Nagana, or African trypanosomiasis of animals, caused by some six species, already named, affects all mammals. Economically it is the most important protozoan disease of livestock. In cattle the species responsible for the disease, in order of importance, are T. congolense,T. vivax, and T. uniforme. The first two account for most of the cases. T. congolense occurs throughout the tsetse fly areas and is the most virulent trypanosome affecting animals. The organisms are found only in the blood. In the case of T. vivax, organisms are not readily found in the blood stream but may generally be demonstrated in a gland smear.

A study of cattle losses in Nigeria revealed that 30 of every 100 deaths were due to nagana.

In horses, the principal species causing nagana are T. brucei and T. congolense. T. vivax sometimes infects horses but rarely causes symptoms. T. brucei, like T. vivax, is more readily found in glandular tissue than in blood. Horses infected with either T. brucei or T. conolense almost always die unless they are adequately treated.

In sheep and goats, nagana is caused by the same species that cause it in horses. T. congolense infections, in contrast to those associated with other trypanosomes, is characterized by a sameness of grave disease in cattle, horses, sheep, and goats.

In swine, the chief pathogen is T. simiae, which has been called the lightning destroyer of pigs. Swine are susceptible to infection with T. brucei and T. congolense, but these species rarely produce symptoms. T. simiae infection Of pigs is extremely acute. Animals in apparently good health are taken ill overnight and die the next day.

Camels and dogs are notably susceptible to trypanosomiases; in them, T. evansi, the cause of surra in horses, produces the same disease. Some authorities regard surra as a predominantly camel disease. Camels and dogs are also victims of severe and fatal disease caused by T. congolense and T. brucei. Camels, but not dogs, are subject to the same hyperacute disease caused by T. simiae that occurs in pigs. This species, as suggested by its name, also causes fatal illness in monkeys.

The transmission of nagana is both biological and mechanical. Tsetse flies are the only biological vectors, but they and other biting flies transmit the infections mechanically. The cyclical development of trypanosomes in tsetse flies is exceedingly complicated, since it varies with different species of trypanosomes and even with the same species in different tsetse fly species. In general, T. congolense, for example, initiates its development in the alimentary tract of the fly. Then elongated organisms move to the hypo-pharynx, where attached intermediate forms and free trypanosomes successively develop. In the case of T. vivax, all development takes place in the mouth parts of the fly. Usually from 2 to 4 weeks are required for multiplication and metamorphosis in the fly.

All forms of nagana are also spread by the interrupted feeding of biting flies, including tsetses, and this may be the normal method of transmission when outbreaks occur. Throughout the tsetse fly region of equatorial Africa, there exist numerous horse flies and other biting flies, notably Chrysops and Haematopota. With reference to these genera, probably all species act as mechanical vectors. Small flies are poorer vectors than large ones.

Extensive studies of nagana seem to warrant the general deduction that mechanical vectors have a large part in the spread of African trypanosomiases but that cyclical development in tsetse flies is essential to the perpetuation of the diseases. Eradication of tsetse flies from any area has always eliminated nagana completely.

Tsetse flies, found only in Africa, owe their importance entirely to the fact that they are vectors of trypanosomes. The principal species, about 20, vary considerably in size, abundance, distribution, habits, susceptibility to adverse environment, and economic importance. They are about the size of house flies. Low mean temperatures generally are unfavorable to them. They cannot endure dry heat or temperature above 106 F., even in areas of high humidity. Vegetation must be ample for the support of reservoir and other host animals, since blood is the sole food of tsetses, but treeless grassland, deciduous bushland, and woodlands with a thick underbrush are unfavorable. Rainfall or fresh-water streams must be abundant where the flies and their mammalian hosts reach maximum populations. Some tsetses in East Africa, however, are well adapted to comparatively and districts. Because of this delicate environmental adjustment, tree clearance, burning of grass and brush, establishment of zones of vegetation-clearance, and like measures have been useful in controlling the flies. Seasonal changes and other natural factors cause expansion and contraction of fly belts.

Unlike mosquitoes, which are the only insects of greater medical importance than the tsetses, males as well as females are bloodsuckers. They feed mainly on large game and domestic animals. Native game, however, are comparatively resistant but serve as reservoirs of trypanosomes. Probably no trypanosome is pathogenic to its normal host. In any event, notwithstanding a complete dependence on large mammals, neither tsetse flies nor trypanosomes are especially host-specific. Some authorities also believe that any species of tsetse fly probably can transmit any species of pathogenic trypanosome with which it comes in common contact.

In addition to peculiar feeding habits and the comparative immunity of native game reservoirs, the method of reproduction of tsetse flies increases the difficulties of control. Females do not lay eggs like most insects. They give birth to live young and deposit the larvae in haunts that are peculiar to the individual species. Larvicides therefore are of no avail, and breeding places cannot be eradicated.

Adult tsetse flies probably do not live longer than 8 or 10 months. Their cycle of development is comparatively simple and direct. Females produce their first larvae about 3 or 4 weeks after mating. One large larva is produced at a time, but a new larva begins its development as soon as one is born. Successive larvae are produced every 9 to 14 days. The larvae pupate promptly in warm, loose soil of protected, shady areas. Pupation lasts 2 weeks to 4 months, and the adults rarely emerge unless the temperature is above 70 and below 87 .

The control of nagana tsetse fly disease, or African animal trypanosomiasis is much more than an entomological or veterinary problem. It is acutely beset with economic and sociological obstacles and with the basic agricultural problems of land usage and soil erosion. But such considerations do not lower the value of continued effort to achieve better control through therapy. prophylaxis, and immunization directed against the trypanosomes, through eradicative and limiting measures directed against tsetse flies, and through modifications of the environment to make it unfavorable for the continuance of trypanosomal diseases. Increased utilization of disease-resistant breeds of livestock, such as the West African Shorthorn cattle, for example, may also be a measure of great potential value.

THE OUTLOOK for better control of insect-borne diseases is bright. The discoveries of new insecticides and the devising of effective formulations and methods of application have in large measure provided the means for a concerted attack upon the insect vectors. New chemicals for treatment of these diseases and methods of immunization against them are also available. Finally, an ever-increasing knowledge of all aspects of insect-borne diseases has provided the foundations essential to the success of applied control measures. Seemingly the major limiting factor in the achievement of unprecedented, constructive victories is the modest economic burden that would be temporarily imposed.

GERARD DIKMAN is a graduate veterinarian from Michigan State College and holder of a master's degree from Minnesota and a doctor's degree from Georgetown University, has been a parasitologist in the Bureau of Animal Industry since 1926. For several years he has been in charge of investigations of ruminant parasites.

A. O. FOSTER, a parasitologist in the Bureau of Animal Industry, is in charge of anthelmintic investigations. He was trained at the Johns Hopkins University School of Hygiene and Public Health and served for 5 years on the stag of the Gorgas Memorial Laboratory of Tropical and Preventive Medicine in Panama.

C. D. STEIN is a veterinarian in the Bureau of Animal Industry. For many years he has contributed important researches on anthrax, equine infectious anemia, and other diseases of large and small animals.

L. T. GILTNER, a veterinarian, is Pathology consultant in the Bureau of Animal Industry. He was assistant chief of the pathological division for many years and has pursued or directed investigations on nearly all aspects of animal diseases.

Camponotus castaneus, a common ant.