Soil-Transmitted Helminthic Zoonoses in Humans and Associated Risk Factors

Abstract

The soil is an important route for transmission of numerous human pathogens, including the five major soil-transmitted helminths (STHs), also known as geohelminths, namely: roundworm (Ascaris lumbricoides), whipworm (Trichuris trichiura), hookworms (Ancylostoma duodenale and Necator americanus), and threadworm Strongyloides stercoralis (Brooker et al., 2006). An estimated one billion people are currently infected with STHs worldwide, particularly in resource-poor settings (WHO, 2011). Although overall mortality due to STH infections is low, morbidity may be significant given the pronounced impact on nutrition, growth, physical fitness, cognitive functions among infected infants, schoolchildren and adults from developing countries (Bethony et al., 2006). In Africa, an estimated 89.9 million children harbor STHs, many of whom are co-infected with two or more STH species (WHO, 2011). Zoonotic agents, comprising a wide variety of bacteria, viruses, and parasites, account for almost two thirds of all known human infections. Some helminthoses that commonly infect canids and felids are typically soil-transmitted. This chapter focuses on two major groups of STHs that cause disease in humans: (a) the ascarids Toxocara canis and T. cati, associated with visceral and ocular larva migrans, and (b) the hookworms Ancylostoma braziliense and Anc. caninum, associated with cutaneous larva migrans. We review the current geographic distribution, laboratory diagnosis and clinical spectrum of these infections, examine the relative contribution of some risk factors for infection and disease, and discuss potential control measures for reducing the burden of disease in companion animals and humans. A third soil-transmitted ascarid species that can cause human disease is Baylisascaris procyonis, commonly found in raccoons in North America. Human infections are characterized by severe neurological disease, leading to death or long-lasting sequelae (Watts et al., 2006). Another nematode species, Gnathostoma spinigerum, has occasionally been found in biopsy sample from patients with suspected VLM. More recently species of Toxocara including T. malayensis, a parasite of the domestic cat, and T. lyncus, which infects the caracal, have been identified, but their role in human disease remains unknown (Despommier, 2003). Soil-transmitted larval infection with other common canine and feline hookworms, such as Anc. ceylanicum, Anc. tubaeforme and Uncinaria stenocephala, can also cause occasional dermatological lesions in humans, and Anc. ceylanicum can readily develop in adults causing severe enteritis (Bowman