HELMINTHS

Background

Soil-transmitted Helminths (STH) infections refer to a group of parasitic diseases caused by nematode worms that are transmitted to humans by fecally-contaminated soil. The STH of major concern to humans are the roundworm (Ascaris lumbricoides), the whip- worm (Trichuris trichiura), and the hookworms (Necator americanus and Ancylostoma duodenale).They are among the most common infections worldwide which heavily affect the poorest and most deprived communities where the sanitation is inadequate and water supplies unsafe. Latest estimates indicate that approximately two billion people (24% of the world's population) are infected with STH infections worldwide. Infections are widely distributed in tropical and subtropical areas, with the greatest numbers occurring in sub-Saharan Africa, the Americas, China and East Asia. Although soil-transmitted helminthiases inflict tremendous disability and suffering, they can be controlled or eliminated.

The global estimate of STH infection was 438.9 million, 819.0 million, and 464.6 million people for hookworm, A. lumbricoides, and Trichuris trichiura, respectively, in 2010. The highest prevalence of STH infections and vast majority of years lived with disability attributable to STH infections occur in Asia and Africa. In sub-Saharan Africa (SSA), there were 866 million people infected by STH as indicated by the World Health Organization estimate 2012: the respective number (prevalence) of people infected by hookworm, A. lumbricoides, and T. trichiura was 117 million (13.6%), 117 million (13.6%), and 100.8 million (11.6%), respectively.

Infection with soil-transmitted helminths (STHs) is common in the world particularly in the rural developing regions and more among school-aged children. Warm climates and adequate moisture, lack of personal or environmental hygiene, sanitation, and education, walking barefoot and poor health or nutritional status could increase the risk of STH infections due to their frequent playing habits and low level of awareness. These risk factors vary with localities and could provide vital information to guide policy makers in designing a more focused preventive approach to control the disease.

In Nigeria, there have been sporadic de-worming programs undertaken by government. However, there has been no national school based parasites or soil-transmitted helminthes control programme. Also, not much has been done to provide data about the demography and hygiene conditions of the children to help guide the development of school health programmes which are required for sustainable control of soil-transmitted helminths in school children.

Transmission

Soil-transmitted helminths are transmitted by eggs that are passed in the faeces of infected people. Adult worms live in the intestine where they produce thousands of eggs each day. In areas that lack adequate sanitation, these eggs contaminate the soil. This can happen in several ways:

- eggs that are attached to vegetables are ingested when the vegetables are not carefully cooked, washed or peeled;

- eggs are ingested from contaminated water sources;

- eggs are ingested by children who play in the contaminated soil and then put their hands in their mouths without washing them.

In addition, hookworm eggs hatch in the soil, releasing larvae that mature into a form that can actively penetrate the skin. People become infected with hookworm primarily by walking barefoot on the contaminated soil. There is no direct person-to-person transmission, or infection from fresh faeces, because eggs passed in faeces need about 3 weeks to mature in the soil before they become infective. Since these worms do not multiply in the human host, re-infection occurs only as a result of contact with infective stages in the environment.

Symptoms

Morbidity is related to the number of worms harboured. People with light infections usually have no symptoms. Heavier infections can cause a range of symptoms including intestinal manifestations (diarrhoea and abdominal pain), general malaise and weakness, and impaired cognitive and physical development. Hookworms cause chronic intestinal blood loss that can result in anaemia.

Diagnosis/Testing

Parasitological Diagnosis:

Infection with soil-transmitted helminthiases (STHs) can be readily diagnosed by detection of helminth eggs in stool samples using microscopic techniques. Such techniques are generally sensitive provided that appropriate methodologies are used.

Antibody Tests:

Except for the case of strongyloidiasis, the use of antibody tests to detect specific antibody responses to STHs has not been considered necessary for diagnostic purposes, and relatively little work has been undertaken to develop and standardize serological assays for hookworm infection, ascariasis or trichuriasis. For strongyloidiasis, a significant amount of work has been undertaken to develop antibody ELISA diagnostic tests, using both crude larval antigen and recombinant proteins.

Antigen Detection:

Some work has been undertaken in the zoonotic hookworm species An. ceylanicum, and in Strongyloides spp. demonstrating the presence of coproantigen in faeces. However, no assay has reached a mature stage of development.

Molecular Diagnosis:

PCR-based diagnosis of hookworm infection has been developed and subject to pilot testing in human populations. A multiplex PCR assay has been described and shows promise for quantifying egg counts for hookworm infection and ascariasis. Such assays promise the ability to undertake multiplexed and quantitative assessment of STHs.

Treatment

The WHO recommended medicines – albendazole (400 mg) and mebendazole (500 mg) – are effective, inexpensive and easy to administer by non-medical personnel (e.g. teachers). They have been through extensive safety testing and have been used in millions of people with very few and minor side-effects.

Prevention and control:

The strategy for control of soil-transmitted helminth infections is to control morbidity through the periodic treatment of at-risk people living in endemic areas.

WHO recommends periodic medicinal treatment (deworming) without previous individual diagnosis to all at-risk people living in endemic areas. Treatment should be given once a year when the prevalence of soil-transmitted helminth infections in the community is over 20%, and twice a year when the prevalence of soil-transmitted helminth infections in the community is over 50%. This intervention reduces morbidity by reducing the worm burden. In addition:

- health and hygiene education reduces transmission and reinfection by encouraging healthy behaviours; and

- provision of adequate sanitation is also important but not always possible in resource-poor settings.

Morbidity control aims to reduce intensity of infection and protect infected individuals from morbidity through periodic treatment of at-risk populations.

Periodic deworming can be easily integrated with child health days or supplementation programmes for preschool children, or integrated with school health programmes.

© NCDC