INTRODUCTION
Eastern equine encephalomyelitis virus (EEEV) cycles through avian (host) and mosquito (vector) populations in New Jersey. Typically, the virus recrudesces during spring in migrant or overwintering birds possibly due to stresses experienced during migration and the subsequent breeding season. Culiseta melanura, an obligate avian feeder, picks up the virus from infected adult birds early in the summer. Birds born in spring (Hatch Year or HY) become infected when fed upon by these mosquitoes. Amplification of the virus occurs as particles are passed between infected and non-infected participants. If viral particle load becomes high enough in these populations, infections may then expand to other mosquito species (bridge vectors) capable of transmitting EEE to dead-end hosts such as horses and humans (Scott and Edman 1991).
The presence of all participants, however, do not ensure that
any site will become an EEE focus. In New Jersey, southern counties
are far more likely than northern counties to have EEE cases even
though many of the same vectors are present throughout the state.
The chronology of the vectors in relation to each other may be
critical in determining how much amplification of EEEV occurs,
i.e., the accessibility of hosts to vectors in time and space
(Scott 1988). Sufficient exposure to each other allows enough
individuals to become infected and thus the virus amplifies. When
amplification is insufficient, then virus is not passed to bridge
vectors. Factors that reduce amplification would include the dispersion
of one or more vectors or hosts from the area, thus shortening
the time that the amplifying participants remain in each other's
presence. In this study, we investigate the chronology of two
EEEV vectors in a southern (focus) and a northern (non-focus)
county in New Jersey. Passerine birds were caught in mist nets
at High Point State Park in northern New Jersey, and the presence
of HY birds was compared with historical bird-banding records
of southern New Jersey as well as the mosquito presence at both
sites.
MATERIALS AND METHODS
Study Sites: We examined the presence and migration of avian species caught at two sites in New Jersey. In the south, historical records from birds caught in Japanese mist nets were analyzed. Six to ten mist nets were run by J. McNelly for W. Crans during the spring, summer, and fall from 1980 to 1984 at Jake's Landing, Dennisville, Cape May County, NJ (hereafter referred to as "Dennisville"). The site is part of Belleplain State Forest and a long-term state site for monitoring EEE at an endemic focus. Nets were either placed along trails or roadways, or lanes were cut into thickets. Birds were caught from dawn to mid-afternoon, occasionally extending to early evening. Birds were identified to species, age, and sex, banded, and then immediately released. Culiseta melanura populations were monitored by resting boxes maintained as part of the state's EEE monitor program. Historical records from the previous 20 years were examined by T. Rainey (pers. comm.) for Cs. melanura population patterns.
The northern site was located at Kuser Natural Area in High Point
State Park in Sussex County, NJ (hereafter referred to as "High
Point"). Eight to 12 Japanese mist nets were run from 02
July 1998 to 04 October 1998. Nets were set up at dawn and run
until midday (noon to 1300 hrs). Nets were placed along trails
only and monitored throughout their operation. Birds caught were
identified to species, age, and sex, if possible, and then banded
and released immediately. T. Rainey monitored adult mosquito populations
through resting boxes.
RESULTS AND DISCUSSION
Fewer bird species were seen at the northern site than at the southern site. Twenty-two species of birds were mist-netted at High Point as compared to the 67 species caught at Dennisville (see Table 1; status derived from Crans et al 1994). This is not surprising given the longer period of capture at Dennisville (over a period of four years versus one season at High Point), coupled with the geographic location. The Cape May peninsula funnels a great number of migrants through a small area, concentrating their numbers and make it all the more likely to show a wider diversity of species (Sibley 1993).
The role of the nulliparous mosquitoes as the replicating and transmitting vector is critical for EEEV amplification. Culiseta melanura adults were not as persistent at High Point as at the more southerly Dennisville. T. Rainey (pers. comm.) observed that adults disappeared from resting boxes at High Point by the end of September while in Dennisville, adults were found on average some two to three weeks later and well into October (Figure 1). Thus, mosquitoes were at the southern site for a longer period of time than at the northern site. In addition, while both sites appear to have nulliparous females present throughout the season, life table data from heat summation models indicate that a second emergence of nulliparous females at Dennisville likely occurs (Mahmood and Crans 1998). Pulses of the vector population that most readily replicates virus are more likely to occur in the southern site than the north. Therefore, not only were mosquitoes in the south present for longer periods of time, but the population also experienced periodic infusions of fresh females throughout the season.
As part of the host population, HY birds are of particular interest in the amplification of virus due to their less effective anti-mosquito behaviors, possible preference by avian-seeking mosquitoes, and their role as unchallenged hosts of the virus (Scott 1988). Thus, the presence of HY birds is important in the role of viral amplification as an egg development source for nulliparous mosquitoes. At High Point, the proportion of HY birds decreased earlier and more dramatically than at Dennisville (Figure 1). Mortality may account for some of this decrease. Differential age migration may also account for the decrease and then increase in the percentage of HY birds observed at both sites. Some species partition the timing of migration among age classes or migration pathways (Stewart, Mewaldt, and Kaiser 1974) or both (King, Farner and Mewaldt 1965). Regardless of the mechanism and although the timing of departure was similar, the proportion of HY birds leaving High Point was much greater than at Dennisville.
Accessibility in space and time (Scott and Edman 1991) is one
component to ensure successful amplification of virus and passage
of encephalomyelitis through bridge vector to dead-end hosts.
In conjunction with accessibility is the idea that enough participants
will be available to ensure amplification. If a potential host
is not present in large enough numbers, nor consistently, that
host will be of minor importance in the transmission of EEEV.
HY birds and Cs. melanura mosquitoes are both present at
High Point and Dennisville, but their exposure to each other is
reduced at the former when both participants leave early. An extended
period of exposure between birds and mosquitoes plus the multiple
infusions of fresh nulliparous females at Dennisville, characteristics
of a hot focus, are lacking in the northern part of New Jersey.
LITERATURE CITED
Crans, W. J., D. F. Caccamise, and J. R. McNelly. 1994.
Eastern equine encephalomyelitis virus in relation to the avian community of a coastal cedar swamp. J. Med. Ent., 31(5): 711-728.
King, J. R., D. S. Farner, and L. R. Mewaldt. 1965.
Seasonal sex and age ratios in populations of the White-crowned Sparrows of the race Gambelii. Condor 67: 489-504.
Mahmood, F. and W. J. Crans. 1998. Effects of temperature on the development of Culiseta melanura (Diptera: Culicidae) and its impact on the amplification of eastern equine encephalimyelitis virus in birds. J. Med. Entomol. 35(6): 1007-1012.
Pyle, P. 1997.
Identification Guide to North American Birds, Part 1: Columbidae to Ploceidae. Slate Creek Press: Bolinas, California.
Scott, T. W. 1988.
Vertebrate host ecology. In The arboviruses: epidemiology and ecology. (T.P. Monath, ed). CRC Press: Boca Raton, Florida.
Scott, T.W. and J. D. Edman. 1991.
Effects of avian host age and arbovirus infection on mosquito attraction and blood-feeding. In Bird-Parasite Interactions (J. E. Loye and M. Zuk, Eds). New York: Oxford University Press.
Sibley, D. 1992.
The Birds of Cape May. New Jersey Audubon Society: Cape May Bird Observatory.
Stewart, R. M., L. R. Mewaldt, and S. Kaiser. 1974.
Age ratios of coastal and inland fall migrant passerines in central
California. J. Field Ornith. 45: 46-47.
Table 1. Avian species mist-netted at two sites in New Jersey.
Data from Dennisville represents 4 years of mist-netting and is
located in the southern end of the state while High Point is in
the north and represents one summer and fall of mist-netting.
| Acadian Flycatcher (Empidonax virescens) | ||
| American Goldfinch (Carduelis tristis) | ||
| American Redstart (Setophagus ruticilla) | American Redstart | |
| American Robin (Turdus migratorius) | American Robin | |
| American Woodcock (Philohela minor) | ||
| Barn Swallow (Hirundo rustica) | ||
| Black-and-White Warbler (Mniotilta varia) | Black-and-White Warbler | |
| Black-capped Chickadee (Poecile atricapillus) | Black-capped Chickadee | |
| Black-throated Blue Warbler (Dendroica caerulescens) | Black-throated Blue Warbler | |
| Blue Jay (Cyanocitta cristata) | Blue Jay | |
| Blue-winged Warbler (Vermivora pinus) | ||
| Brown Creeper (Certhia americana) | ||
| Brown Thrasher (Toxostoma longirostre) | ||
| Brown-headed Cowbird (Molothrus ater) | ||
| Canada Warbler (Wilsonia canadensis) | Canada Warbler | |
| Cape May Warbler (Dendroica tigrina) | ||
| Carolina Chickadee (Poecile carolinensis) | ||
| Carolina Wren (Thryothorus ludovicianus) | ||
| Chestnut-sided Warbler (Dendroica pennsylvanica) | ||
| Common Grackle (Quiscalus quiscula) | ||
| Common Yellowthroat (Geothlypis trichas) | ||
| Cooper's Hawk (Accipter cooperii) | ||
| Downy Woodpecker (Picoides pubescens) | ||
| Eastern Kingbird (Tyrannus tyrannus) | ||
| Eastern Towhee (Pipilo erythrophthalmus) | Eastern Towhee | |
| Eastern Tufted Titmouse (Baeolophus bicolor) | ||
| Eastern Wood-Pewee (Contopus virens) | ||
| Field Sparrow (Spizella pusilla) | ||
| Golden-crowned Kinglet (Regulus satrapa) | Golden-crowned Kinglet | |
| Gray Catbird (Dumetella carolinensis) | ||
| Gray-cheeked Thrush (Catharus minimus) | ||
| Great Crested Flycatcher (Myiarchus crinitus) | ||
| Hairy Woodpecker (Picoides villosus) | ||
| Hermit Thrush (Catharus guttatus) | Hermit Thrush | |
| Hooded Warbler (Wilsonia citrina) | ||
| House Finch (Carpodacus mexicanus) | ||
| House Wren (Troglodytes aedon) | ||
| Indigo Bunting (Passerina cyanea) | ||
| Kentucky Warbler (Oporornis formosus) | ||
| Louisiana Waterthrush (Seiurus motacilla) | ||
| Magnolia Warbler (Dendroica magnolia) | ||
| Myrtle Warbler (Dendroica coronata) | ||
| Nashville Warbler (Vermivora ruficapilla) | ||
| Northern Bobwhite (Colinus virginanus) | ||
| Northern Cardinal (Cardinalis cardinalis) | ||
| Northern Waterthrush (Seiurus noveboracensis) | Northern Waterthrush | |
| Ovenbird (Seiurus aurocapillus) | Ovenbird | |
| Palm Warbler (Dendroica palmarum) | ||
| Pine Warbler (Dendroica pinus) | ||
| Prairie Warbler (Dendroica discolor) | ||
| Prothonotary Warbler (Protonotaria citrea) | ||
| Red-breasted Nuthatch (Sitta canadensis) | ||
| Red-eyed Vireo (Vireo olivaceus) | ||
| Red-winged Blackbird (Agelaius phoeniceus) | ||
| Rose-breasted Grosbeak (Pheucticus ludovicianus) | ||
| Ruby-crowned Kinglet (Regulus calendula) | Ruby-crowned Kinglet | |
| Ruby-throated Hummingbird (Archilochus colubris) | ||
| Savannah Sparrow (Passerculus sandwichensis) | ||
| Scarlet Tanager (Piranga olivacea) | ||
| Sharp-shinned Hawk (Accipter striatus) | Sharp-shinned Hawk | |
| Slate-colored Junco (Junco hyemalis) | Slate-colored Junco | |
| Song Sparrow (Melospiza melodia) | ||
| Swainson's Thrush (Catharus ustulatus) | Swainson's Thrush | |
| Swamp Sparrow (Melospiza georgiana) | ||
| Tennesee Warbler (Vermivora peregrina) | ||
| Veery (Catharus fuscescens) | Veery | |
| White-eyed Vireo (Vireo griseus) | ||
| White-throated Sparrow (Zonotrichia albicollis) | White-throated Sparrow | |
| Willow Flycatcher (Empidonax traillii) | ||
| Wilson's Warbler (Wilsonia pusilla) | ||
| Winter Wren (Troglodytes troglodytes) | Winter Wren | |
| Wood Thrush (Hylocichla mustelina) | Wood Thrush | |
| Worm-eating Warbler (Helmitheros vermivorus) | ||
| Yellow Warbler (Dendroica petechia) | ||
| Yellow-billed Cuckoo (Coccyzuz americanus) | ||
| Yellow-breasted Chat (Icteria virens) | ||
| Yellow-shafted Flicker (Colaptes auratus) | ||
| Yellow-throated Warbler (Dendroica dominica) |
1. M = Migrant; PM = Partial Migrant; PR = Permanent
Resident; SR = Summer Resident; WR = Winter Resident. Partial
migrant refers to two equally strong strategies present in the
area. (derived from Crans et al 1994 and Sibley 1992).