Ochlerotatus j. japonicus in Frederick County, Maryland: Discovery, distribution, and vector competence for West Nile virus

Ochlerotatus japonicus japonicus is reported fur the Ist time south of the Mason-Dixon Line, in Frederick County, Maryland. Fifty-seven oviposition tr ap samples were collected throughout the county between June 30 and August 24. 2000. From 971 larvae reared from the oviposition traps, 5 species were identified: Ochleratatus triseriatus (45%). Oc. j. japonicus (43%), Aedes albopictus (7%), Culex pipiens (4%). and Toxorynchites ritulus septentriona lis (<1%). Ochlerotatus j. japonicus was found widely distributed over the area sampled. This is the 1st record of Ae. albopictus in the county as wel l. Vector competence studies indicated that Oc. j. juponicus is an efficien t laboratory vector of West Nile (WN) virus. Depending on the viral titer a t time of feeding, the estimated transmission rates fur Oc. j. japonicus fo r WN virus were 2-4 times higher than that fur Cx. pipiens. Studies of the viral titer in mosquitoes over time showed that titers in the bodies of inf ected Oc. j. japonicus reached their peak (similar to 10(6.5) plaque-formin g units/mosquito) between 7 and 11 days after taking an infectious blood me al, and that virus became detectable in the legs (an indicator of dissemina ted infection) as early as 3 days after taking an infectious blood meal.