Gastrointestinal nematodes: the Karkar experience

Abstract

This paper reviews our research on the hookworm Necator americanus over several years. Our field site for the research was on Karkar Island, Papua New Guinea, where we found a prevalence of N. americanus infection among adults of nearly 100%. The intensity of infection was related to host age and to the development of iron deficiency anaemia, which occurred at a much lower infection intensity than had been previously reported. We studied the immune response to infection and our results initially suggested that antibody responses and eosinophilia do not protect the host against infection. However, we have more recently found a negative correlation of both IgE and eosinophilia with the weight and fecundity of N. americanus which suggests that the immune response does have some effect on N. americanus and that this immunity is dependent on the Th2 subset of T lymphocytes. Following treatment for hookworm, the prevalence of N. americanus returned almost to pretreatment levels within 2 years, with the rate of acquisition of adult worms independent of host age. A significant predisposition to hookworm infection was demonstrated by individuals. Prevention will result from measures to reduce the transmission and intensity of infection, and can be achieved through improved sanitation or by vaccination. However, vaccination is not yet a viable option because of our limited knowledge about protective immunity.

PIP: Findings are reported from the author's research into the hookworm Necator americanus in the village of Kabasob, Karkar Island, Papua New Guinea, beginning in 1988. N. americanus was determined to be the most prevalent of the identified gastrointestinal nematodes, infesting almost all of the adults with an intensity of 40 worms/host. The intensity of infection was related to host age and to the development of iron deficiency anemia, which occurred at a much lower infection intensity than had been previously reported. Results of an assessment of the immune response to infection initially suggested that antibody responses and eosinophilia do not protect the host against infection. However, the authors have since found a negative correlation of IgE and eosinophilia with the weight and fecundity of N. americanus and that immunity is dependent upon thymus-derived helper 2 lymphocytes. Patients were treated with single 10 mg/kg doses of pyrantel pamoate. However, after 2 years, the prevalence of N. americanus infection had returned almost to pretreatment levels, with the rate of acquisition of adult worms being independent of host age. A highly significant correlation was observed between individuals' pretreatment egg counts and worm burden and those acquired after reinfection. The prevention of infection with N. americanus will come from measures to reduce the transmission and intensity of infection, and can be achieved through improved sanitation or by vaccination. For now, vaccination is not an option because too little is known about protective immunity.