Clearing the jungle along the water courses for some yards beyond the natural range of the fly has proved a very important measure. Castellani recommends that the area be one hundred yards and around a village three hundred yards at least.
Detailed studies of the parasites and the natural enemies of the tsetse-fly are being undertaken and may ultimately yield valuable results.
South American Trypanosomiasis—The tsetse-flies are distinctively African in distribution and until recently there were no trypanosomes known to infest man in America. In 1909 Dr. Chagas, of Rio de Janeiro described a new species, Trypanosoma cruzi, pathogenic to man.
Trypanosoma cruzi is the causative organism of a disease common in some regions of Brazil, where it is known as "opilacao." It is especially to be met with in children and is characterized by extreme anemia, wasting, and stunted development associated with fever, and enlargement of the thyroid glands. The disease is transmitted by the bites of several species of assassin-bugs, or Reduviidæ, notably by Conorhinus megistus. The evolution of the parasite within the bug has been studied especially by Chagas and by Brumpt. From the latter's text we take the following summary.
The adult trypanosomes, ingested by a Conorhinus megistus, of any stage, first change into Crithidia-like forms and then those which remain in the stomach become ovoid and non-motile. Brumpt found these forms in immense numbers, in a Cornohinus which had been infested fourteen months before. The forms which pass into the intestine quickly assume the Crithidia form and continue to develop rapidly under this form. Some weeks later they evolve into the trypanosome forms, pathogenic for man. They then pass out with the excrement of the bug and infect the vertebrate host as soon as they come in contact with any mucous layer (buccal, ocular or rectal). More rarely they enter through the epidermis.
Brumpt showed that the development could take place in three species; bed-bugs (Cimex lectularius, C. hemipterus) and in the tick Ornithodoros moubata. The evolution proceeds in the first two species of bed-bugs as rapidly as in Conorhinus, or even more rapidly, but they remain infective for a much shorter time and hence Brumpt considers that they play a much less important rôle in the spread of the disease.
Conorhinus megistus, like related forms in our Southern States, very commonly frequents houses and attacks man with avidity. Chagas states that the bites are painless and do not leave any traces. They are usually inflicted on the lips, or the cheeks and thus the buccal mucosa of a sleeper may be soiled by the dejections of the insect and the bite serving as a port of entry of the virus, remain unnoticed.
The possibility of some of our North American Reduviidæ playing a similar rôle in the transmission of disease should not be overlooked.
Leishmanioses and Insects—Closely related to the trypanosomes is a group of intracellular parasites which have recently been grouped by Ross under the genus Leishmania. Five species are known to affect man. Three of these produce local skin infestations, but two of them, Leishmania donovani and L. infantum, produce serious and often fatal systemic diseases.
The first of these, that produced by L. donovani, is an exceedingly virulent disease common in certain regions of India and China. It is commonly known as "Kala-azar," or "dum-dum" fever, and more technically as tropical leishmaniasis. Patton (1907) believes that the parasite is transmitted by the bed-bug Cimex hemipterus, and has described a developmental cycle similar to that which can be found in artificial cultures. On the other hand, Donovan was unable to confirm Patton's work and believes that the true intermediate host is a Reduviid bug, Conorhinus rubrofasciatus.