Comparison of chickens and pheasants as sentinels for eastern equine encephalitis and St. Louis encephalitis viruses in Florida

Pheasants and chickens were compared as sentinels for monitoring the transmission of arboviruses of public health significance in Florida during 1991-93. Results suggest that pheasants are better sentinels for eastern equine encephalitis (EEE) virus. They detected virus transmission 3-6 weeks earlier in epizootic years, 2-4 times more frequently during the season, and twice as many birds within a flock seroconverted during a given week. Pheasants detected virus transmission at 2 sites during 2 interepizootic years when chickens failed. Although pheasants detected St. Louis encephalitis (SLE) virus activity somewhat later than did chickens, they had greater seroconversion rates than did chickens. Although both bird species can be used to monitor EEE and SLE viruses, pheasants are more sensitive than chickens as sentinels for EEE virus and as sensitive as chickens as sentinels for SLE virus.

Eastern equine encephalomyelitis virus in relation to the avian community of a coastal cedar swamp

Eastern equine encephalomyelitis virus (EEEV) is perpetuated in eastern North America in a mosquito-wild bird maintenance cycle that involves Culiseta melanura (Coquillett) as the principal enzootic vector and passerine birds as the primary amplifying hosts. We examined the role of birds in the EEEV cycle at a site in southern New Jersey where EEEV cycles annually at high levels. Birds and mosquitoes were sampled during three epiornitics and one season of limited virus activity. We examined antibody prevalence in birds in relation to eight physical and natural history characteristics. Our goal was to compare EEEV cycling in C. melanura and the primary avian hosts better to understand the mechanisms that initiate annual epiornitics. Antibody prevalence was highest in the Blue Jay (62%), Wood Thrush (60%), and Tufted Titmouse (44%). Resident status of birds was the natural history characteristic most closely linked to participation in the EEEV cycle. Species spending the greatest amount of time at our study site (permanent residents, summer residents) had the highest antibody rates. We captured viremic birds as early as 25 May, 51 d before we first detected virus in C. melanura. We recaptured 10 after hatching year adults and one hatching year (HY) bird that seroconverted before we detected virus in C. melanura. We also found EEEV antibody in 15 HY birds up to 31 d before we isolated EEEV from C. melanura. We provide evidence that a cryptic cycle develops weeks before epiornitic cycling is detected in C. melanura by traditional laboratory techniques, indicating that the early season cycle is initiated by the recrudescence of latent virus in previously infected birds.

Rapid assessment of vectorborne diseases during the Midwest flood--United States, 1993

Heavy spring and summer rainfall during 1993 caused the most extensive flash and riverine flooding ever recorded in the upper midwestern United States. In portions of the flood region,* standing water provided large expanses of habitat capable of producing large populations of the mosquitoes Culex pipiens and Cx. tarsalis. These species can rapidly amplify transmission of the arboviruses that cause St. Louis encephalitis (SLE) and western equine encephalitis (WEE). Although information from state health departments in the disaster area indicated minimal SLE or WEE activity in the region before the flooding, large vector populations in certain areas following the flooding increased the potential for exposure of residents and emergency workers to arboviral infection. To determine the risk for arboviral disease in the disaster area, CDC, in collaboration with state and local health departments, conducted surveillance during August-September 1993. This report summarizes the results of the surveillance activity.

Experimental transmission of eastern equine encephalitis virus by strains of Aedes albopictus and A. taeniorhynchus (Diptera: Culicidae)

The vector competence of Aedes taeniorhynchus (Wiedemann) and four strains of Aedes albopictus (Skuse) was assessed for eastern equine encephalitis (EEE) virus isolated from Ae. albopictus collected in Polk County, Florida. Both species became infected with and transmitted EEE virus by bite after feeding on 1-d-old chicks that had been inoculated with EEE virus (viremia = 10(10.1) plaque-forming units [PFU] per ml of blood). However, when fed on an older chick with a lower viremia (viremia = 10(6.1) PFU per ml of blood), Ae. albopictus was significantly more susceptible to infection (90%, n = 61) than was Ae. taeniorhynchus (15%, n = 40). Transmission was also significantly more efficient by Ae. albopictus (36%, n = 44), than by Ae. taeniorhynchus (0%, n = 14). These data, combined with the recent isolation of EEE virus from Ae. albopictus and its opportunistic feeding behavior, indicate that Ae. albopictus could function as a bridge vector between the enzootic Culiseta melanura (Coq.)-avian cycle and susceptible mammalian hosts.

Natural vertical transmission of western equine encephalomyelitis virus in mosquitoes

The mechanism by which western equine encephalomyelitis (WEE) virus and other mosquito-borne alphaviruses (Togaviridae) survive during periods of vector inactivity is unknown. Recently, three strains of WEE virus were isolated from adult Aedes dorsalis collected as larvae from a salt marsh in a coastal region of California. This provides evidence of vertical transmission of WEE virus in mosquitoes in nature. Vertical transmission in Ae. dorsalis and closely related mosquito species may be an important mechanism for the maintenance of WEE virus in temperate regions in North America where horizontal transmission of the virus is seasonal.

Weather factors in the prediction of western equine encephalitis epidemics in Manitoba

Cases of western equine encephalitis in horses in 1987 in western USA and Manitoba, Canada were examined by backward trajectory analysis of winds. Culex tarsalis mosquitoes infected with western equine encephalitis virus could have been carried on southerly winds from Texas and Oklahoma to northern USA and from there to Manitoba. The presence of the Polar front over North Dakota and Minnesota at the end of July would have led to the landing of Cx. tarsalis in Montana and Wisconsin and prevented further carriage into Manitoba. Temperatures in southern Texas during the winter months (average daily maximum temperatures 19.7 degrees C and higher) would have permitted continuous transmission of western equine encephalitis virus by Cx. tarsalis in this area. Weather factors involved in outbreaks from 1975-88 were analysed to see if epidemics in Manitoba (23 or more cases in horses) could be predicted. The conditions for epidemics could be defined as follows: (a) the number of cases in horses in USA was 98 or more, (b) winds were southerly with speeds 45 kmh-1 or higher, and (c) counts of Cx. tarsalis females/light trap per day were 3.2 or higher. There were 3 or fewer cases in Manitoba, when the number of cases in USA was 27 or less, even when Cx. tarsalis counts were higher than 3.2. With Cx. tarsalis counts below 3 and/or unsuitable winds, or the Polar front further south, the number of cases in Manitoba was between 0 and 17, even when the number of cases in USA was from 38-172. Without information on the extent of infection further south, the weather variables would probably be more useful in excluding the possibility of an epidemic in Manitoba than in predicting one.

Effect of temperature on the transmission of western equine encephalomyelitis and St. Louis encephalitis viruses by Culex tarsalis (Diptera: Culicidae)

The extrinsic incubation rate (inverse of the time in days from infection to median transmission) of western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses by laboratory strains of Culex tarsalis Coquillett increased as a linear function of incubation temperatures from 10 to 30 degrees C. The estimated temperatures for zero transmission thresholds (intercept of the X axis) were 10.9 and 14.9 degrees C, and the number of degree days above these thresholds required for median transmission (inverse of the slope) was 67.6 and 115.2, respectively. Although the bodies of most Cx. tarsalis females remained infected and the WEE viral titer did not decrease significantly throughout the incubation periods at all temperatures, transmission rates by females incubated at 20 to 30 degrees C decreased markedly after peaking at 7-10 d after infection. In contrast, midgut escape and salivary gland infection barriers limited the transmission rates of SLE virus at all temperatures, but these rates did not decrease markedly as a function of incubation time, indicating that virus modulation did not occur. Degree-day models were used to calculate monthly changes in the duration of the extrinsic incubation period for WEE and SLE viruses in the San Joaquin and Coachella valleys based on mosquito temperatures estimated by combining nocturnal air and diurnal resting site temperatures. Temperatures in the San Joaquin Valley averaged 5 degrees C cooler than in the Coachella Valley, proportionately shortening the duration of the potential transmission season for WEE virus from 10 to 8 mo and for SLE virus from 8 to 5 mo, respectively.

Susceptibility parameters of Aedes albopictus to per oral infection with eastern equine encephalitis virus

Aedes albopictus (Skuse) mosquitoes were fed on snowy egrets, Egretta thula (Thayer and Bangs), that had been infected by subcutaneous inoculation of eastern equine encephalitis (EEE) virus. Freshly fed mosquitoes were frozen and tested to determine how much virus they had ingested. Other fed mosquitoes from the same lots were incubated for 7 d at 27 degrees C before testing. Seven lots of Ae. albopictus fed on viremic birds. Based on average amounts of virus ingested and day 7 virus infection rates in mosquitoes from the same lots, the amount of virus required to infect 50% of the mosquitoes was calculated to be 10(2.8) Vero cell plaque-forming units (PFU). The infection threshold (i.e., the amount of virus required to infect from 1 to 5% of mosquitoes) was determined to be < or = 10 PFU per blood meal. These parameters indicate that Ae. albopictus is sufficiently susceptible to infection with EEE virus to enable it to acquire infectious doses from a wide variety of viremic birds and possibly from equines.

Laboratory transmission of eastern equine encephalomyelitis virus to chickens by chicken mites (Acari: Dermanyssidae)

Pools of adult female chicken mites, Dermanyssus gallinae (De Geer), were allowed to feed on chicks that had been inoculated with eastern equine encephalomyelitis (EEE) virus and that had a viremia level of 10(6.2)-10(6.6) plaque-forming units per milliliter of blood. Virus remained detectable by plaque assay in samples of these mites for 30 d after the infectious blood meal. Virus was not recovered from any of 151 progeny of virus-exposed female mites. Mites that had fed on viremic chicks were allowed to feed on naive chicks 3, 7, 11, 15, or 30 d later. EEE virus was transmitted to chicks by these mites on days 3 (one transmission in four trials) and 7 (one transmission in four trials). Both transmissions were confirmed by the presence of virus in chick blood 24-72 h after mites had fed, and by plaque-reduction neutralization assays of 21-d convalescent chick sera against the original viral strain.

An assessment of the biological capacity of a Sacramento Valley population of Aedes melanimon to vector arboviruses

Daily survivorship, duration of the gonotrophic cycle, absolute abundance and season-long relative abundance were estimated for Aedes melanimon in the Sacramento Valley of California in 1987 and 1988 using mark-release-recapture (MRR) techniques and by monitoring changes in the abundance and parity rate of the native population. One objective of these studies was to determine the extent to which A. melanimon was biologically capable of serving as a horizontal arbovirus vector. Daily survivorship was estimated to be 0.90 and 0.84 in MRR studies conducted in September 1987 and August 1988, 0.89 based on changes in the parity state and abundance of the native population in August 1988 and 0.82 using summer-long parity data in 1988. Gonotrophic cycle length (GCL) was estimated to be five days in three studies. Aedes melanimon densities were estimated to be approximately 1,000,000 and 15,000 females per hectare in September 1987 and August 1988 respectively. Parous A. melanimon females were collected on each sampling occasion from April to November 1988, suggesting that A. melanimon maintained a continuous presence in the study area throughout the summer. The results of these studies suggest that A. melanimon has the potential to be an efficient horizontal vector of western equine encephalomyelitis (WEE), based on high adult survivorship, short GCL, high abundance and a continuous presence across the summer. This supports the concept of a WEE transmission cycle in the Sacramento Valley involving Ae. melanimon as an important vector. Aedes melanimon also can be an efficient horizontal vector of California encephalitis virus (CE), though the importance of horizontal transmission to the maintenance of CE virus is unclear.

Detection of eastern equine encephalomyelitis virus deposition in Culiseta melanura following ingestion of radiolabeled virus in blood meals

We examined deposition of eastern equine encephalomyelitis virus (EEEV) in the alimentary tract of its enzootic mosquito vector, Culiseta melanura, to detect potential sites of initial infection. Artificial viremias were created by injecting purified [3H]uridine-labeled EEEV intravenously into one-day-old chicks. Mosquitoes were allowed to engorge, incubated 1-2 hr, fixed, and whole abdomens and thoraces of 25 specimens embedded. Tagmata were sectioned, mounted on slides and coated with autoradiographic emulsion. Following exposures of 1-4 months, slides were developed and examined microscopically. In the majority of mosquitoes, label (virus) was detected only within the midgut; most virus was concentrated in a band of expressed serum adjacent to the abdominal midgut epithelium. Small amounts of virus were also deposited within folds in the cardial thoracic midgut of 96% of mosquitoes. Viral penetration into epithelial cells was detected throughout the abdominal midgut in all mosquitoes, and in the thoracic midgut of 20%. A small number (4/25) of mosquitoes examined showed signs of leaky abdominal midguts, with virus detected in the abdominal hemocoel. Concentration of EEEV in expressed serum adjacent to the abdominal midgut epithelium may enhance initial midgut infection. Leaky abdominal midguts exhibited by some mosquitoes may also facilitate rapid systemic infections of Cs. melanura. Deposition of virus in thoracic alimentary tissues suggests the possibility of early EEEV infection of the anterior midgut.

Peroral susceptibility of Aedes albifasciatus and Culex pipiens complex mosquitoes (Diptera: Culicidae) from Argentina to western equine encephalitis virus

The transmission cycle of western equine encephalitis (WEE) virus in South America is unknown. A WEE virus strain was isolated from Aedes albifasciatus in Argentina during the WEE epizootic of 1982-83. Also, Culex pipiens from Argentina was reported to be able to transmit WEE virus experimentally, but other results indicate that Cx. pipiens from the USA is refractory to this virus. We determined the susceptibility of Argentina strains of Ae. albifasciatus and Culex pipiens complex mosquitos to infection by WEE virus by the oral route. Adult females were fed on chicks infected with a WEE virus strain isolated in Cordoba Province, Argentina, or were fed on a blood/virus suspension. Each mosquito ingested between 10(1.6) to 10(6.4) vero cell plaque-forming units of virus. Each of 28 Ae. albifasciatus was positive for virus from the fourth day postfeeding, and there was evidence for virus replication. In contrast, 0/44 Cx. p. quinquefasciatus and only 1/15 Cx. p. pipiens was positive. Aedes albifasciatus is susceptible to infection by WEE virus and should be considered a potential vector of this virus in Argentina. Both subspecies of Cx. pipiens are refractory to peroral infection by WEE virus and probably do not play a role in the WEE virus cycle in Argentina.

Temporal variations in the susceptibility of a semi-isolated population of Culex tarsalis to peroral infection with western equine encephalomyelitis and St. Louis encephalitis viruses

A semi-isolated population of Culex tarsalis in Kern County, CA was found to vary significantly in its seasonal and yearly susceptibility to peroral infection with western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses during the breeding seasons of 1975-1981. Female cohorts of the population were significantly more resistant to WEE virus from 1975 through 1977 than from 1978 through 1981. On the average, females were 40 times more susceptible to WEE virus during May than during August. Increases in resistance correlated significantly with the number of days accrued from April through June, when daily ambient air temperatures equaled or exceeded 26.7 degrees C or 32.2 degrees C, respectively. Analyses of data on WEE viral susceptibility, temperature, and rainfall for the 7 year period suggested that Cx. tarsalis should be most susceptible to infection in years with wet winter-early spring periods followed by cool springs. However, inconsistencies in the expression of WEE viral susceptibility during mid- to late summer of some years indicated that other unidentified extrinsic factors, in addition to temperature and rainfall, may induce changes during preimaginal development that affect the peroral susceptibility of adult females. Seasonal and yearly changes in the susceptibility of the Cx. tarsalis population to per os infection with SLE virus did not necessarily occur at the same time as those observed with WEE virus and did not appear to correlate with changes in ambient air temperature. Thus, extrinsic factors that affect the expression of viral susceptibility of Cx. tarsalis, and perhaps the genetic basis of viral susceptibility, apparently differ for WEE and SLE viruses.

Trajectory analysis of winds and eastern equine encephalitis in USA, 1980-5

Backward trajectories of winds were determined to identify possible sources of eastern equine encephalitis virus associated with isolation of virus from mosquitoes or birds or outbreaks in horses between 1980 and 1985 in Maryland, New Jersey, New York and Michigan, USA. The results of the trajectory analyses suggested that eastern equine encephalitis virus could have been carried by infected mosquitoes on surface winds at temperatures 13 degrees C or higher from North Carolina north-eastwards along the Atlantic Coast to Maryland and New Jersey and thence to upstate New York and from western Kentucky to Michigan. Landing of mosquitoes was associated with the presence of a cold front and rain leading to variations in the location and timing of outbreaks from year to year. The mosquito responsible was most likely to have been Culiseta melanura, but Coquillettidia perturbans and Aedes sollicitans could also have been involved. There may be a continual cycle of eastern equine encephalitis virus in mosquitoes and birds in south-eastern USA, from where the virus could be distributed by infected mosquitoes on the wind along the Gulf and Atlantic Coasts and up the Mississippi Valley.

Eastern equine encephalitis in Quebec and Connecticut, 1972: introduction by infected mosquitoes on the wind?

In 1972 there were outbreaks of eastern equine encephalitis in the Eastern Townships, Quebec, Canada and in Connecticut, USA. Climatic data including Northern Hemisphere synoptic charts were examined. The findings indicate that the virus could have been brought to Lac Brome by infected mosquitoes carried on surface winds from Meriden, Connecticut, on the night of August 22-23, 1972. The distance of 400 km would have been covered in 14-16 h at a speed of 25-30 km h-1 and at a temperature of 15 degrees C and higher. The first case was recorded 13 days later on September 5, 1972. The outbreak at Meriden, Connecticut started on August 21, 1972. On August 7, 1972 southwesterly winds blew along the Atlantic coast at heights up to 1.5 km. Infected mosquitoes could have been carried on the wind from Cape May, New Jersey, Delaware-Maryland-Virginia peninsula, North Carolina or Georgia. Flights would have been at 17 degrees-20 degrees C and lasted 5-6, 9-10, 14-16 and 20-26 h depending on the origin. The arrival on August 8, 1972 coincided with a cold front moving from the northwest through Connecticut. Culiseta melanura is regarded as the mosquito species most likely to have been involved in the transmission of infection.

Impact of climate on western equine encephalitis in Manitoba, Minnesota and North Dakota, 1980-1983

Information was collected on confirmed outbreaks of western equine encephalitis (WEE) in North America east of the Rockies for 1981 and 1983 (epidemic years) and 1980 and 1982 (non-epidemic years). The initial pattern of outbreaks in Manitoba, Minnesota and North Dakota was determined for each year. Backward (and in some instances forward) wind trajectories were computed for each day 4-15 days (incubation period) before the initial outbreaks of WEE in a given area of province or state. During these years the timing and location of WEE outbreaks in horses and man, seroconversion in chickens, the maximum Culex tarsalis counts at Winnipeg and first isolation of WEE virus from C. tarsalis could be correlated with trajectories of winds from states further south within acceptable intervals. It is suggested that C. tarsalis mosquitoes infected with WEE virus are carried on the wind from Texas on the Gulf of Mexico, where they continue to breed during the northern winter months, to northern Texas and Oklahoma in the spring. In May, June and July C. tarsalis are carried north on southerly winds from these states through Kansas and Nebraska to North Dakota, Minnesota, Wisconsin and Manitoba. Distances of 1250-1350 km are traversed in 18-24 h at heights up to 1.5 km with temperatures greater than or equal to 13 degrees C. Landing takes place where the warm southerly winds meet cold fronts associated with rain. Convergence leads to concentration of C. tarsalis and determines where outbreaks occur. It is possible that return of new generations of C. tarsalis to the south may occur later in the year. The development of an epidemic of WEE in the northern states and provinces would appear to depend on (i) suitable trajectories from the south in June and July with temperatures greater than or equal to 13 degrees C meeting cold fronts with rain, (ii) sufficient C. tarsalis infected with WEE virus at source, carried on the wind and locally, (iii) C. tarsalis biting horses and man, (iv) maintenance of local mosquito populations in August and (v) susceptible hosts (birds) at source and susceptible hosts (horses and man) locally. Possible methods of prediction involving determination of trajectories, identification of C. tarsalis blood meals, measuring seroconversion in calves are discussed in addition to the methods already in use.

Factors influencing the transmission of western equine encephalomyelitis virus between its vertebrate maintenance hosts and from them to humans

A simple model is used to explore the extent to which the uniquely comprehensive studies of western equine encephalomyelitis in Kern County, California, by Reeves and his colleagues over many years, explain the dynamics and epidemiology of the infection. It is concluded that not only does this series of integrated field and laboratory studies successfully account for these phenomena, to an extent which is unlikely to be substantially improved upon, not least because of the inherent difficulties in measuring the key factors with greater precision; but it also provides a unique model of the dedication and ingenuity required if comparable levels of understanding of the ecology and epidemiology of other arbovirus infections are to be achieved.

The ecology of western equine encephalomyelitis virus in the Central Valley of California, 1945-1985

Reeves' concept of the summer transmission cycle of western equine encephalomyelitis virus in 1945 was that the virus was amplified in a silent transmission cycle involving mosquitoes, domestic chickens, and possibly wild birds, from which it could be transmitted tangentially to and cause disease in human and equine populations. Extensive field and laboratory studies done since 1945 in the Central Valley of California have more clearly defined the specific invertebrate and vertebrate hosts involved in the basic virus transmission cycle, but the overall concept remains unchanged. The basic transmission cycle involves Culex tarsalis as the primary vector mosquito species and house finches and house sparrows as the primary amplifying hosts. Secondary amplifying hosts, upon which Cx. tarsalis frequently feeds, include other passerine species, chickens, and possibly pheasants in areas where they are abundant. Another transmission cycle that most likely is initiated from the Cx. tarsalis-wild bird cycle involves Aedes melanimon and the blacktail jackrabbit. Like humans and horses, California ground squirrels, western tree squirrels, and a few other wild mammal species become infected tangentially with the virus but do not contribute significantly to virus amplification.

Arbovirus isolations from mosquitoes collected during and after the 1982-1983 epizootic of western equine encephalitis in Argentina

Mosquitoes were collected in Santa Fe and Rio Negro provinces, Argentina, in 1982-1983 during a western equine encephalitis (WEE) epizootic. Totals of 153,084 mosquitoes from Santa Fe Province and 484 from Rio Negro Province were tested for virus in 2,351 pools. Seventeen virus strains were isolated, all from Santa Fe collections, as follows: 4 WEE, 6 Venezuelan equine encephalitis, 1 St. Louis encephalitis, 2 Antequera, 1 Maguari, 1 Melao, 1 new vesiculovirus (Calchaqui), and 1 Gamboa. The WEE virus isolates were from Aedes albifasciatus, Anopheles albitarsis, Mansonia species, and Psorophora pallescens. Collections during the spring and summer (1983-1984) following the epizootic yielded 49,707 mosquitoes from Santa Fe, 15,961 from Rio Negro, and 2,019 from Chubut provinces. Twenty-two virus strains were isolated, all from Santa Fe mosquitoes, as follows: 3 strains of SLE virus and 19 strains of Turlock (TUR) virus. All but one of the TUR virus isolates appear to have come from mosquitoes that engorged on a viremic chicken following entry into a bait trap. The vector relationships of each virus isolated during and after the WEE epizootic are discussed.

Laboratory vector competence of Culex (Melanoconion) cedecei for sympatric and allopatric Venezuelan equine encephalomyelitis viruses

Laboratory vector competence of Culex (Melanoconion) cedecei was examined for Venezuelan equine encephalomyelitis (VEE) viruses. Colonized adult female mosquitoes originating from a southern Florida population were given bloodmeals from viremic hamsters circulating various titers of 3 hemagglutination inhibition (HI) subtypes of VEE viruses. Following extrinsic incubation of about 3 weeks, mosquitoes were allowed to refeed on uninfected hamsters for transmission trials. Cx. cedecei was highly efficient in becoming infected with and transmitting its sympatric, HI subtype II "Everglades" virus. With bloodmeal titers of 10(0.9) chick embryo cell culture (CEC) plaque forming units (PFU), the infection rate was 9% and transmission occurred following extrinsic incubation. Infection rates were greater than or equal to 80% with oral doses of greater than or equal to 10(1.8), and all infected mosquitoes were capable of transmission following incubation. Cx. cedecei was also highly sensitive to infection with allopatric HI subtype IE Middle American VEE virus isolates. Infection rates were greater than or equal to 50% with bloodmeal titers undetectable by CEC assay. Rates were 100% with oral doses of greater than or equal to 10(0.8) CECPFU. Transmission rates were 100% in all experiments. Similar results were obtained with HI subtype IAB "epizootic" VEE virus isolates from the 1969 Middle American outbreak. Infection rates were 100% with oral doses of greater than or equal to 10(1.2), and transmission rates were 100% after extrinsic incubation. Comparisons with laboratory vector competence of the Middle American enzootic VEE virus vector, Culex (Melanoconion) taeniopus, are discussed.

Air conditioning and television as protective factors in arboviral encephalitis risk

In California, the advent of television and air conditioned housing has coincided with a general decrease in mosquito-borne viral encephalitis cases in humans in the past 25 years. During this same period, levels of vector populations, virus activity in vectors, birds, and horses has been high at times. Air conditioning and television encourage persons to remain indoors during summer evenings, the primary time when infected Culex tarsalis transmit western equine encephalomyelitis and St. Louis encephalitis viruses. The attack rates for these 2 diseases in 33 counties in the central valley of California, 1945-1982, were compared with the prevalence of household air conditioning and television. Encephalitis rates were negatively associated with television ownership in both cross-sectional and longitudinal analysis. Moreover, individual counties which experienced the most rapid increase in household television ownership had the greatest decline in encephalitis rates. A telephone survey conducted in Kern County revealed that air conditioner and television utilization times corresponded closely to the feeding times of Cx. tarsalis, and respondents indicated a preference to remain indoors during this time because of these appliances. It is concluded that changed behavioral patterns may protect from vector-borne diseases and be complementary to vector control programs.