Handbook of Medical Entomology

by William A. Riley; O. A. Johannsen

Leishmania infantum is the cause of the so-called infantile splenic leishmaniasis, occurring in northern Africa, Spain, Portugal, Italy, and possibly other parts of Europe. The parasite occurs habitually in the dog and is only accidentally transferred to children. Alvares and da Silva, in Portugal (according to Brumpt, 1913) have found that the excrement of a flea from a diseased dog contains flagellates, and they suggest that the infection may be transmitted by the accidental inoculation of this excrement by means of the proboscis of the flea, as has been thought to occur in the case of the plague. To this Brumpt objects that they and other workers who thought to trace the development of Leishmania infantum were apparently misled by the presence of a harmless Herpetomonas which infests dog fleas in all countries, even where the leishmaniasis is unknown.

Basile (1910 and 1911) however, carried on numerous experiments indicating that the disease was transferred from children to dogs and from dog to dog by the dog flea, and was able to find in the tissues of the insects forms perfectly identical with those found in children and in dogs suffering from leishmaniasis. He also found that Pulex irritans was capable of acting as the carrier.

Of the cutaneous type of leishmaniasis, the best known is the so-called "Oriental sore," an ulcerative disease of the skin which is epidemic in many tropical and subtropical regions. The causative organism is Leishmania tropica, which occurs in the diseased tissues as bodies very similar to those found in the spleen in cases of Kala-azar. The disease is readily inoculable and there is no doubt that it may be transferred from the open sores to abraded surfaces of a healthy individual by house-flies. It is also believed by a number of investigators that it may be transferred and directly inoculated by various blood-sucking insects.

Ticks and Diseases of Man and Animals

We have seen that the way to the discoveries of the relations of arthropods to disease was pointed out by the work of Leuckart and Melnikoff on the life cycle of Dipylidium, and of Fedtschenko and Manson on that of Filaria. They dealt with grosser forms, belonging to well-recognized parasitic groups.

This was long before the rôle of any insect as a carrier of pathogenic micro-organisms had been established, and before the Protozoa were generally regarded as of importance in the causation of disease. The next important step was taken in 1889 when Smith and Kilbourne conclusively showed that the so-called Texas fever of cattle, in the United States, is due to an intracorpuscular blood parasite transmitted exclusively by a tick. This discovery, antedating by eight years the work on the relation of the mosquito to malaria, had a very great influence on subsequent studies along these lines.

While much of the recent work has dealt with the true insects, or hexapods, it is now known that several of the most serious diseases of animals, and at least two important diseases of man are tick borne. These belong to the types known collectively as babesioses (or "piroplasmoses"), and spirochætoses.

The term babesiosis is applied to a disease of man or animals which is caused by minute protozoan parasites of the genus Babesia, living in the red blood corpuscles. These parasites have usually been given the generic name Piroplasma and hence the type of disease which they cause is often referred to as "piroplasmosis." The best known illustration is the disease known in this country as Texas fever of cattle.

Cattle Ticks and Texas Fever—The cattle disease, which in the United States is known as Texas fever, is a widely distributed, exceedingly acute disease. In Australia it is known as redwater fever and in Europe as hæmoglobinuria, due to the fact that the urine of the diseased animals is discolored by the breaking down of the red blood corpuscles infested by the parasite.

In their historical discussion, Smith and Kilbourne, point out that as far back as 1796 it was noted that Southern cattle, in a state of apparent health, might spread a fatal disease among Northern herds. As observations accumulated, it was learned that this infection was carried only during the warm season of the year and in the depth of winter Southern cattle were harmless. Moreover, Southern cattle after remaining for a short time in the North lost their power to transmit the disease, and the same was true of cattle which had been driven for a considerable distance.