California encephalitis virus prevalence throughout the Yukon Territory, 1971-1974

Mosquitoes from Europe Are Able to Transmit Snowshoe Hare Virus

Snowshoe hare virus (SSHV) is a zoonotic arthropod-borne virus (arbovirus) circulating in colder areas of the Northern Hemisphere. SSHV is maintained in an enzootic cycle between small mammals and mosquitoes, assumably of the genera Aedes and Culiseta. Symptoms of SSHV human infection can range from asymptomatic to severe neuroinvasive disease. Studies on SSHV transmission are limited, and there is no information available on whether mosquitoes of the genus Culex are able to transmit SSHV. Therefore, we investigated six mosquito species via salivation assay for their vector competence. We demonstrated that SSHV can be transmitted by the abundant European Culex species Cx. pipiens biotype pipiens, Cx. pipiens biotype molestus, and Cx. torrentium with low transmission efficiency between 3.33% and 6.67%. Additionally, the invasive species Ae. albopictus can also transmit SSHV with a low transmission efficiency of 3.33%. Our results suggest that local transmission of SSHV after introduction to Europe seems to be possible from a vector perspective.

Snowshoe hare virus: discovery, distribution, vector and host associations, and medical significance

Snowshoe hare virus (SSHV), within the California serogroup of the genus Orthobunyavirus, family Peribunyaviridae, was first isolated from a snowshoe hare (Lepus americanus) in Montana, United States, in 1959. The virus, closely related to LaCrosse virus (LACV) and Chatanga virus (CHATV), occurs across Canada and the northern latitudes of the United States, primarily in the northern tier of states bordering Canada. Reports of SSHV in northern Europe and Asia are probably the closely related to CHATV, or the less closely related Tahyna virus. Vertebrate associations include snowshoe hares and ground squirrels, demonstrated by field isolation of virus from wild-caught animals, seroconversion of snowshoe hares, seroconversion of sentinel rabbits, isolation of virus from sentinel rabbits, and experimental infections demonstrating viremia. Isolations of virus from field populations of mosquitoes include primarily univoltine and boreal mosquitoes of the genus Aedes, Culiseta impatiens and Culiseta inornata; and, rarely, certain multivoltine floodwater Aedes species. Experimental transmission studies in mosquitoes show infection in and transmission by boreal Aedes and Culiseta inornata. Isolation of SSHV from larval Aedes on three occasions, and experimentation in Culiseta inornata, reveal transovarial transmission of the virus in mosquitoes. Serosurveys reveal exposure to SSHV in human and domestic animals, with rates of seropositivity commonly high in some settings in Alaska and Canada, but disease in humans or horses has rarely been reported, only in Canada.

California Serogroup Viruses in a Changing Canadian Arctic: A Review

The Arctic is warming at four times the global rate, changing the diversity, activity and distribution of vectors and associated pathogens. While the Arctic is not often considered a hotbed of vector-borne diseases, Jamestown Canyon virus (JCV) and Snowshoe Hare virus (SSHV) are mosquito-borne zoonotic viruses of the California serogroup endemic to the Canadian North. The viruses are maintained by transovarial transmission in vectors and circulate among vertebrate hosts, both of which are not well characterized in Arctic regions. While most human infections are subclinical or mild, serious cases occur, and both JCV and SSHV have recently been identified as leading causes of arbovirus-associated neurological diseases in North America. Consequently, both viruses are currently recognised as neglected and emerging viruses of public health concern. This review aims to summarise previous findings in the region regarding the enzootic transmission cycle of both viruses. We identify key gaps and approaches needed to critically evaluate, detect, and model the effects of climate change on these uniquely northern viruses. Based on limited data, we predict that (1) these northern adapted viruses will increase their range northwards, but not lose range at their southern limits, (2) undergo more rapid amplification and amplified transmission in endemic regions for longer vector-biting seasons, (3) take advantage of northward shifts of hosts and vectors, and (4) increase bite rates following an increase in the availability of breeding sites, along with phenological synchrony between the reproduction cycle of theorized reservoirs (such as caribou calving) and mosquito emergence.

Throw out the Map: Neuropathogenesis of the Globally Expanding California Serogroup of Orthobunyaviruses

The California serogroup (CSG) comprises 18 serologically and genetically related mosquito-borne orthobunyaviruses. Of these viruses, at least seven have been shown to cause neurological disease in humans, including the leading cause of pediatric arboviral encephalitis in the USA, La Crosse virus. Despite the disease burden from these viruses, much is still unknown about the CSG viruses. This review summarizes our current knowledge of the CSG viruses, including human disease and the mechanisms of neuropathogenesis.

The Ecological Significance and Implications of Transovarial Transmission among the Vector-Borne Bunyaviruses: A Review

Transovarial transmission (TOT) is a widespread and efficient process through which pathogens can be passed between generations of arthropod vectors. Many species within the order Bunyavirales utilize TOT as a means of persisting within the environment when classical horizontal transmission is not possible due to ecological constraints. The purpose of this review is to summarize previous findings regarding the ecological significance of TOT among viruses in the order Bunyavirales and identify the gaps in knowledge regarding this important mechanism of arboviral maintenance.

Serologic Survey of Snowshoe Hares (Lepus americanus) in the Greater Yellowstone Area for Brucellosis, Tularemia, and Snowshoe Hare Virus

We examined sera from snowshoe hares (Lepus americanus) livetrapped in the northern Greater Yellowstone Area (GYA), US, for antibodies to Brucella abortus, Francisella tularensis, and snowshoe hare virus (SSHV). Zero of 90, 0 of 67, and 40 of 100 samples were antibody positive for B. abortus, F. tularensis, and SSHV, respectively. Hares were trapped from 2009 to 2012, and of the six animals that were captured twice with at least 1 yr between captures, four developed antibody to SSHV, indicating active exposure to the agent. These findings suggest snowshoe hares in the GYA do not play a significant role as a reservoir of B. abortus, but do maintain the zoonotic, encephalitic SSHV in the population.

Arboviruses in North Dakota, 2003-2006

To investigate arbovirus transmission in North Dakota, we collected and screened mosquitoes for viral infection by Vero cell culture assay. Seven viruses were isolated from 13 mosquito species. Spatial and temporal distributions of the important vectors of West Nile virus (WNV), Cache Valley virus, Jamestown Canyon virus (JCV), and trivittatus virus are reported. Snowshoe hare virus, Potosi virus, and western equine encephalomyelitis virus were also isolated. The risks of Culex tarsalis and Aedes vexans transmitting WNV to humans were 61.4% and 34.0% in 2003-2006, respectively, but in 2003 when the largest epidemic was reported, risks for Ae. vexans and Cx. tarsalis in Cass County were 73.6% and 23.9%, respectively. Risk of humans acquiring an infectious bite was greatest from about the second week of July through most of August. West Nile virus sequences were of the WN02 genotype. Most JCV strains belonged to a single clade of genetically related strains. Cache Valley virus and JCV were prevalent during August and early September and during July and August, respectively.

Infections in the natural environment of British Columbia, Canada

The Canadian province of British Columbia has a luxurious environment, complete with the multitude of wildlife and insects, and would at first glance appear to be suitable for the transmission of diseases in nature communicable to humans. Despite this potential, such diseases are relatively uncommon, although several have the potential for serious consequences. Attention has been recently focused on hantavirus infection, water-borne toxoplasmosis and parasitic diarrheal diseases, cryptococcosis on Vancouver Island, and rabies. West Nile virus has not yet caused endemic human infection in this province as of 2008. We review the cumulative science in this area.

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.

Bunyavirus development in arctic and Aedes aegypti mosquitoes as revealed by glucose oxidase staining and immunofluorescence

Northway virus replication has been detected in salivary glands of wild-caught Culiseta inornata and Aedes communis mosquitoes from the western Canadian Arctic after incubation at 4 degrees C for 9 to 11 months, and after incubation at 13 degrees C for 3 to 4 months after they received virus by oral ingestion or intrathoracic injection. Aedes hexodontus supported Northway virus replication after incubation at 13 degrees C for one month after intrathoracic injection. Aedes aegypti supported Northway virus replication after incubation at 13 degrees C or 23 degrees C for 6 to 28 days following intrathoracic injection. A larval isolate of California encephalitis virus (snowshoe hare subtype) multiplied in all 3 species of arctic mosquito after incubation at 13 degrees C for 1 to 3 months after virus was administered by oral ingestion or intrathoracic injection. Virus was detected in salivary glands of Cs. inornata after 329 days incubation at 4 degrees C after intrathoracic injection. Bunyavirus antigens in salivary glands of arctic and domestic mosquitoes were detected by the glucose oxidase immunoenzyme technique somewhat less frequently than by assay for virus infectivity.

Transmission of Northway and St. Louis encephalitis viruses by arctic mosquitoes

Transmission of a Canadian arctic isolate of Northway virus has been demonstrated after incubation of arctic Aedes communis mosquitoes at 13 degrees C for 27 days after intrathoracic injection of 300 plaque forming units of virus. Replication has also been demonstrated after intrathoracic injection of domestic A. aegypti mosquitoes of this virus. Virions of Northway virus, 84--92 nm diameter were morphologically typical of a bunyavirus after propagation in salivary glands of A. communis or in tissue cultures of baby hamster kidney (BHK-21) cells. An Ontario isolate of St. Louis encephalitis was transmitted by bites of A. communis after 27 days incubation at 13 degrees C after oral ingestion of 3 or 30 mouse LD50 virus. This mosquito species transmitted virus after 13 to 76 days incubation at 13 degrees C following intrathoracic injection of 3 mouse LD50 or higher virus doses.

California encephalitis group viruses isolated from mosquitoes collected in Southern and Arctic Norway

Three virus strains serologically related to the California encephalitis group (Bunyaviridae) of arboviruses were isolated from 7331 mosquitoes collected in Norway in June-August 1975. Two of the isolates (S 548 and S 618) seemed to be closely related and the third, S 568, more distantly related by serological techniques to Tahyna virus. Viruses were found in the mosquito species Aedes sticticus, A. diantaeus and A. hexodontus colllected (in order) from Oyern (59 degrees N, 11 degrees 12' E), Trandum (60 degrees 08' N, 11 degrees 10' E) and Masi (69 degrees 26' N, 23 degrees 39' E). The Masi isolate seems to be the northernmost arbovirus isolate in the world so far. Strain S 568 was from 16 male A. diantaeus, indicating transovarial transmission of the virus. An accidental infection demonstrated the potential human pathogenicity of one of the newly-isolated strains (S 568), and the ability of CE viruses to persistently infect suckling mouse brains was indicated by in vivo findings. The biological characteristics of the new strains so far investigated are consistent with those of the California encephalitis group. During this work the Aerosil absorption method for production of haemagglutinating antigens proved useful for Tahyna virus and the newly-isolated strains. Trypsinization of erythrocytes improved the haemagglutinating capacities of these viruses.

Bunyavirus isolations from mosquitoes in the western Canadian Arctic

Strains of California encephalitis virus (snowshoe hare subtype) were isolated from 8 of 475 pools comprising 23 747 unengorged female mosquitoes of five species collected at three of six locations throughout the Mackenzie Valley of the Northwest Territories, Canada, from latitudes 60 to 69 degrees N between 10 and 24 July 1976. Minimum field infection rates included 1:2734 for Aedes communis, 1:256 to 1:3662 for A. hexodontus and 1:911 to 1:1611 for A. punctor. Northway virus was also isolated from 1 of 3662 A. hexodontus mosquitoes collected at Inuvik (69 degrees N, 135 degrees W). Transmission of CE virus by A. communis infected by feeding on virus in defibrinated blood and incubation at 0, 13 and 23 degrees C for 13-20 days clearly demonstrates the importance of this species as a natural vector, and transmission of CE virus by Culiseta inornata after incubation at 0 and 13 degrees C following intrathoracic injection strengthens evidence of its role as a natural vector. Immunofluorescence was less reliable than imunoperoxidase for detection of CE viral antigen in mosquito salivary glands.

California encephalitis virus transmission by arctic and domestic mosquitoes

A zero passage arctic mosquito isolate of California encephalitis (CE) virus (showshoe hare subtype) was transmitted by wild-caught Aedes communis mosquitoes after 13 days incubation at 13 degrees and 23 degrees C, AFTER 20 days incubation at 13 degrees C, when mosquitoes imbibed 1 mouse LD50 in a blood meal. Transmission occurred after 20 days incubation at 13 degrees and 23 degrees C when mosquitoes were injected intrathoracially with 1 or 0.1 mouse LD50. Virus was also transmitted by A. aegypti 13 days after infection with 100 mouse LD50 by feeding or intrathoracic injection, and incubation at 13 degrees C. Virus antigen was detected in salivary glands of 42 percent virus-positive A communis mosquitoes by direct immunofluorescence, and in 50 percent or less of A aegypti mosquitoes by immunoperoxidase and immunofluorescence, with somewhat greater regularity by the indirect than the direct technique.

Recombination and complementation between temperature-sensitive mutants of a Bunyavirus, snowshoe hare virus

Complementation and recombination have been observed with temperature-sensitive mutants of Bunyavirus snowshoe hare virus.

Arbovirus growth in Aedes aegypti mosquitoes throughout their viable temperature range

After intrathoracic inoculation of laboratory-bred Aedes aegypti mosquitoes with 3 Yukon isolates of California encephalitis (CE) virus (showshoe hare subtype), Northway (NOR) and Murray Valley encephalitis (MVE) viruses, viral replication was observed following incubation at 13, 21, 35 and 39 degrees C, which constituted the full temperature range of viability of A. aegypti. Rates of viral replication were reduced at low temperatures and accelerated at high temperatures. Virus-specific immunoperoxidase staining of mosquito salivary glands occurred regularly after thoraces attained maximum infectivity levels. At 13 and 21 degrees C, mosquitoes were infected by 10 to 100 times less CE and MVE viruses than mice, but about 10 times more NOR virus was required to infect mosquitoes than mice.