Persistence of exotic Newcastle disease virus (ENDV) in laboratory infected Musca domestica and Fannia canicularis

House flies (Musca domestica) and little house flies (Fannia canicularis) were examined for their ability to take up and harbor a velogenic strain of exotic Newcastle disease virus (ENDV) (family Paramyxoviridae, genus Avulavirus). Laboratory-reared flies were allowed to feed on evaporated milk containing ENDV at a virus concentration of 10^8.3 egg infectious dose (EID)₅₀/0.1 ml or on poultry feces containing an ENDV titer of 10^5.8 EID₅₀/0.1 g. Flies exposed to either infectious food source for 24 hr became transiently infected with virus. Virus persisted predominantly in the mid- and hindgut, with relatively little virus isolated from the remainder of the fly body. Virus persisted similarly in both fly species that were fed evaporated milk containing ENDV, with a maximum ENDV titer of 10^5.98 EID₅₀/fly for the house fly and 10 ^4.78 EID₅₀/fly for the little house fly at 1 day postexposure; titers decreased on subsequent days to 10^2.38 EID ₅₀/fly for house fly and ≥1 EID₅₀/fly for little house fly at 5 days postexposure. Both fly species acquired viral titers greater than the infective dose for a susceptible chicken (10^3.0 EID ₅₀-10^4.0 EID₅₀). In addition, flies fed evaporated milk containing a high titer of ENDV maintained viral titers above the infective dose for up to 4 days postexposure to the infectious food source. Flies fed on infective feces retained a chicken infective dose for only one day. The decrease in viral titer over time was significantly explained by logistic regression for both fly species (P < 0.05). The slope of the regression line was not different for the two fly species (P < 0.05), indicating a similar rate of virus loss.