated, 47 control). The initial overall prevalence of Ascaris lumbricoides, Trichuris trichiura and hookworm was 71, 44 and 10% respectively. The final prevalence of infection, estimated from a random sample of 265 children, was A. lumbricoides 6%, T. trichiura 6% and hookworm 2% in the mebendazole group compared with 64, 18 and 19% respectively in the placebo group. Despite the successful treatment of helminths, there was no significant improvement in the growth of treated children compared with their untreated counterparts in terms of the change in z-scores of height-for-age, weight-for-age, weight-for-height and MUAC. The factors which may have contributed to this outcome are discussed.

A quantitative framework for evaluating the effect of community treatment on the morbidity due to ascariasis

There is growing emphasis on the use of community treatment to reduce the level of morbidity caused by helminth infection. The design of chemotherapy programmes, in terms of frequency of treatment and proportion of the community treated, would be assisted by a quantitative framework which enabled the morbidity reduction achieved by different approaches to be compared. The present study describes a model developed for this purpose which embodies two innovative features. First, a quantitative score of morbidity (the proportion of individuals harbouring an intense infection) is used to rate the success of a programme and, second, the distribution of helminths in the host population is generated by a mechanism that allows the distribution to change dynamically as a function of both treatment and reinfection. The model behaviour, using values typical of Ascaris lumbricoides, indicates that the benefit derived from community chemotherapy increases non-linearly with the coverage and efficacy of treatment.

The regulation of gastrointestinal helminth populations

One quarter of the world's human population suffers infection with helminth parasites. The population dynamics of the ten or so species, which cause disease of clinical significance have been well characterized by epidemiological field survey. The parasites are in general highly aggregated between hosts, and their populations seem to be temporally stable and to recover rapidly from perturbation, including interventions designed to alleviate disease. This paper reviews current understanding of the population regulation of helminth species of medical significance. Both empirical (field and laboratory) and theoretical results are included, and we attempt to interpret the findings in the broader context of the population ecology of free-living species. We begin by considering the evidence for regulation from field data concerning the temporal stability of helminth populations within communities and from the results of perturbation experiments. The detection of regulatory processes is then discussed (with regard to statistical and logistical considerations), and the evidence from both the field and laboratory studies reviewed. Deterministic models are described to investigate the possible consequences of regulation imposed at different points in the parasite life-cycle. The causes and consequences of parasite aggregation are considered, and a stochastic model used to investigate the impact of different combination of regulatory processes and heterogeneity generating mechanisms.

The relationship between the frequency distribution of Ascaris lumbricoides and the prevalence and intensity of infection in human communities

Observed field data from a range of geographically distinct human communities suggest a consistent non-linear relationship between prevalence and mean intensity of Ascaris lumbricoides infection. Utilizing the negative binomial distribution as a description of observed aggregation, maximum-likelihood analysis reveals that the degree of aggregation is a negative linear function of mean worm burden. The factors responsible for this relationship in human populations require further study but may involve some combination of (i) density-dependent reduction in worm numbers within individuals, (ii) density-dependent parasite-induced host mortality or (iii) self-treatment by heavily infected hosts. Variability in the degree of aggregation appears dependent on the level of infection in a community and independent of geographical differences in the host or parasite populations.

The weight/length profiles of Ascaris lumbricoides within a human community before mass treatment and following reinfection

Weight and length profiles are presented of 3505 Ascaris worms recovered after mass anthelmintic treatment of a human community in January 1984 and, after an 11 month period of reinfection, in November 1984. Male and female worms recovered after reinfection were significantly heavier and longer than those expelled after initial treatment (P less than 0.0001). Multiple regression models were employed to examine variability in parasite size. A positive influence of host body weight on the weight of parasites recovered in January, but not in November, was observed. No negative relationship was recorded at either date between worm size and the total number of worms harboured, even after controlling for host weight. Striking host age-related variability was observed in the distribution profile of weights and lengths of individual worms recovered in January. Children harboured predominantly smaller worms before initial treatment, while adults expelled mainly heavy worms. In contrast, worms expelled by both children and adults after reinfection were heavier and more homogeneous in size, particularly within the relatively heavily infected group. These patterns argue against a primary role for density-dependent or acquired resistance mechanisms in determining the size of Ascaris in humans. However, the results are consistent with a hypothesis initially suggested by Jung (1954) that established worms may inhibit the development of newly acquired Ascaris larvae, perhaps regulating their own abundance.

Density dependence in establishment, growth and worm fecundity in intestinal helminthiasis: the population biology of Trichuris muris (Nematoda) infection in CBA/Ca mice

The results are presented of an experimental study of the population biology of chronic Trichuris muris (Nematoda) infection in cortisone-treated CBA/Ca mice. Attention is focused upon both the validity of the common use of faecal egg counts to demonstrate density dependence in helminth fecundity, and the identification of other possible density-dependent mechanisms that may regulate worm numbers in chronic trichuriasis. The results show that faecal egg counts, although demonstrating high daily variation, are not an artefact of host faecal output but a significant density-dependent function of worm burden. This finding contrasts with the observations on Heligmosomoides polygyrus infection in outbred MFI mice, but accords with similar studies in a wide variety of host - helminth systems. Worm establishment in the murine host is found to be a density related function of infection dose. This is attributed to the probable existence of a physical gut-carrying capacity in the murine host for T. muris. Worm distribution in the gut is also shown to the density dependent, with worms being displaced from the caecum to the colon at increasing intensities of infection. The sex ratio of the adult parasites, however, is found to be both unitary and independent of worm burden. Evidence for a significant density-dependent decline in female T. muris growth or size is presented. The results also show a significant positive association between female T. muris weight and per capita fecundity. These findings indicate that the stunted growth of individual worms at high parasite densities may be a potential mechanism underlying density dependence in helminth fecundity.

Ascaris lumbricoides infection in pre-school children from Chiriqui Province, Panama

An epidemiological survey of intestinal parasitic infections was conducted in a sample of 203 children aged 3-5 years from a semi-urban and a rural community in Chiriqui, Panama, in 1983-4. On the basis of stool examinations, the prevalences of Ascaris lumbricoides, Trichuris trichiura, hookworm, Giardia intestinalis, Entamoeba spp. and Strongyloides stercoralis were found to be 27, 34, 14, 15, 5 and 4% respectively. The results from children from the two communities were compared. Polyparasitism occurred significantly more often in rural than semi-urban children. Following anthelmintic treatment with levamisole, the numbers of A. lumbricoides passed/child were recorded and the frequency distribution of the parasite was observed to be highly aggregated with a variance to mean ratio of 10.2. For A. lumbricoides, relationships between worm burden, worm biomass and egg production were investigated. In the data analysis, an attempt was made to explore the influence of numbers of male worms on egg production. The results are compared with those obtained during other recent studies on the epidemiology of A. lumbricoides infection in other countries.

Epidemiology and population dynamics of Ascaris lumbricoides and Trichuris trichiura infection in the same community

The gastrointestinal helminth infection status of an age-stratified sample from a single Caribbean community was assessed using anthelmintic expulsion techniques. The same sample was re-assessed in a similar manner after a 17 month period of re-infection. The age-prevalence profile of Ascaris lumbricoides was convex while that of Trichuris trichiura was asymptotic. The age-intensity profiles of both species were convex. These differing patterns are attributed to differences in the absolute worm burdens of the 2 species. The frequency distributions of infection intensity were similar for both species, and largely independent of host age. The basic reproductive rate of A. lumbricoides (Ro = 1-1.8) was similar to that recorded elsewhere and much lower than that of T. trichiura (Ro = 4-6), implying that the latter is intrinsically more resistant to control. Individual hosts were predisposed to high (or low) intensity infection with either species, although predisposition to both species simultaneously was not conclusively demonstrated. Further studies are required to determine the cause of these observations.

Plasma albumin concentrations and intestinal permeability in Bangladeshi children infected with Ascaris lumbricoides

Plasma albumin concentration and intestinal permeability have been investigated in Bangladeshi children before and 9-14 d after successful treatment for ascariasis. Children infected with A. lumbricoides had lower plasma albumin concentrations than counterparts not harbouring this worm and values increased with successful treatment. Intestinal permeability tests indicated that the children had impaired gastrointestinal function and some loss of mucosal integrity; these factors had not improved 9-14 d after A. lumbricoides expulsion. The lowered nitrogen nutritional status implied by the reduced plasma albumin values in infected children, and the improvement following treatment, are in keeping with previous reports that A. lumbricoides impairs protein digestion or absorption. This may be the basis of the better growth rates of dewormed children in this area.

How much Ascariasis is there in Africa?

What is the real burden of parasitic infections? How many people, for example, are infected with the common roundworm Ascaris lumbricoides-usually cited as'number one' in the 'league table' of human parasite prevalence? Extrapolations are often made from figures for helminth infection published in 1947 by Stoll. More recent estimates suggest almost unbelievable numbers of oscoriosis cases - about 1000 million people representing about one quarter of the world population. Such figures are important because they contribute to the perennial debate about: allocation o f scarce health resources in affected countries to infections of greatest public health importance. But where do such figures come from? In this article, David Crompton and Jim Tulley report on their appraisal of Ascaris prevalence data for Africa. Their figures were used to compile the geographic histogram on this month's cover.

Helminth fecundity: Density dependence or statistical illusion?

Density-dependent constraints on fecundity or survival are critical for the regulation and stability o f all populations. Helminth parasites are no exception to this rule. In medical helminthology, it has been widely assumed that the most effective density-dependent constraints act upon parasite fecundity and are the result of intra-specific parasite competition or acquired immunity to infection. In this article, Anne Keymer and Andrew Slater advocate a more detailed examination of the evidence on which this assumption rests.

Faecal egg counts and nematode fecundity: Heligmosomoides polygyrus and laboratory mice

The results are presented of an experimental investigation of the relationship between faecal egg counts and worm burden in outbred MF1 mice infected with Heligmosomoides polygyrus (Nematoda). The sex-ratio of the adult parasites was found to be significantly female biased, but independent of parasite burden. Evidence was also found to suggest a significant association between parasite density and mating success. The linear logarithmic relationship (gradient of approximately 2) between the variance and mean of repeated egg counts from individual mice could not be disrupted by controlled experimental conditions, and is therefore concluded to be an inherent feature of the biology of helminth-host interactions. A significant positive association was detected between host faecal output and parasite fecundity; further investigation of the cause of this association is now required. Average faecal production was found to be inversely related to worm burden. In combination, the above two factors account partly, but not completely, for the observed density dependence in the absolute rate of parasite egg production. Two epidemiological consequences of the relationship between faecal production and parasite fecundity are discussed. First, absolute rates of egg production can only be estimated by temporal egg counts (epd). Secondly, measurements of egg production/g of faecal material (epg) are more reliable indicators of worm burden than measurements of epd. Both faecal output and worm fecundity respond as might be predicted to a period of host food deprivation; faecal egestion and measurements of epd are significantly depressed, and measurements of epg are significantly increased. The data are consistent with the hypothesis that egg production and faecal output are causally associated, but that other factors relating to parasite density also influence egg production. Of the possible causes for the high variability in parasite egg production between mice harbouring low number of worms, the most likely explanation seems to be host heterogeneity (possible genetic in origin). This is discussed in relation to similar patterns observed in data collected from human populations in areas of endemic helminth infection.

The epidemiology and control of intestinal helminths in the Pulicat Lake region of Southern India. I. Study design and pre- and post-treatment observations on Ascaris lumbricoides infection

The study design of a project to investigate the epidemiology, population dynamics and control of intestinal nematode infections in fishing village communities in Southern India is described. The paper focuses on Ascaris lumbricoides infection and describes changes in prevalence and intensity (worm burdens) with host age, the aggregated frequency distributions of parasite numbers per person, a density-dependent relationship between parasite fecundity and worm burden and rates of reinfection following chemotherapeutic treatment. The age-intensity of infection profile is convex in form, where maximum worm burdens are attained in children in the age range five to nine years. On the basis of juvenile to adult worm ratioos, the life expectancy of Ascaris in man is estimated to be of the order of one year. Rates of reacquisition of worms after chemotherapy are shown to be dependent on host age. Wormy individuals with heavy infections are shown to be predisposed to this state such that they reacquire heavier than average worm burdens following treatment.

The clinical significance of gastrointestinal helminths--a review

Gastrointestinal helminths (nematodes, trematodes and cestodes) constitute some of the most common and important infective agents of mankind and are responsible for much morbidity and some mortality. Whereas many symptoms and signs are confined to the intestine and less often the associated digestive organs, systemic manifestations are also numerous; this applies especially to indigenous populations of developing 'Third World' countries. Using a clinical classification these organisms can be broadly separated into those involving the small-intestine and those which have a colo-rectal distribution; of the former, a minority has been causally related to intestinal malabsorption. Clearly, however, not all gastrointestinal helminths are associated with disease and it is important to be able to separate these two groups; when present at high concentration and especially in infants and children some of the least pathogenic are not, however, entirely asymptomatic. Maintenance of a high 'index of suspicion' is necessary and this applied especially to 'western' populations in whom rapid and extensive travel to areas of the world with substandard sanitation and contaminated food and water supplies is now common; first evidence of infection in them may result from serious clinical complications. Recent advances have focussed on treatment, and especially the introduction of the benzimidazole compounds (especially albendazole) for nematode, and praziquantel for cestode, infections. Treatment of strongyloidiasis remains, however, unsatisfactory. Mass elimination of gastrointestinal helminths in developing 'Third World' countries remains a major challenge.

Epidemiological aspects of Trichuris and trichuriasis in Caribbean communities

The review argues for a reappraisal of the health significance of the human whipworm, Trichuris trichiura. Infections with this geohelminth are at least as prevalent as Ascaris lumbricoides in many localities, and are associated with significant morbidity. Infection may result in severe trichuriasis syndrome or, more frequently, in a chronic colitis associated with growth stunting. Under-reporting of the chronic manifestations of disease has resulted in a gross under-estimation of the health impact of trichuriasis. Furthermore, estimation of the population dynamical parameters of T. trichiura transmission suggests that whipworm infections are intrinsically more resistant to control than those of other common geohelminths. A major determinant of the transmission dynamics and morbidity characteristics of this helminthiasis is the aggregation of worm burdens in certain predisposed individuals and age groups. It is suggested that improved understanding of the factors generating this distribution of infection intensity is a pre-requisite for effective control of both infection and morbidity.

Chronic ascariasis and malnutrition

Malnutrition is the most widespread ill of mankind. Its global distribution coincides indistinguishably with that of the common roundworm. Ascaris lumbricoides is common - infecting about one-quarter of the world's population. Acute clinical ascariasis sometimes requires hospital treatment, but chronic infections contribute to long-term malnutrition. This alone should justify greater expenditure on the treatment and control of Ascaris.

Hookworm burdens and faecal egg counts: an analysis of the biological basis of variation

The relationships between various measures of faecal egg output and hookworm burdens were investigated in 84 villagers from West Bengal with mixed Necator americanus and Ancylostoma duodenale infections. The pattern of day to day variability in egg counts from individuals can be characterized by the linear relationship between the logarithms of the variances and means. Egg output is shown to be non-periodic in long runs of day-to-day records. The distribution of worm numbers per host is well described by the negative binomial probability model, and the relationship between per capita egg output and worm burden is non-linear where egg output declines as parasite burden rises. Density-dependent effects on fecundity are shown to act at comparatively low worm burdens in relation to the range of observed parasite loads. Egg output measures are shown to be qualitative as opposed to quantitative measures of worm burdens as a consequence of inherent sampling heterogeneity and variability induced by biological processes, such as density-dependent depression of parasite fecundity. The analyses suggest that it is possible, on the basis of egg counts, to discriminate between individuals with low and high hookworm burdens. The intrinsic per capita fecundities of the two species of hookworms are shown to be similar, but density-dependent constraints on egg production by N. americanus appear to be more severe than those acting on A. duodenale. This observation may help explain why A. duodenale appears to produce larger numbers of eggs per unit of time than N. americanus. The regulatory role of density-dependent fecundity and aggregated distributions of worm numbers per person are discussed in relation to the over-all transmission dynamics of hookworm parasites.

Density-dependent fecundity in Schistosoma mansoni infections in man

An analysis is presented of autopsy data collected by Cheever in 1968 on the association between Schistosoma mansoni worm burdens and faecal egg counts. Significant negative relationships are found between the number of eggs/worm pair/g faeces and both the number of worm pairs and the total number of worms. A general, non-linear, statistical routine is shown to be the most practical method of fitting non-linear models to the observed density-dependent fecundity relationship. The conceptual and practical implications of density-dependence are discussed in the context of epidemiological field studies and chemotherapy programmes.