Download EASTERN EQUINE ENCEPHALITIS VIRUS IN BIRDS: RELATIVE

Am. J. Trop. Med. Hyg., 60(3), 1999, pp. 387–391
Copyright q 1999 by The American Society of Tropical Medicine and Hygiene
EASTERN EQUINE ENCEPHALITIS VIRUS IN BIRDS: RELATIVE COMPETENCE OF
EUROPEAN STARLINGS (STURNUS VULGARIS)
NICHOLAS KOMAR, DAVID J. DOHM, MICHAEL J. TURELL, AND ANDREW SPIELMAN
Harvard School of Public Health, Boston, Massachusetts;
United States Army Medical Research Institute of Infectious Diseases, Fort Detrick, Frederick, Maryland
Abstract. To determine whether eastern equine encephalitis (EEE) virus infection in starlings may be more fulminant than in various native candidate reservoir birds, we compared their respective intensities and durations of
viremia. Viremias are more intense and longer lasting in starlings than in robins and other birds. Starlings frequently
die as their viremia begins to wane; other birds generally survive. Various Aedes as well as Culiseta melanura
mosquitoes can acquire EEE viral infection from infected starlings under laboratory conditions. The reservoir competence of a bird is described as the product of infectiousness (proportion of feeding mosquitoes that become infected)
and the duration of infectious viremia. Although starlings are not originally native where EEE is enzootic, a starling
can infect about three times as many mosquitoes as can a robin.
Precise knowledge of the identity of the avian reservoir
of eastern equine encephalitis (EEE) virus may facilitate efforts to protect the public health. Serologic evidence suggests that numerous species of birds are exposed to this virus, particularly those residing near swamps in which the
Culiseta melanura vector mosquito breeds.1–3 This virus has
been isolated from a similarly broad array of hosts. Indeed,
the virus replicates in virtually all wild birds that have been
infected experimentally.4–10 Diverse species of birds appear
to contribute to the natural transmission cycle of EEE virus.
The intensity of transmission of EEE virus depends in part
on the vertebrate reservoir competence (ability to sustain
infection and present infectious agents to vectors) of local
birds for this agent. Candidate reservoir birds would be those
that roost abundantly in or near swamps during the late summer season of virus amplification.11 Species that roost communally are especially strong candidate reservoirs because
of their tendency to return nightly to stable roost sites.12 In
Massachusetts, these birds mainly include European starlings
(Sturnus vulgaris), common grackles (Quiscalus quiscula),
red-winged blackbirds (Agelaius phoeniceus), and American
robins (Turdus migratorius). Starlings were introduced from
Europe in 1890 into New York City13 and rapidly expanded
their range throughout North America. These gregarious
birds became abundant in the northeastern United States in
the 1920s, shortly before the first recognized epidemic of
human EEE in 1938.14 Reservoir competence of starlings has
not previously been evaluated.
Because starlings are not native to North America, EEE
virus infection may be more fulminant in these birds than in
other birds, a condition that may modify reservoir competence. Accordingly, we compared the intensity and duration
of viremia of EEE virus in starlings to those in such other
likely candidate reservoirs as robins, grackles, and red-wing
blackbirds.
Mitchell (Division of Vector-Borne Infectious Diseases,
Centers for Disease Control and Prevention, Fort Collins,
CO). This virus originated from a 1991 pool of Aedes albopictus collected in Florida and was passed once in Vero
cells prior to use. The EEE virus strain MA93-SF52 originated from a 1993 pool of Cs. melanura collected in Massachusetts and was passed once in a Japanese quail (Coturnix
coturnix). No differences were noted among the strains in
their ability to infect birds or in the duration or intensity of
the resulting viremias.
Source of birds. Birds were captured in Massachusetts by
means of mist nets15 or baited funnel traps. Bird species captured included starling, grackle, red-winged blackbird, robin,
swamp sparrow (Melospiza georgiana), song sparrow (Melospiza melodia), brown-headed cowbird (Molothrus ater),
northern cardinal (Cardinalis cardinalis), mourning dove
(Zenaida macroura), and domestic pigeon (Columba livia).
This activity was authorized by U.S. Fish and Wildlife Service Scientific Collecting Permit PRT-719506 and Massachusetts Division of Fisheries and Wildlife Scientific Collecting Permit for Birds #121.94SCB, #167.95SCB, and
#118.96SCB. Captive birds were maintained in stainless
steel monkey cages or guinea pig cages with food and water
provided ad libitum. Passeriform birds were fed moistened
Sportsmen’s Pride Kennel’s Choice dog food containing 21%
protein (Sunshine Mills, Inc., Red Bay, AL) and mixed
seeds, supplemented with mealworms. Columbiform birds
were fed mixed seeds. Some passerine birds were held temporarily (up to two months) in a flight room (2 3 4 m) prior
to placement within standard cages. The maintenance and
care of experimental animals complied with the National Institutes of Health guidelines for the humane use of laboratory animals.
Source of mosquitoes. The Yale strain of Cs. melanura,16
provided by Dr. John Edman (University of Massachusetts,
Amherst, MA), was used to infect starlings. Our own strain
was colonized in 1995 from about 200 blood-engorged females collected from Kingston, MA. They were permitted
to oviposit on distilled water in standard 30 3 30 3 30 cm3
mosquito cages (BioQuip, Inc., Gardena, CA) at 22–248C
and 70% relative humidity. Larvae were reared in polystyrene mouse cages containing distilled water, with daily food
supplement comprised of two parts liver powder, one part
MATERIALS AND METHODS
Source of virus. The EEE virus strain MA92-1406 was
provided by Dr. Barbara Werner (Massachusetts Department
of Public Health, Jamaica Plain, MA). This virus originated
from a 1992 pool of Cs. melanura collected in Massachusetts, and was passed once in Vero cells prior to use. The
EEE virus strain FL91-4679 was provided by Dr. Carl
387
388
KOMAR AND OTHERS
log-transformed sample means of viremias, expressed as
plaque-forming units (pfu)/ml of blood.19
RESULTS
FIGURE 1. Percentage of Culiseta melanura (Kingston strain) infected with eastern equine encephalitis virus after 18 days of extrinsic incubation as a function of the viremia of ingested starling blood.
PFU 5 plaque-forming units.
brewer’s yeast, and four parts ground rabbit chow. Emergent
adults were reared in 61 3 61 3 61 cm3 mosquito cages
(BioQuip, Inc.) and maintained on distilled water and apple
slices or sugar cubes. Fourteen hours of light was provided
daily with additional 60-min crepuscular periods of 15-watt
white light. Adults of the sixth or greater filial generation
were used in transmission experiments. Mosquitoes used included the Polk XII strain of Ae. albopictus, provided by Dr.
George B. Craig, Jr. (University of Notre Dame, Notre
Dame, IN), the Walton strain of Ae. triseriatus, and the
Rockefeller strain of Ae. aegypti.
Infection of mosquitoes. Mosquitoes were infected by
permitting them to feed within darkened enclosures on birds
that had been anesthetized with ketamine hydrochloride (50
mg/ml) and xylazene (2 mg/ml) administered intramuscularly at a dose of 0.1 ml/100 g of body weight. Some Cs.
melanura were infected by intrathoracic inoculation.17
Plaque assay. Virus concentration in avian blood or triturated mosquitoes was measured by plaque assay by inoculating serial 10-fold dilutions of each sample onto Vero cell
monolayers as described18 except that infected cells were
stained after two days instead of four.
Statistical analysis. Student’s t-test was used to compare
Infectibility of birds by mosquito-borne EEE virus.
First, we determined whether EEE virus readily replicates in
birds bitten by virus-infected Cs. melanura mosquitoes. Each
of four starlings and five robins became infected. Culiseta
melanura-transmitted EEE virus readily replicates in both
starlings and robins.
Susceptibility of mosquitoes to bird-borne EEE virus.
To estimate the EEE virus concentration required to infect
Cs. melanura mosquitoes, we permitted such mosquitoes to
feed on starlings at various time intervals following subcutaneous inoculation of virus. Plaque assay was used to measure the virus content of blood at the time of feeding. After
18 days of extrinsic incubation at 268C, virus was detected
in all mosquitoes that fed on starlings whose blood contained
at least 106.2 pfu/ml (Figure 1). No virus was detected in
those that fed on birds circulating 103.0 or fewer pfu/ml. Virus was detected in the legs of 49 of 55 infected mosquitoes
(89%). Nine of those that harbored disseminated infection
fed subsequently on birds and all (100%) transmitted infection.
Intensity and duration of viremia. To compare the intensity and duration of viremia in starlings and in robins, we
permitted infected Cs. melanura to feed individually on birds
that were then bled daily for five days. Viremia was most
intense on the first day after infection (Figure 2) and more
so in starlings (107.3 pfu/ml) than in robins (105.7) (P 5 0.01).
Viremia in starlings remained sufficiently intense to infect
mosquitoes for three days, compared to just one day for robins. Starlings sustain mosquito-induced viremias of greater
intensity and of longer duration than do robins.
To compare the competence of eight other native bird species against starlings and robins, we determined intensities
and durations of viremias after administering a standard subcutaneous inoculum of 250–1,000 pfu of EEE virus. Virus
was detected in all but one, a robin, of 29 birds tested, rep-
FIGURE 2. Daily means (6 SE) of eastern equine encephalitis viremia in four starlings and five robins following the bites of infected
Culiseta melanura mosquitoes. The dashed line represents the level of viremia required to infect 10% of Cs. melanura that engorge on these
birds. PFU 5 plaque-forming units.
389
RESERVOIR COMPETENCE OF BIRDS FOR EEE VIRUS
FIGURE 3. Daily means (6 SE) of eastern equine encephalitis viremia in 10 species of birds, representing eight passerines and two
Columbiformes, following subcutaneous injection. The dashed line represents the level of viremia required to infect 10% of Culiseta melanura
that engorge on these birds. PFU 5 plaque-forming units.
resenting 10 species (Figure 3). Viremia was most intense
on the first day after infection and more so (P , 0.001) in
starlings (109 pfu/ml) than in other birds (variable, from 103
to 106 pfu/ml). A single mourning dove was the only exception to this rule, becoming viremic at an infectious level (104
pfu/ml) after four days. Although starlings sustained a viremia intense enough to infect mosquitoes for at least two
days, viremias were sustained at this level in other birds for
only one day. As with mosquito-induced infections, needleinduced viremias are more intense and longer lasting in starlings than in other birds tested.
Infectiousness of starlings for Aedes mosquitoes. We
then determined whether various species of Aedes mosquitoes can acquire EEE virus infection after feeding on viremic
starlings. Thus, we permitted Ae. triseriatus, Ae. aegypti, and
Ae. albopictus to feed on starlings two days after they were
inoculated with virus. Engorged mosquitoes were incubated
for 10 days and then individually tested for virus by plaque
assay. Specimens of all three species became infected (Table
1). Starlings infected by EEE virus are infectious for Aedes
mosquitoes.
Virus-induced mortality. Finally, we compared the survival of starlings and robins following EEE virus infection
(Table 2). Seven of eight starlings and two of 10 robins died
2–4 days after they were infected by needle-inoculation. No
mortality was observed in the same number and species of
birds that were mock infected and similarly bled daily from
the jugular vein. Mortality correlated positively with peak
TABLE 1
Infectiousness of eastern equine encephalitis virus–infected starlings
for various Aedes mosquitoes
Mosquitoes engorged
Aedes
Strain
of
virus
Log pfu*
of virus/
ml of blood
No.
%
infected
triseriatus
triseriatus
aegypti
albopictus
MA92-1406
FL91-4679
FL91-4679
FL91-4679
7.6
5.9
6.9
7.5
20
35
10
6
85
34
40
100
* pfu 5 plaque-forming units.
viremia. This experiment was repeated with starlings and
robins that were not bled during the course of observation.
Only one of five infected starlings died. No noninfected
birds and none of three infected robins died. In a third series
of observations, infection was induced by the bites of infected mosquitoes. Blood was sampled from these birds daily. Two of four starlings and none of five such robins died
during the following five days. Starlings die more frequently
than do robins when infected with the strain of EEE virus
used.
TABLE 2
Correlation of eastern equine encephalitis virus peak viremia in birds
with mortality
Bird
Peak viremia
(log pfu* of
virus/ml of blood)
Mortality
(if yes, day
post-inoculation)
Mode
of
inoculation
Starling
Starling
Starling
Starling
Starling
Starling
Starling
Starling
Starling
Starling
Starling
Starling
Robin
Robin
Robin
Robin
Robin
Robin
Robin
Robin
Robin
Robin
Robin
Robin
Robin
Robin
Robin
10.0
9.9
9.4
9.3
8.7
8.6
8.4
7.8
7.7
7.5
7.2
6.6
6.6
6.3
5.7
5.6
5.6
5.4
5.4
5.4
5.2
5.1
5.0
5.0
4.9
4.6
4.3
2
3
3
3
3
3
3
No
5
No
4
No
No
No
No
No
No
3
No
No
No
No
No
No
No
3
No
Needle
Needle
Needle
Needle
Needle
Needle
Needle
Mosquito
Mosquito
Needle
Mosquito
Mosquito
Mosquito
Mosquito
Needle
Needle
Needle
Needle
Needle
Mosquito
Needle
Needle
Mosquito
Mosquito
Needle
Needle
Needle
* pfu 5 plaque-forming units.
390
KOMAR AND OTHERS
DISCUSSION
European starlings and American robins seem particularly
well-suited as reservoirs for EEE virus because their communal roosts contain so many birds and they frequently are
located near the wetland breeding sites of Cs. melanura, the
enzootic vector of this virus. Isolations of EEE virus from
wild robins have been reported in Massachusetts,2 New Jersey,3 and Rhode Island (Gettman A, unpublished data). Similarly, serosurveys of wild birds generally suggest that robins
are exposed frequently to EEE virus throughout the northeastern United States2,3,20–22 Because of their suburban and
urban daily habits, starlings are rarely included in serosurveys of sylvatic bird species. Seropositive starlings, nonetheless, have been discovered in eastern New York.21 Our
observation that EEE virus infection is exceptionally intense
in starlings is consistent with reports that introduced birds
tend to be more vulnerable to EEE disease than are native
birds.6 Starlings, therefore, merit special consideration as
candidate vertebrate reservoir hosts for EEE virus.
Birds that are competent as reservoir hosts for EEE virus,
of course, must be susceptible to infection. We found that
each of 10 diverse avian species become viremic after being
bitten by virus-infected mosquitoes or following subcutaneous injection of about 1,000 pfu. Because virtually all species of birds seem to be uniformly susceptible to infection
by EEE virus,23 our derivation of a quantitative measure of
vertebrate reservoir competence ignores susceptibility as a
parameter.
The intensity of the EEE viremia in a bird contributes
crucially to its ability to infect vector mosquitoes. We find
that Cs. melanura mosquitoes do not become infected unless
they ingest more than three pfu of virus, an inoculum equivalent to that ingested from a bird circulating more than 103.0
pfu/ml of blood. Virtually all become infected after ingesting
4,500 pfu (equivalent to that in 106.2 pfu/ml of blood). These
observations confirm, in general, those of Howard and Wallis.24 Of the birds that we examined, starlings become most
intensely viremic, developing as many as 108 pfu/ml after
mosquito-induced infection and 1010 pfu/ml after infection
by needle. In contrast to the intense viremias that develop
in these introduced birds, those in native adult birds generally never exceed 107 pfu/ml. Because peak intensity of viremia in diverse birds generally varies between 104 pfu/ml
and 1010 pfu/ml, attempts to express reservoir competence
quantitatively must include a measure of viremia. The degree
of infectiousness of a bird, therefore, is expressed as the
proportion of Cs. melanura that become infected after feeding on a bird that is sufficiently viremic to infect mosquitoes.
Starlings would infect more mosquitoes while they are infectious than would native birds.
Finally, the calculation of competence must include the
duration of infectiousness because the number of mosquitoes
acquiring infection from an infectious host accumulates over
time. Although starlings sustain infectious viremias for about
three days, the various native birds that we examined do so
only for a single day. As in the case of viremia intensity,
starlings sustain EEE viremia longer than do other birds.
The competence of diverse species of birds as reservoir
hosts for EEE virus may be described as the product of the
potential for infecting vector mosquitoes throughout the pe-
TABLE 3
Vertebrate reservoir competence of various birds as hosts for eastern
equine encephalitis virus
Bird
Starling
Robin
Starling
Swamp sparrow
Song sparrow
Cardinal
Grackle
Robin
Red-winged blackbird
Brown-headed cowbird
Domestic pigeon
Mourning dove
Mode of
infection
No.
of
birds
Days
infectious
Proportion of
mosquitoes
infected
c*
Mosquito
Mosquito
Needle
Needle
Needle
Needle
Needle
Needle
Needle
Needle
Needle
Needle
4
5
8
2
1
2
1
11
1
1
1
1
4.0
1.4
2.9
1.0
1.0
2.0
1.0
1.4
1.0
2.0
1.0
1.0
0.62
0.62
0.71
1.00
1.00
0.38
0.75
0.50
0.55
0.08
0.08
0.08
2.5
0.9
2.0
1.0
1.0
0.8
0.8
0.7
0.6
0.2
0.1
0.1
* This value, c, is the product of the duration of viremia exceeding 103.0 plaque-forming
units/ml of blood and the proportion of Culiseta melanura mosquitoes that become infected
after feeding on a host sustaining such a viremia.
riod of infectiousness of the virus and the length of that
period. Vertebrate reservoir competence, c, is expressed as
the product of infectiousness (i) and duration of infectivity
(d): c 5 (i)(d) where i represents the proportion of mosquitoes that become infected after feeding during the period of
infectious viremia and d represents the number of days duration of infectious viremia. A c value of unity may represent
a vertebrate host that sustains an infectious viremia for one
day that is sufficiently intense (at least 106.2 pfu/ml) to infect
all vectors that feed within that day, or a two-day infectious
viremia capable of infecting only half of the vector mosquitoes that feed during the two days (mean viremia 5 105.3).
Applying this treatment to our data for starlings and the other birds examined in this study, we find that starlings are at
least twice as virus competent as any native species examined (Table 3). Of these native species, the smallest birds
(swamp and song sparrows) are most competent, followed
by medium-sized birds such as the cardinal, grackle, robin,
and red-winged blackbird. Interestingly, cowbird, pigeon,
and mourning dove appear to be poorly competent as reservoirs. These last three species are all common and roost
communally. If one assumes that all birds provide the same
number of blood meals to mosquitoes, then the number of
mosquitoes acquiring EEE virus infection from an infected
native bird varies with species of bird by an order of magnitude, yet starlings infect at least twice as many vector mosquitoes as does any other candidate reservoir host that has
been evaluated.
Although starlings are far more competent as hosts for
EEE virus than are native birds, they more often die in the
course of experimental infection. Because these birds tend
to die toward the end of their period of infectivity, mortality
would not detract from the number of mosquitoes that these
birds infect. We confirm the observation of Davis that infected birds appear healthy until shortly before death.4 These
birds might then escape predation when they are most infectious to vector mosquitoes.
Because Cs. melanura only rarely feed on mammals in
nature,25–27 risk of human EEE virus infection depends on
the presence of indiscriminately feeding bridge vectors28 that
RESERVOIR COMPETENCE OF BIRDS FOR EEE VIRUS
include various Aedes mosquitoes.29 At least three species of
such mosquitoes become infected after feeding on viremic
starlings. Of these, Ae. albopictus has been infected naturally
in Florida.30 The EEE virus is capable of passing from starlings to various Aedes mosquitoes, perhaps including mosquitoes that may serve as bridge vectors in nature.
We have provided for the first time a quantitative analysis
of relative reservoir competence of avian hosts for EEE virus. The abundance of starlings and their tendency to roost
communally near the breeding sites of the enzootic vectors
of this virus suggest that they may serve effectively as hosts
in which EEE virus may amplify naturally. In spite of their
greater EEE virus-induced mortality compared to robins,
starlings appear to be the most infectious avian hosts available to sylvan mosquitoes and are competent to infect almost
three times as many mosquitoes as are robins.
Acknowledgments: We thank members of Dr. Spielman’s laboratory,
Ed O’Brien (Franklin Park Zoo, Boston, MA), and Jim McNelly
(Cape May County Mosquito Extermination Commission, Cape May
Court House, NJ) for assistance with capture of wild birds, and
Maurice Ndansi for assistance with collecting engorged Cs. melanura that were used to initiate our colony. Jack Ruffa (United States
Army Medical Research Institute of Infectious Diseases) assisted
with animal care duties. Drs. Thomas P. Monath, Heinz Remold. and
Phyllis Kanki (Harvard University) provided critical comments on
early versions of the manuscript.
Financial support: Nicholas Komar was supported by the Massachusetts Department of Public Health, the Massachusetts Health Research Institute, and the Northeastern Mosquito Control Association,
Inc.
Authors’ addresses: Nicholas Komar, Division of Vector-Borne Infectious Diseases, Centers for Disease Control and Prevention, PO
Box 2087, Fort Collins, CO 80522. David. J. Dohm and Michael J.
Turell, United States Army Medical Research Division of Infectious
Diseases, Fort Detrick, Frederick, MD 21702. Andrew Spielman,
Department of Immunology and Infectious Diseases, Harvard
School of Public Health, 677 Huntington Avenue, Boston, MA
02115.
Reprint requests: Nicholas Komar, Division of Vector-Borne Infectious Diseases, Centers for Disease Control and Prevention, PO Box
2087, Fort Collins, CO 80522.
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