INSECTS AS VECTORS OF INFECTIVE AGENTS Two types of the insect transmission of infective agents are discernable, namely biological and mechanical.
i. Biological this group transmission of the infective agent is accomplished by the aid of some blood sucking arthropod in whose body certain definite stages of the infective agents' life cycle are passed. So far as known the infective agents transmitted by this means have no other means of transmission.
Biological transmission of the parasites producing the follow ing diseases is certain, namely; Malaria, Yellow Fever, Sleeping Sickness, Rocky Mountain Spotted Fever, Chagas' disease, Relapsing Fever, Typhus Fever, and Kala Azar. It is prob ably the usual means of transmission of Filiariasis and Dengue. The parasites concerned in this method of transmission are quite diverse, though apparently all belong to the animal kingdom. Thus we have protozoa, filterable viruses and nematodes. The groups of arthropods concerned are also quite diverse, although they all have the common habit of sucking blood, as for example; mosquitoes, biting flies, biting bugs, ticks, and lice. These arthropods are either temporary or permanent ectoparasites of mammals, living a life more or less intimately connected with the mammalian species upon which they prey.
The infective agents concerned undergo only an asexual multiplication in their vertebrate host, hence the vertebrate is known as the intermediate host. On the other hand, sexual development takes place in the arthropod which is known as the definitive host. The relation of the infective agents to the arthropod species concerned in their transmission is usually specific or nearly so. Where not specific we find that only closely allied species of arthropods are involved. Similarly we may find a high degree of specificity on the part of the infective agent and the vertebrate host, although in some in stances several species of mammals may apparently serve equally well.
The influence of temperature and rainfall is noticed in the distribution and prevalence of these diseases, due to the effect which these climatic factors exercise upon the breeding oppor tunities of the insects and hence influence their abundance. Temperature also profoundly influences the development of the protozoan parasites within their insect hosts, for example the distribution of malaria is limited by the annual isotherm of 6o degrees F. due to the inability of the malarial parasites to develope in the mosquitoes at a lower temperature. Thus
low temperature may even inhibit protozoan development or sterilize the insect altogether. On the other hand, with favor able conditions of temperature it would appear that infection of the insect is usually permanent.
With infective agents whose transmission is biological, a definite lapse of time must occur before an insect which has become infected can transmit the infective agents to the host upon which it feeds. This period of noninfectivity is known as the extrinsic period of incubation and represents the time necessary for the infective agent to complete its sexual cycle in the insect and reach the appropriate point of exit.
The routes by which the mature infecting forms of the in fective agents leave their arthropodal hosts varies with different species. In the case of trypanosomes present in Tse-tse flies, departure is by the food canal of the probosis; in the cases of the malarial parasites in the anophelines the departure is by the salivary ducts in the probosis; with the spirochetes of relapsing fever in either lice or bedbugs the departure may be either with the feces through the anus, or by liberation from the body cavity when the insect is crushed. In some few species hereditary infection of the insect host is known, thus for example the piroplasms of Texas Fever pass to the ovaries and young ticks are congenitally infected. The same also occurs in African Relapsing Fever.
As already noted the infectivity of the definitive arthropodal host is influenced by : (i) external temperature, which controls the development of the parasite within the insect; (2) The digestive processes of the insect, which may unfavorably influence the parasites; and (3) the presence and number of mature sexual forms of ihe parasites in the blood of the vertebrate upon which the insect has fed. Each feeding only removes a very small quantity of blood and unless the parasites are sufficiently numerous in the blood abstracted and both sexual forms are present, infection of the insect will not occur. All the chances are against the parasite. Thus we can understand that fre quently only one or two per cent. of anopheline mosquitoes will show malarial parasites, yet under more favorable cir cumstances where heavy gamete carriers are abundant, as many as zo to 25 per cent. of mosquitoes will be infected.