INSECTS AS AGENTS IN SPREAD OF DISEASE 541 



through eating moldy corn or corn products, and investigators went so 

 far as to describe a certain fungus as the specific cause of the disease. 

 This hypothesis was never satisfactorily in accord with the facts, and 

 has been abandoned very generally in favor of a belief that pellagra is 

 insect-borne. This has not yet been sustained by actual proof, and is 

 far from being generally accepted, but Sambon and others have adduced 

 much evidence to show that the "black-fly" (Simulium) may be the 

 carrier for the virus of pellagra. These flies are widely distributed 

 throughout the world, always occurring in proximity to rapidly flowing 

 streams of water in which the larva? live. The adults, though small, are 

 vicious biters. They appear mainly in the spring, more rarely in the 

 fall, and agree in seasonal distribution with the incidence of pellagra. 

 The causal organism has never been found and is evidently an ultra- 

 microscopical or filterable virus. 



One of the best known insect-borne diseases, and one which is of 

 great importance in many parts of our own country is malarial fever, 

 variously termed ague, chills and fever, etc. This was the first human 

 disease traced directly to insect carriers and gave the impetus which 

 has led to the unraveling of the facts connected with other insect-borne 

 diseases. There are many types of malarial fevers, due to a number of 

 similar but different blood parasites and the disease is most common in 

 tropical regions, although in our own country it extends well into the 

 northern states, even quite commonly into Massachusetts. The proto- 

 zoan blood parasites that cause malaria were first demonstrated many 

 years ago, in 1880, by a French surgeon, Laveran, who discovered them in 

 the blood of persons suffering from malaria. Five years later an Italian, 

 Golgi, distinguished three kinds, each associated with one of the more 

 familiar types of malaria. They were found to go through a regular 

 life cycle in the red blood corpuscles and, from analogy with other 

 known Protozoa, it was suspected that in addition to their non-sexual 

 generations in the human blood there must be a sexual development in 

 some cold-blooded animal. Manson was led to suspect that some insect 

 might be the secondary host and, working on this hypothesis, Eoss in 

 India first found the malarial parasites in a certain kind of mosquito 

 in 1898. He had worked for nearly three years on a common mosquito 

 belonging to the genus Culex without result, but finally in a mosquito 

 of the genus Anopheles was able to trace the development of the parasite. 

 His epoch-making discovery has been since amply confirmed and ex- 

 tended by experimental proof till we now know that the various types of 

 malarial blood parasites complete their life-cycles in anopheline mosqui- 

 toes, the latter acting as the sole carriers of the disease. 



The details of growth and development of these parasites, which 

 belong to the Protozoan genus Plasmodium, are extremely interesting, 

 but far too complicated to discuss briefly. In general it may be said 



