Intermediate Hosts of Parasites

Tapeworms (cestodes), flukes (trematodes), roundworms (nematodes), and thorny-headed worms (acanthocepha-lans) of many species that infest domestic and wild animals use beetles as intermediate hosts. Animals become infested by ingesting parasitized beetles that contaminate feed or bedding (tenebrionids, carabids) or that are attracted to animal dung (scarabaeids), or by ingesting water in which infecdve beetles have disintegrated.

Two tapeworms that infest the small intestines of poultry are the broad-headed tapeworm (Raillietina cesticillus) and Choanotaenia infundibulum. Both parasites cause enteritis and hemorrhaging in chickens, turkeys, pheasants, and guinea fowl. A few tenebrionids and scarabaeids and more than 35 species of carabid beetles, notably in the genera Amara and Pterostichus, are intermediate hosts for R. cesticillus (Cheng 1973). Some tenebrionid and dermestid species, including the lesser mealworm beetle, are intermediate hosts for C. infundibulum. Proglot-tids or tapeworm eggs ingested by beetle larvae or adults develop into cystercerci (encysted larvae) that can then infest birds that eat the beedes. Chicks are most susceptible to serious infestations and often die from worm burdens.

The beef tapeworm (Taenia saginata) can use dung beedes and carabids as intermediate hosts, although they are not essential for transmission. Beedes associated with infective dung or debris can ingest proglottids or eggs, as in the case of poultry worms. Catde and humans infested with the tapeworm may show mild symptoms such as weight loss, abdominal pain, and increased appetite.

The dwarf tapeworms (Hymenolepis nana and H. diminuta) that usually infest rodents, especially rats and mice, can infest humans when the intermediate host beetles are accidentally ingested. Tenebrio molitor may act as an intermediate host for H. nana, although this worm is readily transmitted directiy from one vertebrate host to another. Several species of tenebrionids (Tenebrio spp. and Tribolium spp.) are required intermediate hosts for H. diminuta. Larval and adult beedes infesting grain and cereals ingest worm eggs that develop into cysticercoid stages that infest rodents or humans, usually children, who ingest the beetles. Dwarf tapeworms produce minimal symptoms in rodents and people, although heavy infestations in children may cause abdominal pain, diarrhea, convulsions, and dizziness.

Beedes are known to be intermediate hosts for only a few trematodes. These are parasites of frogs that become infested by ingesting parasitized dytiscid beedes and pose no problem for other vertebrate animals.

Many nematodes infest livestock and wildlife, but only a few use beetles as intermediate hosts. Spirurid nematodes of various species infest livestock and, rarely, humans. Physocephalus sexalatus and Ascarops strongylina eggs develop in many species of scarabaeid dung beetles (Geotrupes spp., Onthophagus spp., and Scar abacus spp.) that then may be ingested by pigs. Both wild and domestic swine can be infested with these stomach worms, which can cause digestive problems in heavily infested young animals. Gongylonema pulchrum is a parasite of the upper digestive tract of sheep, catde, goats, and other ruminants, as well as horses, dogs, and humans. The worms burrow in the mucosa and submucosa of the oral cavity and esophagus and may cause bleeding, irritation, numbness, and pain in the mouth and chest. Scarabaeid and tenebrionid beedes serve as intermediate hosts for the larvae. Physaloptera caucasica, another spirurid, often parasitizes monkeys in tropical Africa, where humans are also commonly infested. This nematode causes digestive distress by infesting the alimentary tract from the esophagus to the terminal ileum. Scarabaeid dung beedes are its intermediate hosts.

The acanthocephalans, aptly named for their thorny heads, include species found worldwide infesting swine, rodents, and carnivores, such as dogs. Macracan-thorhynchus hirudinaceus, which attaches to the small intestines of swine, causes enteritis and produces intestinal nodules that lower the value of these tissues when they are sold to make sausage casings. Eggs of this parasite are ingested by scarab beede larvae of species of various genera (Phyllophaga, Melolontha, Lachnosterna, Cetonia, Scarabaeus, and Xyloryctes), including May and June beetles, leaf chafers, dung beedes, and rhinoceros beedes. Infested beetle larvae, as well as the pupae and adults that develop from them, are infective to both pigs and humans. Humans and pigs often show no symptoms. However, in cases of heavy infestations, both human and porcine hosts may experience digestive problems, such as abdominal pain, loss of appetite, and diarrhea, which can lead to emaciation.

Two other acanthocephalan worms that parasitize the small intestines of their hosts use scarab beetles or tene-brionids as intermediate hosts. They are M. ingens, which infests raccoons and occasionally dogs and humans, and Moniliformis moniliformis, a parasite of rodents and dogs.

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