Insects and Helminths

Everett E. Wehr, John T. Lucker.

Many species of the helminths, or parasitic worms, of livestock and poultry can pass through certain of their early stages only within the body of an insect. These species are transmitted, in the true sense, by insects. Beetle mites or grass mites similarly transmit others. One species is transmitted by a tick as well as by insects. These particular species of worms are obligatory parasites of insects, or their allies, just as truly as they are obligatory parasites of farm animals or birds. For their continued existence and propagation, for their survival as species, they depend equally upon insect and upon avian or mammalian hosts.

The life cycle of a helminth of one of these species, like the life cycle of all other parasitic helminths, is initiated by the eggs or microscopic larvae produced by the mature female or hermaphroditic individual. But depending on its specific identity, its eggs or larvae are infectious only to an insect or a mite or perhaps a tick. If ingested by a suitable insect, for example, each egg or larva gives rise to a more advanced developmental stage of the parasite, which takes up its abode in some part of the insect's body. There, however, the development of the worm stops at a stage far short of reproductive maturity. Unless this arrested- developmental stage gains access to the body of a suitable vertebrate animal, the life cycle of the parasite cannot be completed. Obviously, therefore, any step that can be taken to destroy infected insects will aid in preventing the infection of livestock and poultry with worms that have this type of life cycle. The world-wide extermination of the insect vectors, were this possible, would result automatically in the extermination of a goodly proportion of the species of worms that now afflict man and his domestic animals and many others that live in wild animals and birds.

Worms that are obliged to undergo development in two or more hosts are called heteroxenous parasites. The hosts in which they can reach reproductive maturity are called final or definitive hosts. Hosts in which their larval stages must develop before the parasites can take up life in a final host are called intermediate hosts.

Some instances of the transmission of parasitic worms by insects were discovered before it was learned that insects are also vectors of some of the most devastating protozoal and infectious diseases known to medical and veterinary science. Some years before the transmission of malaria or yellow fever by mosquitoes or of southern cattle fever by ticks was discovered, it had been demonstrated that the larvae of a nematode worm, Wuchereria bancrofti, which causes human filiariasis, could be sucked up by a feeding mosquito and would undergo developmental transformation in its body.

Very few kinds of parasitic worms can multiply that is, reproduce through successive generations entirely within the body of the animal in which they mature. The eggs or larvae of nearly all species must leave the host's body to perpetuate the parasite.

The eggs or larvae of worms that live in the digestive tract or in an organ or system ( such as the liver or respiratory system) that communicates with the digestive tract or in the urinary system ordinarily pass from the host with its feces or urine. The presence of the progeny of the worms in those substances, which in natural circumstances are deposited by livestock and poultry on the ground, leads to their ingestion by various kinds of invertebrate and vertebrate animals. Although the insects and their close relatives are perhaps the most ubiquitous of the invertebrates and are of outstanding importance as vectors of parasitic worms, the eggs and larvae of some of the heteroxenous worms of farm animals and birds are not infectious to them. Other arthropods, snails, slugs, earthworms, or other animals serve as intermediate hosts in those instances.

Some of the insect vectors of worms that produce eggs, which leave the definitive host's body in the manner described, habitually feed upon the excrement of higher animals. In the process they ingest the worm eggs and thus become infected. Others are not susceptible as adults to infection or at least do not become infected. They habitually deposit their eggs in excrement or in materials contaminated by it. The larvae that hatch from their eggs ingest the worm eggs and are susceptible to infection by them. In other instances the insects involved cannot be classified as coprophagous dung eating nor do they customarily or preferentially breed in manure. But natural forces continually scatter worm eggs into their habitats. They ingest quite incidentally the worm eggs that contaminate their normal food supply.

Some of the vectors are themselves ectoparasites of farm animals. They normally feed upon the cellular debris or detritus on the skin of their hosts. They take in worm eggs when the skin is contaminated with fecal matter or crushed parts of worms.

Some of the heteroxenous worms live in situations, such as the circulatory system or subcutaneous tissues, that have no connection with the external body openings of the host. They include several species of viviparous roundworms, or Nematoda, which eject the larvae they produce into their host's blood, or lymph, or dermal skin layers. There the larvae remain, ultimately to perish unless they are ingested by a biting or bloodsucking insect.

Not only do the habits, habitats, and structural modifications of the various insects and certain of their close relatives lead these arthropods to ingest the microscopic progeny of parasitic worms of many kinds. The insects like-wise afford an almost ideal means of transport of the infectious stages of the worms back to the definitive hosts, livestock or poultry. Many of them form part of the normal diet of birds. In grazing, swine, sheep, cattle, and horses cannot avoid taking in beetles, mites, and similar insects along with the herbage they consume. A dog or cat suffering from infestation by fleas or lice, bites and licks at the noxious creatures and swallows some of them. A female mosquito must have a blood meal before it can lay fertile eggs and a further blood meal between every two batches of eggs it lays. If, between meals, infectious worm larvae have developed in its body, it injects these into the blood of the next animal it bites.

Many adaptations exist among insects, parasitic worms, and the definitive hosts of the worms. Farmers can take advantage of some of these adaptations to protect livestock and poultry against the inroads of insect-borne worm infections,

INSECTS FREQUENT FECAL and related waste materials because those substances are essential for their growth and development or because they contain something that attracts insects a bright or moving object, for example. Segments of tapeworms, because of their bright color or ability to move, readily attract insects and mites and often are eaten by them.

Insect-borne worms of livestock and poultry include representatives of all four of the major groups of helminths: Roundworms (Nematoda), tapeworms (Cestoda), thorny-headed worms Acanthocephala), and flukes (Trematoda) . Those that are transmitted by habitual or accidental dung feeders inhabit the digestive tract of the definitive host or organs that communicate with this tract; as has been noted, the eggs or larvae of worms living in these situations occur in the host's feces.

Various species of tumble bugs and dung beetles are intermediate hosts for worms occurring in swine, sheep, cattle, poultry, cats, and dogs. Two stomach worms, Ascarops strongylina and Physocephalus sexalatus, of swine, and the gullet worm, Gongylonema putchrum, which occurs in swine, sheep and cattle, utilize such coprophagous beetles as Copris, Aphodius, Passalurus, Onthophagus, Scarabaeus, Gymnopleurus, Ataenius, Canthon, Phanaeus, and Geotrupes as intermediate hosts. The German cockroach also serves as an intermediate host of the gullet worm. The eggs ingested by the insects contain well-developed embryos at the time of oviposition. On hatching in the insect's gut, the larvae first enter the abdominal cavity of the intermediate host and finally come to rest in the walls of the Malpighian tubules or musculature, where they become encysted. Completely formed cysts are usually found free in the abdominal part of the body cavity. The larvae become infective in the intermediate host in a month or so.

The larvae of the esophageal worm, Spirocerca lupi, of the dog develop to the infective stage in the beetle, Scarabaeus saver, and other beetles. The infective larvae become encysted in these insects, chiefly on the tracheal tubes. If such beetles are swallowed by an unsuitable host, such as a frog, snake, bird, or a small mammal, the larval worms become encysted again in the esophagus, mesentery, or other organs of these animals. This phenomenon is also known to occur in the case of the swine stomach worm, Physocephalus sexalatus, the larvae of which have been found naturally reencysted in the wall of the digestive tract of such birds as the loggerhead shrike, screech owl, and red-tailed hawk in southern Georgia and northern Florida. Reencystment of the larvae was found in experiments to occur in many different animals, including birds, mammals, and reptiles, to which beetles containing infective larvae were fed.

One of the commonest species of tapeworms, Hymenolepis carioca, found in the domestic fowl, is transmitted by beetles (Aphodius, Choeridium, Hister, and maybe Anisotarsus).

Another species of tapeworm, Hymenolepis cantaniana, found in chickens, turkeys, and quail of the Eastern States, develops in the beetles Ataenius and Choeridium. Its development in its intermediate host is unusual. The larva elongates to form a somewhat branched myceliumlike structure; buds along the branches develop into the cysticercoids, or small larval forms, which contain the tapeworm heads. Tapeworms belonging to the genera Joyeuxlella and Diplopylidlum, which are closely related to Dipylidium, occur in cats and apparently develop in dung beetles and related insects. It takes about 3 weeks to 2 months, depending on temperature, for the cysticercoids to develop within the insect host. Completely developed cysts are found in its body cavity. The tapeworm Metrollasthes lucida, commonly found in the small intestine of the domestic and wild turkey, is reported to have the grasshoppers Melanoplus sp., Chorthippus longicornis, and Paroxya clavuliger as intermediate hosts. Guinea fowls are also susceptible to infection with this tapeworm.

Dermestid beetles, darkling beetles, fungus beetles, and other groups of beetles and several species of grasshoppers have been infected experimentally, or found to be infected naturally with the larvae of the gizzard worm of poultry, Cheilospirura hamulosa.