Mosquitoes and the incidence of encephalitis

Outbreak Investigation: Jamestown Canyon Virus Surveillance in Field-Collected Mosquitoes (Diptera: Culicidae) From Wisconsin, USA, 2018-2019

During the summers of 2017–2019, 60 human cases of Jamestown Canyon virus-associated disease were reported in the State of Wisconsin, U.S.A; by comparison, there were 28 cases in the 5 years prior. Jamestown Canyon virus (JCV, Peribunyaviridae: Orthobunyavirus) is a zoonotic, mosquito-borne virus that is endemic throughout North America. The proposed transmission cycle for JCV involves horizontal transmission by a variety of mammal-feeding mosquito species and deer hosts, and transseasonal maintenance by vertical transmission in Aedes mosquito species. Although some of the earliest work on JCV transmission and disease was done in Wisconsin (WI), little is known about the spectrum of mosquitoes that are currently involved in transmission and maintenance of JCV, which is key to inform the approach to control and prevent JCV transmission, and to understand why case numbers have increased dramatically in recent years. Therefore, we undertook an intensive surveillance effort in Sawyer and Washburn counties, WI between April and August of 2018 and 2019, in an area with a concentration of JCV human cases. Larval and adult stages of mosquitoes were surveyed using larval dippers and emergence traps, light traps, resting boxes, a Shannon-style trap, and backpack aspirator. In total, 14,949 mosquitoes were collected in 2018, and 28,056 in 2019; these specimens represent 26 species in 7 genera. Suspect vector species were tested for JCV by polymerase chain reaction (PCR); of 23 species that were tested, only Aedes provocans yielded JCV positive results. In 2018, a single pool of Ae. provocans tested positive. In 2019, with more focused early season surveillance, we detected JCV in 4 pools of adult mosquitoes, and one pool that consisted of lab-raised adults that were collected as larvae. Material from all of these PCR-positive samples also yielded infectious virus in cell culture. Overall, these data provide new insight into the seasonality and habitat preferences for 26 mosquito species in Northern WI, which will be useful to inform future surveillance efforts for JCV. The results underscore the importance of Ae. provocans as a vector species involved in transseasonal maintenance of JCV in this region.

Public health consequences of global climate change in the United States--some regions may suffer disproportionately

Current risk assessments of the likely regional health impacts of global climate change (GCC) are hindered by two factors. First, dose-response relationships between weather parameters and many of the likely health effects have not been developed, and second, reliable estimates of future regional climates across the United States are still beyond the scope of current modeling efforts. Consequently, probabilistic risk estimates of most of the likely regional health impacts of GCC have such a high degree of uncertainty that their usefulness to health officials dealing with regional issues is very limited. With the numerous pressures on today's health care systems, it is understandable that the possible consequences of GCC have received scant attention from regional health care decision makers. Indeed, the consensus among this community appears to be that any increases in health effects associated with GCC will be easily handled by the current health care system. However, such a position may be naive as the potential exists that an unequal distribution of such effects could overwhelm some regions, whereas others may feel little or no impact. This review of the likely regional impacts of GCC has been structured as a semianalytical look at this issue of distributional effects. Because of the lack of dose-response information and reliable estimates of future regional climates, however, it takes a historical perspective. That is, it assumes that the quality and quantity of health risks a region faces under GCC will be directly related to its recent history of health risks from warm weather/climate-related diseases as well as to the size, characteristics, and distribution of the sensitive subpopulations currently residing within its borders. The approach is semiquantitative; however, it uses national data gathered on a regional level and as such should only be used to generate a hypothesis rather than test it. When applied to the United States, its outcome leads to the hypothesis that if indeed history repeats itself, some states or regions may be more greatly affected by GCC than others, not only because historically they are more prone to summer weather/climate-related diseases, but also because they contain a greater proportion of the sensitive subpopulations in the United States.

La Crosse and other forms of California encephalitis

The California serogroup viruses are mosquito viruses that cause human infections on five continents. They are maintained and amplified in nature by a wide variety of mosquito vectors and mammalian hosts; they thrive in a remarkably wide variety of microclimates (eg, tropical, coastal temperate marshland, lowland river valleys, alpine valleys and highlands, high boreal deserts, and arctic steppes). In 1993, California serogroup viruses caused 71% of all cases of arboviral illness in the United States, principally La Crosse encephalitis. The 30 to 180 annual cases of La Crosse encephalitis represent 8% to 30% of all cases of encephalitis, rendering this illness the most common and important endemic mosquito-borne illness in the USA. Subclinical or mild infections are much more common. Methods and results acquired from intense study of California serogroup viruses have been applied, with benefit, to the study of the ecology and pathogenesis of many more serious human arboviral illnesses. The evolutionary potential of viruses, with particular reference to the development of more virulent strains, has been studied more closely in the California serogroup viruses than in almost any other agent of human disease.

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.

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.

The pathogenesis of Japanese encephalitis virus in Culex tritaeniorhynchus mosquitoes

Culex tritaeniorhynchus were inoculated intrathoracically with mosquito and human strains of Japanese encephalitis virus and maintained at 26 degrees C or 32 degrees C. Virus titration and localization of viral antigen by avidin-biotin immunoperoxidase staining were done at intervals up to 21 days. Marked differences were noted in the tempo of organ involvement at the 2 temperatures; at both there was initial infection of fat body cells followed by selective infection of the central nervous system (CNS), with consistent infection of cells of the compound eye, patchy involvement of cephalic, thoracic and abdominal ganglia and no infection of Johnston's organ. CNS infection was always present 4 days after infection, when salivary gland involvement was first seen at 32 degrees C; at 26 degrees C CNS infection preceded salivary gland infection by 2 weeks. Late involvement of gut cells, pericardial cells and oviducts was also found, with no involvement of muscle.

Evolution of bunyaviruses by genome reassortment in dually infected mosquitoes (Aedes triseriatus)

Aedes triseriatus mosquitoes became dually infected after ingesting two mutants of LaCrosse (LAC) virus simultaneously or after ingesting, by interrupted feeding, the two viruses sequentially within a 2-day period. After 2 weeks of incubation, approximately 25 percent of the vectors contained new virus genotypes as the result of RNA segment reassortment. New viruses were transmitted when the mosquitoes fed on mice. Viruses ingested more than 2 days after the initial infecting virus did not cause superinfection of the mosquito vectors.