Parasites: the hidden hunters By George Emery

Categorisation table

Tapeworms Cestoda (Cestoidea) is the name given to a class of parasitic flatworms, commonly called tapeworms, of the phylum Platyhelminthes. Its members live in the digestive tract of vertebrates as adults, and often in the bodies of various animals as juveniles. Over a thousand species have been described, and all vertebrate species can be parasitised by at least one species of tapeworm. Several species parasitise humans after being consumed in underprepared meat such as pork (T. solium), beef (T. saginata), fish (Diphyllobothrium spp.), or in food prepared in conditions of poor hygiene (Hymenolepis spp. or Echinococcus spp.). T. saginata, the beef tapeworm, can grow up to 12 m (40 ft); other species may grow to over 30 m (100 ft).[

Roundworms The nematodes (n ɛ məto ʊ dz) or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode species might be approximately 1,000,000. Unlike cnidarians or flatworms, roundworms have a digestive system that is like a tube with openings at both ends.

Flukes The trematodes or flukes are estimated to include 18,000 to 24,000 species, and are divided into two subclasses. Nearly all trematodes are parasites of mollusks and vertebrates. The smaller Aspidogastrea, comprising about 100 species, are obligate parasites of mollusks and may also infect turtles and fish, including cartilaginous fish. The Digenea, which constitute the majority of trematode diversity, are obligate parasites of both mollusks and vertebrates, but rarely occur in cartilaginous fish.

In general Parasitic worms are sequential hermaphrodites and reproduce depending on the species of worm, either with the presence of a male and female worm, joining sperm and eggs, producing fertile eggs, such as hookworms, or by breaking off segments that contain both male and female sex organs that are able to produce fertile eggs without the presence of a male or female (e.g., tapeworms). All worm offspring are passed on through poorly-cooked meat, especially pork, wild fish, and beef, contaminated water, faeces and mosquitoes. However, it is estimated that 40 million Americans are infected with the most common roundworm in the United States, the pinworm. Worm eggs or larvae or even adults enter the human body through the mouth, anus, nose, or skin, with most species attaching themselves to the intestinal tract. With the presence of digestive enzymes, worm egg shells are dissolved, releasing a brand-new worm; unlike its egg shell, the parasitic worm is protected from the body's powerful digestive enzymes by producing a protective keratin layer.