D. W. T. Crompton

Ascaris and ascariasis.

In recent years much new information has been obtained about the epidemiology, population biology and public health significance of infections of Ascaris lumbricoides in humans. Results from experimental infections of A. suum in pigs have helped to elucidate the observations made in the community on human ascariasis. The main purpose of the review is to see how new information may contribute to further acceptance of ascariasis as a serious contributor to ill-health and so to the design and implementation of sustainable control programmes intended to reduce the morbidity due to infection with A. lumbricoides. Eradication is neither a realistic nor prudent aim given the current shortage of appropriate sanitation in many countries where ascariasis is endemic. A substantial body of evidence shows that for the four common species of soil-transmitted nematode, including A. lumbricoides, regular administration of broad-spectrum anthelminthic drugs to children attending primary schools is a cost-effective means of controlling the infections. Anthelminthic drugs must be of proven quality and efficacy and health professionals should be prepared to detect and manage drug resistance should that emerge. Despite a deeper understanding of the immune response of a variety of hosts to infections with either A. lumbricoides or A. suum there is at present little prospect of an effective vaccine against ascariasis. The relationship between A. lumbricoides and A. suum is addressed, particularly since both species, if they are indeed separate species, occur in people and their pigs in many communities.

Control strategies for human intestinal nematode infections.

In recent years significant progress has been made in understanding the ecology, epidemiology and related morbidity and development of new tools for the control of soil-transmitted helminths. Such knowledge has recognized the impact of helminth infections on the health of infected groups and has created a rational basis for their control. Schoolchildren harbour some of the most intense helminthic infections, which produce adverse effects on health, growth and scholastic performance. However, although great effort has been put into targeting school-age children, women of child-bearing age and pre-school children are two other groups at high risk of morbidity due to intestinal nematode infections. Highly effective and safety-tested, single-dose anthelminthic drugs are now available, permitting periodical deworming of schoolchildren and other high-risk groups at affordable prices. Four anthelminthics against all intestinal nematodes are included in the WHO Essential Drug List (albendazole, levamisole, mebendazole and pyrantel). Recently ivermectin has also been registered for use against Strongyloides stercoralis in humans. Several well-monitored country experiences have shown that chemotherapy-based control of morbidity due to soil-transmitted helminths is possible and highly cost-effective.

The epidemiology of Ascaris lumbricoides and other soil-transmitted helminths in primary school children from Ile-Ife, Nigeria

An epidemiological survey of intestinal helminthiases was conducted on 766 primary school children aged 5-16 years from Ile-Ife, Nigeria. On the basis of stool examinations, the prevalence of Ascaris lumbricoides, Trichuris trichiura, hookworm and Strongyloides stercoralis was 88.5, 84.5, 33.1 and 3% respectively. Intensity of infection was measured indirectly by egg counts for each species of helminth and also by counting worms passed after chemotherapy in the case of A. lumbricoides. The influence of host age and sex on infection levels was assessed. Relationships between the intensities of A. lumbricoides, T. trichiura and hookworm in individual children were identified. After anthelmintic treatment with levamisole, the frequency distribution of A. lumbricoides per host and the relationship between parasite fecundity and worm burden were investigated. Reinfection patterns of A. lumbricoides were assessed at two 6-monthly intervals and even within the narrow age range described, differences were found. In addition, evidence was obtained for predisposition of individuals to heavy or light infection with A. lumbricoides.

Intervention for the Control of Soil-Transmitted Helminthiasis in the Community

The global strategy for the control of soil-transmitted helminthiasis, based on regular anthelminthic treatment, health education and improved sanitation standards, is reviewed. The reasons for the development of a control strategy based on population intervention rather than on individual treatment are explained. The evidence and experience from control programmes that created the basis for (i) the definition of the intervention package, (ii) the identification of the groups at risk, (iii) the standardization of the community diagnosis and (iv) the selection of the appropriate intervention for each category in the community are discussed. How to best deliver the appropriate intervention, the impact of the control measures on morbidity and on indicators such as school attendance, cognitive development and productivity are presented. The factors influencing the cost-benefits of helminth control are also considered. The recent progress on the control of soil-transmitted helminth infections is illustrated. Research needs are analysed in relation to the most recent perceptions from private-public partnerships involved in helminth control. The way forward for the control of soil-transmitted helminth infections is described as a multi-disease approach that goes beyond deworming and fosters a pro-poor strategy that supports the aims of the Millennium Development Goals.

Haemoglobin concentrations and concomitant infections of hookworm and Trichuris trichiura in Panamanian primary schoolchildren

The blood haemoglobin concentrations and intestinal helminth infections were surveyed in 658 children aged between 6 and 12 years attending 4 primary schools in Coclé Province, Panama, in November 1987. 147 (22.3%) of the children were judged to have haemoglobin concentrations indicative of iron-deficiency anaemia. Blood haemoglobin concentrations were significantly lower in children with heavier Trichuris trichiura infections (> 5000 eggs/g) (P = 0.014), and in children with dual infections of both hookworm and T. trichiura (P = 0.005). Children with concomitant T. trichiura and hookworm infections were also significantly more likely to have blood haemoglobin levels indicative of anaemia than children who were uninfected or had single infections with either of these helminths (P < 0.005). In a longitudinal study involving 171 children selected from the original 658, blood haemoglobin concentrations were measured again in November 1988. Children who were considered to show an improvement in both hookworm and T. trichiura infections over this 12 month period showed a significantly greater increase in blood haemoglobin concentrations than children who had remained uninfected with either of these helminths throughout the study period (P < 0.05).

Hookworm infections and human iron metabolism

Ancylostoma duodenale and Necator americanus are extremely common species of soil-transmitted helminth which flourish where poverty and malnutrition prevail. Hookworms contribute significantly to iron-deficiency anaemia, which remains one of the worlds major nutritional problems, through the feeding activities of intestinal stages leading to chronic blood loss into the gut. In this article, a mathematical model is proposed to explain how human iron metabolism may respond to hookworm infection of varying intensity. The model draws attention to the importance of the regulation of stored iron levels in the process. The results from the model are presented for the effects of hookworm infection on the iron metabolism of a healthy adult male. Calculations are also presented in which the effects of hookworms on the iron metabolism of a non-pregnant woman are compared with those of a pregnant woman. Use of the model may help develop a better understanding of the pathology of hookworm disease.

Intestinal helminthiases in relation to the socioeconomic environment of Panamanian children.

A cross-sectional investigation was made into ascariasis and nutritional status in Panamanian preschool children from October 1983 to July 1984. Within this framework, an analysis was undertaken of possible relationships between a range of intestinal helminthiases and the socioeconomic status of the participants. Attention was paid to caregivers education, parental occupation, family earnings, quality of housing and sanitation in the assessment of socioeconomic status. On this basis, strong associations were established between the socioeconomic status of the children and infection with Ascaris lumbricoides, Trichuris trichiura and hookworm. In general, the prevalence of single and multiple helminth infections was significantly higher in children living in housing made of wood or bamboo than in those living in housing made of concrete blocks. The same pattern applied to levels of sanitation. Ascaris lumbricoides occurred more frequently in children of mothers with the least formal education and in children living in relatively crowded conditions. Evidence was also obtained to indicate that the intensity of the intestinal helminth infections was greater in the children from the poorer environment. Since children from poorer socioeconomic conditions might be more exposed to infective stages than those from a better environment, the possible role of socioeconomic factors in contributing to the predisposition of some individuals to harbour large worm burdens of intestinal helminths was briefly discussed.

Community control of Ascaris lumbricoides in rural Oyo State, Nigeria: mass, targeted and selective treatment with levamisole

A study to compare effects of mass, targeted and selective chemotherapy with levamisole (Ketrax) as an action for the control of Ascaris lumbricoides was carried out in three communities in rural Oyo State, Nigeria. Selective treatment was applied in one village by treating the most heavily infected 20% of the inhabitants, targeted treatment in the second village involved children aged 2-15 years, while mass treatment was offered to all inhabitants excluding infants under 1 year and pregnant women in the third village. Recommended doses of levamisole were given in the villages, as described above, at 3-monthly intervals during a period of 1 year. Prevalence and intensity (epg) of A. lumbricoides infection were determined immediately before and 3 months after the period of intervention using a modified Kato-Katz technique. In the selective treatment village, no significant differences were found between the pre- and post-treatment egg counts (mean (+/- S.D.) epg 6776 +/- 10,791 versus 4259 +/- 10,909 respectively) of A. lumbricoides in the total population. In the targeted treatment village, significant differences were recorded in pre- and post-treatment egg counts for the total population (9057 +/- 15,797 versus 2579 +/- 6529) among the children alone (10,935 +/- 20,094 versus 992 +/- 3175) and among the untreated adults (7742 +/- 9782 versus 4561 +/- 8798). In the mass treatment village, significant differences in pre- and post-treatment egg count values were also recorded (11,907 +/- 17,220 versus 1489 +/- 5165). The intensity of Trichuris trichiura and hookworm infections among the villagers before and after intervention were not observed to have changed significantly regardless of selective, targeted or mass treatment.

Changes in metabolism and behaviour of the freshwater copepod Cyclops strenuus abyssorum infected with Diphyllobothrium spp

In a population of copepods (Cyclops strenuus abyssorum), with a naturally high prevalence of infection with procercoids of Diphyllobothrium spp., no difference in body size was found between infected and uninfected hosts. However, a significant reduction in the reproductive capacity of infected females was observed, 87% of uninfected females having developed eggs in their gonads and sacs compared with 21% of infected females. The feeding rate of infected copepods was relatively high soon after infection occurred, but gradually decreased to less than half that of uninfected animals. Respiration rate was also lower in infected copepods. Infected copepods showed reduced motility and impaired escape responses, which is likely to make them more susceptible to predation.

Nutritional aspects of infection

Current knowledge is examined about the means whereby ascariasis, hookworm disease, strongyloidiasis and trichuriasis may contribute to the aetiology of human malnutrition. Results from experiments with related parasites in the laboratory have demonstrated the role of gastrointestinal helminthiases in animal malnutrition. Some evidence shows that in children, infection with the intestinal stages of Ascaris lumbricoides is associated with reduced growth rate, disturbed nitrogen balance, malabsorption of vitamin A, abnormal fat digestion, lactose maldigestion and an increased intestinal transit time. The main impact of hookworm infection is its relationship with iron-deficiency anaemia which may have effects at the community level as regards work and productivity in adults and learning and school performance in children. More research is needed to extend knowledge of the nutritional impact of ascariasis and hookworm disease in order to establish their public health significance. Research is needed also to identify the range of nutritional effects on man that occur as a result of trichuriasis and strongyloidiasis. The significance of less prevalent and more localized gastrointestinal helminthiases should not be ignored.