Handbook of Medical Entomology Part 24

_Aedes calopus_, more commonly known as _Stegomyia fasciata_ or _Stegomyia calopus_ (fig. 134) is a moderate sized, rather strikingly marked mosquito. The general color is dark-brown or reddish-brown, but the thorax has a conspicuous broad, silvery-white curved line on each side, with two parallel median silvery lines. Between the latter there is a slender, broken line. The whole gives a lyre-shaped pattern to the thorax. The abdomen is dark with silvery-white basal bands and silvery white spots on each side of the abdominal segments. Legs black with rings of pure white at the base of the segments.

Size of the female 3.3 to 5 mm.; male 3 to 4.5 mm.

[Ill.u.s.tration: 134. The yellow fever mosquito (Aedes calopus). (7).

After Howard.]

It is preeminently a domesticated species, being found almost exclusively about the habitation of man. "Its long a.s.sociation with man is shown by many of its habits. It approaches stealthily from behind. It retreats upon the slightest alarm. The ankles and, when one is sitting at a table or desk, the underside of the hands and wrists are favorable points of attack. It attacks silently, whereas other mosquitoes have a piping or humming note. The warning sound has doubtless been suppressed in the evolutionary process of its adaptation to man. It is extremely wary. It hides whenever it can, concealing itself in garments, working into the pockets, and under the lapels of coats, and crawling up under the clothes to bite the legs. In houses, it will hide in dark corners, under picture moldings and behind the heads of old-fashioned bedsteads.

It will enter closets and hide in the folds of garments."--Howard.

It was claimed by the French Commission, and subsequently often stated in discussions of the relation of the mosquito to yellow fever that the mature _Aedes calopus_ will bite only at night. If this were true it would be of the greatest importance in measures to avoid the disease.

Unfortunately, the claim was illy founded and numerous workers have clearly established that the exact converse is more nearly true, this mosquito being pre-eminently a day species, feeding most actively in early morning, about sunrise, and late in the afternoon. On cloudy days it attacks at any time during the day. Thus there is peril in the doctrine that infected regions may be visited with perfect safety during the daytime and that measures to avoid the mosquito attack need be taken only at night.

[Ill.u.s.tration: 135_a_. Aedes calopus. Pupa. After Howard.]

Dr. Finlay maintained that the adult, even when starved, would not bite when the temperature was below 23 C, but subsequent studies have shown that this statement needs modification. The French Commission, working at Rio de Janeiro, found that _Aedes calopus_ would bite regularly at temperatures between 22 and 25 and that the optimum temperature was between 27 and 30 C, but their experiments led them to believe that it would bite in nature at a temperature as low as 17 C.

The yellow fever mosquito breeds in cisterns, water barrels, pitchers and in the various water receptacles about the house. In our own Southern States it very commonly breeds in the above-ground cisterns which are in general use. Often the larvae (fig. 135b) are found in flower vases, or even in the little cups of water which are placed under the legs of tables to prevent their being overrun by ants. They have been repeatedly found breeding in the holy water font in churches. In short, they breed in any collection of water in close proximity to the dwellings or gathering places of man.

The life cycle under favorable conditions is completed in from twelve to fifteen days. These figures are of course very dependent upon the temperature. The Army Commission in Cuba found that the cycle might be completed in as brief a period as nine and a half days. Under less favorable conditions it may be greatly lengthened.

The adults are long lived. We have seen that during the experimental work in Cuba specimens were kept in captivity for sixty-nine and seventy-one days, respectively, and that they were proved to retain their infectivity for at least fifty-seven days. Dr. Guiteras subsequently kept an infected adult for one hundred and fifty-four days.

Low temperatures have a very great effect not only on development, but on the activity and even life of the adults. Long before the method of transmission of yellow fever was discovered it was well known that the epidemics were brought to a close by heavy frosts, and it is now known that this is due to the killing of the mosquitoes which alone could spread the disease.

[Ill.u.s.tration: 135_b_. Aedes calopus; larva. (7). After Howard.]

_Aedes calopus_ has a very wide distribution since, as Howard says, being a domestic mosquito, having a fairly long life in the adult stage, and having the custom of hiding itself in the most ingenious ways, it is particularly subject to carriage for long distances on board vessels, in railway trains, even packed in baggage. In general, its permanent distribution is from 40 degrees north lat.i.tude to 40 degrees south lat.i.tude (Brumpt), in a belt extending around the world. In the United States it breeds in most of our Southern States.

Thus, as in the case of malaria, there are many places where the insect carrier is abundant but where yellow fever does not occur. Such, for instance, are Hawaii, Australia and Asia. An outbreak may occur at any time that a patient suffering from the disease is allowed to enter and become a source of infection for the mosquitoes. In this connection various writers have called attention to the menace from the Panama Ca.n.a.l. When it is completed, it will allow of direct pa.s.sage from regions where yellow fever is endemic and this will greatly increase the possibility of its introduction into these places where it is now unknown. The result, with a wholly non-immune population, would be appalling.

On the other hand, there are places wholly outside of the normal range of _Aedes calopus_ where the disease has raged. Such are New York, Boston, and even Philadelphia, which have suffered notable epidemics.

These outbreaks have been due to the introduction of infected mosquitoes during the heat of summer, when they have not only conveyed the disease but have found conditions favorable for their multiplication. Or, uninfected mosquitoes have been thus accidentally brought in and developed in large numbers, needing then only the accidental introduction of cases of the disease to start an epidemic.

Methods of control of various diseases have been revolutionized by the discovery that they were insect-borne, but in no other case has the change been as radical or the results as spectacular as in the case of yellow fever. The "shot-gun quarantine," the sufferings and horrors, the hopelessness of fighting absolutely blindly have given way to an efficient, clear-cut method of control, based upon the knowledge that the disease is carried from man to man solely by the mosquito, _Aedes calopus_. The lines of defense and offense are essentially as follows:

In the first place, when a case of yellow fever occurs, stringent precautions must be adopted to prevent the infection of mosquitoes and the escape of any already infected. This means that the patient must be removed to a mosquito-proof room, or ward beyond reach of the insects, and that the infected room must be thoroughly fumigated at once, to kill the mosquitoes hiding within it. All cracks and openings should be closed with strips of paper and fumigation with burning sulphur or pyrethrum carefully carried out.

It should be remembered that if the first case noted is that of a resident rather than imported, it means that the mosquito carriers became infected more than two weeks before the case was diagnosed, for as we have seen, the germ must undergo a twelve-day period of development within its insect host. Therefore a careful search must be made for mild cases which, though unrecognized, may serve as foci for the spread of the disease.

In face of a threatened epidemic one of the most essential measures is to educate the citizens and to gain their complete cooperation in the fight along modern lines. This may be done through the schools, the pulpit, places of amus.e.m.e.nt, newspapers and even bulletin boards.

Emphasis should be placed on the necessity of both non-immunes and immunes using mosquito curtains, and in all possible ways avoiding exposure to the mosquitoes.

Then the backbone of the fight must be the anti-mosquito measures. In general, these involve screening and fumigating against adults, and control of water supply, oiling, and drainage against the larvae. The region involved must be districted and a thorough survey undertaken to locate breeding places, which must, if possible, be eradicated. If they are necessary for water supplies, such as casks, or cisterns, they should be carefully screened to prevent access of egg-laying adults.

The practical results of anti-mosquito measures in the fight against yellow fever are well ill.u.s.trated by the cla.s.sic examples of the work in Havana, immediately following the discoveries of the Army Commission and by the stamping out of the New Orleans epidemic in 1905.

The opportunities for an immediate practical application of the theories of the Army Commission in Havana were ideal. The city had always been a hotbed of yellow fever and was the princ.i.p.al source from which the disease was introduced year after year into our Southern States. It was under martial law and with a military governor who was himself a physician and thoroughly in sympathy with the views of the Commission, the rigid enforcement of the necessary regulations was possible. The story of the first campaign has been often told, but nowhere more clearly than in Dr. Reed's own account, published in the _Journal of Hygiene_ for 1902.

Closer home was the demonstration of the efficacy of these measures in controlling the yellow fever outbreak in New Orleans in 1905. During the spring and early summer of the year the disease had, unperceived, gained a firm foothold in that city and when, in early July the local Board of Health took cognizance of its existence, it was estimated that there had been in the neighborhood of one hundred cases.

Conditions were not as favorable as they had been under martial law in Havana for carrying on a rigid fight along anti-mosquito lines. The densely populated city was unprepared, the public had to be educated, and an efficient organization built up. The local authorities actively began a general fight against the mosquito but in spite of their best efforts the disease continued to spread. It was recognized that more rigid organization was needed and on August 12th the United States Public Health and Marine Hospital Service was put in absolute charge of the fight. Up to this time there had been one hundred and forty-two deaths from a total of nine hundred and thirteen cases and all of the conditions seemed to threaten an outbreak to exceed the memorable one of 1878 when, as we have seen there were four thousand and forty-six deaths.

With the hearty cooperation of the citizens,--physicians and laymen alike,--the fight was waged and long before frost or any near approach thereto the disease was stamped out,--a thing unheard of in previous epidemics. The total loss of life was four hundred and sixty--about 11 per cent as great as that from the comparable epidemic of 1878. If the disease had been promptly recognized and combated with the energy which marked the fight later in the summer, the outbreak would have made little headway and the great proportion of these lives would have been saved.

CHAPTER IX

ARTHROPODS AS ESSENTIAL HOSTS OF PATHOGENIC PROTOZOA (Continued)

INSECTS AND TRYPANOSOMIASES

By trypanosomiasis is meant a condition of animal parasitism, common to man and the lower animals, in which trypanosomes, peculiar flagellate protozoa, infest the blood. Depending upon the species, they may be harmless, producing no appreciable ill-effect, or pathogenic, giving rise to conditions of disease. A number of these are known to be transferred by insects.

In order that we may consider more fully the developmental stage of these parasites within their insect host, it is necessary that we describe briefly the structure of the blood-inhabiting stage.

[Ill.u.s.tration: 136. Trypanosoma brucei. After Bruce.]

The trypanosomes are elongated, usually pointed, flagellated protozoa (fig. 136) in which the single flagellum, bent under the body, forms the outer limit of a delicate undulating membrane. It arises near one end of the organism from a minute centrosome-like body which is known as the blepheroplast, and at the opposite end extends for a greater or less distance as a free flagellum. Enclosing, or close beside the blepheroplast is the small kinetonucleus. The princ.i.p.al nucleus, round or oval in form, is situated near the center of the body. As.e.xual reproductions occurs in this stage, by longitudinal fission, the nucleus and the blepheroplast dividing independently of one another. From the blepheroplast of one of the daughter cells a new flagellum is formed.

Among the pathogenic species are to be found the causative organisms of some of the most serious diseases of domestic animals and even of man.

It is probable that these pathogenic species secrete a specific poison.

The majority of them are tropical in distribution.

Though we are concerned especially with the species which infest man, we shall first consider two of the trypanosomes of lower animals, known long before any of those of man had been found.

FLEAS AND LICE AS CARRIERS OF TRYPANOSOMA LEWISI.--_Trypanosoma lewisi_, the first mammalian trypanosome known, is to be found in the blood of wild rats. Like its host, it appears to be cosmopolitan in distribution, having been reported from several localities in the United States, Brazil, Argentine, England, Germany, France, Italy, Russia, Asia and Africa.

This species is usually regarded as non-pathogenic, but in experimental work, especially with white rats, heavy infestations often result fatally, and naturally infested specimens sometimes show evidence of injury. Rats which have been infested exhibit at least temporary immunity against new infection.

_Trypanosoma lewisi_ is transmitted from rat to rat by fleas and by lice. Rabinowitsch and Kempner (1899) first found that healthy rats which were kept with infested rats, showed trypanosomes in their blood after about two weeks. They found the trypanosomes in the alimentary ca.n.a.l of fleas which had fed on the diseased rats. On teasing such fleas in physiological salt solution and inoculating them into fresh rats they were able to produce the infection. Finally, they showed that the fleas which had fed upon infested rats were able to carry the parasites to healthy rats. Corresponding experiments with lice were not successful.

Prowazek (1905) found in the rat louse (_Haematopinus spinulosus_) organisms which he regarded as developmental stages of the _Trypanosoma lewisi_. He believed that the s.e.xual cycle was undergone in this insect.

Nuttall (1908) readily transmitted the trypanosomes through the agency of fleas, (_Ceratophyllus fasciatus_ and _Ctenopthalmus agyrtes_). He believes that these insects are probably the chief transmitters of the parasite. He was also able to transmit it from diseased to healthy rats through the agency of the rat louse. He was unable to trace any developmental stages in the louse and inclined to the opinion that Prowazek was deceived by the presence of extraneous flagellates such as are known to exist in a number of blood-sucking arthropods.

Nuttall concludes that since three distinct kinds of blood-sucking insects are capable of transmitting _Trypanosoma lewisi_ it appears doubtful that this flagellate is a parasite of the invertebrate "host"

in the sense claimed by Prowazek and other investigators.

TSETSE-FLIES AND NAGANA--One of the greatest factors in r.e.t.a.r.ding the development of certain regions of Africa has been the presence of a small fly, little larger than the common house-fly. This is the tsetse-fly, _Glossina morsitans_ (fig. 165) renowned on account of the supposed virulence of its bite for cattle, horses and other domestic mammals.

The technical characteristics of the tsetse-flies, or Glossinas, and their several species, will be found in a later chapter. We need emphasize only that they are blood-sucking Muscidae and that, unlike the mosquitoes, the s.e.xes resemble each other closely in structure of the mouth-parts, and in feeding habits.

In 1894, Colonel David Bruce discovered that the fly was not in itself poisonous but that the deadly effect of its bite was due to the fact that it transmitted a highly pathogenic blood parasite, _Trypanosoma brucei_. This trypanosome Bruce had discovered in the blood of South African cattle suffering from a highly fatal disease known as "nagana".