Ecology
Reservoir and Reservoir Distribution: 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.
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