Surveillance of Arbovirus Activity in Iowa during 1972 - 19751

During a 4-year period (1972-1975), mosquitoes were collected, identified, and processed for virus isolation. Laboratory examinations of clinical specimens from suspected encephalitis cases were made to provide etiologic diagnosis. Arbovirus was isolated from 102 pools of mosquitoes (2 LaCrosse, 2 Cache Valley, 10 Flanders, 3 St. Louis Considerable interest in arbovirus surveillance activity was generated in Iowa during the 1975 encephalitis season by the simultaneous occurrence of St. Louis encephalitis (SLE), Western equine encephalomyelitis (WEE), and California (LaCrosse) encephalitis (LAC). During August, and September, there was increased demand for entomological and epidemiological data as well as immediate need for laboratory diagnosis of clinically defined cases. Intensification of interest in arbovirus activity during an encephalitis outbreak is expected. However, arbovirus surveillance has been a i continuous long-term activity of our laboratories. These surveillance activities for 1965 to 1971 have been reported (Wong etal. 1970, Wong etal. 1973). A statewide mosquito surveillance-arbovirus isolation program was conducted in 1971 and 1972 as a cooperative effort of the State Hygienic Laboratory, University of Iowa, and the Department of Entomology, Iowa State University. The results of the 1971 study have been summarized (Rowley et al. 1973). In 1973 and 1974, our efforts were directed toward the mosquito Aedes trivittatus and its relationship with Trivittatus virus (TVT). The State Forest Nursery in Ames was the principal site for mosquito collection during this period. There have ' Journal Paper No. J-8764 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project 2053. encephalitis, 83 Trivittatus, and 2 Western equine encephalitis viruses). Laboratory results confirmed recent infections with arbovirus in 44 human cases of diseases of the central nervous system. Chronological and geographical data, information on the mosquito species collected, and the viruses isolated are presented. been fluctuations in our mosquitoarbovirus surveillance because of budget limitations; but diagnostic services for the detection and laboratory confirmation of arbovirus encephalitis have remained available and unrestricted. This report summarizes mosquito-arbovirus research for the period from 1972 to and including 1975. METHODS AND MATERIALS Mosquitoes were collected in state parks, cities, recreational areas, and farms, using battery-operated CDC light traps baited with dry ice. Mosquito collection procedures and the processing and handling of mosquitoes were done according to standard techniques (Sudia and Chamberlain 1967). The mosquitoes were processed in pools of 1 to 100. The procedure described in an earlier paper (Wong et al. 1971) was followed for arbovirus isolation with use of suckling mice. The field infection ratio (FIR) for each mosquito species from a collection site was calculated by dividing the total number of specimens of a given species collected each month by the number of virus isolations from that species during the month. Studies of human cases of encephalitis were initiated upon receipt of clinical specimens for examination from suspected cases of infection of the central nervous system. Sera were tested for LAC, SLE, and WEE complement fixing 2ib Mosquito News Vol. 38, No. 2 (CF) and neutralizing (N) antibodies. The microtiter LBCF test, using sucroseacetone extracted infected mouse brain antigens, was used to detect CF antibody (USPHS 1965). N antibodies were determined by means of a microtiter tissue culture system using baby hamster kidney (BHK 21) cells (Lennette and Schmidt 1969). Occasionally, blood clots from young patients were inoculated into suckling mice for arbovirus isolation. RESULTS AND DISCUSSION During the 4 years discussed in this report, 136,807 mosquitoes were tested in 2,492 pools for the presence of arbovirus. Table 1 shows the number of mosquitoes and the collection sites for each of the 4 years. Approximately 80% of the total number of mosquitoes were collected and tested in 1972. About 20,000 were collected at the State Nursery in Ames during 1973 and 1974. Collections during these 2 years were made primarily to study host-parasite relationships of Ae. trivittatus and TVT virus. In 1975, selection of mosquito collection sites was made after the appearance of human cases of encephalitis. Tables 2, 3, and 4 show the numbers and pools of each mosquito species collected by month for 1972, 1973, and 1975. In 1974, 7,933 (27 pools) Ae. trivittatus and 1,211 (13 pools) Ae. vexans were collected at the State Nursery in Ames. Arbovirus was isolated from 102 pools of mosquitoes collected and examined during the 4-year period. Two LAC isolates, 2 Cache Valley (CV), 10 Flanders (Fl), 3 SLE, 83 TVT, and 2 WEE viruses were identified. The two LAC isolates were from mosquitoes collected in areas where LAC virus had not been found in previous years. The virus was isolated from a pool of Ae. triseriatus collected in Black Hawk State Park in Waterloo, Iowa, in August 1972. The other LAG isolate was from mosquitoes collected in August 1975 in Sioux City. Laboratory-confirmed cases of LAC encephalitis have occurred in these areas. The FlR's for LAC virus in Ae. triseriatus ranged from 1:10 to 1:86, with isolations from pools of 3 and 19 mosquitoes. Three isolations of SLE were made from Culex tarsalis and Cx. pipiens collected during August and September 1975. The calculated FlR's for SLE virus in Cx. pipiens ranged from 1:19 at Des Moines to 1; 158 at Ames. The FIR was 1:47 in Cx. Table 1. Summary of moisquitoes collected in Iowa for virus isolation, 1972-1975, Location 1972 1973 1974 1975 Total Clive, (lies Moines) 4,098(92)* 188(9) 4,286(101) Eldora 13,250(206) 13,250(206) Lost Island Lake, Clay Co. 13,081(181) 13.081(181) Malvern 227(57) 227(57) Yellow River Forest 909(36) 909(36) Iowa City 2.485(58) 2,485(58) Ventura 15,705{206) 15,705(206) Waterloo 9.215(243) 9,215{243) Homick 6,737(178) 228(22) 5.965(200) Ames 43,837(583) 11,673(203) 9,144(140) 5.125(128) 69.779(1,054) Berne Farm, Allamakee Co. 218{32) 261(4) 479(36) Lake MacBride State Park 297(34) 56(4) 353(38) Lacey-Keosauqua State Park 213(26) 213(26) Sioux City 611(34) 611(34) Des Moines 249(16) 249(16) TOTAL 109.272(1.932) 11,729(207) 9,144(140) 6,662(213) 138,807(2.492) * Number of mosquitoes (pools). June, 1978 Mosquito News 247