Immunosuppression in Helminth Infection

1.1 Parasitism Parasitism is an antagonistic relationship between organisms of different species where the parasite benefits at the expense of the host. Helminths are long-living, multicellular parasites. There are two major phylla of helminths; Nematodes and Platyhelminthes. The nematodes contain the intestinal worms known as soil-transmitted helminths including hookworms, whipworms and the filarial worms that cause lymphatic filariasis and onchocerciasis. The Platyhelminthes, known as flatworms, include the flukes and the tapeworms. Both nematodes, flukes and tapeworms widely infect humans and animals (Hotez & Kamath, 2009). Most of the parasitic species causing weakness and disease survive in and explore the host as natural environment. Helminths can be found in a great variety of tissue niches, and although they cause very high morbidity, direct mortality of the host species remains low (Brooker, 2010). Human hookworm infection is a common soiltransmitted helminth infection that is caused by the nematode parasites Necator americanus and Ancylostoma duodenale. Hookworm infections are asymptomatic however substantially contributes to the incidence of anemia and malnutrition in developing nations (de Silva et. al 2003, WHO 2010). Filarial diseases are rarely fatal and morbidity of human filariasis results mainly from the host reaction to microfilariae or developing adult worms in different areas of the body. Most of the filarial infected individuals have a subclinical condition associated with patent infection, and acute manifestations which are rarely life threatening. However, chronic manifestations, such as lymphedema (elephantiasis) and hydrocele, are debilitating (Keiser et al., 2002). Schistosomes, the blood flukes reside in the mesenteric and vesical venules. They have a life span of many years and daily produce large numbers of eggs, which must traverse the gut or bladder tissues on their way to the lumens of the excretory organs. Many of the eggs remain in the host tissues, inducing immunologically mediated granulomatous inflammation and fibrosis (Warren, 1982). The relationship between the presence of schistosome infection and clinical morbidity revealed schistosomiasis-related disease and associated death (Van der Werf et al., 2003). Worldwide, many cestode infestations occur with very low prevalence of infections and are asymptomatic. Nevertheless some of the more serious infestations result in symptoms from mass effects on vital organs, inflammatory responses, nutritional deficiencies, and the potential of fatal anaphylaxis (Del Brutto, 2005; Morar & Feldman, 2003; Ozturk et al., 2007).