This chapter deals with the structure and development of the three major groups of helminths. These groups are subdivided for convenience according to the host organ in which they reside, e.g., lung flukes, extraintestinal tapeworms, and intestinal roundworms. Roundworms are nematodes (nemato from the Greek root meaning “thread”). In medically oriented schemes the flatworms or platyhelminths (platy from the Greek root meaning “flat”) include flukes and tapeworms. The helminths are invertebrates characterized by elongated, flat or round bodies. However, low-intensity or early infections may be asymptomatic.Helminth is a general term meaning worm. Symptoms generally include abdominal pain and related digestive complaints. The distribution of Moniliformis moniliformis is not known but is likely cosmopolitan.Ĭlinical symptoms of acanthocephaliasis are often severe, due in part to the mechanical damage caused by the insertion of the armed proboscis into the lumen of the host’s intestine.
Macracanthorynchus ingens is highly endemic in raccoons from the southeastern United States, and the recorded human cases originate from Texas and Florida. Macracanthorynchus hirudinaceus is found wherever wild or domestic swine occur. Cases of acanthocephaliasis more commonly occur in areas where insects are eaten for dietary or medicinal purposes or in children who consume insects. Geographic DistributionĪcanthocephala are widely distributed.
ingens, and beetles or cockroaches for M. The insect intermediate host varies by species but is usually scarabaeoid or hydrophilid beetles for M. Natural definitive hosts include rats ( Moniliformis moniliformis), swine ( Macracanthorynchus hirudinaceus), and raccoons ( Macracanthorynchus ingens). In humans the worms seldom develop to full maturity or produce eggs. In the definitive host, liberated juveniles attach to the wall of the small intestine, where they mature and mate in about 8–12 weeks. The definitive host becomes infected upon ingestion of intermediate hosts containing infective cystacanths.
After 6–12 weeks, the worm reaches the infective stage called a cystacanth. Within the hemocoelom of the insect, the acanthor molts into a second larval stage, called an acanthella. The eggs are ingested by an intermediate host, which is always an insect. The eggs contain a fully developed acanthor when shed in feces. Eggs are shed in the feces of the definitive hosts.
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