Ecology

Reservoir and Reservoir Distributions: United States

All hantaviruses known to cause hantavirus pulmonary syndrome (HPS) are carried by New World rats and mice of the family Muridae, subfamily Sigmodontinae. The subfamily Sigmodontinae contains at least 430 species, which are widespread in North and South America. The rodent hosts of HPS are generally not associated with urban environments, although some species, including the deer mouse (Peromyscus maniculatus), and white-footed mouse (Peromyscus leucopus), will enter human habitation in rural and suburban areas.

Several hantaviruses that are pathogenic for humans have been identified in the United States. In general, each virus has a single primary rodent host. Other small mammals can be infected as well but are much less likely to transmit the virus to other animals or humans. The deer mouse is the host for Sin Nombre virus (SNV), the primary causative agent of HPS in the United States. The deer mouse is common and widespread in rural areas throughout much of the United States. Although prevalence varies temporally and geographically, on average about 10% of deer mice tested throughout the range of the species show evidence of infection with SNV.

Other hantaviruses associated with sigmodontine rodents and known to cause HPS include New York virus, which is hosted by the white-footed mouse; Black Creek Canal virus, which is hosted by the cotton rat (Sigmodon hispidus); and Bayou virus, which is hosted by the rice rat (Oryzomys palustris). Nearly the entire continental United States falls within the range of one or more of these host species. Several other sigmodontine rodent species in the United States are associated with additional hantaviruses that have yet to be implicated in human disease.

Recent studies have confirmed that infected deer mice are present in every habitat type-from desert to alpine tundra, although the prevalence of infection is higher in certain middle-altitude habitats. Surveys of rodents throughout the United States suggest that SNV is distributed in all locations where P. maniculatus is found. Related hantaviruses are also found throughout the geographic range of their rodent carriers. Given that P. maniculatus and P. leucopus are commonly found in the peridomestic setting and typically have higher population densities than other rodents, cases of HPS can be expected to occur throughout the range of these rodent species. Other implicated species, such as S. hispidus and O. palustris, generally do not live in such close proximity to human habitats, and this factor may decrease the probability of human exposure to viruses shed by these rodents.

Lower population density, a lesser propensity for peridomestic encroachment and a narrower geographic and ecologic distribution (and perhaps differing virulence) may explain the lack of human disease associated with hantaviruses (or genetic sequences thought to represent additional hantaviruses) from meadow and California voles (Microtus pennsylvanicus and californicus, respectively), the western harvest mouse (Reithrodontomys megalotis), and the brush mouse (Peromyscus boylii).

Reservoir Distribution Outside the United States

HPS is more common in South America than in North America. Cases have been identified in Argentina, Chile, Uruguay, Paraguay, Brazil, and Bolivia. Andes virus causes HPS in Argentina and Chile and is the only hantavirus known to have been transmitted from person to person. Andes, Bermejo, Hu39694, Lechiguanas, Maciel, Oran, and Pergamino viruses have been linked to HPS cases in Argentina. Bermejo and Laguna Negra virus cause HPS in Bolivia, and Laguna Negra virus is also linked to HPS in Paraguay. Araraquara, Castelo dos Sonhos, and Juquitiba viruses have been associated with HPS in Brazil.

In 1999, an outbreak in Panama marked the first cases of HPS identified in Central America. This outbreak led to the identification of another hantavirus, Choclo virus, which is associated with the rodent host Oligoryzomys fulvescens. The broad geographic distribution of sigmodontine rodents suggests that human cases of HPS will eventually be identified from all countries in the Americas.

Numerous other New World hantaviruses associated with sigmodontine rodents have been identified by molecular methods, but so far, few of them have not been linked to human illness. It is likely that HPS does not occur in the Old World and is confined to the New World distribution of Sigmodontine rodents.

Infection in the Host

Hantaviruses do not cause overt illness in their reservoir hosts. Although infected rodents shed virus in saliva, urine, and feces for many weeks or months or for life, the quantity of virus shed can be at its greatest approximately 3-8 weeks after infection. Field data suggest that transmission in host populations occurs horizontally and that this occurs more frequently among male than female rodents. Transmission from rodent to rodent is believed to occur primarily after weaning and through physical contact, perhaps through aggressive behavior, such as fighting. Studies of the genomic sequences indicate that the virus has probably evolved concurrently with its rodent host over a long period of time.

Occasional evidence of infection (antibody) is found in numerous other species of rodents and their predators (e.g., dogs, cats, and coyotes), indicating that many (perhaps any) mammal species coming into contact with an infected host might become infected. No evidence supports the transmission of infection to other animals or to humans from these “dead-end” hosts. However, domestic cats and dogs may bring infected rodents into contact with humans.

Ticks, fleas, mosquitoes, and other biting insects have not been implicated in the transmission of HPS. Nevertheless, species of Peromyscus in the western United States are susceptible to infection with the plague bacterium (Yersinia pestis), and may act as hosts for plague-carrying fleas. Control of rodents without concurrent control of fleas might therefore increase the risk of human plague as the rodent fleas seek an alternative food source.

Transmission

Human infection occurs most commonly through the inhalation of infectious aerosolized saliva or excreta. Persons visiting laboratories where infected rodents were housed have been infected after only a few minutes of exposure to animal holding areas. Transmission can occur when dried materials contaminated by rodent excreta are disturbed and inhaled, directly introduced into broken skin or conjunctiva, or possibly, when ingested in contaminated food or water. Persons have also acquired HPS after being bitten by rodents. High risk of exposure has been associated with entering or cleaning rodent-infested structures.

Person-to-person transmission has not been associated with HPS cases in the United States. However, person-to-person transmission, including nosocomial transmission of Andes virus, was well documented for a single outbreak in southern Argentina, and it was suspected to have occurred much less extensively in another outbreak in Chile that was associated with the same virus. Therefore, universal precautions are recommended for healthcare workers treating HPS patients.

Related Links

• How People Get Hantavirus Pulmonary Syndrome (HPS)
• Reported Cases of HPS