Bunyaviruses

Hantaviruses

A wide diversity of hantavirus species have been discovered around the world ( Table 219.1 ). Those that are pathogenic to humans are divided among Old World and New World hantaviruses. All have been associated with rodent reservoirs and are distributed according to their reservoir. Periodic increases in human cases have been associated with ecological conditions that promote rodent population growth and increased contact between rodents and humans. In the dry southwestern US, Sin Nombre virus disease cases have been shown to increase during El Nino years, in a direct relationship with rainfall and the population of the deer mouse ( Peromyscus maniculatus ) reservoir. ^, Similarly, trends of Puumala cases in Finland correlate with changes in the bank vole ( Myodes glareolus ) population.

A wide variety of New World hantaviruses can cause disease in humans. All have rodent reservoirs, and pathogenic viruses are known to exist in the US, Canada, Panama, and across a large portion of South America. Within the US, most cases are due to Sin Nombre virus infection and occur in the western portion of the country. ^, A number of hantavirus species are associated with hantavirus pulmonary syndrome across South America. ^, The route of infection is typically through inhalation of rodent urine or feces or by a bite from an infected rodent. Infections often are associated with cleaning or other activities that result in the aerosolization of infectious material from buildings or structures with rodent contamination.

Pathogenic Old World hantaviruses are found across Europe and Asia, with Hantaan and Seoul viruses associated primarily with China, Korea, and far east Russia; Dobrava and Puumala viruses are found in western Russia and Europe. The globally distributed Norway rat ( Rattus norvegicus ) is the reservoir for the Seoul virus, and cases of Seoul virus have been found across the world. Hantavirus infections resulting in hemorrhagic fever with renal syndrome also occur through inhalation of rodent excreta or by rodent bite. Domestically acquired cases due to Seoul virus have been documented in North America, and serologic evidence suggests the presence of the virus in rodent populations in a number of cities in North and South America. In 2017, an outbreak of Seoul virus affecting pet Norway rats and their breeders spread through 11 US states and Canada. The resulting investigation identified 18 human cases of Seoul virus infection and three hospitalizations.

Rift Valley Fever

RVF is known for explosive outbreaks in animals and people. The causative virus is maintained through transovarial transmission in mosquitoes (most commonly Aedes species), and infected eggs can remain dormant for many years in dried waterbeds and flood plains, with no evidence of activity. Small numbers of wild ungulates may become infected in this enzootic cycle. Outbreaks typically occur after heavy rains and flooding, where expanded wetland areas lead to increased mosquito breeding, resulting in large numbers of infected mosquitoes from reservoir pools. Domestic ruminants (e.g., cattle, sheep, goats, and camels) become clinically affected, with high fatality and abortions observed due to infections. Evidence of RVF activity in animals often precedes evidence of human infections during outbreaks. ^,

Farmers, abattoir workers, and other persons with occupational exposures to animals are at greater risk for developing RVF because infections occur through direct contact with blood or bodily fluids of infected animals, often through slaughtering or handling meat. ^, Although less common, humans also can be infected through a mosquito bite. Additionally, there are concerns that RVF virus also may be transmitted through the consumption of raw milk from infected animals, but additional studies are needed. ^, There is no evidence of person-to-person or nosocomial transmission. RVF is endemic across a large portion of Africa, and in recent decades, outbreaks also have occurred in the Middle East (Saudi Arabia and Yemen). ^, Major outbreaks have occurred recently in Kenya, Tanzania, Sudan (2006–2007), Madagascar (2008–2009), South Africa (2010), Mauritania (2012–2015), Niger (2016), Angola (2016), Kenya (2018), Mayotte (2019), and Uganda (2016–2019).

Crimean-Congo Hemorrhagic Fever

CCHF virus is transmitted primarily by Hyalomma species ticks, which are distributed across much of Europe, Africa, and Asia. Cases or seroprevalence has been documented in central Africa, South Africa, the Middle East, central Asia, and southeastern Europe. Although not apparent before 2002, thousands of cases occur each year in Turkey. CCHF virus is maintained through a 2-year transmission cycle in which ticks acquire the nutrients they need to transition through each developmental stage by taking a blood meal from a vertebrate. Overt symptomatic disease is known to occur only in humans. Human infection results from the bite of an infected tick or by contact with tissues, blood, or milk from infected animals. Persons who work closely with livestock are at greater risk for developing CCHF, either through direct contact with the animals or tissues or from bites of ticks or handling ticks associated with the livestock. Person-to-person transmission also has been reported, including vertical and nosocomial transmission resulting from direct contact with the blood or body fluids of infected individuals. ^,

California Serogroup Viruses

Several California serogroup viruses are endemic in the US (i.e., La Crosse, Jamestown Canyon, snowshoe hare, Trivittatus, Keystone, and California encephalitis viruses); however, most human disease is associated with La Crosse and Jamestown Canyon viruses. La Crosse virus is the most common cause of arthropod-borne viral (arboviral) infections of the central nervous system among children in the US. On average, 80–100 cases of La Crosse virus disease are reported annually in the US, with almost 90% of them occurring in children. ^, La Crosse virus infections among adults are uncommon but have been reported and may be under-recognized due to limited awareness or testing. La Crosse virus is transmitted by Aedes triseriatus , a tree-hole breeding mosquito; small rodents (e.g., grey squirrels and chipmunks) serve as the primary amplifying hosts. ^{, ,} The virus is endemic in the eastern half of the US, with the highest numbers of cases in Ohio, West Virginia, North Carolina, and Tennessee. ^{, ,} Infections peak in the summer months of July to September. One study identified an increased likelihood of disease among family members living in the same household or unrelated persons living in the same building at a later date, suggesting ongoing environmental risks in some focal locations.

Jamestown Canyon virus is another mosquito-borne California serogroup orthobunyavirus that is closely related to La Crosse virus. ^, Aedes and Ochlerotatus spp. mosquitoes likely are the primary vectors, and deer appear to be important amplifying hosts. ^, Although Jamestown Canyon virus is widely distributed throughout temperate North America, until recently, reports of human Jamestown Canyon virus infection had been rare, in part owing to a lack of available diagnostic testing. From 2010 to 2019, 208 cases of Jamestown Canyon virus disease were reported to the US Centers for Disease Control and Prevention (CDC). Most cases (90%) occurred from April through September, but, in contrast to La Crosse virus, only 13 (7%) occurred in children. Overall, 125 (69%) of the reported cases were male. Almost all of the cases (98%) were identified since 2013, the first year CDC implemented routine Jamestown Canyon virus antibody testing, suggesting the disease has been historically under-recognized. ^, Although cases were reported from 22 states, 161 (77%) were residents of Wisconsin or Minnesota. Recent enhanced surveillance activities have identified additional cases in upper Midwest and northeastern states. ^, However, for some of these cases, it is not clear if the finding of Jamestown Canyon virus antibodies in serum reflects recent or previous infection or is due to cross-reactivity with another California serogroup virus.