CHAPTER VII. Arthropods As Essential Hosts of Pathogenic Organisms

ARTHROPODS AS ESSENTIAL HOSTS OF PATHOGENIC ORGANISMS

We now have to consider the cases in which the arthropod acts as the essential host of a pathogenic organism. In other words, cases in which the organism, instead of being passively carried or merely accidentally inoculated by the bite of its carrier, or vector, is taken up and undergoes an essential part of its development within the arthropod.

In some cases, the sexual cycle of the parasite is undergone in the arthropod, which then serves as the definitive or primary host. In other cases, it is the asexual stage of the parasite which is undergone, and the arthropod then acts as the intermediate host. This distinction is often overlooked and all the cases incorrectly referred to as those in which the insect or other arthropod acts as intermediate host.

We have already emphasized that this is the most important way in which insects may transmit disease, for without them the particular organisms concerned could never complete their development. Exterminate the arthropod host and the life cycle of the parasite is broken, the disease is exterminated.

As the phenomenon of alternation of generations, as exhibited by many of the parasitic protozoa, is a complicated one and usually new to the student, we shall first take up some of the grosser cases illustrated by certain parasitic worms. There is the additional reason that these were the first cases known of arthropod transmission of pathogenic organisms.

INSECTS AS INTERMEDIATE HOSTS OF TAPEWORMS

A number of tapeworms are known to undergo their sexual stage in an insect or other arthropod. Of these at least two are occasional parasites of man.

Dipylidium caninum (figs. 113 and 114), more generally known as Taenia cucumerina or T. elliptica, is the commonest intestinal parasite of pet dogs and cats. It is occasionally found as a human parasite, 70 per cent of the cases reported being in young children.

In 1869, Melnikoff found in a dog louse, Trichodectes canis, some peculiar bodies which Leuckart identified as the larval form of this tapeworm. The worm is, however, much more common in dogs and cats than is the skin parasite, and hence it appears that the Trichodectes could not be the only intermediate host. In 1888, Grassi found that it could also develop in the cat and dog fleas, Ctenocephalus felis and C. canis, and in the human flea, Pulex irritans.

The eggs, scattered among the hairs of the dog or cat, are ingested by the insect host and in its body cavity they develop into pyriform bodies, about 300u in length, almost entirely destitute of a bladder, but in the immature stage provided with a caudal appendage (fig. 115). Within the pear-shaped body (fig. 116) are the invaginated head and suckers of the future tapeworm. This larval form is known as a cysticercoid, in contradistinction to the bladder-like cysticercus of many other cestodes. It is often referred to in literature as Cryptocystis trichodectis Villot.

As many as fifty of the cysticercoids have been found in the body cavity of a single flea. When the dog takes up an infested flea or louse, by biting itself, or when the cat licks them up, the larvae quickly develop into tapeworms, reaching sexual maturity in about twenty days in the intestine of their host. Puppies and kittens are quickly infested when suckling a flea-infested mother, the developing worms having been found in the intestines of puppies not more than five or six days old.

Infestation of human beings occurs only through accidental ingestion of an infested flea. It is natural that such cases should occur largely in children, where they may come about in some such way as illustrated in the accompanying figures 117 and 118.

Hymenolepis diminuta, very commonly living in the intestine of mice and rats, is also known to occur in man. Its cysticercoid develops in the body cavity of a surprising range of meal-infesting insects. Grassi and Rovelli (abstract in Ransom, 1904) found it in the larvae and adult of a moth, Asopia farinalis, in the earwig, Anisolabis annulipes, the Tenebrionid beetles Akis spinosa and Scaurus striatus. Grassi considers that the lepidopter is the normal intermediate host. The insect takes up the eggs scattered by rats and mice. It has been experimentally demonstrated that man may develop the tapeworm by swallowing infested insects. Natural infection probably occurs by ingesting such insects with cereals, or imperfectly cooked foods.

Hymenolepis lanceolata, a parasite of geese and ducks, has been reported once for man. The supposed cysticercoid occurs in various small crustaceans of the family Cyclopidae.

Several other cestode parasites of domestic animals are believed to develop their intermediate stage in certain arthropods. Among these may be mentioned:

Choanotaenia infundibulformis, of chickens, developing in the house-fly (Grassi and Rovelli);

Davainea cesticillus, of chickens, in some lepidopter or coleopter (Grassi and Rovelli);

Hymenolepis anatina, H. gracilis, H. sinuosa, H. coronula and Fimbriaria fasciolaris, all occurring in ducks, have been reported as developing in small aquatic crustaceans. In these cases, cysticercoids have been found which, on account of superficial characters, have been regarded as belonging to the several species, but direct experimental evidence is scant.

ARTHROPODS AS INTERMEDIATE HOSTS OF NEMATODE WORMS

FILARIASIS AND MOSQUITOES--A number of species of Nematode worms belonging to the genus Filaria, infest man and other vertebrates and in the larval condition are to be found in the blood. Such infestation is known as filariasis. The sexually mature worms are to be found in the blood, the lymphatics, the mesentery and subcutaneous connective tissue. In the cases best studied it has been found that the larval forms are taken up by mosquitoes and undergo a transformation before they can attain maturity in man.

The larvae circulating in the blood are conveniently designated as microfilariae. In this stage they are harmless and only one species, Filaria bancrofti, appears to be of any great pathological significance at any stage.

Filaria bancrofti in its adult state, lives in the lymphatics of man. Though often causing no injury it has been clearly established that they and their eggs may cause various disorders due to stoppage of the lymphatic trunks (fig. 119). Manson lists among other effects, abscess, varicose groin glands, lymph scrotum, chyluria, and elephantiasis.

The geographical distribution of this parasite is usually given as coextensive with that of elephantiasis, but it is by no means certain that it is the only cause of this disease and so actual findings of the parasites are necessary. Manson reports that it is "an indigenous parasite in almost every country throughout the tropical and subtropical world, as far north as Spain in Europe and Charlestown in the United States, and as far south as Brisbane in Australia." In some sections, fully 50 per cent of the natives are infested. Labredo (1910) found 17.82 per cent infestation in Havana.

The larval forms of Filaria bancrofti were first discovered in 1863, by Demarquay, in a case of chylous dropsy. They were subsequently noted under similar conditions, by several workers, and by Wucherer in the urine of twenty-eight cases of tropical chyluria, but in 1872 Lewis found that the blood of man was the normal habitat, and gave them the name Filaria sanguinis hominis. The adult worm was found in 1876 by Bancroft, and in 1877, Cobbold gave it the name Filaria bancrofti. It has since been found repeatedly in various parts of the lymphatic system, and its life-history has been the subject of detailed studies by Manson (1884), Bancroft (1899), Low (1900), Grassi and Noe (1900), Noe (1901) and Fulleborn (1910).

The larvae as they exist in the circulating blood, exhibit a very active wriggling movement, without material progression. They may exist in enormous numbers, as many as five or six hundred swarming in a single drop of blood. This is the more surprising when we consider that they measure about 300u x 8u, that is, their width is equal to the diameter of the red blood corpuscle of their host and their length over thirty-seven times as great.

Their organs are very immature and the structure obscure. When they have quieted down somewhat in a preparation it may be seen that at the head end there is a six-lipped and very delicate prepuce, enclosing a short "fang" which may be suddenly exserted and retracted. Completely enclosing the larva is a delicate sheath, which is considerably longer than the worm itself. To enter into further details of anatomy is beyond the scope of this discussion and readers interested are referred to the work of Manson and of Fulleborn.

One of the most surprising features of the habits of these larvae is the periodicity which they exhibit in their occurrence in the peripheral blood. If a preparation be made during the day time there may be no evidence whatever of filarial infestation, whereas a preparation from the same patient taken late in the evening or during the night may be literally swarming with the parasites. Manson quotes Mackenzie as having brought out the further interesting fact that should a "filarial subject be made to sleep during the day and remain awake at night, the periodicity is reversed; that is to say, the parasites come into the blood during the day and disappear from it during the night." There have been numerous attempts to explain this peculiar phenomenon of periodicity but in spite of objections which have been raised, the most plausible remains that of Manson, who believes that it is an adaptation correlated with the life-habits of the liberating agent of the parasite, the mosquito.

The next stages in the development of Filaria nocturna occur in mosquitoes, a fact suggested almost simultaneously by Bancroft and Manson in 1877, and first demonstrated by the latter very soon thereafter. The experiments were first carried out with Culex quinquefasciatus (= fatigans) as a host, but it is now known that a number of species of mosquitoes, both anopheline and culicine, may serve equally well.

When the blood of an infested individual is sucked up and reaches the stomach of such a mosquito, the larvae, by very active movements, escape from their sheaths and within a very few hours actively migrate to the body cavity of their new host and settle down primarily in the thoracic muscles. There in the course of sixteen to twenty days they undergo a metamorphosis of which the more conspicuous features are the formation of a mouth, an alimentary canal and a trilobed tail. At the same time there is an enormous increase in size, the larvae which measured .3 mm. in the blood becoming 1.5 mm. in length. This developmental period may be somewhat shortened in some cases and on the other hand may be considerably extended. The controlling factor seems to be the one of temperature.

The transformed larvae then reenter the body cavity and finally the majority of them reach the interior of the labium (fig. 120). A few enter the legs and antennae, and the abdomen, but these are wanderers which, it is possible, may likewise ultimately reach the labium, where they await the opportunity to enter their human host.

It was formerly supposed that when the infested mosquito punctured the skin of man, the mature larvae were injected into the circulation. The manner in which this occurred was not obvious, for when the insect feeds it inserts only the stylets, the labium itself remaining on the surface of the skin. Fulleborn has cleared up the question by showing that at this time the filariae escape and, like the hookworm, actively bore into the skin of their new host.

Once entered, they migrate to the lymphatics and there quickly become sexually mature. The full grown females measure 85-90 mm. in length by .24-.28 mm. in diameter, while the males are less than half this size, being about 40 mm. by .1 mm. Fecundation occurs and the females will be found filled with eggs in various stages of development, for they are normally viviparous.

Filaria philippinensis is reported by Ashburn and Craig (1907) as a common blood filaria in the Philippine Islands. As they describe it, it differs from Filaria bancrofti primarily in that it does not exhibit periodicity. Its development has been found to occur in Culex quinquefasciatus, where it undergoes metamorphosis in about fourteen or fifteen days. There is doubt as to the species being distinct from bancrofti.

Several other species occur in man and are thought to be transferred by various insects, among which have been mentioned Tabanidae and tsetse-flies, but there is no experimental proof in support of such conjectures.

Filaria immitis is a dangerous parasite of the dog, the adult worm living in the heart and veins of this animal. It is one of the species which has been clearly shown to undergo its development in the mosquito, particularly in Anopheles maculipennis and Aedes calopus (= Stegomyia). The larval form occurs in the peripheral blood, especially at night. When taken up by mosquitoes they differ from Filaria bancrofti in that they undergo their development in the Malpighian tubules rather than in the thoracic muscles. In about twelve days they have completed their growth in the tubules, pierce the distal end, and pass to the labium. This species occurs primarily in China and Japan, but is also found in Europe and in the United States. It is an especially favorable species for studying the transformations in the mosquito.

Filariae are also commonly found in birds, and in this country this is the most available source of laboratory material. We have found them locally (Ithaca, N. Y.) in the blood of over sixty per cent of all the crows examined, at any season of the year, and have also found them in English sparrows.

In the crows, they often occur in enormous numbers, as many as two thousand having been found in a single drop of the blood of the most heavily infested specimen examined. For study, a small drop of blood should be mounted on a clean slide and the coverglass rung with vaseline or oil to prevent evaporation. In this way they can be kept for hours.

Permanent preparations may be made by spreading out the blood in a film on a perfectly clean slide and staining. This is easiest done by touching the fresh drop of blood with the end of a second slide which is then held at an angle of about 45^o to the first slide and drawn over it without pressure. Allow the smear to dry in the air and stain in the usual way with haematoxylin.

OTHER NEMATODE PARASITES OF MAN AND ANIMALS DEVELOPING IN ARTHROPODS

Dracunculus medinensis (fig. 121), the so-called guinea-worm, is a nematode parasite of man which is widely distributed in tropical Africa, Asia, certain parts of Brazil and is occasionally imported into North America.

The female worm is excessively long and slender, measuring nearly three feet in length and not more than one-fifteenth of an inch in diameter. It is found in the subcutaneous connective tissue and when mature usually migrates to some part of the leg. Here it pierces the skin and there is formed a small superficial ulcer through which the larvae reach the exterior after bursting the body of the mother.

Fedtschenko (1879) found that when these larvae reach the water they penetrate the carapace of the little crustacean, Cyclops (fig. 122). Here they molt several times and undergo a metamorphosis. Fedtschenko, in Turkestan, found that these stages required about five weeks, while Manson who confirmed these general results, found that eight or nine weeks were required in the cooler climate of England.

Infection of the vertebrate host probably occurs through swallowing infested cyclops in drinking water. Fedtschenko was unable to demonstrate this experimentally and objection has been raised against the theory, but Leiper (1907), and Strassen (1907) succeeded in infesting monkeys by feeding them on cyclops containing the larvae.

Habronema muscae is a worm which has long been known in its larval stage, as a parasite of the house-fly. Carter found them in 33 per cent of the house-flies examined in Bombay during July, 1860, and since that time they have been shown to be very widely distributed. Italian workers reported them in 12 per cent to 30 per cent of the flies examined. Hewitt reported finding it rarely in England. In this country it was first reported by Leidy who found it in about 20 per cent of the flies examined at Philadelphia, Pa. Since then it has been reported by several American workers. We have found it at Ithaca, N. Y., but have not made sufficient examinations to justify stating percentage. Ransom (1913) reports it in thirty-nine out of one hundred and thirty-seven flies, or 28 per cent.

Until very recently the life-history of this parasite was unknown but the thorough work of Ransom (1911, 1913) has shown clearly that the adult stage occurs in the stomach of horses. The embryos, produced by the parent worms in the stomach of the horse, pass out with the feces and enter the bodies of fly larvae which are developing in the manure. In these they reach their final stage of larval development at about the time the adult flies emerge from the pupal stage. In the adult fly they are commonly found in the head, frequently in the proboscis, but they occur also in the thorax and abdomen. Infested flies are accidentally swallowed by horses and the parasite completes its development to maturity in the stomach of its definitive host.

Gigantorhynchus hirudinaceus (= Echinorhynchus gigas) is a common parasite of the pig and has been reported as occurring in man. The adult female is 20-35 cm. long and 4-9 mm. in diameter. It lacks an alimentary canal and is provided with a strongly spined protractile rostrum, by means of which it attaches to the intestinal mucosa of its host.

The eggs are scattered with the feces of the host and are taken up by certain beetle larvae. In Europe the usual intermediate hosts are the larvae of the cockchafer, Melolontha vulgaris, or of the flower beetle, Cetonia aurata. Stiles has shown that in the United States the intermediate host is the larva of the June bug, Lachnosterna (fig. 124). It is probable that several of the native species serve in this capacity.

A number of other nematode parasites of birds and mammals have been reported as developing in arthropods but here, as in the case of the cestodes, experimental proof is scant. The cases above cited are the better established and will serve as illustrations.