Emerging Infectious Diseases
[Volume 5 No.1 / January - February 1999]

Suggested Citation

Hantavirus

Natural History of Sin Nombre Virus in Western Colorado

Charles H. Calisher,* William Sweeney,* James N. Mills, and Barry J.
Beaty,*
*Colorado State University, Fort Collins, Colorado, USA; and Centers for
Disease Control and Prevention, Atlanta, Georgia, USA

---------------------------------------------------------------------------
      A mark-recapture longitudinal study of immunoglobulin G (IgG)
      antibody to Sin Nombre virus (SNV) in rodent populations in
      western Colorado (1994results summarized to October 1997)
      indicates the presence of SNV or a closely related hantavirus
      at two sites. Most rodents (principally deer mice, Peromyscus
      maniculatus, and pinyon mice, P. truei) did not persist on the
      trapping webs much beyond 1 month after first capture. Some
      persisted more than 1 year, which suggests that even a few
      infected deer mice could serve as transseasonal reservoirs and
      mechanisms for over-winter virus maintenance. A positive
      association between wounds and SNV antibody in adult animals at
      both sites suggests that when infected rodents in certain
      populations fight with uninfected rodents, virus amplification
      occurs. At both sites, male rodents comprised a larger
      percentage of seropositive mice than recaptured mice, which
      suggests that male mice contribute more to the SNV epizootic
      cycle than female mice. In deer mice, IgG antibody prevalence
      fluctuations were positively associated with population
      fluctuations. The rates of seroconversion, which in deer mice
      at both sites occurred mostly during late summer and midwinter,
      were higher than the seroprevalence, which suggests that the
      longer deer mice live, the greater the probability they will
      become infected with SNV.

To monitor Sin Nombre virus (SNV) dynamics in natural rodent communities,
we established longitudinal studies at two sites in western Colorado, each
near a location where human hantavirus infections occurred in 1993. This
article provides a summary of the data collected during the first 3 years
of the studies. The results indicate that rodent populations in western
Colorado have decreased since 1993; SNV or an SNV-like hantavirus persists
at these sites; and prevalence of immunoglobulin G (IgG) antibody to SNV
fluctuates with time and perhaps with weather patterns that modify the
ecosystem.

The Study

Selected Sites

Each study area was selected on the basis of its proximity to residences of
hantavirus pulmonary syndrome (HPS) case-patients, convenience for field
work, and guaranteed cooperation by land managers. Sites at Fort Lewis (La
Plata County, southwest Colorado) (N 37 13' 30.9" latitude, W 108 10'
51.1" longitude, altitude 2,438 m) and Molina (Mesa County, west central
Colorado) (N 39 09' 45.8" latitude, W 108 03' 18.4" longitude, altitude
1,951 m) were within a few kilometers of case-patient residences.

Fort Lewis (approximately 22 km west of Durango, 8 km south of Hesperus) is
10 km north of Red Mesa, Southern Ute Indian Reservation, Colorado, where
rodent trapping in 1993 showed that deer mice had an antibody prevalence
rate of 50% to SNV and near where one of the persons who later died of HPS
had been infected with SNV (1). Seroprevalence in Peromyscus maniculatus,
the principal rodent reservoir of SNV, was approximately 50% to 19%,
respectively, near study sites in La Plata and Mesa Counties (1).

We established trapping webs (Mills et al., this issue, pp. 95-101) in two
protected areas (Fort Lewis A and Fort Lewis B) of the 2,550 ha-Colorado
State University San Juan Basin Research Center, which serves as a model
for cattle breeders and livestock geneticists. The natural characteristics
of these sites have been preserved.

Fort Lewis is in the drainage of the La Plata River, south of Mount
Hesperus in the La Plata Mountains. The general ecosystem of the area is
montane shrubland (2) superimposed on intrusive igneous rocks forming
laccoliths (3). The overstory vegetation at Fort Lewis A is predominately
ponderosa pine (Pinus ponderosa) and Gambel's oak (Quercus gambeli);
understory vegetation is primarily blue grama (Bouteloua gracilis), black
grama (B. eriopoda), and floral components also seen at Fort Lewis B. At
Fort Lewis B, 500 m from Fort Lewis A, overstory is essentially all
Gambel's oak; understory is composed of blue and black grama or there is no
overstory, with the microcommunity composed primarily of blue and black
grama, small soapweed (Yucca glauca), tree cholla (Opuntia imbricata), and
pasture sagebrush (Artemesia frigida).

The trapping sites near Molina (approximately 60 km east of Grand Junction)
are within 2 km of the home of a 1993 case-patient. In 1993, deer mice had
an antibody prevalence rate of 19% to SNV (4).

At Molina we established webs in two areas (Molina A and Molina B, 500 m
apart) that are privately owned and have not been grazed by cattle for many
years. The sites have no standing water sources, but an irrigation ditch,
containing rapidly running water, flows during the summer at the west and
north edges of Molina A.

The general ecosystem of the area is semidesert shrubland (2) superimposed
on Mancos shale (3). At Molina A, we found principally Rocky Mountain
juniper (Juniperus scopulorum), pinyon pine, small soapweed, and pasture
sagebrush. Molina B is characterized by pasture sagebrush, Rocky Mountain
juniper, Parry's rabbitbrush (Chrysothamnus paryii), and pinyon pine at the
periphery.

All field data were recorded on hard copy and entered into EPI-5, a
database and statistical program available from the Centers for Disease
Control and Prevention (CDC) (5).

Sampling Methods

All materials were transported to the study sites or were available in
towns near the sites. Under license of the State of Colorado's Department
of Natural Resources, sampling was done every 6 weeks, weather permitting.
Trapping webs were established according to methods agreed upon by
collaborating groups (Mills et al., this issue, pp. 95-101). In brief, each
web comprised 12 rows of 12 Sherman traps (7.6 cm x 8.9 cm x 22.9 cm; H.B.
Sherman Traps, Inc., Tallahassee, FL) each, the first four traps in each
row being placed 5 m apart, the next eight placed 10 m apart; rows were 30
degrees from each other. The location of each trap was marked with a
construction flag. Rodents were anesthetized with Metaphane
(methoxyflurane, Pitman-Moore, Mundelein, IL) during processing, marked
with sequentially numbered stainless steel ear tags, and released at the
capture site.

Webs A and B at each location were sampled for 2 or 3 consecutive nights,
but rodents were neither bled nor swabbed at webs B until October 1996,
when animals from both sites were sampled. The original intent had been to
not take blood or oropharyngeal swab samples at either web B to determine,
by comparison with data from the corresponding web A, the impact of these
invasive procedures on the rodent populations. Because the death rates at
webs A and B were essentially the same after 2 years (6; C.H. Calisher and
B.J. Beaty, unpub. data), in October 1996, we began to take blood samples
from rodents at both webs and to no longer collect oropharyngeal swabs.
Rodents, principally deer mice, were processed and samples were placed on
dry ice (-70C), returned to the laboratory in Fort Collins, and placed in
a mechanical freezer (-80C) until they were tested for IgG antibody.

Sampling was conducted according to standardized protocols (Mills et al.,
this issue, pp. 95-101). To compare age categories, in the field we
empirically classified captured animals as juvenile, subadult, or adult,
according to Fitzgerald, Meaney, and Armstrong (2). For final
determination, we separated animals into weight classes (10% to 40% of
adult mean weight = juvenile, 41% to 80% = subadult, and 81% to 100% =
adult).

After being tested at Colorado State University, blood samples and
oropharyngeal swabs were shipped to Atlanta, Georgia, where confirmatory
testing for IgG antibody to SNV was conducted with blood samples, and
oropharyngeal swabs were stored for possible future testing.

Enzyme-Linked Immunosorbent Assays (ELISA) for IgG Antibody to SNV

ELISA was performed at Colorado State University as described (Mills et
al., this issue, pp. 95-101). Results presented here were obtained at
Colorado State University; testing at CDC provided confirmation. We
initially screened whole blood samples at 1:100; antibody-positive samples
were titrated to determine end points.

Population Densities

We estimated the population size at each sampling period by calculating the
minimum number of rodents alive (7). The minimum number of rodents alive
for a given trapping session was calculated by taking the total number of
rodents captured during that session and adding to that sum all rodents
that had been captured on at least one previous and one subsequent
occasion. The minimum number of antibody-positive rodents was calculated
similarly, and the estimated standing prevalence was calculated as minimum
number of antibody-positive rodents/minimum number of rodents alive.

Findings

Over the 41-month trapping period at Fort Lewis and the 37-month trapping
period at Molina, antibody reactive with SNV was detected in 29 (9.6%) of
302 deer mice at Fort Lewis and 36 (9.4%) of 385 at Molina; 4 (2.6%) of 155
of pinyon mice at Molina also had antibody (Table 1). For comparison, in
1993, prevalence of antibody to SNV in P. maniculatus was approximately 50%
near Fort Lewis (La Plata County) and 19% near Grand Junction (Mesa County)
(1). Of 112 least chipmunks (Tamias minimus), two Colorado chipmunks (T.
quadrivittatus), and two western harvest mice (Reithrodontomys megalotis),
none had antibody to SNV.

Table 1. Antibody (enzyme-linked immunosorbent
assay for immunoglobulin G) to Sin Nombre virus,
Fort Lewis and Molina, Colorado, 1994-1997
--------------------------------------------------
                               No.         %
				    positive/  Antibody-
Location	Species	   No. tested  positive
--------------------------------------------------
Fort       Peromyscus        29/302       9.6
Lewis       maniculatus
	     Tamias minimus     0/48        0
	     P. truei           1/3        33

Molina     P. maniculatus    36/385       9.4
	     P. truei           4/155       2.6
	     P. leucopus        1/2        50
           Reithrodontomys    0/2         0
		megalotis
 	     T. minimus         0/64        0
	     T. quadrivittatus  0/2         0
--------------------------------------------------

At Fort Lewis, trapping success (number of animals per total number 
of trap nights) was 0.3% to 7.6%, depending on the season (lowest 
rates, AprilJune; highest, AugustOctober). Antibody-positive deer  
mice were found in 13 of 21 trapping intervals Antibody prevalence     
(calculated when more than four deer mice were caught in a given
trapping period) was 0%  Molina to 42.9% with a mean of           
29 (9.5%) of 302. Antibody to SNV was detected in adult (10.5%), 
subadult (9.8%), and juvenile (12.5%) deer mice; the stages represented                
63.9%, 13.1%, and 23%, respectively, of the deer mice captured.
Males represented 48.8% of the deer mice (and 47.8% of recaptured deer
mice) but 58.3% of the antibody-positive rodents.

At Molina, trapping success was 2.6% to 17.9% and, as at Fort Lewis,
depended on the season (lowest rates, MayJune; highest, JulyOctober).
Antibody-positive deer mice were found in 12 of 17 trapping intervals.
Antibody prevalence was 0% to 33% in deer mice (mean 9.4%) and 0% to 18.2%
in pinyon mice (mean 2.6%). Antibody to SNV was detected in adult (11.3%),
subadult (1.7%), and juvenile (4.4%) deer mice; the stages represented
73.1%, 15.3%, and 11.7%, respectively, of the mice captured. Males
represented 45.5% of the deer mouse population, 46.3% of the recaptured
deer mice, and 60% of the antibody-positive mice. Antibody was detected in
four adult (three male, one female) pinyon mice (P. truei). Of 118 pinyon
mice collected, 62 (50.8%) were female and 56 (49.4%) were male. We
detected seropositive pinyon mice only during May and June 1995 and April
1996.

Wounds and Antibody

Because we were working with a large number of anesthetized rodents, we did
not closely examine each animal for wounds, as had been done by Glass et
al. (8). However, we noted the most obvious wounds (ear nicks, torn ears,
scarred tail) and those likely not to have been caused by trapping,
tagging, or processing, and we evaluated the data for deer mice at webs A
for Fort Lewis and Molina.

Of 233 adult deer mice at Fort Lewis, 20 had both antibody and wounds, 76
had no antibody but had wounds, 4 had antibody and no wounds, and 133 had
neither antibody nor wounds; thus, wounds were associated with antibody to
SNV among adult deer mice (Yates-corrected chi-square 17.71, p = <0.001).
At Molina, of 339 adult deer mice, 8 had antibody and wounds, 23 had no
antibody but had wounds, 21 had antibody and no wounds, and 287 had neither
antibody nor wounds; again wounds were associated with antibody to SNV
(Yates-corrected chi-square 10.67, p = <0.001).

Seroconversion

Fifteen deer mice and one pinyon mouse seroconverted (i.e., seronegative to
seropositive or a fourfold or greater increase in titer) between captures
(Figure 1). At Fort Lewis, 302 deer mice (150 female and 152 male) were
captured. Of these, 37 female and 37 male mice were recaptured at least
once. Five male and three female deer mice at Fort Lewis seroconverted. One
deer mouse had antibody for the first time 14 months after it was initially
captured. At Molina, 385 deer mice (212 female, 173 male) and 155 pinyon
mice (85 female, 70 male) were captured. Of these, 33 female and 30 male
deer mice and 12 female and 10 male pinyon mice were recaptured at least
once. Five male and two female deer mice and one male pinyon mouse
seroconverted. An additional three deer mice (two male, one female) at
Molina were recaptured and had significant (3,200 to 25,600) but stable IgG
antibody titers; we did not consider these as having seroconverted. The
five male mice seroconverted at Fort Lewis during the summer (one between
July and September 1994, two between July and September 1995, one at
[estimated] midsummer 1995, and one between June and September 1997); two
female mice seroconverted between October 1994 and May 1995, and one female
mouse seroconverted during late summer (September to October) 1997. At
Molina, one male deer mouse seroconverted in late spring (estimated May)
1995, one in late fall 1995, two male deer mice and a male pinyon mouse
during the winter or early spring of 1995 to 1996, and one male deer mouse
during late spring 1996; one female deer mouse seroconverted in late summer
1995 and one during the winter 1995 to 1996. Seropositive samples were
titrated by IgG ELISA with fourfold dilutions. Titers were 100 to 102,400,
with most of them at 6,400 to 25,600.

[Fig]
Figure 1. Approximate month of seroconversion in Peromyscus
species at Fort Lewis and Molina, Colorado, June 1994October 1997, 
by sex.
                           
Incidence Rates

We calculated incidence rates of IgG antibody to SNV in deer mice recaptured
and sampled at least twice at Fort Lewis and Molina (Table 2). At Fort Lewis A,
the overall incidence was 4.6 new infections per 100 mice per month (4.8
for male, 4.4 for female); at Fort Lewis B, the overall incidence was 10.91 (9.1
for male, 18.2 for female); and at the two sites combined, the overall 
incidence was 6.1 (6.3 for male, 5.8 for female). At Molina A, the overall 
incidence rate in deer mice was 2.8 new infections per 100 mice per month 
(3.2 for male, 2.3 for female); at Molina B, no new infections were detected 
during the observation period; the incidence at the two sites combined was 2.6 
(3.0 for male, 2.2 for female). Because sufficient numbers of pinyon mice were 
captured and a seroconversion was detected at Molina A, we were able to calculate 
the incidence of seroconversion: 0.6 overall (1.4 for male, 0 for female); the
incidence at the two sites combined was 0.5 (1.3 for male, 0 for female).

Table 2. Incidence of immunoglobulin G antibody reactive with Sin Nombre
virus in Peromyscus maniculatus (deer mice) recaptured and sampled at
least twice at Fort Lewis (June 1994October 1997) and P. maniculatus and
P. truei (pinyon mice) recaptured and sampled at least twice at Molina,
western Colorado (October 1994October 1997)
---------------------------------------------------------------------------
Species/       No. at     No.of new     Cum. %     Mouse mos. of
location Sex  risk(sup a)infections antibody-pos.observation(sup b)Incidence(sup c)
 ---------------------------------------------------------------------------
Deer    Grid A    38        4           10.5          88           4.6
mice/    male     17        2            5.9          42           4.8
Ft.      female   21        2            9.5          46           4.4
Lewis   Grid B    11        3           27.3          27.5        10.9
         male      6        2           33.3          22           9.1
         female    5        1           20.0          5.5         18.2
        Grids A+B 49        7           14.3         115.5         6.1
         male     23        4           17.4          64           6.3
         female   26        3           11.5          51.5         5.8

Deer    Grid A    59        5            8.5         179.5         2.8
mice/    male     33        3           11.0          94           3.2
Molina  female    26        2            7.7          85.5         2.3
        Grid B     7        0            0            12.0          0
         male      4        0            0             7.5          0
         female    3        0            0             4.5          0
        Grids A+B 66        5            7.6         191.5         2.6
         male     37        3            8.1         101.5         3.0
         female   29        2            6.9          90           2.2

Pinyon  Grid A    22        1           4.5           157          0.6
mice/    male     10        1           10.0           71          1.4
Molina   female   12        0            0             86           0
        Grid B    15        0            0            33.5          0
         male      7        0            0             9            0
         female    8        0            0            24.5          0
        Grids A+B 37        1            2.7         190.5         0.5
         male     17        1            5.9          80           1.3
         female   20        0            0           110.5          0
---------------------------------------------------------------------------
(sup a)No. of mice antibody-negative at first capture.
(sup b)Total time intervals between successive captures when mice were
antibody-negative, plus half the interval when mice changed from
antibody-negative to antibody-positive.
(sup c)New infections per 100 mice per month.

Longevity

By recapturing animals, we were able to estimate the longevity of infected
and uninfected mice at the sites. Most Peromyscus spp. (75.7% at Fort
Lewis, 66.2% at Molina) were not recaptured after they were first caught.
At Fort Lewis, of 118 female and 117 male deer mice, 79 and 83,
respectively, were caught only once; 61 were recaptured only within a
5-month period, eight within 6 to 9 months, and four 11 to 14 months after
they were first captured. At Molina, of 164 female and 150 male deer mice,
115 and 93, respectively, were caught only once; 89 were recaptured only
within a 4-month period, 15 only within 5 to 9 months, and 2 as long as 10
months after they were first captured. Of 63 female and 59 male pinyon
mice, 49 and 43, respectively, were caught only once; 21 were recaptured
only within a 4-month period, 5 only within 5 to 9 months, and 1 each for
10, 11, 19, and 20 months after they were first captured.

Longevity data of seropositive and seronegative deer mice at Fort Lewis and
Molina are summarized in Table 3. Eighteen deer mice had antibody at two or
three bleeding intervals from 1 to 7 months after first capture (mean = 2.4
months). Totals do not match the text above because blood samples were not
collected from captured rodents at sites B until October 1996 and because
we included separately periods of seronegativity and seropositivity for
deer mice that seroconverted. Differences between mean longevities by sex,
site, or antibody status were not statistically significant
(Yates-corrected chi-square, p = >0.2).

 Table 3. Longevity of hantavirus-infected and -uninfected male and female
 Peromyscus maniculatus at Fort Lewis and Molina, Colorado, June
 1994October 1997 expressed as number of months between first and last
 capture
 -----------------------------------------------------------------------
                                       No. months between first
                                          and last capture
                   Sero-    Total
 Site       Sex    status    No.   1   2 3  4 5  6 7 8  9 10  11  Mean
 -----------------------------------------------------------------------
 Fort Lewis F        +        3    2               1              3
            M        +        6    3     2         1              2.7
 Molina     F        +        3    2     1                        1.7
            M        +        6    4     1         1              2.3
 Fort Lewis F                27   15  5 2       2 1 1    1       2.5
            M                22   7   4 2  2 1  1 2    2 1       3.7
 Molina     F                37   24  2 4  2    1 2    1     1   2.4
            M                27   13  1 4  2    3 1 1  2         3.2
 -----------------------------------------------------------------------

[Fig]
Figure 2. Minimum number of deer mice alive (MNA) (the number of
individual mice captured in a month plus those mice captured 
on at least one previous and one subsequent occasion) and
estimated standing prevalence (ESP) (minimum number infected
divided by MNA), Fort Lewis, June 1994October 1997.

[Fig]
Figure 3. Minimum number of deer mice alive (MNA) (the number of
individual mice captured in a month plus those mice captured
on at least one previous and one subsequent occasion) and
estimated standing prevalence (ESP) (minimum number infected
divided by MNA), Molina, June 1994October 1997.

Population Densities

Deer mouse populations and prevalence of antibody to SNV at Fort Lewis 
(Figure 2)were relatively low throughout this study, except in May and 
June 1995 when samples included only four deer mice and one deer mouse, 
respectively. Mean minimum number of rodents alive was 28 in 1994 but 
lower from 1995 to 1997(10.8, 13.4, and 16.4, respectively). At Molina, 
populations were relatively stable between 1995 and 1997 (only one
collection made in 1994), with mean minimum number of rodents alive values
of 31.2, 20.4, and 25.4, respectively. As at Fort Lewis, estimated standing
prevalence values were commensurately low (Figure 3).

Conclusions

On the basis of the high antibody titers of these seropositive samples, 
our findings elsewhere in Colorado (Calisher, Beaty, and Mills, unpub.
data), and the findings of others studying hantaviruses in the Southwest
(9), we presumed that IgG antibody to SNV in deer mice indicated infection
with SNV and not with El Moro Canyon or another hantavirus. Although 
we did not attempt to isolate or detect hantaviral RNA in blood or other 
tissues from mice with antibody, the only hantavirus specifically identified 
in deer mice in western Colorado has been SNV (10).

The presence of IgG antibody to hantaviruses in rodents is presumed to
indicate past infection and present infection, at least in the primary
vertebrate hosts of hantaviruses (Mills et al., this issue, pp. 135-142).
That is, rodents infected with hantaviruses with which they appear to be
closely associated coevolutionarily (e.g., deer mice and SNV, Western
harvest mice and El Moro Canyon virus, rice rats [Oryzomys palustris] and
Bayou virus, Black Creek Canal virus and cotton rats [Sigmodon hispidus])
do not appear ill or otherwise affected by hantaviruses specific to them.
In host-virus associations that have been studied, the specific hosts
become infected early or later in life, are viremic for a short period, and
excrete virus in their saliva, urine, and feces, perhaps for life (11-14).

Fighting (including exchange of blood and saliva) between infected and
uninfected adult rodents has been suggested as the primary mechanism by
which hantaviruses are amplified epizootically (8). Infected rodents become
viremic and viruric and serve as subsequent sources of infection for others
in the population. Earlier studies using Seoul virus and laboratory rats as
a model system had indicated that while in newborn rats infection became
persistent, in older rats it was transient (15). However, evidence using
Black Creek Canal virus and adult hispid cotton rats, Hantaan virus and
Apodemus agrarius, and Puumala virus and Clethrionomys glareolus indicates
that whereas viremia may diminish over time, virus can still be detected in
various organs, including the salivary gland, for several months after
infection (11-14). Given the relatively brief life span of rodents,
infection and concomitant infectivity for a few weeks or months would
provide a mechanism for seasonal, albeit not transseasonal, persistence of
hantaviruses. Passive acquisition of maternal antibody may protect the
offspring of infected dams early in their lives, but when antibody wanes,
they enter the adult population as susceptibles. Infected later in life,
they can become persistent shedders of virus and sources of infection for
others in the population.

Deer mice infected with SNV when very young likely are able to serve as
reservoirs of the virus for the remainder of their lives. Although our
studies do not distinguish between death and dispersal, the life span of
many deer mice at these sites may not be much more than a month. However,
because some deer mice live for 1 or 2 years, longevity of even a small
proportion of the deer mouse population may provide a transseasonal
mechanism for virus persistence.

A second mechanism of virus transmission, an epizootic one, depends on
short-term infections of deer mice infected as subadults or as adults. At
periods of deer mouse population peaks (e.g., at the end of the breeding
season, in late summer and fall, and during period of decreased
availability of food), male mice fight one another for breeding partners,
food, and territory. This premise is supported by results of serologic
tests of recaptured deer mice at Fort Lewis and at Molina. At Fort Lewis,
48.8% of the deer mice and 47.8% of the recaptured deer mice were male, but
58.3% of the seropositive deer mice were male. At Molina, 45% of the deer
mice and 46.3% of the recaptured deer mice were male, but 60% of the
seropositive deer mice were male. These data support the hypothesis that
male deer mice contribute more to the epizootic cycle of SNV than female
deer mice. However, the lack of association between sex, wounds, and
antibody at either Fort Lewis or Molina indicates that individual mice of
either sex may fight and, through this mechanism or another, become
infected with a hantavirus. That most mice with antibody to SNV are male
supports the suggestion that fighting among mice, biting, and scratching
can lead to hantavirus transmission from an infected to an uninfected,
wounded mouse (4). The limited time these mice may be able to transmit
virus might be sufficient to maintain virus infection in the population.

When deer mouse populations decrease precipitously because of decreased
availability of food and water, the likelihood that SNV will disappear from
the population increases. However, a few long-lived, persistently infected
deer mice can serve as reservoirs until conditions are suitable for the
populations to recover.

Our data appear to support such a unified hypothesis. Fluctuations in IgG
antibody prevalence in deer mice at Fort Lewis and at Molina have lagged
somewhat behind but have been similar to fluctuations in deer mouse
population. In male deer mice at Fort Lewis and at Molina, most
seroconversions (recent infections) occurred during the summer or fall,
whereas in female deer mice, most occurred between fall and spring. During
winter, Colorado deer mice reduce their home range, aggregate in nests, and
enter short-term torporstrategies that together temper reduced food
availability and energy loss due to cold (2). Although we did not find deer
mice that had been infected for more than 3 months, we recovered a few more
than 1 year (some nearly 2 years) after they first were trapped; thus,
under natural conditions and despite the usual declines caused by
predation, cold, heat, and decreases in food, deer mice that reach
adulthood can live as long as 2 years (2), a period sufficient to allow SNV
to survive adverse conditions of low populations and the resulting
decreased number of susceptibles. Furthermore, whereas the overall
seroprevalence of IgG antibody to SNV in deer mice at Fort Lewis was 6.8%
(12 of 165) and in deer mice and pinyon mice, respectively, at Molina 7.2%
(15 of 193) and 5.5% (3 of 52), the rate of seroconversion among deer mice
at Fort Lewis was 16.3% (8 of 41) recaptures, and among deer mice and
pinyon mice, respectively, at Molina 9.9% (7 of 64) and 3.3% (1 of 29).
These results suggest that the longer deer mice live, the greater the
cumulative probability they will become infected with SNV.

The deer mouse, the most numerous mammal in North America, often described
as a "quintessential generalist," can survive on any dry land habitat in
its range and invade and exploit areas disturbed by flood, fire, avalanche,
landslides, mining, construction, extreme grazing, or land development. In
ecologically stable areas, deer mice may be limited by the presence of more
specialized rodent species (2), but they are found from forests to
grasslands, canyons to deserts, farmlands to farm houses and suburban
homes, moving into the latter more often in fall but able to take up
residence whenever an opportunity presents itself. Thus, the movement of
SNV-infected deer mice into human residences itself creates a risk factor
for HPS. The Fort Lewis and Molina sites have not been affected by obvious
ecosystem perturbations in recent years, and deer mouse populations at
these sites are not high, yet are considerably decreased from the
apparently inordinately high levels of 1993 (J. Mills, pers. comm. 1997).
Each site seems ecologically stable, but subtle changes may have gone
unnoticed.

Deer mice are omnivores, storing food for winter consumption but known to
feed on acorns, nuts, insects, other small invertebrates, carrion, fungi,
bone, and various plant parts, including seeds, leaves, and bark, roots,
and tubers (2,16). In one study, seeds accounted for 69% to 76% of stomach
contents of deer mice in Colorado, insects for 14% to 25% (2). This
proportion depends on the season (i.e., availability of food supply); deer
mice are more likely to feed on insects and insect larvae in spring, seeds
and berries in fall.

Notwithstanding the nature of deer mice to consume a variety of foods, they
rely heavily on acorns, when oaks (Quercus sp.) comprise a significant
proportion of their habitat (16). Further, in the northeastern United
States, the quantity of mast seems directly related to population size of
white-footed mice (P. leucopus) and eastern chipmunks (T. striatus) (17).
Although Gambel's oak is abundant at the Fort Lewis trapping sites and
copious numbers of acorns were attached to the trees and on the ground in
1994, we did not observe acorns on trees there between spring 1995 and fall
1997. The chipmunk population at this site was, with the exception of a
transient, moderate increase in June 1996, never high during our study
period (June 1994 to October 1997). At the Molina sites, which do not have
oaks, the chipmunk population declined considerably after June 1996 and did
not return to its previous level. Chipmunks may serve as an indicator for
the ready availability of acorns and other nuts or food in general.

The continued low population densities of deer mice at Fort Lewis and
Molina are puzzling. If, for example, deer mouse population densities in
surrounding areas are higher than at the study sites, one might expect deer
mice from those areas to move into the area with the low population.
However, at Fort Lewis 59% of adult and 5% of subadult deer mice were
recaptured; at Molina 86% of adults and no subadult or juvenile deer mice
were recaptured as adults. These data indicate that few, if any, young deer
mice are immigrating to these sites or if they are, they did not survive
long enough to be captured, and the survival rate of this species' young is
not high.

Temperature fluctuations that affect habitat characteristics can influence
rodent breeding seasons (e.g., rate of plant growth, total available
nutritional biomass). However, analyses of available data (not presented)
did not provide obvious evidence for such direct relationships. In
contrast, a paucity of precipitation at Fort Lewis, between March 1995 and
October 1996, and at Molina between May 1995 and April 1997, coincided with
the usual breeding season of deer mice, least chipmunks, and other rodents
at these sites, and with lower rodent population densities between the end
of 1995 and the end of 1997; antibody prevalence fluctuated in parallel.
Whether the two consecutive relatively wet years 1996 and 1997 will bring
about conditions suitable to increase rodent populations near Fort Lewis
and Molina and lead to an increase in HPS in the near term has not been
determined. Availability of water may be the sine qua non of plant food
availability, reproductive preparedness, gravidity rates, and attendant
intraspecific fighting among individual mice within an increased
population. From data (not shown) collected at these sites since October
1997, indications are that both rodent population densities and antibody
prevalence are increasing at both sites. If precipitation excess correlates
with rodent population, density increases, and the prevalence of
hantaviruses, we will be able to predict increases in risk for hantavirus
infection in the human population.

Acknowledgments

      We thank the following for directly assisting us in the field or with
logistics, encouragement, and the odd cool drink: David W. Schafer, Mr. and
Mrs. Roger Szczecinski, Catherine Crabb, Robin Carns, and Marcia Patterson,
Heather Clifton, Ted Davis, Ed Kuhn and Gordon Smith, Brendan Wolff, and
Edgar C. de Van III.

      Funding for this work was provided by the U.S. Centers for Disease
Control and Prevention, Atlanta, GA, under cooperative agreement No.
U50/ccu809862-03.

      Dr. Calisher is professor of microbiology, Arthropod-Borne and
Infectious Diseases Laboratory, Department of Microbiology, Colorado State
University. His areas of expertise are arboviruses, hantaviruses, and other
rodent-borne viruses. His research focuses on hantaviruses, arboviruses,
arenaviruses, and epidemiology.

      Address for correspondence: Charles H. Calisher, Arthropod-borne and
Infectious Diseases Laboratory, Foothills Campus, Colorado State
University, Fort Collins, CO 80523, USA; fax: 970-491-8323; e-mail:
calisher@usa.healthnet.org.

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Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

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