Guest Parasites

GUEST PARASITES (xenoparasites), a term denoting true parasites that have ended up not in their natural host and yet live within it. The concept of guest parasites is applicable only in relation to parasites with a limited range of hosts. An example is the cucumber tapeworm (Dipylidium caninum), which normally lives in the tapeworm stage in the intestine of a dog or cat. Cases of finding Dipylidium caninum in children are known. Infection of the latter can only take place on the condition that they swallow lice or fleas containing the cysticercoids of the worm. The environment of the human intestine proves suitable for the development of the cucumber tapeworm within it, and man, therefore, can effectively become its primary host. At the same time, it is undoubted that D. caninum is a biologically foreign parasite to man, since its entire cycle of generation alternation was phylogenetically established on the basis of the relationships between dogs (cats) and their ectoparasites. Such cases of parasitism are, in essence, a variety of true parasitism. Therefore, it is completely incorrect to call such parasites (as is sometimes done in the literature) false parasites, since the latter are free-living animals that accidentally enter the human (or animal) organism and inhabit it for some time (e.g., see Geophilus). There are other parasitic worms that are guest parasites of humans, such as Hymenolepis diminuta—the rat tapeworm, which normally parasitizes the small intestines of rats and mice. For human infection with this parasite, it is necessary to swallow one of the intermediate hosts with H. diminuta cysticercoids (see Hymenolepis). Among the thorny-headed worms, a guest parasite of humans can be the moniliform thorny-headed worm—Gigantorhynchus moniliformis (an intestinal parasite of various rodents—rats, mice, hamsters). Roundworms (nematodes) also provide cases of guest parasitism: the mouse nematode—Syphacia obvelata—was discovered in a child on the Philippine Islands; dog (Toxascaris limbata) and cat (Belascaris mystax) ascarids are found in rare cases in humans as well; Gongylonema, which inhabits the esophagus or the thickness of its wall in pigs, has been discovered several times in the submucosa of the lips and cheeks in humans; Gnathostomum spinigerum (stomach of various wild felines) can be a wandering subcutaneous guest parasite of humans, causing the so-called "oedeme ambulant siamois," etc. Various arthropods also provide many examples of guest parasitism in the human body. Linguatula rhinaria—the tongue worm (Linguatulidae, Arthropoda)—is often encountered in the form of encapsulated larvae in the liver or other human organs, where these larvae eventually perish due to the deposition of lime in the wall of the capsule. Among mites, guest parasites of humans include the itch mites of horses (Sarcoptes scabiei var. equi) and other animals, which transfer upon close contact of a human with the true hosts of Sarcoptes. Diverse cases of guest parasitism are provided by the stomach botflies of horses (Gastrophilus equi, see). Their adult larvae have been discovered in the stomach of dogs and even birds; the larvae of the first phase of transformation often live in the human epidermis, causing a disease called "hair," creeping disease, larva migrans, etc. Under human skin, the larva of the botfly Hypoderma, which normally lives in cattle, can act as a guest parasite, and others. It is undoubted that with the passage of time, more and more new examples of guest parasitism will be discovered, especially since various patients (in particular, the mentally ill) sometimes swallow diverse objects; among them, various hosts of parasites that a normal person would not eat may be ingested. At the same time, to recognize the authenticity of such cases, it is necessary to exclude the possibility of mystification of the physician by the patient.