vertebrate host. Diagnosis is made on the basis of the recovery and identification of the larval worms (microfilariae) in the blood, other body fluids, or skin. While circulating in peripheral blood or cutaneous tissues, the microfilariae can be ingested by blood-sucking insects. After the larvae mature, they can escape into the vertebrate host’s skin when the arthropod takes its next blood meal. The severity of disease due to these nematodes varies; however, elephantiasis may be associated with some of the filarial worms. Specific organisms include Wuchereria bancrofti, Brugia spp., Loa loa, Mansonella spp., Onchocerca volvulus, and Dirofilaria spp.
Cestodes (Intestinal)
The adult form of the tapeworm is acquired through ingestion of the larval forms contained in poorly cooked or raw meats or freshwater fish (Taenia spp., Diphyllobothrium spp.). Dipylidium caninum infection is acquired by the accidental ingestion of dog fleas infected with the larval tapeworms. Both Hymenolepis nana and H. diminuta are transmitted via ingestion of certain infected arthropods (fleas and beetles). Also, H. nana can be transmitted through egg ingestion (its life cycle can bypass the intermediate beetle host). The adult tapeworm consists of a chain of egg-producing units called proglottids, which develop from the neck region of the attachment organ, the scolex. Food is absorbed through the worm’s integument. The intermediate host contains the larval forms, which are acquired through ingestion of the adult tapeworm eggs. Humans can serve as both the intermediate and definitive hosts in H. nana and Taenia solium infections.
Cestodes (Tissue)
The ingestion of certain tapeworm eggs or accidental contact with certain larval forms can lead to tissue infection with Taenia solium, other Taenia spp., Echinococcus spp., Diphyllobothrium spp., and Spirometra mansonoides.
Trematodes (Intestinal)
Trematodes are flatworms and are exclusively parasitic. With the exception of the schistosomes (blood flukes), flukes are hermaphroditic. They may be flattened, and most have oral and ventral suckers. All of the intestinal trematodes require a freshwater snail to serve as an intermediate host; these infections are foodborne (freshwater fish, mollusks, or plants) and are emerging as a major public health problem (more than 40 million people are infected with intestinal and liver/lung trematodes). Specific examples include Fasciolopsis buski, Heterophyes heterophyes, and Metagonimus yokogawai.
Trematodes (Liver and Lungs)
The hermaphroditic liver and lung trematodes also require a freshwater snail to serve as an intermediate host; these infections are foodborne (acquired by ingestion of freshwater fish, crayfish or crabs, or plants). Public health concerns include cholangiocarcinoma associated with Clonorchis and Opisthorchis infections, severe liver disease associated with Fasciola infections, and the misdiagnosis of tuberculosis in those infected with Paragonimus spp.
Trematodes (Blood)
The sexes of blood trematodes (schistosomes) are separate, and infection is acquired by skin penetration by the cercarial forms that are released from freshwater snails. The males are characterized by having an infolded body that forms the gynecophoral canal in which the female worm is held during copulation and oviposition. The adult worms reside in the blood vessels over the small intestine, large intestine, or bladder. Although these parasites are not endemic within the United States, patients are seen who may have acquired schistosomiasis elsewhere. Schistosomiasis is a chronic disease, and many infections are subclinically symptomatic, with mild anemia and malnutrition being common in areas where the infection is endemic. Acute schistosomiasis (Katayama’s fever) may occur weeks after the initial infection, especially infection by S. mansoni and S. japonicum. Signs may include abdominal pain, cough, diarrhea, high eosinophilia, fever, fatigue, and hepatosplenomegaly. Representative species include S. mansoni, S. japonicum, and S. haematobium.
Pentastomids
Pentastomids are found in a separate phylum, Pentastomida, and are called tongue worms. Human infections have been reported from Africa, Europe, Asia, and the Americas. When humans serve as the intermediate hosts, the infective larvae die in situ. However, when mature larvae (often encysted) are ingested, they may migrate from the stomach, attach themselves to nasopharyngeal tissues, develop into adult pentastomids, and cause symptoms of the halzoun syndrome. Symptoms include throat discomfort, paroxysmal coughing, sneezing, and occasionally dysphagia and vomiting. Pentastomids isolated from humans include Armillifer spp., Linguatula serrata, and Sebekia spp.
Rare eye infections have been reported; inflammation is minimal, and the infection is probably the result of direct eye contact with water containing pentastomid eggs. Even more rare is human infection caused by a pentastomid larva belonging to the genus Sebekia. The adult worms are parasites found in the respiratory tract of reptiles, and the larval forms are found in the viscera, in the muscles, and along the spinal cord of freshwater fish. There may be serpiginous burrows surrounded by an intense erythematous zone and a 30% eosinophilia. The infection was possibly acquired by the ingestion of water containing eggs of the parasites or the ingestion of raw or improperly cooked fish.
Diagnosis is made by identifying the pentastomid in a biopsy specimen or at autopsy.
Acanthocephala
The Acanthocephala (thorny-headed worms) are closely related to the tapeworms. These worms are diecious and tend to have a retractable proboscis which is usually armed with spines. The larvae require an arthropod intermediate host, and the adult worms are always parasites in the intestine of vertebrates. Two of these organisms are parasitic in humans: Macracanthorhynchus hirudinaceus and Moniliformis moniliformis. Human infection is acquired from the ingestion of infected insects (various beetles and cockroaches). Very few cases have been reported in the literature; however, symptoms included abdominal pain and tenderness, anorexia, and nausea. In some cases, adult worms have been passed in the stool.
Suggested Reading
Beaver, C. B., R. C. Jung, and E. W. Cupp. 1984. Clinical Parasitology. Lea & Febiger, Philadelphia, PA.
Garcia, L. S. 2007. Diagnostic Medical Parasitology, 5th ed. ASM Press, Washington, DC.
Gibson, D. I. 1998. Nature and classification of parasitic helminths, p. 453–479. In L. Collier, A. Balows, and M. Susman (ed.), Topley & Wilson’s Microbiology and Microbial Infections, 9th ed. Oxford University Press, New York, NY.
Goddard, J. 2007. Arthropods of Medical Importance, 5th ed. CRC Press, New York, NY.
Murray, P. R., E. J. Baron, J. H. Jorgensen, M. L. Landry, and M. A. Pfaller (ed.). 2007. Manual of Clinical Microbiology, 9th ed. ASM Press, Washington, DC.
