Necator americanus: population structure, distribution, and fluctuations in population densities of infective larvae in contaminated farmlands

Edaphic and climatic factors influence on the distribution of soil transmitted helminths in Kogi East, Nigeria

The need for a reliable risk map in the control of soil-transmitted helminths (STHs) in Kogi East, North Central Nigeria is very important. This study was carried out to determine the effect of environmental risk factors on geospatial distribution of STHs. Epidemiological data were obtained from a district-wide survey conducted in 2018 in Kogi East. Edaphic and climatic factors were downloaded as spatial layers from international recognised health data resources centres. A total of 24 environmental factors were used in determining the risk map of STHs using MaxEnt tool. The predicted high-risk areas of A. lumbricoides, hookworms and S. stercoralis were the central part of Kogi East covering parts of Dekina, Ofu, Igalamela-Odolu, Olamaboro and Omala LGAs with probability of 0.8 to 1.00. Among the factors investigated; Temperature [mean diurnal temperature range (BIO2), temperature annual range (BIO7) and maximum temperature of the warmest month (BIO5)], precipitation [precipitation of the wettest quarter (BIO16)], and soil clay contents were the five factors that exerted most significant influence on the geospatial distribution of STHs in Kogi East, Nigeria. Public health control programmes on STHs should target high-risk areas by including them in mass drug administration, health education as well as provision of water, sanitation and hygiene infrastructures.

Infectious helminth ova in wastewater and sludge: A review on public health issues and current quantification practices

Raw and partially treated wastewater has been widely used to maintain the global water demand. Presence of viable helminth ova and larvae in the wastewater raised significant public health concern especially when used for agriculture and aquaculture. Depending on the prevalence of helminth infections in communities, up to 1.0 × 10³ ova/larvae can be presented per litre of wastewater and 4 gm (dry weight) of sludge. Multi-barrier approaches including pathogen reduction, risk assessment, and exposure reduction have been suggested by health regulators to minimise the potential health risk. However, with a lack of a sensitive and specific method for the quantitative detection of viable helminth ova from wastewater, an accurate health risk assessment is difficult to achieve. As a result, helminth infections are difficult to control from the communities despite two decades of global effort (mass drug administration). Molecular methods can be more sensitive and specific than currently adapted culture-based and vital stain methods. The molecular methods, however, required more and thorough investigation for its ability with accurate quantification of viable helminth ova/larvae from wastewater and sludge samples. Understanding different cell stages and corresponding gene copy numbers is pivotal for accurate quantification of helminth ova/larvae in wastewater samples. Identifying specific genetic markers including protein, lipid, and metabolites using multiomics approach could be utilized for cheap, rapid, sensitive, specific and point of care detection tools for helminth ova and larva in the wastewater.

Methods for Quantification of Soil-Transmitted Helminths in Environmental Media: Current Techniques and Recent Advances

Limiting the environmental transmission of soil-transmitted helminths (STHs), which infect 1.5 billion people worldwide, will require sensitive, reliable, and cost-effective methods to detect and quantify STHs in the environment. We review the state-of-the-art of STH quantification in soil, biosolids, water, produce, and vegetation with regard to four major methodological issues: environmental sampling; recovery of STHs from environmental matrices; quantification of recovered STHs; and viability assessment of STH ova. We conclude that methods for sampling and recovering STHs require substantial advances to provide reliable measurements for STH control. Recent innovations in the use of automated image identification and developments in molecular genetic assays offer considerable promise for improving quantification and viability assessment.

Influence of physico-chemistry and mineralogy on the occurrence of geohelminths in geophagic soils from selected communities in the Eastern Cape, South Africa, and their possible implication on human health

Geophagic soils from selected communities in Eastern Cape, South Africa were characterised to determine their properties and geohelminth content. The soils were coarse-textured with cation exchange capacity values ranging from 6.35 to 18.94 cmol (+)/kg. Quartz was the dominant mineral in the samples with SiO2, Al2O3, and Fe2O3(t) having the highest concentrations among major element oxides. The soil properties, mineralogical composition, and low amounts of particle binding substances may favour the survival of geohelminth ova in the soils. Seven of the samples contained at least one of the following geohelminths: Ascaris lumbricoides, Trichuris trichiura, Necator americanus, Ancylostoma duodenale and Strongyloides stercoralis. The presence of these geohelminths in the soils was attributed to agricultural and sanitary practices inherent in the communities and the soil properties. Communities need to be sensitised on the importance of safe sanitary and animal husbandry practices to reduce the prevalence of helminth infection among geophagists.

The mucosal response of hamsters exposed to weekly repeated infections with the hookworm Ancylostoma ceylanicum

An experiment was carried out to assess mucosal changes in hamsters exposed to weekly repeated low-intensity infections with the hookworm Ancylostoma ceylanicum. The experiment included control groups of naïve, uninfected hamsters and groups that received a single-pulse primary infection. Changes in the intestinal architecture and in the density of inflammatory cells in the mucosa, including mast cells, goblet cells, Paneth cells and eosinophils were examined in relation to changes in hookworm burdens. As in the single-pulse primary infection, hamsters exposed to repeated infections responded with marked changes in the intestinal architecture and in mucosal populations of inflammatory cells. However, there were distinct differences in the kinetics of the responses to these two types of infection (primary single-pulse and repeated). The reduction in villous height and the increase in crypt depth in animals exposed to repeated infections were both initially slower but eventually equalled and exceeded the responses in hamsters given a chronic primary infection, despite the presence of fewer adult worms in the former. Similarly, changes in the mitotic figures of epithelial cells in the mucosa and the mast cell response were both initially slower and less intense in repeatedly infected hamsters, but eventually exceeded the response to primary infection. Furthermore, the eosinophil response was found to be initially greater in repeated infections and overall more persistent. In contrast, both goblet and Paneth cell responses were less marked in repeatedly infected animals compared to those carrying a primary infection. These results are discussed in the context of host protective resistance to infection with A. ceylanicum.

Epidemiology and control of human gastrointestinal parasites in children

Parasites found in the human gastrointestinal tract can be largely categorized into two groups, protozoa and helminths. The soil-transmitted helminths (Ascaris lumbricoides, hookworm and Trichuris trichiura) are the most prevalent, infecting an estimated one-sixth of the global population. Infection rates are highest in children living in sub-Saharan Africa, followed by Asia and then Latin America and the Caribbean. The current momentum towards global drug delivery for their control is at a historical high through the efforts of numerous initiatives increasingly acting in coordination with donors, governments and local communities. Together, they have delivered enormous quantities of drugs, especially anthelmintics to children through nationwide annual or biannual mass drug administration largely coordinated through schools. However, a much larger and rapidly growing childhood population in these regions remains untreated and suffering from more than one parasite. Mass drug administration has profound potential for control but is not without considerable challenges and concerns. A principal barrier is funding. Stimulating a research and development pipeline, supporting the necessary clinical trials to refine treatment, in addition to procuring and deploying drugs (and sustaining these supply chains), requires substantial funding and resources that do not presently exist. Limited options for chemotherapy raise concerns about drug resistance developing through overuse, however, satisfactory pharmaco-epidemiology and monitoring for drug resistance requires more developed health infrastructures than are generally available. Further, the limited pharmacopeia does not include any effective second-line options if resistance emerges, and the research and development pipeline is severely depressed. Herein, we discuss the major gastrointestinal protozoa and helminths reviewing their impact on child health, changing epidemiology and how this relates to their control.

Global epidemiology, ecology and control of soil-transmitted helminth infections

Soil-transmitted helminth (STH) infections are among the most prevalent of chronic human infections worldwide. Based on the demonstrable impact on child development, there is a global commitment to finance and implement control strategies with a focus on school-based chemotherapy programmes. The major obstacle to the implementation of cost-effective control is the lack of accurate descriptions of the geographical distribution of infection. In recent years, considerable progress has been made in the use of geographical information systems (GIS) and remote sensing (RS) to better understand helminth ecology and epidemiology, and to develop low-cost ways to identify target populations for treatment. This review explores how this information has been used practically to guide large-scale control programmes. The use of satellite-derived environmental data has yielded new insights into the ecology of infection at a geographical scale that has proven impossible to address using more traditional approaches, and has in turn allowed spatial distributions of infection prevalence to be predicted robustly by statistical approaches. GIS/RS have increasingly been used in the context of large-scale helminth control programmes, including not only STH infections but also those focusing on schistosomiasis, filariasis and onchocerciasis. The experience indicates that GIS/RS provides a cost-effective approach to designing and monitoring programmes at realistic scales. Importantly, the use of this approach has begun to transition from being a specialist approach of international vertical programmes to becoming a routine tool in developing public sector control programmes. GIS/RS is used here to describe the global distribution of STH infections and to estimate the number of infections in school-age children in sub-Saharan Africa (89.9 million) and the annual cost of providing a single anthelmintic treatment using a school-based approach (US$5.0-7.6 million). These are the first estimates at a continental scale to explicitly include the fine spatial distribution of infection prevalence and population, and suggest that traditional methods have overestimated the situation. The results suggest that continent-wide control of parasites is, from a financial perspective, an attainable goal.

Intestinal helminthiases among rural and urban schoolchildren in south-western Nigeria

The prevalences and intensities of intestinal helminthic infection among primary schoolchildren were determined in rural and urban communities of the Ife Central local government area, in south-western Nigeria. After the schools in the study area had been stratified into urban and rural, a random sample from each stratum was selected for further study. In each study school, a single fresh stool specimen was obtained from each pupil in randomly selected classes and checked, using a quantitative method (Stoll's dilution egg-count technique), for helminth eggs and larvae. Overall, 366 rural and 383 urban schoolchildren were investigated and 30.0% of them (36.1% of the rural and 24.3% of the urban; P<0.001) were found to be harbouring at least one species of intestinal helminth. The mean intensities of infection, in terms of excreted eggs/g faeces of those infected, were 2371.4 for Ascaris, 1070.6 for hookworm, and 500 for Trichuris. Although the mean intensities of Ascaris infection were significantly lower among the infected rural children than among the infected urban (2025.7 v. 2791.4; P=0.014), the corresponding difference in the hookworm infections did not reach statistical significance (1458 v. 666.7; P=0.063). The means of the log-transformed egg outputs, in eggs/g faeces of those infected, also showed similar trends: a significant difference between the rural and urban Ascaris infections (3.1877 v. 3.3340; P=0.002) but not between the rural and urban hookworm infections (2.9667 v. 2.8027; P=0.453). Intestinal helminthiases still clearly represent a common childhood health problem in the study area, particularly in the rural communities.

Human hookworm infection in the 21st century

The scientific study of human hookworm infection began at the dawn of the twentieth century. In recent years, there have been dramatic improvements in our understanding of many aspects of this globally widespread parasite. This chapter reviews recent advances in our understanding in the biology, immunology, epidemiology, public health significance and control of hookworm, and to look forward to the study of this important parasite in the 21st century. Advances in molecular biology has lead to the identification of a variety of new molecules from hookworms, which have importance either in the molecular pathogenesis of hookworm infection or in the host-parasite relationship; some are also promising vaccine targets. At present, relatively little is known about the immune responses to hookworm infection, although it has recently been speculated that hookworm and other helminths may modulate specific immune responses to other pathogens and vaccines. Our epidemiological understanding of hookworm has improved through the development of mathematical models of transmission dynamics, which coupled with decades of field research across multiple epidemiological settings, have shown that certain population characteristics can now be recognised as common to the epidemiology, population biology and control of hookworm and other helminth species. Recent recognition of the subtle, but significant, impact of hookworm on health and education, together with the simplicity, safety, low cost and efficacy of chemotherapy has spurred international efforts to control the morbidity due to infection. Large-scale treatment programmes are currently underway, ideally supported by health education and integrated with the provision of improved water and sanitation. There are also on-going efforts to develop novel anthelmintic drugs and anti-hookworm vaccines.

The potential of geographical information systems and remote sensing in the epidemiology and control of human helminth infections

Geographic information systems (GIS) and remote sensing (RS) technologies are being used increasingly to study the spatial and temporal patterns of infectious diseases. For helminth infections, however, such applications have only recently begun despite the recognition that infection distribution patterns in endemic areas may have profound effects on parasite population dynamics and therefore the design and implementation of successful control programmes. Here, we review the early applications of these technologies to the major human helminths (geohelminths, schistosomes and the major lymphatic filarial worms), which demonstrate the potential of these tools to serve as: (1) an effective data capture, mapping and analysis tool for the development of helminth atlases; (2) an environment for modeling the spatial distribution of infection in relation to RS and environmental variables, hence furthering the understanding of the impact of density-independent factors in underlying observed parasite spatial distributions and their effective prediction; and (3) a focal tool in parasite control programming given their abilities to (i) better define endemic areas, (ii) provide more precise estimates of populations-at-risk, (iii) map their distribution in relation to health facilities and (iv) by facilitating the stratification of areas by infection risk probabilities, to aid in the design of optimal drug or health measure delivery systems. These applications suggest a successful role for GIS/RS applications in investigating the spatial epidemiology of the major human helminths. It is evident that further work addressing a range of critical issues include problems of data quality, the need for a better understanding of the population biological impact of environmental factors on critical stages of the parasite life-cycle, the impacts and consequences of spatial scale on these relationships, and the development and use of appropriate spatially-explicit statistical and modeling techniques in data analysis, is required if the true potential of this tool to helminthology is to be fully realized.

The epidemiology of human hookworm infections in the southern region of Mali

Two surveys of hookworm (Necator americanus) infections, conducted three years apart (December 1994 and January 1998) in a village in the Sikasso region of Mali, revealed that overall prevalence of infection was 68.7% and 53%, respectively. In both years there was a highly significant difference between the sexes in the prevalence and abundance of infection, with male subjects carrying heavier infections than females. Both prevalence and abundance of infection increased with age, although in 1998 there was a strong interaction between sex and age, arising from the declining egg counts among 16-20-year-old females and the continuing increase among males, reinforced by the subsequent reduction among the older males (> or = 61 years) and concomitant increase among females. After controlling for the effects of age, sex and their interaction, a highly significant positive relationship was detected between faecal egg counts of individuals who were examined in both 1994 and 1998 (n = 134), indicating predisposition to infection. This relationship remained significant in each of 4 age classes spanning 7-79 years. The members of some family compounds were shown to carry heavier infections than expected whilst others were less infected, suggesting compound-related clustering of hookworm infections. The use of footwear increased with age but there was no significant relationship between the extent of use of footwear and the abundance of hookworm infection. Eyesight deteriorated with age and impaired vision was particularly prominent among the older sectors of the community, a legacy from the time when onchocerciasis was widely prevalent in the region. Although men with partially damaged eyes carried lower infections than expected for their age, no overall significant relationship was found between quality of vision and hookworm infections. These results are discussed in relation to hookworm epidemiology in general and in Mali in particular.

The distribution of common intestinal nematodes along an altitudinal transect in KwaZulu-Natal, South Africa

In order to study the effect of climate and topography on the distribution of common, intestinal nematodes in schoolchildren, changes in prevalence were investigated along an altitudinal transect from approximately 50 m above sea level (asl), near the coast, to approximately 1700 m asl, in the foothills of the Drakensberg Mountains, KwaZulu-Natal, South Africa. These changes were related to several permutations of temperature, rainfall and evaporation, using univariate and multiple regression analyses. A total of 693 primary schoolchildren aged between 7 and 15 years was examined from six communities along the 150-km transect. Changes in the prevalence of Trichuris trichiura were significantly correlated with temperature-derived variables whereas those of Ascaris lumbricoides and Necator americanus were correlated with rainfall-derived variables. A total of 17 parasite species was recovered along the transect but polyparasitism was negatively correlated with altitude.

The human environment, occupation, and possible water-borne transmission of the human hookworm, Necator americanus, in endemic coastal communities of the Niger Delta, Nigeria

A 15-month longitudinal study was carried out in four riverine communities of the Niger Delta to ascertain the impact of the environment and occupation on the transmission and infection pattern of the human hookworm Necator americanus. A total of 925 (50.10%) individuals were found to be infected. Male and female infection rates were 54.26% and 47.96% respectively. The difference was statistically significant (P less than 0.05). The prevalence of infection, the egg counts, and the seasonal incidence were related to the occupations and mode of transmission. The highest occupation prevalence of 60.92% occurred among the fishermen through water-borne transmission, while prevalence rates of 52.96% and 52.4% were recorded respectively through soil transmission. The difference between fishermen's prevalence and farmers' prevalence was statistically significant (P less than 0.01). Farmers' egg counts varied from 1,500 eggs per gram (epg) in the dry season to 7,500 epg in the rainy season. Fishermen's egg counts ranged from 3,000 epg in the rainy season to 8,000 epg in the dry season. The difference was not significant (P greater than 0.05). Among farmers the highest monthly incidence of 25% occurred in June, while the highest incidence of 35% occurred in fishermen in April. Incidence of infection among fishermen remained significantly higher (P less than 0.05) than in other occupation groups during the dry season months. Water-borne transmission of hookworm probably increases the incidence and egg counts among fishermen during the dry season.

Experimental and field studies on the viability of eggs, and the infectivity of third-stage larvae of the human hookworm Necator americanus recovered from the creek waters of the Niger Delta, Nigeria, and their epidemiological significance

In a combined field and laboratory study, the eggs and 3rd-stage infective larvae of the human hookworm Necator americanus were recovered from the creek waters of the Niger Delta, Nigeria. They were tested for viability and infectivity. Eggs recovered from fresh water creeks showed a viability or hatching index of between 17.5 and 23.7%. Eggs recovered from brackish water showed a viability index of 18.8-21.4% in 'normal' cultures, and a maximum index of 6.3% when cultured in brackish water. The differences in the viability indices of these eggs were not significant (P greater than 0.05). Infective 3rd-stage larvae were recovered from both fresh creek water (205-258 L3/litre of water), and brackish water (45 L3/litre of water). Larvae hatched from brackish water eggs that were cultured in the brackish water were 20% infective by 3 days, but lost all infective potential by 7 days post-hatching. Larvae from fresh water eggs, cultured in fresh water and 'normal' laboratory cultures reached 50% infectivity in 3-5 days, losing potential infectivity in 11-15 days post-hatching. The infectivity patterns of larvae recovered directly from fresh and brackish waters did not differ significantly (P greater than 0.05) from the infectivity patterns of larvae recovered from laboratory cultures of eggs from all the experimental sources. Although larvae recovered directly from fresh and brackish waters survived in these respective media under laboratory conditions, there was a significant difference (P less than 0.05) in their survival patterns. The survival and infectivity rates of field larvae were considerably reduced over time relative to control larvae.(ABSTRACT TRUNCATED AT 250 WORDS)

Population biology of hookworms in west Bengal: analysis of numbers of infective larvae recovered from damp pads applied to the soil surface at defaecation sites

The aims of this study were to analyze the seasonal distribution of infective larvae on the soil surface, to determine whether numbers of infective larvae near faeces were related to the faecal egg count of individuals, and to relate the distribution of larvae to environmental characteristics. Larvae were recovered from damp pads, applied to the soil surface in an annulus around fresh, identified stools of individuals who were participating in a larger epidemiological study. This provided an estimate of exposure to infection at the time of defaecation. Transmission was restricted to the rainy season and large aggregations of larvae were encountered earlier rather than later in the rainy season. Frequency distributions for the number of larvae extracted from each pad showed a high degree of aggregation, with most monthly counts showing good fits to the negative binomial probability distribution. Despite variations in monthly sampling means, the degree of aggregation in the population of larvae was remarkably stable over the 18 month sampling period (k of negative binomial = 0.01 to 0.08). Estimates of the degree of aggregation of the parasites in the human population were also available, and comparisons suggest that the infective larvae were much more aggregated than the parasitic stages. There was no relationship between the mean daily egg output of individuals and the number of larvae which developed and were recovered from the soil surface near the faeces. Thus, people who contribute large numbers of eggs to the environment are not necessarily those who are the greatest source of infection for others.(ABSTRACT TRUNCATED AT 250 WORDS)

Do hookworms elicit protective immunity in man?

The two main species of human hookworm, Ancylostoma duodenale and Necator americanus (Table 1), are together believed to infect about 900 million people - mainly in tropical countries where adequate sanitary facilities may be lacking. But interactions between the two species, and their relative contributions to observed age-related infection patterns and seasonal cycles of transmission, continue to engender controversy. People tend to remain susceptible to infection through-out life, even with constant exposure to the infective stages. So what role does human immunity or resistance play in the epidemiology and control of infection? In this article, Jerzy Behnke reviews the epidemiology of hookworm infection in the light of current understanding of mechanisms involved in host responses to infection and hookworm evasion of those responses. As he stresses, much further work is required.

Necator americanus: temperature, pH, light, and larval development, longevity, and desiccation tolerance

The effect of incubation temperature and pH on the hatch rate of eggs of Necator americanus, and the desiccation tolerance of the resulting infective stage-3 larvae were investigated in the laboratory under controlled conditions. Hatching did not occur below 15 C and above 35 C. A 21% hatch rate was obtained at 15 C while a 10.6% hatch rate was obtained at 35 C. The highest hatch rate (93.7%) was obtained at 30 C. The optimum pH for hatching was 6.0, but the larvae did not reach the infective stage. Incubation temperature of the eggs affected the longevity and desiccation tolerance of resultant infective larvae. Larvae hatched at 30 C and maintained at 26 C under bright fluorescent light had a 50% survival time (S50) of 4 days. In the dark or shade, the S50 for larvae raised at 30 C was 5 weeks, while that of larvae hatched at 20 C was 7 weeks. Incubation temperature also affected the desiccation tolerance of larvae. Larvae developed at 20 C were more resistant to desiccation at various relative humidity values than larvae hatched at 30 C.

Changes in lipid contents of infective third-stage larvae of Necator americanus during desiccation and revival

Changes in lipid content of infective third-stage larvae of Necator americanus were investigated after short periods of induced desiccation and revival. A fall in lipid reserve from an outset level of 86% to 74% was recorded in the first 2 h of desiccation. With increased desiccation, lipid reserves did not show significant decline, probably as a result of decreased lipid metabolism in the desiccated larvae. During revival, there was a drastic fall in lipid reserves as a result of increased lipid utilisation by the reviving larvae. The results showed that desiccated larvae with lipid levels less than 10% did not revive. The presence of lipid did not appear to prevent desiccation but was an essential factor for revival. The ecological significance of these findings in field larvae is discussed.

Necator americanus infection: a longitudinal study of an urban area in Nigeria

A longitudinal study was carried out in Owerri, capital of Imo State, Nigeria, to assess the pattern of Necator americanus infection during a 27-month period. The mean annual prevalence fell from 59.0% in 1978, to 55.5% in 1979 and 51.1% in 1980. The egg count (apparent intensity of infection) fell in a similar pattern. Maximum and minimum conversion rates were 55.2 and 13.7%, respectively, and conversion rates fell by approximately 20% between 1979 and 1980. The highest reversion rate was 55.0% between 1979 and 1980. The population density of available third-stage larvae fluctuated seasonally and this was significant in determining the pattern of infection.