Carriers of Human Diseases

F. C. Bishopp, Cornelius B. Philip.

Through the centuries people have been plagued by insects and have died by the millions from diseases carried by them. Man is gradually gaining mastery over them, but the battle is long and expensive, the burden is too heavy for the poor in many parts of the world, and we still have much to learn about these agents of death.

Probably 10,000 kinds of mites, ticks, and insects infect man directly or indirectly with disease. Most of them are only occasional and accidental carriers. Many spread diseases among livestock and wildlife and carry them from the animal reservoir's of infections back to persons.

INSECTS TRANSMIT DISEASE in many complex ways.

First, their mere presence or attack, without the transfer of germs, may produce a disease or harmful condition. Itch mites and screw-worms that invade the tissues are of this type. Some insects cause accidental injury to sense organs. Others produce intense itching and allergies, such as are caused by body lice, bee stings, and bites of chiggers and ticks. Some persons have idiosyncrasies that intensify their reaction to such attacks.

A fly or other insect that walks over and feeds on filth and then deposits the germ-laden contaminants on food by crawling over it, vomiting on it, or defecating on it is spoken of as a mechanical carrier. An insect, such as a horse fly, is also a mechanical carrier when it picks up germs by biting a diseased animal or person and then carries the germ on its beak until it bites a healthy individual.

More complex is the relationship among insect, disease, and man when the disease germ multiplies in the insect but does not change greatly in form. That occurs in fleas when they ingest plague organisms with the blood of a plague-stricken rat.

The most complex relationship is illustrated by anopheline mosquitoes in transmitting malaria. The malaria organisms in the blood of man at times produce male and female cells. The mosquito ingests the cells when it bites. The cells mate in the mosquito's stomach and develop into active ookinetes, which penetrate the stomach wall of the mosquito and thereon form cysts. Cell division takes place in the cysts, and hundreds of small, spindle-shaped sporozoites are formed.

The greatly enlarged oocyst then bursts open within the insect's body cavity. The active sporozoites swarm out, soon reach and penetrate the salivary glands, and are ready to pass into the blood stream of the next person the mosquito bites. This cycle, which takes 7 to 10 days, is called the essential or sexual cycle. Upon entering the blood stream, the minute malaria organisms the sporozoites enter such organs as the liver. In a few days they attack the red blood cells, in which they go through another cycle of growth and multiplication. Some ultimately become sexually mature, ready for other mosquitoes to ingest, and so repeat the sexual cycle in the insect.

Many variations occur in this method of disease transmission, which is called obligatory or cyclic because the disease organism is dependent on an insect for its continued natural transmission.

INSECTS CARRY DISEASE ORGANISMS of many types, among them microscopic viruses, bacteria, and protozoa and the larger roundworms and tapeworms. Ways by which disease organisms are kept alive in higher animals and insects and are passed from one generation to another frequently are very complex. Unraveling them has often required great scientific imagination and patient skill. We give some examples later. Sometimes disease organisms are carried from one stage of an insect host to another, with the intermediate stage or stages not transmitting infection or even living as parasites. In many instances the disease agents pass through the egg from one host generation to the next.

The disease cycle can be broken by destroying the insect vector, by using drugs to kill or suppress the disease organisms in the human host, or by immunization. Most successful usually is a combination of the three, plus isolation of infected persons (to prevent the insect vector from acquiring the disease organism) and such sanitary measures as screening to protect healthy persons.

Does the disease make the insect sick or kill it? Sometimes the infected insect is not injured in any way; apparently it has become tolerant. Sometimes its life span may be shortened. Occasionally it may be killed when that happens, that particular kind of insect is not a usual or well-adapted carrier of those particular disease germs.

INSECTS OF THE ORDER DIPTERA, or two-winged flies, perhaps are responsible for more human illness and death than any other group. They may rank with the world's top killers of man. Mosquitoes inhabit practically all parts of the earth except the polar regions. They alone carry malaria, yellow fever, dengue, and bancroftian and malayan filariasis. They also carry certain types of encephalitis and may be involved occasionally in the mechanical transfer of tularemia and anthrax.

Malaria, the great disabler, prevails throughout the Tropics and much of the temperate regions. Outbreaks have occurred in Canada and as far north as Archangel in Soviet Russia. Species of dapple-winged Anopheles mosquitoes are the carriers of human malaria.

Large areas of the United States once were malarious, but as the swamps were drained and the land tilled and people got into screened houses, the malady was pushed southward. There the mild climate and abundant water areas gave opportunity for mosquitoes to breed in numbers during the long summer. The malaria parasites also developed in the mosquitoes and people were more exposed, because they spent more time outside during the warm evenings. Poorly built and unscreened houses sometimes permitted infection to occur even indoors. Since 1943 the disease has been further reduced by the use of DDT. There is little malaria now in the United States.

About a dozen species of Anopheles mosquitoes occur in the United States, but only one in the Eastern and Southern States has been important in transmitting malaria. Likewise in the Pacific States a single species, but a different one, is the natural carrier. Scores of different kinds of Anopheles exist in various parts of the world. Their varying breeding and biting habits determine which control measures are instituted in any area. Some carry malaria. Others have no part in infecting man.

Yellow fever, or yellow jack, periodically put terror in the hearts of our people, especially in the South in the early days. When Reed, Carroll. Lazear, and Agramonte proved in 1901 that a semi-domestic mosquito, now known as the yellow-fever mosquito, was the vector, some of the terror disappeared. But although we know how to control or eradicate the mosquito, and although a protective vaccine has been developed, the disease is still regarded as a serious threat to this country and to many other warmer parts of the world. A deadly virus disease, it still lurks in the jungles of South America and Africa. To start serious trouble, the virus needs only to be transferred by jungle species of mosquitoes from an infected monkey to a man, who in turn may infect the yellow-fever mosquito in a populous area. Indeed, this insidious disease has suddenly flared up since 1950 in the jungles of Panama and Costa Rica, where it was thought to be stamped out, resulting in somewhat hysteric, unfounded reports even in Mexico.

The yellow-fever mosquito lives close to humans. It breeds in water in old tin cans, flower vases, and discarded tires. It is seldom found more than a quarter of a mile from a house. Only female mosquitoes bite. Females of the yellow-fever species slip out of hiding places at twilight, find exposed ankles or arms, and dart away at the slightest motion.

The Nobel Prize in Medicine for 1951 was awarded Max Theiler for development of a vaccine of living, attenuated virus, which has not only protected thousands of exposed civilians and troops but has undoubtedly been instrumental in keeping this dread disease out of the Far East, despite the increase in travel by air.

Dengue, or breakbone fever, also carried by the yellow-fever mosquito, is a painful and debilitating but not fatal virus disease that strikes occasionally. In an epidemic in 1922, Texas had more than a half million cases. For short periods it incapacitated large numbers of our troops on Guam and other Pacific islands during the Second World War.

ENCEPHALITIS, caused by several kinds of viruses that attack the central nervous systems of vertebrates, is transmitted by several species of mosquitoes. One species may be a vector of one virus strain and not of another. An outbreak of the so-called St. Louis type of encephalitis in 1933 is thought to have been carried by the northern house mosquito. A strain of the disease that has caused several hundred cases of human encephalitis each year is carried primarily by Culex tarsalis. Several species of mosquitoes can transmit the serious "Japanese B" encephalitis, which has caused serious epidemics in Japan and adjacent areas. Two types of equine sleeping sickness that have killed thousands of horses in the United States also cause illness in man and are probably transferred by a number of our common species of mosquitoes. Some infected parasitic bugs and bird mites have been found in the wild.

Elephantiasis, a disfiguring malady of people in the Tropics and subtropics, is carried by mosquitoes. The extremities and genitals often become greatly swollen because of small roundworms that establish themselves in the lymph glands. Into the blood stream the worms discharge eggs, which, after developing to active embryos known as microfilariae, are picked up by mosquitoes when biting. Some strains of the young worms swarm in the blood near the surface of the body at the time of the day or night when the favored species of mosquito is likely to bite.

Upon reaching the stomach of the mosquito, the young worms wiggle out of their saclike sheaths in an hour or so. They work through the stomach wall and into the thoracic muscles. There they grow for 2 or 3 weeks. Then they migrate to the beak of the mosquito, curl up, and await a chance to gain entrance to the skin of a person when the mosquito again bites. The worms, about one-twentieth inch long, burrow into the skin, reach the capillaries, and are carried in the blood stream to a lymph gland, where they develop to maturity. The female worms are 3 to 4 inches long. The males are about half that size. The cycle is complete when mating takes place and production of microfilariae begins.

Elephantiasis does not necessarily follow infestation from an infected mosquito bite, but skin irritation and fever are often manifest. Infection by these little worms is called filariasis. The malady occurred a number of years in the vicinity of Charleston, S. C., but it appears to have died out. No other endemic foci are known in the United States, although the carrier, the southern house mosquito, is widely distributed in the South.