Handbook of Medical Entomology

by William A. Riley; O. A. Johannsen

The disease is confined to South America and to definitely limited areas of those countries in which it does occur. It is especially prevalent in some parts of Peru.

The causative organism and the method of transfer of verruga are unknown. Castellani and Chalmers pointed out in 1910 that the study of the distribution of the disease in Peru would impress one with the similarity to the distribution of the Rocky Mountain fever and would lead to the conclusion that the ætiological cause must in some way be associated with some blood-sucking animal, perhaps an arachnid, and that this is supported by the fact that the persons most prone to the infection are those who work in the fields.

More recently, Townsend (1913), in a series of papers, has maintained that verruga and Carrion's disease are identical, and that they are transmitted to man by the bites of the Psychodid fly, Phlebotomus verrucarum. He succeeded in producing the eruptive type of the disease in experimental animals by injecting a physiological salt trituration of wild Phlebotomus flies. A cebus monkey was exposed from October so to November 6, by chaining him to a tree in the verruga zone, next to a stone wall from which the flies emerged in large numbers every night. Miliar eruption began to appear on the orbits November 13 and by November 21, there were a number of typical eruptions, with exudation on various parts of the body exactly like miliar eruptive sores commonly seen on legs of human cases.

An assistant in the verruga work, George E. Nicholson, contracted the eruptive type of the disease, apparently as a result of being bitten by the Phlebotomus flies. He had slept in a verruga zone, under a tight net. During the night he evidently put his hands in contact with the net, for in the morning there were fifty-five unmistakable Phlebotomus bites on the backs of his hands and wrists.

Townsend believes that in nature, lizards constitute the reservoir of the disease and that it is from them that the Phlebotomus flies receive the infection.

Cancer

There are not wanting suggestions that this dread disease is carried, or even caused, by arthropods. Borrel (1909) stated that he had found mites of the genus Demodex in carcinoma of the face and of the mammæ. He believed that they acted as carriers of the virus.

Saul (1910) and Dahl (1910) go much further, since they attribute the production of the malignant growth to the presence of mites which Saul had found in cancers. These Dahl described as belonging to a new species, which he designated Tarsonemus hominis. These findings have since been confirmed by several workers. Nevertheless, the presence of the mite is so rare that it cannot be regarded as an important factor in the causation of the disease. The theory that cancer is caused by an external parasite is given little credence by investigators in this field.

In conclusion, it should be noted that the medical and entomological literature of the past few years abounds in suggestions, and in unsupported direct statements that various other diseases are insect-borne. Knab (1912) has well said "Since the discovery that certain blood-sucking insects are the secondary hosts of pathogenic parasites, nearly every insect that sucks blood, whether habitually or occasionally, has been suspected or considered a possible transmitter of disease. No thought seems to have been given to the conditions and the characteristics of the individual species of blood-sucking insects, which make disease transmission possible."

He points out that "in order to be a potential transmitter of human blood-parasites, an insect must be closely associated with man and normally have opportunity to suck his blood repeatedly. It is not sufficient that occasional specimens bite man, as, for example, is the case with forest mosquitoes. Although a person may be bitten by a large number of such mosquitoes, the chances that any of these mosquitoes survive to develop the parasites in question, (assuming such development to be possible), and then find opportunity to bite and infect another person, are altogether too remote. Applying this criterion, not only the majority of mosquitoes but many other blood-sucking insects, such as Tabanidæ and Simuliidæ, may be confidently eliminated. Moreover, these insects are mostly in evidence only during a brief season, so that we have an additional difficulty of a very long interval during which there could be no propagation of the disease in question." He makes an exception of tick-borne diseases, where the parasites are directly transmitted from the tick host to its offspring and where, for this reason, the insect remains a potential transmitter for a very long period. He also cites the trypanosome diseases as possible exceptions, since the causative organisms apparently thrive in a number of different vertebrate hosts and may be transmitted from cattle, or wild animals, to man.