Parasites and Worms

Abstract

A parasite is an organism that lives on or in a host and gets its food from or at the expense of its host. Worms or helminths either live as parasites or free of a host in aquatic and terrestrial environments. Parasites and worms are found worldwide but mainly in the tropics. It is estimated that 20% of immigrants from endemic countries may have helminthic infections at their arrival to the UK. These people could be asymptomatic, but tend to present with unexplained symptoms, especially gastrointestinal in nature or eosinophilia. Travellers to endemic countries tend to be newly infected and have greater immune response and pronounced eosinophilia in some but not all parasitic infections. Parasites that can cause disease in humans fall under three classes: protozoa, helminths, and Ectoparasites Protozoa are microscopic, one- celled organisms that can be free living or parasitic in nature. Transmission of protozoa that live in a human’s intestine to another human typically occurs through a faeco-oral route (for example, contaminated food or water, or person- to-person contact). Protozoa that live in the blood or tissue of humans are transmitted to other humans by an arthropod vector (for example, through the bite of a mosquito or sand fly). Helminths are large, multicellular organisms that are generally visible to the naked eye in their adult stages. Like protozoa, helminths can be either free living or parasitic. There are three main groups of helminths that parasitize humans: cestodes, trematodes, and nematodes. These are flat worms that comprise Echinococcus species: intestinal tapeworms and neurocysticercosis (Taenia solium) These are leaf- shaped, and they vary in length from a few millimetres to 8 cm. They include: ■ Liver fluke: Clonorchis sinensis, Fasciola hepatica ■ Intestinal fluke: Fasciola buski, Heterophyes heterophyes, ■ Lung fluke: Paragonimus westernmani ■ Blood flukes: Schistosoma species These are cylindrical in structure. Blood- sucking arthropods such as mosquitoes are considered as ectoparasites because they depend on blood meal for their survival. Narrowly speaking, ectoparasites include organisms like ticks, fleas, lice, and mites (scabies) that attach or burrow into the skin and remain there for relatively long periods of time (e.g. weeks to months).