Experimental infection of potential reservoir hosts with Venezuelan equine encephalitis virus, Mexico.
ABSTRACT In 1993, an outbreak of encephalitis among 125 affected equids in coastal Chiapas, Mexico, resulted in a 50% case-fatality rate. The outbreak was attributed to Venezuelan equine encephalitis virus (VEEV) subtype IE, not previously associated with equine disease and death. To better understand the ecology of this VEEV strain in Chiapas, we experimentally infected 5 species of wild rodents and evaluated their competence as reservoir and amplifying hosts. Rodents from 1 species (Baiomys musculus) showed signs of disease and died by day 8 postinoculation. Rodents from the 4 other species (Liomys salvini, Oligoryzomys fulvescens, Oryzomys couesi, and Sigmodon hispidus) became viremic but survived and developed neutralizing antibodies, indicating that multiple species may contribute to VEEV maintenance. By infecting numerous rodent species and producing adequate viremia, VEEV may increase its chances of long-term persistence in nature and could increase risk for establishment in disease-endemic areas and amplification outside the disease-endemic range.
Article: Association of Venezuelan equine encephalitis virus subtype IE with two equine epizootics in Mexico.[show abstract] [hide abstract]
ABSTRACT: Two outbreaks of encephalitis consistent with an etiology of Venezuelan equine encephalitis (VEE) virus occurred in equines on the Pacific coast of southern Mexico in 1993 (Chiapas State) and in 1996 (Oaxaca State). In Chiapas, there were 125 cases, of which 63 were fatal and in Oaxaca, there were 32 cases and 12 fatalities. Virus was isolated from two horses from each outbreak, including three brain isolates and one from blood. Virus isolates (93-42124, ISET-Chi93, Oax131, and Oax142) were shown by indirect immunofluorescence, hemagglutination inhibition, monoclonal antibody ELISA, and nucleotide sequencing to be VEE virus, subtype IE, a type previously thought to be equine-avirulent. Genetic characterization and phylogenetic analysis indicated that the outbreak viruses were identical or nearly identical to one another and that they were closely related to equine-avirulent IE strains from Guatemala and the Gulf coast of Mexico. In a plaque-reduction neutralization test, sera collected from healthy horses in Chiapas and Oaxaca reacted significantly better with isolate 93-42124 than with Guatemala IE isolate 68U201, suggesting that subtle genetic changes may have resulted in alteration of neutralization domains. It is not clear whether these differences may also influence equine virulence. However, renewed VEE virus subtype IE activity in Mexico, and its apparent conversion to equine virulence, underscores the need for increased surveillance, additional laboratory and epidemiologic studies in VEE-endemic regions, and possibly new vaccines.The American journal of tropical medicine and hygiene 08/1998; 59(1):100-7. · 2.59 Impact Factor
Article: ARBOVIRUS VECTOR ECOLOGY STUDIES IN MEXICO DURING THE 1972 VENEZUELAN EQUINE ENCEPHALITIS OUTBREAK[show abstract] [hide abstract]
ABSTRACT: Virus vector studies were conducted in the States of Durango, Chihuahua, and Tamaulipas, Mexico, in June and July 1972. Apparently only a low level of Venezuelan equine encephalitis (VEE) virus transmission to equines occurred at the time of the study, and the infection was restricted to areas which had not experienced overt activity during the preceding year. The low level of infection was associated with a scarcity of mosquitoes. The IB (epidemic) strain of VEE virus was isolated from two pools of Anopheles p. pseudopunctipennis (Theo.) and the blood of one symptomatic equine. The low mosquito population, the relatively few equine cases observed, and the absence of reports of VEE human disease from the outbreak area suggested VEE virus persistence through a low-level mosquito-equine transmission cycle. Other studies have already indicated that wild vertebrates play no more than a minor role in outbreaks of epidemic VEE. Mosquito collections made in areas of the states of Durango, Chihuahua, and Tamaulipas, where considerable epidemic activity of VEE had occurred in 1971, failed to reveal evidence of VEE virus persistence. Twenty-nine isolations of other arboviruses were also made in these studies: including 22 of St. Louis encephalitis virus (SLE), 2 of Flanders virus, 1 of Turlock virus, 1 of Trivittatus virus of the California Group, 1 of western equine encephalitis virus (WEE), and 2 (from Santa Rosa) which possibly represent a hitherto unknown virus in the Bunyamwera Group. These are the first reports of SLE virus isolations from mosquitoes in Mexico, and the first demonstration of Trivittatus, WEE, Turlock and Flanders viruses in Mexico from any source.
[show abstract] [hide abstract]
ABSTRACT: Everglades virus (EVEV), an alphavirus in the Venezuelan equine encephalitis (VEE) serocomplex, circulates among rodents and vector mosquitoes and infects humans, causing a febrile disease sometimes accompanied by neurologic manifestations. EVEV circulates near metropolitan Miami, which indicates the potential for substantial human disease, should outbreaks arise. We characterized EVEV infection of cotton rats in South Florida, USA, to validate their role in enzootic transmission. To evaluate whether the viremia induced in cotton rat populations regulates EVEV distribution, we also infected rats from a non-EVEV-endemic area. Viremia levels developed in rats from both localities that exceeded the threshold for infection of the vector. Most animals survived infection with no signs of illness, despite virus invasion of the brain and the development of mild encephalitis. Understanding the mechanisms by which EVEV-infected cotton rats resist clinical disease may be useful in developing VEE therapeutics for equines and humans.Emerging infectious diseases 01/2005; 10(12):2182-8. · 6.17 Impact Factor
In 1993, an outbreak of encephalitis among 125 af-
fected equids in coastal Chiapas, Mexico, resulted in a 50%
case-fatality rate. The outbreak was attributed to Venezu-
elan equine encephalitis virus (VEEV) subtype IE, not previ-
ously associated with equine disease and death. To better
understand the ecology of this VEEV strain in Chiapas, we
experimentally infected 5 species of wild rodents and evalu-
ated their competence as reservoir and amplifying hosts.
Rodents from 1 species (Baiomys musculus) showed signs
of disease and died by day 8 postinoculation. Rodents from
the 4 other species (Liomys salvini, Oligoryzomys fulves-
cens, Oryzomys couesi, and Sigmodon hispidus) became
viremic but survived and developed neutralizing antibod-
ies, indicating that multiple species may contribute to VEEV
maintenance. By infecting numerous rodent species and
producing adequate viremia, VEEV may increase its chanc-
es of long-term persistence in nature and could increase
risk for establishment in disease-endemic areas and ampli-
fi cation outside the disease-endemic range.
tions of North, South, and Central America between the
tropics of Cancer and Capricorn) that can cause outbreaks
involving hundreds of thousands of humans and equids.
VEE virus (VEEV; Togaviridae: Alphavirus) strains are
categorized as either epizootic (associated with equine dis-
ease and major epidemics of human disease through equine
enezuelan equine encephalitis (VEE) is a potentially
fatal, reemerging disease in tropical America (the por-
amplifi cation), or enzootic (not known to cause equine
disease). Most VEEV strains, both epizootic and enzootic,
have been associated with human disease (1). VEEV is
also of biodefense importance; it has been developed as a
biological weapon, mainly because it is highly infectious
by aerosol transmission and can infect humans with a rela-
tively low dose (2).
During the mid-1990s, 2 epizootic equine outbreaks
occurred in coastal Oaxaca and Chiapas states in Mexico;
the causative agent was determined to be VEEV subtype IE
(VEEV-IE), which was previously considered to be not vir-
ulent for equids (1). On the basis of the spread of a VEEV
subtype IAB epizootic/epidemic through Mexico and into
Texas in 1971 (3), the 1993 and 1996 outbreaks were con-
sidered to have the potential to spread to other regions of
Mexico or the United States. To prevent, detect, and evalu-
ate potential reemergence of this virus in the United States,
we need to understand the factors that govern circulation
and persistence of this virus in its enzootic foci and epi-
Enzootic strains of VEEV are maintained naturally by
transmission between mosquitoes of the subgenus Culex
(Melanoconion) and wild rodents (4). These viruses are
thought to circulate continuously among mosquitoes and
their principal vertebrate amplifying hosts, whereas horses
and humans are considered spillover, dead-end hosts not
required for maintenance of the natural cycle. Several stud-
ies have shown that terrestrial mammals of 5 genera (Di-
delphis, Oryzomys, Proechimys, Sigmodon, and Zygodon-
tomys) are susceptible to VEEV-IE infection; they develop
viremia suffi cient to infect mosquito vectors, yet they usu-
ally survive infection (5–10).
Several species of wild rodents captured in coastal
Chiapas have VEEV-specifi c antibodies (11). To address
which of these species are likely to play a role as reser-
Experimental Infec tion of Potential
Reservoir Hosts with Venezuelan
Equine Enc ephalitis V irus, Mexic o
Eleanor R. Deardorff, Naomi L. Forrester, Amelia P. Travassos da Rosa, Jose G. Estrada-Franco,
Roberto Navarro-Lopez, Robert B. Tesh, and Scott C. Weaver
Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 15, No. 4, April 2009 519
Author affi liations: University of Texas Medical Branch, Galves-
ton, Texas, USA (E.R. Deardorff, N.L. Forrester, A.P. Travassos da
Rosa, J.G. Estrada-Franco, R.B. Tesh, S.C. Weaver); and Comi-
sion Mexico–Estados Unidos para la Prevencion de la Fiebre Af-
tosa y Otras Enfermedades Exoticas de los Animales, Mexico City,
Mexico (R. Navarro-Lopez)
voir and/or amplifi cation hosts, we captured rodents from
5 genera (Baiomys, Liomys, Oligoryzomys, Oryzomys, and
Sigmodon) and transported them to the laboratory for ex-
perimental infection studies. Our goals were to evaluate the
role of various vertebrate species in VEEV-IE maintenance
and to help interpret seroprevalence data gathered in the
Materials and Methods
During October 2007, wild rodents of 5 species were
collected from coastal Chiapas, Mexico: Baiomys muscu-
lus (southern pygmy mouse), Liomys salvini (Salvins spiny
pocket mouse), Oligoryzomys fulvescens (fulvus pygmy
rice rat), Oryzomys couesi (Coues’ rice rat) and Sigmodon
hispidus (hispid cotton rat). All animals were captured
from an overgrown fi eld surrounding a stream in Mapaste-
pec municipality, ≈2 km from the Pacifi c coast (15.413°N
and 093.070°W) by using live-capture Sherman traps (H.B.
Sherman Traps, Tallahassee, FL, USA). Species identifi ca-
tion was based initially on morphologic features (12) and
later confi rmed genetically by using cytochrome-B gene
sequences (13). Animals were housed individually and
transported in Taconic Transit Cages (Taconic Farms, Inc.,
Hudson, NY, USA) to the Animal Biosafety Level 3 Facil-
ity at the University of Texas Medical Branch in Galves-
ton, Texas, USA. Animals were captured under permit
number SGPA/DGVS/03858/07 Julio 2 de 2007, issued to
J.G.E.-F.; all studies were approved by the University of
Texas Medical Branch Institutional Animal Care and Use
Virus and Infection
Immediately before rodents were inoculated with vi-
rus, a baseline serum sample was taken from each rodent
for subsequent antibody assays. For inoculation we used
VEEV strain MX01-22 (subtype IE). This strain had been
isolated in 2001 from a sentinel hamster in coastal Chiapas,
Mexico, and passaged once in Vero cells to generate a suf-
fi cient volume of high-titer virus for experimentation. We
chose this strain because it is the most recent low-passage
isolate of VEEV from the outbreak area and because trans-
mission of this strain by VEEV mosquito vector species
from this area has been studied (14,15). Additionally, this
strain is genetically highly similar to the equine-virulent
strains that were isolated during the 1993 outbreak (11) and
caused encephalitis in horses (R. Bowen, pers. comm.).
All animals were inoculated subcutaneously in the right
thigh with 3.2 log10 PFU of virus, a dose that approximates
the maximum amount of VEEV transmitted by a mosquito
bite (16). After inoculation, all animals were weighed daily
for 1 week and observed for signs of illness for 2 weeks.
Blood was collected daily for the fi rst 7 days after in-
oculation, then on days 10, 14, 28, 42, and 66. After the ani-
mals were anesthetized with inhaled isofl urane, retroorbital
sinus blood was collected in heparinized glass capillary
tubes and transferred to 5 volumes of phosphate-buffered
saline (PBS). Erythrocytes were removed by centrifugation
to yield an ≈1:10 dilution of serum, which was stored at
–80ºC. Viremia titers were determined by plaque assay on
Vero cells (17).
Necropsy was performed on all animals, and tissues
were frozen at –80ºC. Using a TissueLyser (QIAGEN Inc,
Valencia, CA, USA), we homogenized ≈2–10 mg of tissue
in minimal essential medium (Eagle) supplemented with
20% fetal bovine serum, L-glutamine, penicillin, strepto-
mycin, gentamicin, and fungizone. Tissue virus titers were
determined by plaque assay on Vero cells.
To detect VEEV-IE–specifi c antibodies, we performed
hemagglutination inhibition assays (17) using antigen de-
rived from the same VEEV strain used for infection (MX01-
22) as well as from 3 other arboviruses: Eastern equine en-
cephalitis virus (TenBroeck strain), West Nile virus (strain
385-99), and St. Louis encephalitis virus (strain TBH28).
Briefl y, 4–8 units of hemagglutinin antigen were reacted
with heat-inactivated test serum in various concentrations
in PBS. Failure to hemagglutinate goose erythrocytes was
considered a positive result. Antibody titers were confi rmed
by plaque reduction neutralization tests (17). Test serum
samples were serially diluted in PBS and heat inactivated
at 56°C for 1 h, then mixed with ≈100 PFU of virus and
incubated at 37°C for 1 h. The mixture was inoculated onto
Vero cells. Dilutions resulting in >80% reduction in virus
titer were considered positive; titers were reported as the
reciprocal of the endpoint dilution.
Clinical Responses and Survival Rates
Of the 5 rodent species examined, only those of species
B. musculus showed signs of disease with neurologic mani-
festations. These animals began to exhibit tremor, lethargy,
dehydration, hunching, and staggering during days 4–6
postinoculation. By day 8, all 4 (100%) of these B. mus-
culus rodents had died or were euthanized after becoming
moribund (Figure 1, panel A). Rodents of this species were
the only ones that lost body weight after inoculation (aver-
age 22% loss; Figure 1, panel B).
No animal from the other 4 species exhibited weight
loss or outward signs of illness after inoculation. Most of
these rodents survived until the end of the experiment, day
66 postinoculation. However, during the fi rst 2 weeks af-
520 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 15, No. 4, April 2009
Potential Reservoir Hosts, VEE, Mexico
ter inoculation, 9 animals died without weight loss or signs
of illness. These animals did not have high levels of virus
in their tissues (Table) and are considered to have died of
stress from daily manipulations rather than of VEEV infec-
tion. To address this possibility, a subcohort of 2 L. salvini
and 3 O. fulvescens rodents, the 2 species that had had the
most manipulation-related deaths, were inoculated and ob-
served for 15 days without daily blood sampling. All 5 ani-
mals survived with little to no illness; they were found to
have seroconverted by day 15 (reciprocal mean titer = 2.7 ±
2.3 log10, standard error) and remained seropositive through
day 42 (3.0 ± 2.9 log10). Similar deaths of wild rodents in
the absence of an infectious cause have been encountered
Of 35 animals tested, 22 (comprising all 5 species) had
measurable virus levels during the fi rst week after inocula-
tion (limit of detection was 1.5 log10 PFU/mL) (Figure 2).
Viremia (>2.7 log10 PFU/mL) developed in all (100%) O.
fulvescens, L. salvini, and B. musculus rodents and lasted as
long as 4, 5, and 8 days, respectively. Conversely, detect-
able viremia developed in only 60% of the cohort of S. his-
pidus rodents (3/5 animals), lasting as long as 4 days, and
in only 39% of the O. couesi cohort (7/18 animals), lasting
as long as 2 days.
In the cohorts of L. salvini, O. fulvescens, and O. coue-
si rodents, maximum viremia occurred on day 1 postinocu-
lation; mean titers were 3.4 ± 0.6 (SEM), 3.3 ± 0.2, and 2.5
± 0.6 log10 PFU/mL, respectively (Figure 2). In S. hispidus
rodents, the cohort peak viremia occurred on day 2 posti-
noculation; mean was 2.9 log10 ± 0.9. In the cohort of B.
musculus rodents, peak viremia occurred on day 3; mean
was 5.5 ± 0.4 PFU/mL (Figure 2).
Of the 40 animals used in this study, only 1 (S. his-
pidus) was found to have preexisting VEEV antibodies.
This animal had a hemagglutination inhibition reciprocal
antibody titer of 2.8 log10 on day 0 and 2.2 log10 on day 6,
when it died during anesthesia and blood collection. For
rodents of all 4 surviving species, antibodies were detect-
able by day 5 and lasted through the end of the experiment
An unanticipated cohort of 3 juvenile rodents (O.
couesi) provided an opportunity to examine whether age
affected outcome of VEEV infection. The species of these
3 animals was initially identifi ed as O. fulvescens but later
determined, based on cytochrome-B gene sequencing, to
be juvenile O. couesi (13). Age at infection was ≈2 weeks,
determined on the basis of growth of 3 litters of O. couesi
rodents born in captivity.
No differences were found between the juvenile and
the adult O. couesi rodents in terms of survival rates, vire-
mia levels, or antibody responses (Figures 1, 2). Viremia
was detected in 1 (33%) of 3 juvenile and 6 (40%) of 15
adult O. couesi rodents. Mean maximum viremia was 2.3
log10 PFU/mL for the juveniles and 2.6 ± 0.6 log10 PFU/mL
for the adults. No viremia was detected after day 1 for either
juveniles or adults, except for 1 adult that had a titer of 2.6
log10 on day 2. Antibody responses were inconsistent among
animals from both groups. Several animals from each group
Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 15, No. 4, April 2009 521
Figure 1. Survival rates and weight change of wild rodents from
Chiapas, Mexico, after experimental infection with 3 log10 PFU of
Venezuelan equine encephalitis virus subtype IE, strain MX01-22.
A) Survival rates. Black and yellow lines represent animals whose
brains yielded live virus after necropsy. Red, green, blue, and purple
lines indicate animals whose death was attributed to manipulation
and/or stress, not to VEEV infection. B) Weight change. Mean
cohort weight (grams) divided by mean cohort starting weight
(day 0). Weight gain or loss was used as an indicator of disease.
Only Baiomys musculus rodents showed weight loss during
acute infection. Data for days 42 and 66 (not shown) did not differ
signifi cantly from that for day 28. Error bars indicate SEM.
showed weak antibody responses of short duration, delayed
onset, or both, after having no detectable viremia.
Reservoir Status and Potential
Of the 5 species of rodents evaluated in this study, only
S. hispidus rodents have been included in previous experi-
mental VEEV infection studies. In Panama (10) and Florida
(5,7), S. hispidus rodents are considered to be competent,
mostly disease-resistant reservoir hosts for disease caused
by sympatric VEE complex alphaviruses. In 2007, Carrara
et al. (7) infected 3 geographically distinct populations of S.
hispidus rodents with 2 enzootic VEEV strains and found
that only the population from a VEE complex alphavirus–
endemic region (Florida) survived infection; cohorts from
the 2 non–virus-endemic populations succumbed to dis-
ease. For this reason, we used a sympatric VEEV strain for
In addition to S. hispidus rodents, 3 other species
(Proechimys semispinosus, Zygodontomys microtinus, and
Oryzomys capito) had viremia suffi cient to infect at least
some mosquito vectors and survive after inoculation with
sympatric strains of VEEV (8–10). Our results support the
hypothesis that enzootic VEEV selects for resistance to dis-
ease in its sympatric reservoir host populations (10).
Several fi eld studies in Mexico have reported VEEV-
specifi c antibodies in a variety of wild vertebrate species.
522 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 15, No. 4, April 2009
Table. Viremia in rodents that died 1–14 days after inoculation with 3.2 log10 PFU of Venezuelan equine encephalitis virus subtype IE
Tissue virus content (log10 PFU/g)†
*Not shown are 3 Liomys salvini, 2 Oligoryzomys fulvescens, and 1 Sigmodon hispidus rodents. These animals died on days 5–10 postinoculation and
showed no detectable virus in any organs tested. dpi, days postinoculation.
†Limit of detection was 1 plaque in150 ?L homogenate. Tissue sample weight varied between 0.002 and 0.01 grams.
n = 4/4 n = 1/4
n = 4/4 n = 7/7
n = 4/4 n = 6/6
Oryzomys couesi (adult)
n = 6/15 n = 9/15
Oryzomys couesi (juvenile)
n = 1/3 n = 3/3
n = 3/5 n = 3/5
log10 mean serum viremia PFU/mL
log10 mean reciprocal HI antibody
Figure 2. Mean viremia profi le (red lines) and mean hemagglutination inhibition (HI) antibody profi le (blue lines) of 5 species of wild rodents
after experimental infection with 3 log10 PFU of Venezuelan equine encephalitis virus type-IE, strain MX01-22. Black dashed lines indicate
approximate mosquito infection viremia threshold for the enzootic vector Culex (Melanoconion) taeniopus. Fractions represent proportion
of total cohort that had measurable response. Data for days 42 and 66 (not shown) did not differ signifi cantly from data for day 28. Error
bars indicate SEM.
Potential Reservoir Hosts, VEE, Mexico
Aguirre et al. (18) found 7 species of wild mammals and 17
species of wild birds that were seropositive against VEEV-
IE in 1992. In the same area from which the animals for our
study were captured, VEEV-neutralizing antibodies were
detected in wild S. hispidus, Oryzomys alfaroi, and Didel-
phis marsupialis rodents (11). In an extensive fi eld study in
southern Mexico during the 1960s, Scherer et al. (6) found
29 species of wild birds, 10 genera of terrestrial mammals,
and 3 genera of bats with serologic evidence of natural
VEEV infection. Evidence of similar broad host ranges
of VEEV has been found in coastal Guatemala, where 7
genera of terrestrial mammals and 11 species of birds had
VEEV-specifi c antibodies (19). After the 1971 epidemic
of VEEV-IAB that started in Central America and reached
southern Texas, extensive fi eld studies were conducted to
determine whether the virus would or could establish a new
enzootic focus (20). In that study, mammals of 10 genera
had VEEV-specifi c antibodies. In 2 follow-up studies in
which wild mammals and wild birds were infected with a
strain of VEEV-IB isolated during the outbreak, viremia
and mortality rates for rodents were high (21,22). In a lon-
gitudinal fi eld study performed concurrent with the study
reported here, seroprevalance for wild rodents was found to
be much lower than previously found for this area (11).
Viremia and Immunologic Response
All 5 of the species tested produced viremia titers suf-
fi cient to infect the proven enzootic mosquito vector Cx.
(Mel.) taeniopus. Of these 5 species, the lowest and shortest
lasting viremia was found in O. couesi rodents; however,
even these reached levels that are considered adequate to
infect a proportion of Cx. (Mel.) taeniopus (23). The other 4
species all exhibited viremia titers well above the minimum
infection threshold for this vector. Therefore, assuming that
they are bitten by Cx. (Mel.) taeniopus mosquitoes, which
are known to be universal feeders and have been recently
found in higher numbers than previously reported in the
area where these animals were captured, all 5 species we
studied should be able to infect this mosquito (11,24).
The uniform susceptibility of B. musculus rodents to
VEE disease was an unexpected result and appeared to
contradict the hypothesis that VEEV circulation selects for
resistance to disease in wild rodents. This difference is evi-
dently not refl ective of the taxonomic relatedness of these
5 species (Figure 3). A different potential explanation is
the lack of temporal overlap of activity between B. mus-
culus rodents and the enzootic vector, Cx. (Mel.) taenio-
pus. Baiomys spp. rodents are diurnally active (12), but Cx.
(Mel.) taeniopus mosquitoes are nocturnal feeders (24,25).
Although the rodents and mosquito vectors coexist spatial-
ly, they are not active at the same time of day, which may
limit their contact. This lack of contact time may preclude
the selection for resistance to VEE that is manifested in the
other 4 rodent species, which are nocturnal and presumably
regularly exposed to bites from this vector. Experimental
infection of other diurnal species from the study area, or
similar studies in another VEE-endemic area, could be used
to test this hypothesis. Of the 5 species, B. musculus ro-
dents were the only species not encountered in previous
capture-and-release studies; however, because of the sever-
ity of disease in this species, seropositive individuals would
be unlikely to survive (and thereby be caught) in the wild.
We ended our study at 66 days postinoculation for the
original cohort and 42 days postinoculation for the subco-
horts of L. salvini and O. fulvescens that survived. The an-
tibody responses for all animals that developed measurable
viremia persisted through the end of the experiment. The
only exception was several O. couesi animals that did not
develop viremia but did demonstrate brief, low-titer (<1.6
log10) antibody responses. Wild rodents have been shown to
remain seropositive for as many as 6 months postinocula-
tion with VEEV (7). For some species with short life spans
in the wild, this antibody response is tantamount to life-
long immunity offering protection against reinfection and
affording more opportunity for the animal to reproduce.
Although the ability of laboratory experimentation to
elucidate natural processes is limited, data gathered in the
laboratory are sometimes more complete and detailed than
fi eld data. In this study, 5 of the most commonly captured
rodent species in coastal Chiapas, Mexico, were evaluated
for their ability to participate in the natural transmission cy-
cle of enzootic VEEV-IE. S. hispidus and O. capito rodents
have previously been implicated in amplifi cation of other
VEE subtypes, ID, IE, and II (7–9), but the other 3 species
(B. musculus, L. salvini, and O. fulvescens) had been stud-
ied little or not at all. Rodents of all 5 species developed
viremia titers suffi cient to infect the enzootic mosquito vec-
tor, Cx. (Mel.) taeniopus. However, only 4 of the 5 species
Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 15, No. 4, April 2009 523
GENUS Proechym ys
GENUS Zygodontom ys
Figure 3. Relatedness of 7 wild rodent genera that have been
experimentally evaluated for suitability as amplifying hosts in
enzootic transmission cycles of Venezuelan equine encephalitis
virus. The 5 genera included in this study are presented in boldface;
the 3 novel genera are underlined.
survived infection with the potential to reproduce, a trait
considered critical for true reservoir status in that it avoids
population declines that might jeopardize long-term virus
History has shown that an outbreak of highly virulent
VEEV in southern Mexico can easily and rapidly spread
into the United States, as it did in 1971. Therefore, a better
understanding of VEEV ecology in Mexico is essential for
assessing the risk for widespread disease. Our results sup-
port the conclusions of Scherer et al. (6) that VEEV has a
wide range of mammalian hosts that may participate in the
natural transmission cycle. This strategy may be an adap-
tive one that affords greater population stability than does
specialization for 1 amplifying host species. By being able
to infect numerous rodent species and produce adequate
viremia for mosquito transmission, VEEV may increase its
chances of long-term persistence in nature when weather or
environmental conditions affect some but not all reservoir
host populations. This ability could also increase the risk
for endemic establishment as well as amplifi cation when
outbreaks spread outside their disease-endemic range.
We thank Justin Darwin and Estella Abadia-Cruz for their
help in collecting the rodents, Hilda Guzman and Collette Keng
for help preparing the viral antigen, Nicole Arrigo and Paige Ad-
ams for help with sampling, and Judy Barnett and Don Bouyer for
assistance with access to the Biosafety Level 3 facilities.
This research was supported by contract N01-AI25489 from
the National Institutes of Health (NIH) and from grants provided
to J.G.E.-F. by the Inter-American Institute for Cooperation on
Agriculture and the Pan American Health Organization in Wash-
ington, DC, and Mexico. E.R.D. was supported by the James W.
McLaughlin predoctoral fellowship award and by NIH grant T32-
Ms Deardorff is a doctoral student in the Experimental Pa-
thology Department at the University of Texas Medical Branch.
Her research interests include zoonotic disease, viral evolution
and ecology, and wildlife conservation biology.
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Address for correspondence: Scott C. Weaver, University of Texas
Medical Branch, 301 University Blvd, Galveston, TX 77555, USA; email:
Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 15, No. 4, April 2009 525
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