Soil-Transmitted Helminths: World's Neglected Tropical Diseases
CHAPTER ONE
INTRODUCTION
1.0. Soil – transmitted helminths
Soil–transmitted helminths (STHs) are included in the list of the world’s neglected tropical diseases (Molyneux et al., 2005). STHs are major public health problem especially in the developing nations. The species of soil-transmitted helminths that infect humans are concentrated in the developing world, and these include the large roundworm Ascaris lumbricoides, the whipworm Trichuris trichiura, and the hookworm Ancylostoma duodenale and (Hotez et al., 2003). The disease burden depends on the regional ecological conditions and also on the local standards of the people (Ukpai et al., 2003). Soil-transmitted helminths (STHs) are group of parasitic nematode worms that afflict humans through the ingestion of infective eggs or contact with larvae (Nava and Brown, 1994; Nokes and Bundy, 1994). Nematodes have a widespread prevalence and distribution that result in hundreds of millions of human infections (Hotez et al., 2003).
1.1. Biology of STHs and transmission
The life cycles of the major STHs i.e. hookworm, A. lumbricoides and T. trichiura have many features in common. The adult parasites inhabit the human intestinal tract. For hookworms, humans acquire the infection when third-stage larvae in soil penetrate their skin while for A. lumbricoides and T. trichiura, infection takes place through the oral route by contaminated food and water. In some settings, transmission can occur through the practice of eating soil or geophagia (Geissler et al., 1998). Larvae of STHs migrate through the vasculature and are carried to the muscles of the heart and the lungs. From the lung tissues, the larvae ascend through the alveoli, bronchioles, bronchi and trachea from where they are coughed and swallowed into the intestinal tract. In the tissues of the intestines, they develop into adults and lay eggs. Except for A. lumbricoides which does not develop in soil, worm eggs shed into the soil through faeces hatch into
INTRODUCTION
1.0. Soil – transmitted helminths
Soil–transmitted helminths (STHs) are included in the list of the world’s neglected tropical diseases (Molyneux et al., 2005). STHs are major public health problem especially in the developing nations. The species of soil-transmitted helminths that infect humans are concentrated in the developing world, and these include the large roundworm Ascaris lumbricoides, the whipworm Trichuris trichiura, and the hookworm Ancylostoma duodenale and (Hotez et al., 2003). The disease burden depends on the regional ecological conditions and also on the local standards of the people (Ukpai et al., 2003). Soil-transmitted helminths (STHs) are group of parasitic nematode worms that afflict humans through the ingestion of infective eggs or contact with larvae (Nava and Brown, 1994; Nokes and Bundy, 1994). Nematodes have a widespread prevalence and distribution that result in hundreds of millions of human infections (Hotez et al., 2003).
1.1. Biology of STHs and transmission
The life cycles of the major STHs i.e. hookworm, A. lumbricoides and T. trichiura have many features in common. The adult parasites inhabit the human intestinal tract. For hookworms, humans acquire the infection when third-stage larvae in soil penetrate their skin while for A. lumbricoides and T. trichiura, infection takes place through the oral route by contaminated food and water. In some settings, transmission can occur through the practice of eating soil or geophagia (Geissler et al., 1998). Larvae of STHs migrate through the vasculature and are carried to the muscles of the heart and the lungs. From the lung tissues, the larvae ascend through the alveoli, bronchioles, bronchi and trachea from where they are coughed and swallowed into the intestinal tract. In the tissues of the intestines, they develop into adults and lay eggs. Except for A. lumbricoides which does not develop in soil, worm eggs shed into the soil through faeces hatch into