Population dynamics of human helminth infections: control by chemotherapy

The evidence for predisposition to trichuriasis in humans: comparison of institutional and community studies

This article compares the results of two studies of Trichuris trichiura infection, one conducted in a children's home and the other in a village. In both, the intensity of infection of a cohort of children was determined by antihelminthic expulsion initially, and again after a period of re-infection. The cohort of village children showed a predisposition to a particular intensity of infection. An individual with a heavy infection initially was likely to re-acquire a heavier than average worm burden. No such correlation was observed for the cohort of institutionalized children. It is suggested that the causation of predisposition is multifactorial, and that the contrasting results of the two studies may be due in part to differing levels of heterogeneity in exposure to infection, the village children being exposed to dissimilar domestic environments and the institutionalized children to only one.

Egg laying capacity of Fibricola seoulensis in mice and rats

The egg laying capacity of Fibricola seoulensis was observed in mice and rats. In mice, the mean number of eggs produced by an adult worm a day was 116 at the first week after infection, 123 at the 2nd week and 42 at the 3rd week. Thereafter, the capacity of about 20 eggs/worm/day was maintained up to one year period. In rats, it generally produced more eggs; 49 at 1 week, 250 at 2 weeks, 216 at 4 weeks, 327 at 6 months and 11 at one year after infection. It survived longer than a year in mice and rats.

Reinfection with Ascaris lumbricoides after chemotherapy: a comparative study in three villages with varying sanitation

This study examined the effect of the 2 major means of control of Ascaris lumbricoides--chemotherapy and sanitation. About 200 pre-school Caribbean children living in 3 villages with varying sanitation were studied by quantitative stool examination for the presence of the eggs of helminths. Children with Ascaris eggs were treated with piperazine. Over a 2-year period this procedure was repeated after every 6 months of natural re-infection. Results showed that (i) the rate of reinfection was on average 20% higher than the rate of new infection; (ii) there was a highly significant correlation between the results of children's initial stool examination and that 6 months later; (iii) children with high Ascaris egg counts also frequently had high Trichuris egg counts; (iv) during the 6-month intervals, 36% of the infected children were not reinfected after treatment; the difference in reinfection rates between villages with and without sanitation was 48%. Regression analysis indicated that, after several socio-economic variables were controlled, only sanitation and crowding remained significantly associated with reinfection. The implications of these findings in formulating control strategies are discussed.

Predisposition of humans to infection with Schistosoma mansoni: evidence from the reinfection of individuals following chemotherapy

In a study of faecal egg counts of Schistosoma mansoni from 359 people of all ages from a rural Kenyan community, a positive association was demonstrated between infection intensity in individuals before treatment and reinfection intensity in the same individuals 9 months after treatment in certain age groups of the sampled population. Consequences and possible causes of these observations are discussed in terms of the epidemiology and control of schistosomiasis.

Microfilaria density distribution in the human population and its infectivity index for the mosquito population

A new method of computing the infectivity index of microfilariae (mf) for the mosquito population is proposed using the estimated mf density distribution in the human population. the observed density distribution is considered a compound of the Poisson and the gamma distributions. The former distribution describes the probability of a specimen containing a specified number of mf and the latter describes the density distribution of mf in the host population. The mf infectivity index is the probability that a blood meal will include at least 1 mf, conditional on the population-density distribution of mf as specified by the gamma distribution. Actual data indicate that this population-density-based infectivity index can be considerably different from the conventional index based on the survey-density distribution. The level of the carrier rate of mf in a survey is greatly influenced, apart from the sample variation, by the average volume of blood taken from each person. The rate computed on the estimated population-density distribution of mf is convertible to any base amount of blood.

Clustering of Ascaris lumbricoides and Trichuris trichiura infections within households

A survey of 428 households in a shanty town in Coatzacoalcos, Mexico, revealed high prevalences of Ascaris lumbricoides and Trichuris trichiura. The data were analysed separately for A. lumbricoides and T. trichiura in order to investigate the spatial distribution of heavy infections through the town. Within each age class, those individuals with egg counts in the upper 20% of the range for that age were classified as "heavily infected". When the data were stratified by household size, it was found that the distribution of "heavily-infected" individuals was not random in the community. "Heavily-infected" individuals were found together in households; fewer household units had a single heavily-infected individual than would be expected by chance. Such a pattern could result either from genetic similarities among family members influencing their ability to mount an effective immunological response to infection, or focal transmission in the vicinity of the home, or both. This result may have important implications for the development of community control programmes.

Age-related prevalence, intensity and frequency distribution of gastrointestinal helminth infection in urban slum children from Kuala Lumpur, Malaysia

The gastrointestinal helminth infection status of 1574 children living in a slum area of Kuala Lumpur, Malaysia was assessed by quantitative coprology. Almost two-thirds were infected with Trichuris trichiura, 49.6% with Ascaris lumbricoides, and 5.3% with hookworm. Infection prevalence rose rapidly to a stable asymptote at 7 years of age, and the age-intensity profile was convex with maximal values in the 5-10 year age classes. This pattern was the same for males and females, but differed markedly between different ethnic groups. The frequency distributions of A. lumbricoides and T. trichiura were highly overdispersed (k values were 0.21 and 0.27, respectively), and age-dependent over the 0-8 year age classes. This suggests that the force of infection with these nematodes is lower in infants than in older children.

The importance of host age and sex to patterns of reinfection with Ascaris lumbricoides following mass anthelmintic treatment in a South Indian fishing community

Analysis of egg and worm counts of Ascaris recorded at various intervals following a mass anthelmintic treatment programme in a South Indian fishing community is presented. Three indices of infection in the community are compared, namely the prevalence and intensity of egg output (at 2, 6 and 11 months following treatment) and the number of worms expelled following an 11 month period of reinfection. Detailed examination of these measurements revealed significant associations with patient sex and age. The age-prevalence profile of Ascaris infection changed little over time (except immediately following treatment) with the peak prevalence found in the 5-9 year age group. Although 85% of both males and females harboured Ascaris initially, the prevalence following 11 months reinfection was decreased, due to a significantly lower proportion of males being reinfected. By the 11th month of reinfection, the age-intensity profiles of egg output were similar to those observed at initial treatment in the older age groups (10 years and above) and in male children (less than 10 years). However, a dramatic increase in the egg output of female children, greatly exceeding the initial mean, was observed within a 6 month period of reinfection. The intensity of egg output did not accurately reflect the abundance of Ascaris recovered via drug-induced expulsion following an 11 month period of reinfection. Although the egg output attained preintervention levels, the average worm intensity reached only half the initial value. The trends in the sex- and age-intensity profiles were consistent at the two sampling dates and showed similar patterns to the egg output curves. The relevance of the results to helminth control and the monitoring of reinfection is discussed.

Schistosomiasis in childhood

Schistosomiasis is a parasitic disease of the tropics which is estimated to affect up to 300 million people worldwide. In endemic areas the childhood age group has the highest prevalence and intensity of infection. There are several distinct species of schistosomes. The principal organ system involved in Schistosoma haematobium infection is the urinary tract since parasite eggs penetrate the bladder and are excreted in the urine. Hematuria, proteinuria, leukocyturia and symptoms like dysuria or nocturia are the most common clinical presentations. Heavily infected patients show obstructive uropathy of different severity which may lead to renal failure. Intestinal schistosomiasis is caused by Schistosoma mansoni infection. Initial symptoms can be diarrhea and blood-tinged stool. Chronic infection is characterized by fibrotic involvement of the liver and consecutive portal hypertension. The diagnosis of schistosomiasis depends on the demonstration of schistosome eggs in human excreta or biopsy material. Imported cases of schistosomiasis to Europe show an increasing tendency due to expanding international travel. Furthermore imported cases are usually not diagnosed until years after the patients have left an endemic area. The treatment of choice is a single dose of praziquantel 40 m/kg bodyweight resulting in cure rates of around 90% and considerable reversibility of pathological abnormalities due to schistosome infections.

Necator americanus in neonatally infected hamsters. The time-course of infection and antibody response to the surface antigens of L4 and adult worms

Neonatal hamsters were infected with a hamster-adapted strain of Necator americanus, and the time-course of infection was followed by worm and faecal egg counts. Parasite eggs were first recorded during the 6th week of infection, increasing rapidly thereafter to peak in weeks 7-10. Male hamsters excreted more eggs than females, but both sexes were equally susceptible to infection and harboured comparable worm burdens. Faecal egg counts declined from week 10 onwards and this was associated with a loss of worms from animals with heavy infections. Low level infections were stable over the first 114 d of infection but worm fecundity nevertheless still declined over this period. Both hamster sexes responded similarly to surface antigens on adult worms, the antibody levels rising from week 5 onwards to reach a plateau in weeks 6-7, which persisted until the experiments were terminated. The major antigens recognised on the surface of adult worms had molecular masses corresponding to 25 kDa, 32 kDa, a doublet with the heaviest polypeptide resolving at 46 kDa, and a triplet with the heaviest at 67 and 93kDa. In contrast L4 had only 2 major cuticular antigens resolving at 41 and 93kDa. The 93kDa molecule on L4 and adult worms may be antigenically related.

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.

Evidence for predisposition of individual patients to reinfection with Schistosoma mansoni after treatment

Statistical analysis of the relationship between intensities of infection before treatment and during reinfection after treatment in a sample of 119 Kenyan schoolchildren demonstrated a positive association, indicating that the individuals differed consistently in their tendency to become infected. This association was stronger in young children but the trend was detectable in older individuals. Possible reasons for this variation and for its apparently greater influence in younger age groups are discussed.

Predisposition to Trichuris trichiura infection in humans

The study examines the distribution of Trichuris trichiura infection in a village community in St Lucia, West Indies. The infection intensity of the same age-stratified population was assessed (by drug expelled worm burden and faecal egg count) at the initiation of the study, and after 17 months of reinfection following treatment. The frequency distribution of worm numbers per person was similar at both periods of sampling. There was a significant correlation between the initial infection intensity of an individual, and the intensity acquired by the same individual following the 17 month period of reinfection. This relationship was observed in a broad range of host age classes. The study provides firm evidence that individuals are predisposed to heavy (or light) T. trichiura infection.

Population dynamics in echinococcosis and cysticercosis: mathematical model of the life-cycles of Taenia hydatigena and T. ovis

It is shown that under the conditions that prevailed in New Zealand in the late 1950s, Taenia hydatigena was hyperendemic, the life-cycle being regulated by a density-dependent constraint in the form of acquired immunity, and T. ovis was rare. The control measures that caused Echinococcus granulosus, which was endemic at the time, to decline towards extinction reduced T. hydatigena and T. ovis to endemic status only. A non-linear integrodifferential equation model, which was previously linearized to describe the life-cycle of E. granulosus in dogs and sheep in New Zealand, is used to describe the life-cycles of T. hydatigena and T. ovis. The model is then used to compare and contrast the population dynamics of these three species. The model is used to demonstrate that the endemic steady state is structurally unstable, and may be asymptotically unstable to small perturbations. It is also shown that despite the lower infection pressure experienced by the intermediate host in the endemic state, the numbers of larvae in sheep may be higher than in the hyperendemic state. Finally it is shown that the partial success of the control measures against T. hydatigena may have caused an increase in the numbers and prevalence of T. ovis larvae in sheep due to the reciprocal immunity between the two species.

Age-related prevalence and intensity of Trichuris trichiura infection in a St. Lucian community

Age-related changes in the average worm burden and the prevalence of Trichuris trichiura infection, in a village community in St. Lucia, were examined by field studies based on worm expulsion techniques. Horizontal age-intensity profiles were convex in form with peak parasite loads occurring in the 2 to 15-year-old children. Prevalence is shown to be a poor indicator of changes in average worm load with age. Faecal egg counts (epg and epd) provide a qualitative measure of worm burdens since fecundity is shown to be approximately independent of worm load. The parasites were highly aggregated within the study community, with most people harbouring low burdens while a few individuals harboured very heavy burdens. Of the total parasite populations in the study sample, 84% were harboured by the 2 to 15-year-old children. Of those individuals harbouring 100 worms or more, 87% were in the 2 to 10-year-old age range. Crude estimates of population parameters (basic reproductive rate, 4-5; rate of reinfection, 90 year-1) suggest that the rate of reinfection is higher than for other helminth parasites of man. The control of morbidity and parasite transmission is discussed in the context of targeting drug treatment at the child segment of the study population.

The prevalence and intensity of intestinal parasites in two Somalian communities

About 85% of the population of two Somali communities harboured soil-transmitted intestinal nematodes and/or protozoa. The commonest parasite (75% in the Lafoole institution and 59% in the Afgoye institution) was Trichuris trichiura. Mixed infections were common. The source of infection is contaminated fields around dwelling quarters, because of indiscriminate defaecation. One of the factors responsible for the higher incidence of hookworm in Lafoole (45%) compared with Afgoye (1.5%) may be the different soil character of the surrounding fields.

Measurement of incidence of the human liver fluke, Opisthorchis viverrini, in northeast Thailand

A survey of a community in northeast Thailand where Opisthorchis viverrini is endemic permitted comparison of two methods of measuring incidence: direct determination of the rate at which an uninfected group became infected between two surveys a year apart, and estimation of incidence from age-specific prevalence data using a logarithmic regression method. Both methods revealed that incidence increased with age in young children, and estimation from age-prevalence data showed that the increase was roughly linear from near birth to about age 5 years, beyond which no clear trend in incidence was evident. A catalytic infection model incorporating an infection rate increasing from birth to age 5, and remaining constant thereafter, gave an excellent fit to age-prevalence profiles. Both methods of determining incidence are sensitive to errors in diagnosis, but the direct determination method is more sensitive to the presence of false negatives. A method for correcting this bias is given. The regression method, which was less sensitive to yearly variation in incidence and is easier to use, is recommended for preliminary surveys to identify villages with high transmission intensity.

The population dynamics of acquired immunity to helminth infection

Based on experimental evidence we develop and exploit a mathematical model to describe the role of acquired immunity in host-helminth interactions. We show that the model gives a quantitative description of the population dynamics in a laboratory situation involving a host immunological response. We make practical predictions based on these results.

The recognition of antigens on the surface of adult and L4 Necator americanus by human and hamster post-infection sera

The surface antigens of adult Necator americanus were recognized by post-infection hamster sera and resolved at molecular weight 93,000, 67,000, 46,000, 43,000, 32,000 and 25,000. L4 larvae in contrast had one major surface antigen, resolving at 93,000. These antigens were also recognized by a range of human sera, although on a differential basis. This suggests that the human sera tion. However, the results do indicate that the hamster model might be of immunological relevance to the human disease state, in that infected hamster recognized the full cuticular antigen spectrum of adult Necator. This, at least, gives the experimenter a convenient reference point from which to conduct further experiments incorporating parameters such as re-infection, anthelmintic treatment and genetic variability to study the effect of these modifications on the serological response.

Population dynamics in echinococcosis and cysticercosis: mathematical model of the life-cycle of Echinococcus granulosus

A mathematical model of the life-cycle of Echinococcus granulosus in dogs and sheep in New Zealand is constructed and used to discuss previously published experimental and survey data. The model is then used to describe the dynamics of transmission of the parasite, and the means by which it may be destabilized. It is found that under the conditions that prevailed in New Zealand during the late 1950s, at the time of surveys of this parasite, the dog-sheep life-cycle was not regulated by any effective density-dependent constraint. In contrast there was evidence for an effective acquisition of immunity to reinfection by cattle. The long time to maturity of the cyst in sheep, together with the practice of feeding aged sheep to dogs, provides a time delay in the intermediate host. By comparison, the time to maturity of the adult stage in dogs is short, but it is of sufficient magnitude to be a key factor in the destabilization of the system by a regular dog-dosing programme. The model used to describe the life-cycle is a linear integrodifferential equation of the Volterra type. Such equations are intrinsically unstable in that a small perturbation in parameters can drive a previous equilibrium solution to zero. At the time of the surveys, the value of the basic reproductive rate, R0, was close to 1, and it has since been reduced below 1 by control measures.

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.

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.

The population dynamics and epidemiology of intestinal nematode infections

The paper reviews recent studies on the population biology, transmission dynamics and epidemiology of intestinal nematode infections. Particular attention is given to patterns of change in average intensity of infection with age, the possible role of acquired immunity assessed via reinfection studies after chemotherapy, evidence of predisposition to heavy (or light) infection by single and multiple species of parasites and control by mass, selective and targetted chemotherapy.

Prevalence, worm burden and other epidemiological parameters of Ascaris lumbricoides infection in rural communities in Korea

The epidemiological status of ascariasis was analyzed in 8 rural villages in Korea, through observation of its epidemiological parameters such as prevalence, worm burden and basic reproductive rate. Total 978 inhabitants were subjected to stool examination and recovery of worms after chemotherapy with pyrantel pamoate. The results were as follows: The worm positive rate in each village was 16.5~79.5%, while the egg positive rate was 9~18% lower, 3.3~66.7%. The average worm burden(among all inhabitants) ranged from 0.21 to 8.44 by villages and the frequency of cases with each worm burden showed negative binomial distributions with 'k' values of 0.38-0.54. The prevalence rates (worm) in each village was almost identical with the theoretical ones from Anderson and May's equation; p=1-(1+M(*)/k)**(-k), where 'p' is worm prevalence and 'M(*)' equilibrium average worm burden. The basic reproductive rate 'R' was calculated from 1.03 to 2.11. It is suggested that, although 'R' in lower endemic areas is approaching to the breakpoint of reinfection (R=1), control programs of ascariasis in Korea should be continued until it becomes below the level nationwidely.

Vaccination and herd immunity to infectious diseases

An understanding of the relationship between the transmission dynamics of infectious agents and herd immunity provides a template for the design of effective control programmes based on mass immunization. Mathematical models of the spread and persistence of infection provide important insights into the problem of how best to protect the community against disease.

A simple model for the spatial spread and control of rabies

A simple mathematical model for the spatial spread of rabies is presented. It models the dynamics of the front of an epizootic wave. We show how the model can be used to estimate the minimum width (in kilometers) of a break, that is, a region in which a control scheme is employed in order to stop the spatial progression of the rabies wave front. A simple expression is derived for the surviving fox population, after the passage of the epizootic, in terms of measurable parameters of the model.

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.

New methods of drug application for control of helminths

Several new ways have been developed of delivering anthelmintics to ruminants aimed particularly at reducing labour. For single doses, these include a semi-automatic rumen injector for giving insoluble drugs to cattle more conveniently and efficiently than by oral dosing, and the dermal application of levamisole which is, however, subject to seasonal variation in absorption. Sustained administration offers potentially a high level of preventive control but carries a greater risk of developing drug resistance. Even with the best available methods of group administration in feed supplements or drinking water, there remains some uncontrollable variation in individual intake. Intraruminal sustained release devices largely overcome this problem and constitute the most important new technology. They are represented at present by the commercially successful morantel sustained release bolus, and the more versatile Laby capsule which is under development for anthelmintic delivery. Other new applications include the possible development of synergists, potentiators and drug combinations, the special features of drugs which bind strongly to plasma proteins, and the new possibilities offered by a drug highly effective against all stages of Fasciola hepatica. In the general approach to anthelmintic application in helminth control, there have been advances in knowledge of helminth population biology which can lead to better timing of strategic dosing programs, which show that single treatments can have persistent benefits without a simultaneous reduction in infection rate, and which question the conventional view that control schemes require the treatment of all animals in the group.

Predisposition to hookworm infection in humans

Frequency distributions of parasitic helminths within human communities are invariably highly aggregated, the majority of worms occurring in relatively small fractions of the host populations. It has been suggested that the heavily infected individuals are predisposed to this state, not by chance, but by as yet undefined genetic, ecological, behavioral, or social factors. Analyses of individual post-treatment patterns of hookworm reinfection among 112 villagers in an endemic area of West Bengal provide quantitative evidence of predisposition to heavy infection. This observation has implications for the design of control programs based on chemotherapy because of the potential economic advantage of selective or targeted treatment as opposed to mass or blanket treatment.

Herd immunity to helminth infection and implications for parasite control

Despite much research on immunological responses to helminth parasites, knowledge of the dynamic interplay between levels of herd immunity in humans and the rates of exposure, establishment and mortality of parasites remains limited. We describe here a simple mathematical model for the population dynamics of helminth infections which mirrors the development of a degree of acquired immunity within populations which are genetically heterogeneous with respect to immunological responsiveness. We interpret observed patterns in the age-specific intensity of infection and attempt to understand the possible effects of control measures based on chemotherapy and vaccination. Mass chemotherapy can, in some circumstances, reduce the level of herd immunity such that average worm burdens in the adult age classes rise above their precontrol levels. When certain individuals or groups are predisposed to heavy infection, selective or targeted drug treatment can have significantly greater impact than mass or random application. Conversely, model predictions suggest that effective parasite control by vaccination (if and when vaccines become available) is difficult to achieve in communities that are genetically heterogeneous in their ability to mount protective responses to infection.

Community control of helminth infections of man by mass and selective chemotherapy

The design of mass and targetted community-based chemotherapy programmes for the control of the major helminth infections of man is discussed in relation to the population and transmission dynamics of the parasites. Rapid reinfection following a single mass or targetted anthelmintic application is shown to be a universal feature of helminth transmission, as a consequence of the regulatory or feed-back mechanisms controlling population abundance within both individuals and the community as a whole. Control of reinfection requires repeated community treatment where the intensity of application and the interval between treatments are dependent on the reproductive life-expectancy of the adult worm, the net force of transmission prior to control (the basic reproductive rate) and the factors which create aggregation in the distribution of parasite loads within the population. Selective or targetted treatment is shown to be most effective for the control of morbidity as opposed to the control of transmission. The impact of targetted treatment depends critically on the factors that generate heterogeneity in parasite burdens and on whether or not selectivity is based on a single or repeated identification of the 'wormy' fraction of the community. Monte Carlo simulation studies are employed to assess the likely impacts of different control strategies on average parasite abundance/person and the distributions of parasite loads within populations. Future epidemiological research needs are discussed in relation to theoretical work and recent field studies of predisposition to heavy infection.

Rate of expulsion of Trichuris trichiura with multiple and single dose regimens of albendazole

The efficacy of multiple and single dose regimens of albendazole on Trichuris trichiura infection was evaluated by counting the number of worms expelled/day from two pair-matched groups of children, for nine days following therapy. The temporal patterns of worm expulsion were similar whether the children received a single 400 mg dose or two consecutive doses: no worms were passed before the second day, or after the sixth day, after intervention, and the maximum worm expulsion rate was attained on the fourth day. A second treatment six days after the first expelled no more worms. The results obtained here resemble those obtained previously with a three-day (600 mg) regimen of mebendazole in a study of heavily infected children. We conclude: that irrespective of dose, benzimidazole carbamates require the gut transit time plus 48 hours to immobilize T. trichiura; and that a single dose of albendazole is effective against light infections of T. trichiura but requires further evaluation with high intensity infections.

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.

Population distribution of Trichuris trichiura in a community of Jamaican children

The Trichuris trichiura worm burdens of 23 children living in a Place-of-Safety in Kingston, Jamaica, were assessed by stool collection for more than five days after treatment with mebendazole. This procedure was repeated after a seven-month period of natural re-infection. For both collections the maximum rate of worm expulsion was achieved on the fourth day after starting treatment. The worm population distributions were overdispersed and well described by the negative binomial probability model (k = 0.29) in each case. For any one individual, the number of worms passed on the first expulsion was unrelated, absolutely or relatively, to the number passed on the second. These data suggest that: knowledge of the time dependency of helminth expulsion is essential for the accurate estimation of worm burdens by this method; populations of Trichuris are more highly aggregated than those of Ascaris and may thus be more susceptible to control by selective rather than random chemotherapy; and the inherent predisposition of hosts to infection may be of minor importance in determining the distribution of worms in the population-heavily infected hosts appear no more or less likely to acquire large worm burdens on subsequent exposures.

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.

The quasispecies (extremely heterogeneous) nature of viral RNA genome populations: biological relevance--a review

We review evidence that cloned (or uncloned) populations of most RNA viruses do not consist of a single genome species of defined sequence, but rather of heterogeneous mixtures of related genomes (quasispecies). Due to very high mutation rates, genomes of a quasispecies virus population share a consensus sequence but differ from each other and from the consensus sequence by one, several, or many mutations. Viral genome analyses by sequencing, fingerprinting, cDNA cloning etc. indicate that most viral RNA populations (quasispecies) contain all possible single and double genomic site mutations and varying proportions of triple, quadruple, etc. site mutations. This quasispecies structure of RNA virus populations has many important theoretical and practical implications because mutations at only one or a few sites may alter the phenotype of an RNA virus.

Population dynamics and chemotherapeutic control of Trichuris trichiura infection of children in Jamaica and St. Lucia

Population dynamical parameters of Trichuris trichiura infections in children were estimated from longitudinal intensity and prevalence data from a population (n = 23) in a children's home in Jamaica. The theoretical predictions of a deterministic model incorporating these parameters were approximated to observed horizontal-age prevalence data from a naturally infected population (n = 203) of children in a St. Lucian village, and a rough estimate of the basic reproductive rate (Ro = 8-10) of T. trichiura obtained. The findings suggest that T. trichiura populations are intrinsically more difficult to control by traditional mass-treatment chemotherapy (eradication requires greater than 91% of the population to be treated every 6 months for greater than 5 years) than are populations of Ascaris, but may be more susceptible to selective chemotherapy programmes which aim to treat only the most heavily infected individuals.

Therapy of parenchymal brain cysticercosis with praziquantel

Twenty-six patients with cysticercosis of the brain parenchyma were treated with the antihelmintic agent praziquantel (50 mg per kilogram of body weight daily for 15 days). During treatment a strong inflammatory reaction occurred, as evidenced by increased protein and cells in the cerebrospinal fluid. This finding correlated with headache, exacerbation of neurologic symptoms, and edema and inflammation around cystic lesions. After three months of treatment all patients had improved clinically, and 13 (50 per cent) were asymptomatic. The total number of cysts on CT scans had decreased from 152 at the beginning of treatment to 51, and the mean diameter of cysts was reduced by 72 per cent. CT scans showed improvement in 25 of the 26 patients, with total remission of all cysts in nine. Seventeen control patients followed with CT studies for a mean of 9 +/- 2 months had no spontaneous remission of lesions, and in many cases the scans showed worsening during the observation period. Our results indicate that praziquantel is effective in cysticercosis of the brain parenchyma.

Epidemiology and transmission dynamics of Ascaris lumbricoides in Okpo village, rural Burma

A horizontal study of the prevalence of Ascaris infection was carried out on the total population of Okpo village near Rangoon, Burma, where a similar study had been conducted 13 years earlier. Ascaris eggs in faeces were counted after treatment with levamisole on a random sample of 50% of the infected population to give information to the numbers of epg of stool, the mean worm burden per host and the distribution of worms in the community. The information on prevalence is compared with that of the previous survey. Various population parameters of Ascaris were calculated to estimate the transmission dynamics of A. lumbricoides. In addition, chemotherapeutic regimes, assessed on the proportion of the human population to be treated and the time interval between treatments, are proposed to reduce transmission below a critical threshold. The findings are compared with those of other studies and the probable mode of occurrence and maintenance of Ascaris infection in Okpo village are discussed.

Selected economic issues in helminth control

This paper examines four main economic issues in the formulation of helminth control policies: whether, what, how and with whose resources to control helminthiasis. The paper argues that (i) although helminth control would have a negligible impact on mortality, its nutrition-mediated effects on improved labor productivity and intellectual performance may be significant; (ii) that reduction of helminth disease rather than infection should be the target of control policy, although the preferred intervention may still be eradication rather than continuous control; and (iii) that although the case for public subsidy of helminth control interventions is strong, the existence of serious fiscal constraints and some evidence of private willingness-to-pay for anthelminthic chemotherapy indicates a potential for partial cost recovery which should be explored.