Dosage effect study of the microsporidian Octosporea muscaedomesticae in the adult black blowfly Phormia regina,with reference to the reproductive cycle of the pathogen

The quantitative pathogenicity of the microsporidian Octosporea muscaedomesticae in adult Phormia regina was studied. Dosage levels ranging from 10² to 10⁶ spores per fly were administered to five and six groups of newly emerged, starved adult flies in two trials. Rates of mortality and infection were determined. A direct relationship between number of spores ingested and subsequent infection rate was found in a 4-day trial while no such relationship was found in an 18-day trial, using the same source of inoculum and host flies from the same colony. The lack of a direct relationship between spore dose and rate of infection in the 18-day trial is explained on the basis of the short spore-to-spore development time of the parasite. New generations of spores formed within the host tissues obscure the results in relation to the spore dose initially administered. An appreciable number of spores in the inoculum is needed to initiate frank infection. The ID₅₀ (median infective dose) was 4.4 × 10⁴ spores per fly after 4 days.