The need for a more adequate index of experimental infection in laboratory studies of mosquito hosts and filarial parasites is noted.
The concept of host efficiency is discussed and it is suggested that recent attempts to combine into a single formula an evaluation of early as well as late host-parasite relations render the host efficiency ratio less useful as a numerical symbol. Accordingly, an expression for host efficiency is proposed as a ratio of the mean number of third stage larvae per mosquito surviving at the termination of the filarial incubation period to the mean number of microfilariae per mosquito in samples shortly after feeding upon an infected vertebrate host.
Two additional components are suggested as follows: 1) the survival rate of the host, defined as the percentage of engorged female mosquitoes surviving the period of extrinsic incubation of the parasite; and 2) the infection rate of the host, derived by dividing the number of mosquitoes with third stage larvae at the termination of the parasite's incubation period by the number of surviving mosquitoes at the end of incubation.
It is suggested that the appropriate effects of the above three components may be reflected by an index relating them in the following manner:
Index of experimental infection = (survival rate) × (infection rate) × (efficiency ratio)
The above formula is applied to experiments comparing the susceptibility of various species of mosquitoes to Dirofilaria immitis. On this basis, it is suggested that the present index, although subject to certain limitations, appears to be of practical value insofar as it succeeds in evaluating simultaneously certain factors which already have been demonstrated to affect the likelihood that a given species of mosquito will be a good host for a filarial parasite.