Aedes vexans and Other Potential Mosquito Vectors of Dirofilaria immitis in Minnesota

Abstract

Investigations were conducted during two summers, and the information derived from two 11-week periods of trapping several species of mosquitoes found in Minnesota was incorporated. Two types of traps were used, a stable trap in which the mosquitoes were given an opportunity to obtain a blood meal from an infected dog, and a New Jersey type light trap which yielded naturally infected specimens. The results indicated that Aedes vexans was the best potential vector of Dirofilaria immitis in this northern area. Complete larval development of D. immitis was observed in Anopheles walkeri. This mosquito may prove to be a vector in areas where it occurs in sufficient numbers. Information on the vectors of canine dirofilariasis is not complete, even though over a century has elapsed since Dirofilaria immitis was first described. Many species of mosquitoes and several species of fleas have been reported as vectors of this parasite. However, the work of Newton and Wright (1956) cast doubt as to the validity of the flea as a vector. It is now fairly well established that mosquitoes are the sole intermediate hosts for this parasite. Table I lists 48 species of mosquitoes in which the larvae of D. immitis have reportedly completed development to the third stage. Studies on the mosquito vectors of canine dirofilariasis in Minnesota have been carried on during the past two summers in an attempt to determine which mosquitoes in this northern area are the best potential vectors of this parasite and which mosquito is the primary vector. Yen (1938) suggested several potential vectors in his study of mosquitoes in Minnesota, but did not attempt any field studies. When he was conducting his work he brought in an infected animal from the southern United States for experimental use since animals infected with D. immitis were not obtainable locally. It is no longer necessary to import infected dogs since the parasite is now enzootic in this area. That the situation has changed Received for publication 27 February 1965. *This work was supported by Public Health Service Research Grant AI-03165, from the NIAID, Public Health Service. Paper No. 5626 Scientific Journal Series, Minnesota Agricultural Experiment Station, St. Paul, Minnesota. has been shown by Schlotthauer and Griffiths (1964). They reported 4,164 cases of canine dirofilariasis in Minnesota from 1950 through 1960; 3,579 of these occurred in the metropolitan area of Minneapolis-St. Paul. Examination of the information concerning the known intermediate hosts of D. immitis in Table I indicates that ten potential vectors were known to be present in this enzootic area prior to our investigations. It was decided initially to find out which of the various local species feed readily on the dog. These species could then be considered more thoroughly and an attempt made to determine if any of these mosquitoes were a suitable arthropod vector according to the criteria of Yen (1938) and of Barnett (1960). MATERIALS AND METHODS The initial part of this investigation was undertaken to find which of the local mosquitoes would feed on a dog. A screened trap, as used by Magoon (1935) to accommodate large livestock, was modified to fit a dog. The trap was essentially a small screened house with a 3/-inch opening in a Vshaped slot extending completely around the cage 28 inches above the ground, to allow access for the mosquitoes. Plywood was substituted for galvanized metal and 2by 2-inch for 2by 4-inch braces. The side panels were 6 by 5 ft and 5 by 5 ft, the ends 3 ft wide and the door 30 by 59 inches, which allowed sufficient space for an individual to work comfortably within. The side panels were divided into two sections to make the trap more portable. To observe the feeding habits of mosquitoes an uninfected dog was housed in the stable trap 24 hr per day. Female mosquitoes were collected about 7 AMN each morning, killed, identified, and