Handbook of Medical Entomology Part 20

The procedure during an outbreak of the human plague was well ill.u.s.trated by the fight in San Francisco. The city was districted, and captured rats, after being dipped in some fluid to destroy the fleas, were carefully tagged to indicate their source, and were sent to the laboratory for examination. If an infected rat was found, the officers in charge of the work in the district involved were immediately notified by telephone, and the infected building was subjected to a thorough fumigation. In addition, special attention was given to all the territory in the four contiguous blocks.

By measures such as these, this dread scourge of the human race is being brought under control. Incidentally, the enormous losses due to the direct ravages of rats are being obviated and this alone would justify the expenditure many times over of the money and labor involved in the anti-rat measures.

CHAPTER VII

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 pa.s.sively 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.

[Ill.u.s.tration: 113. Dipylidium caninum. The double pored tapeworm of the dog.]

In some cases, the s.e.xual cycle of the parasite is undergone in the arthropod, which then serves as the _definitive_ or _primary host_. In other cases, it is the as.e.xual 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 ill.u.s.trated 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 s.e.xual 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 cuc.u.merina_ 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, Gra.s.si found that it could also develop in the cat and dog fleas, _Ctenocephalus felis_ and _C.

canis_, and in the human flea, _Pulex irritans_.

[Ill.u.s.tration: 114. Dipylidium caninum. Rostrum ev.a.g.i.n.ated and inv.a.g.i.n.ated. After Blanchard.]

[Ill.u.s.tration: 115. Dipylidium caninum. Immature cysticercoid. After Gra.s.si and Rovelli.]

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 300 in length, almost entirely dest.i.tute of a bladder, but in the immature stage provided with a caudal appendage (fig. 115).

Within the pear-shaped body (fig. 116) are the inv.a.g.i.n.ated 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 s.e.xual 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.

[Ill.u.s.tration: 116. Dipylidium caninum. Cysticercoid. After Villet.]

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 ill.u.s.trated 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. Gra.s.si 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_. Gra.s.si 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.

[Ill.u.s.tration: 117. One way in which Dipylidium infection in children may occur. After Blanchard.]

_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.

[Ill.u.s.tration: 118. The probable method by which Dipylidium infection usually occurs.]

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 (Gra.s.si and Rovelli);

_Davainea cesticillus_, of chickens, in some lepidopter or coleopter (Gra.s.si 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 s.e.xually 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 s.c.r.o.t.u.m, 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.

[Ill.u.s.tration: 119. Elephantiasis in Man. From "New Sydenham Society's Atlas."]

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), Gra.s.si 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 300 8, 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 ca.n.a.l 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.

[Ill.u.s.tration: 120. Filaria in the muscles and labium of Culex. After Blanchard.]

Once entered, they migrate to the lymphatics and there quickly become s.e.xually 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_.