Population dynamics of human helminth infections: control by chemotherapy

Incidence and intensity of Babesia spp. sporokinetes in engorged Boophilus microplus from a dairy herd in Venezuela

The objective of this study was to determine the incidence and intensity of Babesia spp. sporokinetes in hemolymph of engorged females (EF) Boophilus microplus monthly detached from two herds of a local dairy breed of cattle named Carora breed. The experimental herds were located in 2 ranches, 45 km apart, having different management of tick control (TC). Proper methods of TC were commonly used at ranch "A," whereas ineffective methods were used at ranch "B," Babesia spp. sporokinetes were diagnosed in EF B. microplus by microscopic exam of tiny smears of hemolymph stained with Giemsa. A total of 537 smears were checked, and 108 were positive to Babesia spp. (20.1%). The incidence of Babesia spp. was higher (p < 0.01) in ranch "B," 22.3%, compared with the value detected in ranch "A," 6.5%. Incidence of Babesia spp. was greater in the dry season (Dec-Apr) compared with the values detected during the rainy season (May-Nov). The intensity of Babesia spp. was 3.43 at ranch "B" (p < 0.01), and 0.32 at ranch "A" during the rainy season. The values detected for intensity kept no correlation with those two seasons, although it is known that the incidence and seroprevalence of bovine babesiosis are greater during the dry season. The importance of these parameters in the epizootiology of bovine babesiosis in Venezuela is discussed.

A longitudinal study of enterobiasis in three day care centers of Havana City

Pinworm infection was prospectively studied during one year in 469 children attending three day care centers. Each child was examined at six months intervals using up to three perianal swabs with adhesive tape. Those found infected were treated with mebendazole. At the beginning of the study we found a prevalence of 28% that dropped to 13% and 12% in the following study periods. The reinfection rate was twice the incidence rate in both study periods. We also found a small percentage (10%) of the children reinfected in most or all study periods. There was a high correlation between reinfection and perianal itching. Our results add further knowledge to the epidemiology of intestinal parasites in day care centers.

Worm burdens in schistosome infections

Schistosomiasis, caused by fluke worms of Schistosoma spp, is one of the most common tropical diseases. Despite decades of research and progress towards the control of the disease, many aspects of the dynamics of infection and immunity remain unresolved. There is, in fact, not even an approximate measure of how many worms are harboured by infected humans. Epidemiological, mathematical and biomedical arguments indicate that individual worm burdens in endemic areas number hundreds to thousands of adult schistosomes, instead of the few to dozens generally assumed on the basis of available autopsy data. As Bruno Gryseels and Sake de Vlas here discuss, this hypothesis has important consequences for research and control, as many constants in schistosomiasis research have to be reconsidered.

The development of an age structured model for schistosomiasis transmission dynamics and control and its validation for Schistosoma mansoni

Mathematical models are potentially useful tools to aid in the design of control programmes for parasitic diseases. In this paper, a fully age structured epidemiological model of human schistosomiasis is developed and parameterized, and used to predict trends in infection prevalence, intensity and prevalence of heavy infections over age and time during several rounds of mass and age targeted treatment. The model is validated against data from a Schistosoma mansoni control programme in Kenya.

A model for the control of Echinococcus multilocularis in France

In some areas of France the prevalence of Echinococcus multilocularis in foxes is as high as 50%, whereas less than one in a thousand voles (principally Microtus arvalis) are infected. In these regions the control of rabies in foxes is achieved by using helicopters to spread bait containing oral vaccine in capsules. A mathematical model has been constructed in an attempt to determine if the addition of praziquantel to bait would be effective in eradicating E. multilocularis, or at least achieve a useful measure of control. It has been shown that the qualitative population dynamics of E. multilocularis are not affected by the detail of its epidemiology in the intermediate host population. The model is, however, sensitive to assumptions about the distribution and longevity of the adult worm in the definitive host. Given these assumptions, a method is provided that determines the feasibility of eradication conditional on the pre-control prevalence in foxes, or predicts the post-control prevalence if eradication is not feasible. If experiments could be designed to provide better information about the biological factors that determine the epidemiology of this parasite in the definitive host, a more reliable assessment of the feasibility of control would be achieved.

Control of Ascaris infection by chemotherapy: which is the most cost-effective option?

Cost-effectiveness analysis is used to predict the optimal design of mass chemotherapy strategies in controlling Ascaris lumbricoides infection. The question of who to treat, how many to treat, and how often to treat are addressed using a population dynamic model of helminth transmission that assesses effectiveness in terms of disease reduction, combined with cost data from an actual control programme. Child-targeted treatment can be more cost-effective than population treatment in reducing the number of disease cases. The model also implies that, in the circumstances described here, enhancing coverage is a more cost-effective approach than increasing frequency of treatment.

Economic development and tropical disease

Development stands for an improved quality of life through gains in health, education, living standards, and higher income. Development is based on economic growth. Although all indicators of development: GDP per capita, average life expectancy at birth, and per capita income have been rising in almost all countries, including the "developing" countries over the past decades, economic growth did not lead to the expected disappearance of infectious diseases. Economic growth is associated with severe degradation of the natural world. Though economic growth is essential for development, it is a highly imperfect proxy for it. The people who suffer from the adverse effects of economic growth are often different from those who benefit from it. Economic development shows two faces with regard to tropical diseases: it is essential for their prevention and cure and it contributes to their transmission and severity through its impact on the environment. The pivotal point is poverty. If economic growth leads to improved education and social organization, even adverse effects can be mastered by the community.

The population dynamics of parasitic helminth communities

This paper describes a mathematical model which allows us to compare the data collected from short-term cross-sectional surveys with the population dynamics of host and parasite populations over longer periods of time. The model extends an earlier framework for two parasite species in one host, to one for an arbitrary number of parasite species. We show that the conditions necessary for the coexistence of two parasite species extend to expressions for the coexistence of three or more parasite species. Furthermore, the model suggests that those species which form the 'core' of the parasite community are those whose high fecundity and transmission efficiency permit them to colonize hosts readily. In contrast, those species which are classified as 'satellite' species of the community are either species with low fecundity, or low transmission efficiencies. This work confirms earlier studies that suggest that increasing degrees of aggregation are crucial in allowing several species of parasites to coexist in the same species of hosts. The properties of the model are compared with patterns observed in data collected for helminth parasites of Anolis lizards, wood mice and eels. This combined theoretical and empirical approach confirms the importance of the life history strategies of the parasite in determining the abundance of each species in the community. It suggests that studies of parasite community structure have to pay more attention to the strategies pursued by each individual species before interactions between species are considered.

Aggregation and predisposition to Ascaris lumbricoides and Trichuris trichiura at the familial level

This study examines the persistence of familial aggregation and familial predisposition to Ascaris lumbricoides and Trichuris trichiura infection over 2 periods of treatment and reinfection, in an urban community in Kuala Lumpur, Malaysia. Both parasite species were shown to be aggregated (assessed by the variance to mean ratio) within families at all 3 interventions, although no consistent trend in aggregation was observed over the period of the study. Associations between mean A. lumbricoides and T. trichiura infection levels of families, at all 3 interventions, were highly significant (P < 0.0001), suggesting persistent predisposition at the family level.

A synoptic inventory of needs for research on women and tropical parasitic diseases. I. Application to urinary and intestinal schistosomiasis

The determinants of major parasitic infections in women have never been studied systematically. Much work remains to be done before vital gaps in our knowledge will be filled. The protocol presented here categorizes the determinants of parasitic diseases, and functions as an inventory for research needs and priorities. In this paper, the synoptic inventory is tested by matching it against the scientific knowledge of schistosomiasis that is at hand. Available data and existing lacunae are discussed. We conclude that the environmental, economic, socio-cultural, nutritional, genetic, biological and immunological factors which determine schistosomiasis in women are largely unknown. There is an urgent need for systematic and interdisciplinary investigations before appropriate and sustainable interventions can be initiated.

Immunological modulation and evasion by helminth parasites in human populations

Helminth parasites are highly prevalent in human communities in developing countries. In an endemic area an infected individual may harbour parasitic worms for most of his or her life, and the ability of these infections to survive immunological attack has long been a puzzle. But new techniques are starting to expose the diverse mechanisms by which these agents modulate or evade their hosts' defences, creating a dynamic interaction between the human immune system and the parasite population.

Experimental evidence of genetic determinism in high susceptibility to intestinal pinworm infection in mice: a hybrid zone model

In the hybrid zone of the two mouse subspecies Mus musculus musculus and Mus musculus domesticus, mice with hybrid genotypes harbour, on the average, more helminth parasites (cestodes and nematodes) than mice of the two parental taxa. In order to determine the roles played by genetic parameters in this phenomenon, mice with recombined and parental genotypes were experimentally infected with the intestinal pinworm Aspiculuris tetraptera, a natural parasite of the house mouse. The results showed that the high susceptibility of the hybrid zone mice is genetically determined. In addition, this study shows the occurrence of variability among resistant parental populations.

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.

Relationship between the degree of poverty and the IgE response to Ascaris infection in slum children

We examined a group of urban slum children in Caracas, Venezuela, and demonstrated the relationship that exists between poverty, conditions of hygiene and the prevalence of helminth infection. Concordant with the high prevalence of helminth infection in these children, the total serum levels of immunoglobulin E (IgE) and blood eosinophilia were elevated and were directly related to the degree of poverty and lack of sanitary facilities. However, in contrast, the reverse pattern was observed for the specific allergic response to these parasites. Thus, the immediate hypersensitivity skin test reactivity to Ascaris antigens, and the serum levels of specific anti-Ascaris IgE antibody, were lowest in the poorest children, who had the highest prevalence of helminth infection and the highest total IgE levels. As allergic-type reactions may participate in protective mechanisms against helminths, these results suggest that poverty and poor sanitary conditions may, by influencing the IgE response, compromise the resistance of such children to parasitic infections.

Predisposition to reinfection by intestinal helminths after chemotherapy in south Thailand

Rates of reinfection by the intestinal helminths hookworm, Ascaris lumbricoides and Trichuris trichiura after chemotherapy were studied in two villages in Phang-Nga Province, southern Thailand. It was found that intensity of infection levels attained after reinfection correlated positively with pretreatment intensities of infection for all parasites. This implies that certain persons in the community are predisposed to receiving high numbers of worms, due either to environmental or personal risk factors. Therefore, it would be advantageous to identify such persons and treat them preferentially. Targeted chemotherapy, however, should be combined with efforts to identify the risk factors that vary within the community and direct educational efforts or environmental intervention towards the section of the community most affected by the parasites.

Treatment of Onchocerciasis with ivermectin in Central America

The success o f ivermectin in controlling transmission of Onchocerca volvulus in Guatemala suggests a broader role for this drug than morbidity control alone. In this article Ed Cupp reviews recent findings from a three-year pilot study in that country which evaluated the effects of recurrent semiannual treatment on several important factors associated with the biology o f river blindness. These results illustrate the range o f beneficial effects that occur not only for this disease but for other parasitisms when this drug is administered at the community level.

Intensity of reinfection with Ascaris lumbricoides and its implications for parasite control

Intestinal helminths are among the most common and widespread of human infections. Because it is typical to find that most worms are aggregated in a few potential hosts it has been suggested that some individuals are predisposed to heavy infections and that morbidity could be controlled by the treatment of heavily infected individuals only. We have studied the prevalence and intensity of reinfection with the intestinal nematode Ascaris lumbricoides among people living in Dhaka, Bangladesh. 880 people were treated with pyrantel pamoate three times at six month intervals, and on each occasion they collected all their stools for 48 h after treatment. Worms expelled by each subject were counted and weighed. The prevalence of infection at round 1 of treatment was 89% and the mean burden was 18.5 worms. Reinfection was rapid and at rounds 2 and 3 the prevalence was 82% and 80%, respectively, with mean burdens of 14.0 and 11.5 worms. The intensity of reinfection was not random: more subjects than expected became heavily reinfected (greater than or equal to 15 worms) and more subjects than expected remained lightly infected (less than or equal to 14 worms) (p less than 0.001). Worms were highly aggregated at each round of treatment but although just over 10% of all subjects were heavily infected at each and every round of treatment, over 60% of all subjects were heavily infected at least once. The findings show that some individuals seem to be susceptible to heavy infection whereas others are not, that deworming has a greater effect on the intensity of infection than on the prevalence, and that mass chemotherapy is likely to be a more effective means to control morbidity than is selective treatment of heavily infected individuals only.

Renal involvement in prolonged Salmonella bacteremia: the role of schistosomal glomerulopathy

Renal involvement has been well documented in patients with hepatosplenic schistosomiasis and in patients with prolonged Salmonella bacteremia (PSB). Whether there is a specific renal lesion related to PSB or the chronic bacterial infection aggravates a pre-existing schistosomal glomerulopathy has been a matter of controversy. To analyze the clinical manifestations and histopathological findings of the renal involvement, 8 patients with hepatosplenic schistosomiasis and PSB (group I) were compared with 8 patients with schistosomal glomerulopathy (group II) matched by sex and glomerular disease. The mean age in group I was 17.7 years. All patients presented with hematuria, in 4 cases associated with non-nephrotic proteinuria. In group II the mean age was 23 years; nephrotic syndrome was the clinical presentation in 7 of the 8 patients in the group. All patients in group I experienced remission of the clinical and laboratory abnormalities as the salmonella infection was cured; in group II the patients had persistent, steroid-resistant, nephrotic syndrome. On histological examination, no difference was noted between the two groups, except for pronounced glomerular hypercellularity and interstitial mononuclear cell infiltration in group I. These observations strongly suggest that PSB exacerbates a pre-existing sub-clinical schistosomal glomerulopathy by the addition of active lesions directly related to the prolonged bacteremia.

The effect of repeated chemotherapy on age-related predisposition to Ascaris lumbricoides and Trichuris trichiura

This study examines the persistence of predisposition to Ascaris lumbricoides and Trichuris trichiura during repeated chemotherapy in an urban community in Kuala Lumpur, Malaysia. Significant predisposition was observed over 2 periods of reinfection with and without age-standardization of data. Analysis of different age groups indicated that predisposition was most strongly detectable in the younger age classes. The intensities of infection with both parasites were strongly correlated at each cycle of intervention, suggesting that individuals were similarly predisposed to both species.

On the application of mathematical models of schistosome transmission dynamics. II. Control

Mathematical models have considerable potential as aids to the design of schistosome control programmes. This is because of the complex nature of the schistosome transmission cycle and the variety of control measures available, which make the comparative effectiveness of different control options extremely difficult to predict. This review aims to demonstrate how control can be incorporated in models of schistosome transmission dynamics, and to make explicit the assumptions and limitations of the models and their relationships with field data. A basic model is described which considers changes in the mean number of schistosomes per person. The criteria for the eradication of endemic infection and the potential for reducing levels of infection are discussed. The treatment of various control measures within this framework is reviewed. These include: chemotherapy (mass, selective and targeted), molluscicide application (blanket and focal) and other snail control measures, larval stage control, improved water supplies and sanitation, and health education. The incorporation of economic variables is also discussed. The choice between different control options depends on the relationships between schistosome epidemiology and the costs and effects of control. The evaluation of cost-effectiveness is a dynamic problem, and the outcome will depend on local conditions and constraints. Very general recommendations for the design of schistosome control programmes are unlikely to prove useful.

The epidemiology and population biology of Necator americanus infection in a rural community in Zimbabwe

Baseline data from an epidemiological study of hookworm infection in a rural community in Zimbabwe are presented. The infection status of an age-stratified sample of the community was assessed using anthelmintic expulsion techniques. Necator americanus was the only helminth parasite found to be present. The age-prevalence and intensity profiles rose asymptotically to an adult prevalence of about 80% and adult mean burden of 7.7 worms per host. The overall mean burden was 4.8 worms per host. The frequency distribution of N. americanus was overdispersed and well described by the negative binomial distribution with a value for the aggregation parameter, k, of 0.346. Separate estimates of k were lower in males and older hosts. The distribution patterns were difficult to reconcile with any simple process of age-dependent acquisition of an effective immune response. A significant negative correlation was recorded between per caput fecundity and worm burden, providing evidence for a density-dependent regulation of female worm fecundity. The basic reproductive rate (R0 congruent to 2) was found to be similar to estimates from other geographical areas.

Comparison of different chemotherapy strategies against Schistosoma mansoni in Machakos District, Kenya: effects on human infection and morbidity

A comparison was made of the long-term impact of different methods of administration of chemotherapy (oxamniquine, 30 mg/kg in divided doses; or praziquantel, 40 mg/kg) on prevalence and intensity of Schistosoma mansoni infection in four areas in Kangundo Location, Machakos District, Kenya. In Area A, treatment was offered in October 1983 and again in April 1985 to all infected individuals. In Area H, treatment was offered in April 1985 to individuals excreting greater than or equal to 100 eggs per gram (epg) of faeces. In Area S, treatment was offered in April 1985 to all infected school children, within the framework of the primary schools. In the witness area, Area W, treatment was given in April 1985, for ethical reasons, to a small number of individuals excreting greater than or equal to 800 epg. Prevalence and intensities of infection were subsequently monitored at yearly intervals for three complete post-treatment years. In the Area S schools, clinical examination was also carried out at yearly intervals. Treatment of all infected individuals on two occasions (Area A) was the most effective and long-lasting way of reducing prevalence and intensity of infection. In this area, however, some earlier interventions had been carried out and pre-treatment intensities were lower than in the other areas. Treatment only of infected schoolchildren (Area S) also had a marked and prolonged effect, comparable to or better than treatment of individuals with heavy infections (Area H). Treatment of infected schoolchildren also caused a persistent reduction in the prevalence of hepatomegaly, and there was suggestive evidence from intensities of infection in community stool surveys (but not from incidence rates) of an effect on transmission. In all study areas, reinfection was most rapid and most intense among children. These findings are discussed in the light of theoretical considerations and of results from other studies, both on schistosomiasis and on intestinal helminths. We conclude that, in areas of low morbidity such as Kangundo, chemotherapy of schoolchildren only, at intervals of up to 3 years, is a satisfactory way of producing a long-term reduction in both intensity of infection and morbidity.

Age-dependency of infection status and serum antibody levels in human whipworm (Trichuris trichiura) infection

This study examines the age-dependency of the relationships between human infection with whipworm (Trichuris trichiura) and parasite-specific antibody level measured by ELISA against an extract of adult worms after preincubation of the sera with Ascaris lumbricoides adult worm extract. The convex age-profile of parasite infection intensity is shown to be mirrored by an age-dependent change in age-class mean levels of IgG (all subclasses except IgG3), IgA, IgM and IgE. Mean antibody levels rise with increasing acquisition of infection in childhood and decline as the intensity of infection falls in adulthood. Immunoblot analysis of selected sera from different age-classes indicates that antigen recognition is similarly dependent on infection intensity. In individual children, antibody levels correlate positively with acquisition of infection, consistent with a simple model of antigen dosage specifying the magnitude of the humoral immune response. In adults, IgG4 correlates positively and IgA negatively with intensity of infection, suggesting involvement of these isotypes in functional roles of immune blockade or effector mechanisms, respectively.

On the application of mathematical models of schistosome transmission dynamics. I. Natural transmission

The many mathematical models of the transmission dynamics of schistosomes that have been published since 1965 have had little impact on field studies or on the design of schistosome control programmes. At least in part, this is due to limited interaction between theoretician and field worker, resulting in unrealistic models that are not easily applied to field data. This review aims to make explicit the assumptions and limitations of existing models and their relationships with field data. A basic model is described which considers the mean number of schistosomes per person and the prevalence of patent infections of snails. Various modifications to this model are introduced. These include: prepatent infections of snails; loss of infection of snails; the effects of snail population dynamics; the effects of miracidia and cercariae population dynamics; miracidia searching efficiency; reservoir hosts; heterogeneous patterns of transmission; seasonality; and predisposition to infection. Variation in levels of infection with age and the effects of acquired immunity to infection are also considered. Published models of schistosome transmission dynamics are reviewed within this framework. Approaches to the modelling of schistosome control measures are considered in a companion paper. It is suggested that future theoretical studies give greater attention to the details of snail population dynamics, heterogeneous patterns of transmission and the effects of acquired immunity. There is a need for field studies explicitly designed to provide estimates of transmission parameters and for studies of the epidemiological effects of acquired immunity.

The impact of three-monthly age-targetted chemotherapy on Ascaris lumbricoides infection

The impact of 3 chemotherapeutic regimens at 3-monthly intervals against Ascaris lumbricoides infection, targetted at under 15 years old children with and without initial mass chemotherapy, and at 2-12 years old children, was studied in three Myanmar [Burmese] rural communities over a period of 2 years. Computation was made of prevalence and intensity (worm burden) of Ascaris infection, and other parameters for estimation of basic reproductive rate (R0) of the parasite and of the proportion of target age group to be treated 3-monthly (g) by employing the mathematical model for targetted chemotherapy. The 3 treatment regimens were almost equally effective in reducing prevalence and intensity in both the targetted and non-targetted age groups. Ascaris transmission in each of the 3 communities was interrupted, as indicated by the values of mean worm burden per person. The findings are compared with those of other similar studies and the reasons for the impact are discussed. The possible impact in similar endemic areas of applying the mathematical model predictions for age-targetted chemotherapy in controlling ascariasis is also discussed.

Equine helminth infections: control by selective chemotherapy

A programme of selective anthelmintic therapy was used in a herd of 31 horses. Faecal egg counts were done during the months of September, November, January, March, May and the following September. Horses with greater than or equal to 100 eggs per gram (epg) were treated with ivermectin, and those with less than 100 epg were not treated. The criteria for adequate internal parasite control in the herd was a median herd faecal egg count of less than or equal to 100 epg. Effectiveness of selective therapy was assessed by faecal egg count after nine months of treatment and was determined to be adequate when a median herd egg count of 0 epg was obtained. However, on returning from pasture the following September, median herd egg count had risen to 325 epg. A statistically significant correlation was seen in the paired September faecal egg counts of the horses in that initial September faecal egg count was predictive for the following September. Initial September faecal egg count was related to the number of anthelmintic treatments required during the period of selective therapy, whereas age of horse was not. We propose that faecal egg counts be incorporated into strategic anthelmintic programmes as an economical tool for identifying and targeting herd members predisposed to shedding elevated numbers of helminth eggs.

Frequency distribution of Brugia malayi microfilariae in human populations

This study examines the effects of host age and sex on the frequency distribution of Brugia malayi infections in the human host. Microfilarial (mf) counts for a large data base on the epidemiology of brugian filariasis in Shertallai, Kerala, South India are analysed. Frequency distributions of microfilarial counts partitioned by age are successfully described by zero-truncated negative binomial distributions, fitted by maximum likelihood. This analysis provides estimates of the proportion of mf-positive individuals who are identified as negative due to sampling errors, allowing the construction of corrected mf age-prevalence curves, which indicate that the observed prevalence may under-estimate the true figures by between 18 and 47%. There is no evidence from these results for a decrease in the degree of over-dispersion of parasite frequency distributions with host age, such as might be produced by the acquired immunity to infection. This departure from the pattern in bancroftian filariasis (where there is evidence of such decreases in over-dispersion; Das et al. 1990) is discussed in terms of the long history of filariasis control (and consequently low infection prevalence) in Shertallai.

Epidemiology and control of ascariasis in Korea

In view of the epidemiology and control strategy, ascariasis was reviewed with special reference to the data obtained for the past years in Korea. The range of length and prepatent period of A. lumbricoides to become fertile were 12.5-13.5 cm and 1.8-2.1 months for females, and 10.5-11.5 cm and 2.1-2.5 months for males, respectively. In the female worm burden 1 per case, the egglaying capacity steadily increased from 12.6 cm of their length up to 25.0 cm and then decreased significantly after their growth to 27.5 cm. Analysed the egg discharging pattern, it was turned out that the cases with six or more worms have no probability to be false negative or unfertilized ova passer, and 52% of all false negatives were found infected only with male(s). The sex ratio was in the range of 0.74-0.82 (male/female). Basic reproductive rate was calculated in the range of 1.16-2.11 in rural areas, but it was approaching nearly to the 'break point' in some areas. It was observed that the 'U-rate' was increased from 19.4% (1973) to 61.1% (1989) for the past seventeen years. According to the survey in 6 rural areas, the average worm burden was 2.2 per population and 4.5 per infected. The frequency distribution pattern of A. lumbricoides per person in a rural community was well fitted to the negative binomial distribution. Seasonal fluctuation was shown in two, smaller and larger, peaks. The annual prevalence in student group was initially 55.4% in 1969, but decreased to 0.3% in 1989, and it is evident that the decrease has been greatly owing to the national control project. It was proved that the efficacy of a quarter dose of pyrantel pamoate used in long term control programme was almost equal to that of conventional dose (10 mg/kg). The comparatie efficacy of various interval mass chemotherapy schemes was evaluated through 2 year observations, and it was confirmed that at least biannual mass chemotherapy is necessitated to expect gradual lowering of reinfection. In the case of blanket treatments with 2-month interval in a village, all of inhabitants have been free from reinfection for the period of 28 months after the initial treatment. It was fully recognized that without specific legislation and organization supported by the government, the national mass control programme has almost no chance of success, especially in developing countries.(ABSTRACT TRUNCATED AT 400 WORDS)

Frequency distribution of Wuchereria bancrofti microfilariae in human populations and its relationships with age and sex

This paper examines the effects of host age and sex on the frequency distribution of Wuchereria bancrofti infections in the human host. Microfilarial counts from a large data base on the epidemiology of bancroftian filariasis in Pondicherry, South India are analysed. Frequency distributions of microfilarial counts divided by age are successfully described by zero-truncated negative binomial distributions, fitted by maximum likelihood. Parameter estimates from the fits indicate a significant trend of decreasing overdispersion with age in the distributions above age 10; this pattern provides indirect evidence for the operation of density-dependent constraints on microfilarial intensity. The analysis also provides estimates of the proportion of mf-positive individuals who are identified as negative due to sampling errors (around 5% of the total negatives). This allows the construction of corrected mf age-prevalence curves, which indicate that the observed prevalence may underestimate the true figures by between 25% and 100%. The age distribution of mf-negative individuals in the population is discussed in terms of current hypotheses about the interaction between disease and infection.

Reinfection of Somali children with Trichuris trichiura after chemotherapy: relevance of immunostimulation

The intestinal helminth status of an age-stratified sample (6 to 20 years old) from a Somalian community has been assessed and the typical pattern of highly aggregated parasite distribution found. A reinfection study on a sample of 40 children (treated and untreated with a pentapeptide identical to the active site of the thymic hormone thymopoietin) seemed to indicate that immunological factors play a significant role in modulating the population dynamics of infection in endemic communities.

Control of intestinal nematode infections by chemotherapy: mass treatment versus diagnostic screening

There is increasing interest in the implementation of programmes to control nematodiases by chemotherapy, but no consensus on whether diagnostic screening before treatment is an essential component of such programmes. It is estimated that a screening component increases costs by a factor of 2-6 and may be beyond the economic and skill resources of endemic areas, but accords with accepted medical practice since only infected individuals are treated. In an attempt to examine this issue quantitatively, the relative risks of mass treatment and morbidity due to helminthiasis are compared. Age- and locality-targeting of mass treatment at populations at high risk of morbidity are proposed as procedures for further enhancing this risk differential. It is suggested that resolving the question of screening versus mass treatment requires more quantitative information than is currently available on the risk of side effects with treatment and the risk of morbidity with variable intensities of infection.

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.

Effect of selection pressure for thiabendazole resistance on fitness of Haemonchus contortus in sheep

A strain of Haemonchus contortus showing moderate levels of resistance to thiabendazole (TBZ) (egg hatch assay LC50 2.1 microM) was subjected, in alternating sequence, to 5 in vitro and 4 in vivo selections with TBZ. This resulted in a significant increase in resistance (egg hatch assay LC50 2.8 microM). The isolates obtained after each combined in vitro and in vivo selection were compared in terms of egg hatch assay, establishment, net egg output, per capita egg output and degree of pathology caused in infected hosts. The increase in resistance was accompanied by a significant increase in larval establishment in sheep (22.8-39.2%) and was associated with more severe pathology, higher egg count and worm recovery 3 months post-infection. The resistant strains were also compared with a TBZ-susceptible strain (egg hatch assay LC50 1.0 microM). The TBZ-susceptible strain had the highest net egg output, it caused the most severe pathology and it resulted in higher worm recovery than any of the TBZ-resistant strains, although the most resistant strain was very similar to the TBZ-susceptible strain. Neither the per capita egg output nor biological features of the free-living stages were significantly affected by the degree of resistance. The implications of these observations for anthelmintic use and for management of nematode anthelmintic resistance in the field are discussed.

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

Intensity of Ascaris lumbricoides infection was measured in terms of egg counts and worm burden in children 2-10 years of age. The expulsion of A. lumbricoides with a 3-day treatment of mebendazole occurred over 8 days, beginning on the second day of treatment. Ninety-seven percent of the worms were expelled between the second and seventh days. A rapid means of estimating eggs per gram (epg) by the Kato Katz technique correlated well with the method described by Martin & Beaver (1968). In spite of apparent density dependence in egg production, A. lumbricoides egg counts correlated well with worm burdens. It is concluded that, within the context of community surveys, epg is a reasonable means of identifying heavily infected individuals and that epg can be estimated rapidly by a slight modification to the standard Kato Katz technique.

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.

Control of geohelminths by delivery of targeted chemotherapy through schools

This study describes the age-targeted chemotherapeutic control of geohelminthiasis in the total population (n = 11,500) of the island of Montserrat, West Indies. The intervention programme involved the treatment with single dose albendazole of all children aged 2-15 years (approximately 2500) in 4 sequential cycles at intervals of 4 months. Infection status was monitored by an initial coprological survey of an age-stratified sample (11.5%) of the population, and by surveys of smaller samples (4-5%) after 2 and 4 cycles of treatment (7 and 15 months respectively). The programme delivered treatment to greater than 90% of the target population in each cycle, and reduced the prevalence and intensity of Ascaris lumbricoides and Trichuris trichiura infection in the target age-class. A decline in infection was also observed in the 16-25 year age-class, even though less than 4% of adults received treatment. The study demonstrates that chemotherapy targeted only at children can be implemented within an existing health infrastructure, and can achieve an overall reduction in the prevalence and intensity of geohelminth infection.

Ascaris lumbricoides: detecting its metabolites in the urine of infected people using gas-liquid chromatography

Substances dissolved in the urine of people infected with Ascaris lumbricoides were extracted and detected by gas-liquid chromatography. The ratios of the areas of the peaks produced by two substances extracted from urine to the area of the peak of solvent were found to be significantly correlated with the worm burden. A chemical analysis of the predominant substance by infrared spectrophotometry and proton magnetic resonance spectrometry indicated that it was 2-methyl-butyramide. The chemical properties of the other substance indicated that it was 2-methyl-valeramide. These substances are likely to be derivatives of two acids known to be end products of the carbohydrate metabolism of Ascaris.

Bancroftian filariasis in Pondicherry, south India: 1. Pre-control epidemiological observations

A 5-year Integrated Vector Management (IVM) project was implemented in Pondicherry, South India, for the control of Bancroftian filariasis. The efficacy of the IVM strategy was compared with routine control strategy under the national programme. The present paper describes the pre-control epidemiological features of filariasis as determined by a mass blood survey in 1981. Of 24946 persons examined 8.41% were microfilaraemic. Microfilaraemia prevalence was homogeneous throughout the study area. The prevalence and intensity of microfilaraemia were age dependent, and increased monotonically until about 20 years, following which there was a decline until about 40 years to become relatively stable in older age classes. The gender profiles of both prevalence and intensity of microfilaraemia showed no difference between the sexes until about 15 years of age, following which both were higher in males compared to females. The distribution of microfilarial counts was overdispersed, indicating aggregation of adult worms.

Renal function in Sudanese school children with Schistosoma mansoni infection

Renal function was investigated in 218 school children with Schistosoma mansoni infection in the Providence of Gezira in central Sudan and in 65 Sudanese and 65 German age-matched controls. Serum creatinine was normal in all children. A pathological urinary protein-creatinine ratio was found in 3% of S. mansoni-infected children and in 5% of Sudanese controls but in none of the European children. Characterization of pathological proteinuria using albumin nephelometry, alpha-1 microglobulin immunodiffusion and SDS-polyacrylamide gel electrophoresis in these children showed glomerular, tubular or mixed glomerulotubular patterns. One, 4 and 6 months following treatment of schistosomiasis with praziquantel, stools were re-examined; 57% of patients were cured, 16% were found to be reinfected and 27% had persistent egg excretion. Six months after therapy, pathological urinary protein-creatinine ratios were encountered in 3% of S. mansoni patients and in none of the 34 reinvestigated controls. Proteinuria was similar in patients with persistent S. mansoni egg excretion and in children cured of schistosomiasis infection. It is concluded that there was no evidence for S. mansoni associated glomerulonephritis in this group of Sudanese children. The high rate of pathological proteinuria in S. mansoni-infected and non-infected Sudanese children may be due to other causes.

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.

Prevalence, incidence, intensity and associated morbidity of intestinal helminths in south Thailand

Intensive surveys for Ascaris lumbricoides, Trichuris trichiura and hookworm were carried out in two villages in Phang Nga Province, southern Thailand, in order to measure prevalence, estimate incidence and determine the relation between intensity of infection and morbidity before and after chemotherapy. The study populations were a small upland village community (Nai Tone) and a grade school in a small coastal village (Boh Saen). About half of the Nai Tone villagers were given a broad spectrum antihelminthic (albendazole), and the Boh Saen students were all treated successively with three drugs: piperazine citrate to treat for Ascaris, pyrantel pamoate to treat for hookworm, and mebendazole to treat for Trichuris. Stool examinations were made using the quick Kato smear technique, questionnaires were administered concerning a variety of possible symptoms, and anthropometric and blood biochemical parameters were measured both before and after treatment. The prevalence of Ascaris was 31.0 and 22.6%, hookworm was 89.1 and 88.0% and Trichuris was 59.7 and 77.8% in the Nai Tone and Boh Saen study populations, respectively. Average intensity of Ascaris was highest in the 0-9 year age class (greater than 32,000 epg) in Nai Tone Village. Hookworm intensity of infection was higher in males than in females in all age classes, and in Nai Tone Village at least 25% of males and 20% of females had 8000 or more epg of faeces. Trichuris intensity of infection was highest between 5 and 10 years of age in both populations. The only signs or symptoms showing a significant (P less than 0.05) difference between high and low classes of intensity of infection and a significant improvement (P less than 0.01) after drug treatment, were headache and flatulance in the case of hookworm infection in Boh Saen School. The presence of multiple infections made testing of hypotheses concerning particular parasite species difficult.

Reinfection after treatment of schistosome infections

Today, chemotherapy has a central role in the control of schistosome infections. Although the costs involved may be high in relation to local expenditures on health, externally funded mass treatment programmes can lead to large reductions in the prevalence and intensity of schistosome infections. But the benefits of treatment to a community that has been involved in a mass chemotherapy programme, or to an individual patient seen in a health centre, will be limited if reinfection after treatment is rapid and intense. Despite the efficacy of the available drugs few, if any, control programmes based on mass chemotherapy have interrupted transmission and come anywhere near to eradicating schistosome infection.

Non-linear phenomena in host-parasite interactions

The paper examines non-linear dynamical phenomena in host-parasite interactions by reference to a series of different problems ranging from the impact on transmission of control measures based on vaccination and chemotherapy, to the effects of immunological responses targeted at different stages in a parasite's life-cycle. Throughout, simple mathematical models are employed to aid in interpretation. Analyses reveal that the influence of a defined control measure on the prevalence or intensity of infection, whether vaccination or drug treatment, is non-linearly related to the magnitude of control effort (as defined by the proportion of individuals vaccinated or treated with a drug). Consideration of the relative merits of gametocyte and sporozoite vaccines against malarial parasites suggests that very high leves of cohort immunization will be required to block transmission in endemic areas, with the former type of vaccine being more effective in reducing transmission for a defined level of coverage and the latter being better with respect to a reduction in morbidity. The inclusion of genetic elements in analyses of the transmission of helminth parasites reveals complex non-linear patterns of change in the abundance of different parasite genotypes under selection pressures imposed by either the host immunological defences or the application of chemotherapeutic agents. When resistance genes are present in parasite populations, the degree to which abundance can be suppressed by chemotherapy depends critically on the frequency and intensity of application, with intermediate values of the former being optimal. A more detailed consideration of the impact of immunological defences on parasite population growth within an individual host, by reference to the erythrocytic cycle of malaria, suggests that the effectiveness of a given immunological response is inversely related to the life-expectancy of the target stage in the parasite's developmental cycle.

Effect of repeated anthelmintic treatment on ability to detect predisposition of mice to Heligmosomoides polygyrus and Aspiculuris tetraptera (Nematoda) infections

Several recent studies have demonstrated predisposition of certain humans to heavy gastro-intestinal nematode infections. This finding has been used to support the concept of community control of nematodes based on selective treatment of the few heavily infected individuals in the community. A mouse model has been used in this study to determine whether those individuals found to be 'predisposed' to heavy infection continue to regain heavy nematode infections following repeated anthelmintic treatments. Re-infection was examined over 3 intervals of 8 weeks in 52 mice and individual worm burdens attained following drug-induced expulsion were analysed. In the case of Heligmosomoides polygyrus, parasite numbers obtained after first treatment were only significantly correlated with those obtained after the second treatment. Re-infection levels in mice during the second and third re-infection periods did not correlate with parasite loads at the beginning of the study. In the case of Aspiculuris tetraptera, correlations after each re-infection period remained significant, although the magnitude of the correlation coefficient decreased with successive re-infection intervals. The data suggest that a selective control programme based on repeated screening to identify heavily infected individuals prior to each treatment would reduce the number of H. polygyrus in the host population 17.6% more than a single screening at the beginning of the control programme, and would reduce the number of A. tetraptera by 14% more than a single screening. Re-infection rates for H. polygyrus were shown to decrease with increasing mouse age whereas re-infection rates for A. tetraptera increased with increasing mouse age.

Longitudinal studies on human schistosomiasis

A major difficulty in understanding the epidemiology of human schistosomiasis has been to distinguish between acquired immunity and reduced exposure as possible reasons for an observed decline, in older individuals, of levels of superinfection or of reinfection after chemotherapy. A series of studies of Schistosoma mansoni infections in Kenya has been undertaken to approach this problem, by investigation of intensities of reinfection after treatment of individuals whose levels of contact with contaminated water is subsequently observed. Intensities of reinfection are highest among younger children, thereafter declining sharply. This decline can be attributed only in part to age-related changes in the duration and nature of exposure; there is also evidence for the development of an acquired resistance to reinfection that is dependent both on age and on previous experience of infection, and that may be immunologically mediated. Evidence has been obtained that the slow development of this acquired immunity with age may be associated with the early development and subsequent slow decline of inappropriate immune responses that 'block' the effect of potentially protective responses. Implications of these findings for immunological intervention through vaccination are discussed.

Repeated mass treatment of schistosomiasis mansoni: experience in hyperendemic areas of Brazil. II. Micro-level evaluation of results

As shown by a recent survey in northeast Brazil, an overall (macro-level) assessment of control campaigns for schistosomiasis mansoni will tend to ignore localized variations in endemicity, thus resulting in misleading information. Evaluation on a micro-level, on the other hand, allows the identification of individuals who, despite repeated courses of chemotherapy, continue to present substantial reinfection rates. Such cases occur in a definite spatial pattern, which in turn points to the localization of residual foci of transmission. It is believed that in the absence of wide-reaching social investments the control of schistosomiasis by means of mass treatment will not be successful, unless a policy of targeted action, both in chemotherapy as well as snail control, is adopted. Obviously, the search for these reinfection clusters will require the development of sampling techniques suitable for wide-scale use.

Predisposition of mice to Heligmosomoides polygyrus and Aspiculuris tetraptera (Nematoda)

The purpose of this study was to determine whether predisposition to Heligmosomoides polygyrus and Aspiculuris tetraptera (Nematoda) exists within a naturally infected population of mice. A breeding mouse population was housed in a spacious arena in which endemic infections of H. polygyrus and A. tetraptera were present. H. polygyrus were over-dispersed in the mouse population. Prevalence reached 100% by the age of 3 weeks; intensity of infection increased to a peak in the 10 to 15-week-old mice, and remained high throughout life. A group of 73 mice was treated with pyrantel pamoate, and the expelled worms were counted. Mice were returned to the arena. Daily egg production was monitored 4, 8, 12 and 14 weeks after treatment. Mice were then killed and numbers of H. polygyrus and A. tetraptera were counted. Significant positive correlations were detected between numbers of H. polygyrus at first treatment and at necropsy, indicating the existence of predisposition. Similar results were obtained for A. tetraptera. Correlations improved when data were analysed by age class of mice. Analyses based on egg-count data during reinfection did not support the hypothesis of predisposition, however. A. tetraptera and H. polygyrus burdens were significantly correlated only in 3 to 4-week-old mice at the time of the treatment.