1 East Tennessee Regional Office, Tennessee Department of Health, Knoxville, TN.
2 Tennessee Department of Health, Nashville, TN.
3 Department of Entomology and Plant Pathology, University of Tennessee, Knoxville, TN.
4 East Tennessee Children's Hospital, Knoxville, TN.
5 Division of Vector-Borne Infectious Diseases, Centers for Disease Control and Prevention, Fort Collins, CO.
6 Department of Preventive Medicine, Vanderbilt University School of Medicine, Nashville, TN.
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ABSTRACT |
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Aedes; arbovirus infections; arboviruses; central nervous system infections; immunologic surveillance; virus diseases
Abbreviations: CDC, Centers for Disease Control and Prevention
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INTRODUCTION |
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Numerous studies have established Ochlerotatus (= Aedes) triseriatus (the eastern tree hole mosquito) as the primary vector for La Crosse virus (4, 11
). This species is native to the geographic areas in which La Crosse virus infections have been reported. The virus can be transmitted within this mosquito species both vertically (transovarially) and horizontally (venereal); in addition, La Crosse virus may overwinter in Oc. triseriatus (12
14
). A variety of mammals may serve as amplification hosts, including the eastern chipmunk (Tamias striatus), grey squirrel (Sciurus carolinensis), and red fox (Vulpes fulva) (4
). Humans and white-tailed deer (Odocoileus virginianus) are considered incidental, or "dead-end," hosts that do not support viremia sufficiently well enough to infect other mosquitoes; however, since a blood meal may result in a gonadotropic cycle whereby eggs are infected, even these incidental hosts may serve to amplify the virus (15
).
Recently, La Crosse virus was isolated for the first time from naturally infected Aedes albopictus (Asian tiger mosquito) reared from eggs collected around residences in eastern Tennessee and western North Carolina where La Crosse encephalitis cases have been documented (16). This is a nonnative mosquito but one that has spread rapidly after it was introduced in Texas in 1985 (17
, 18
).
This article describes the first known blinded cohort study examining the clinical, environmental, and entomological characteristics of the disease in an area in which it is now emerging. Data were collected immediately after a case of the disease was suspected but had not yet been confirmed.
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MATERIALS AND METHODS |
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Parents who gave their consent were interviewed by a public health nurse within 1 week of the child's hospital discharge. These interviews focused on personal or household characteristics such as use of insect repellent, length of time children spent outdoors, and presence of window screens in the residence. Clinical data were obtained from hospital records. For La Crosse virus cases only, information on symptoms at discharge was obtained from medical records.
Convalescent-phase serum specimens were obtained 24 weeks following onset of illness. Laboratory testing of paired acute-phase and convalescent-phase serum samples was performed at the Knoxville Regional Laboratory of the Tennessee Department of Health. All samples were tested for immunoglobulin G antibodies to La Crosse virus, St. Louis encephalitis, eastern equine encephalitis, and western equine encephalitis viruses by use of a commercially available indirect immunofluorescent antibody test for arboviruses (MRL Diagnostics, Cypress, California). All samples testing positive were subsequently tested for La Crosse virus at the CDC, Division of Vector-Borne Infectious Diseases, Fort Collins, Colorado, by using both enzyme-linked immunosorbent assay and plaque reduction neutralization tests.
In addition to the tests for La Crosse virus infection, some combination of serologic and cerebrospinal fluid studies was conducted for most children enrolled in the study, including those for antibodies to Coxsackieviruses, cytomegalovirus, echoviruses, herpes, influenza, mumps, measles, and varicella-zoster. For a number of these children, their cerebrospinal fluid was also tested for enteroviruses and herpes polymerase chain reaction.
Environmental assessments were conducted around the residences of study enrollees within approximately 1 week following hospital discharge. These assessments focused on characteristics such as the numbers and types of potential mosquito breeding containers, the maintenance condition of the house and grounds, and the predominant surrounding habitat (e.g., suburban, pasture, successional forest). Mosquitos were gathered by using standard oviposition traps to collect eggs (to rear to adults) and to collect larvae from water-filled containers (both artificial and natural); carbon-dioxide-baited traps from the CDC were used to gather adult mosquitos.
Detailed methodologies for the use of oviposition traps and for mosquito rearing from eggs have been described elsewhere (16). Briefly, 10 oviposition traps were left in place at each residence for 1 week, and one carbon-dioxide-baited CDC trap was operated for a single 24-hour period at each site. Artificial and natural containers (but not oviposition traps) were sampled for mosquito larvae, and collected larvae were removed to the laboratory for rearing to adults. Eggs were gathered from the oviposition traps only. Adult mosquitoes, reared from eggs and larvae, and adults collected in the CDC traps were separated by species and sex and then into pools of not more than 50 mosquitoes, which were frozen at -80°C for virus isolation at the CDC Division of Vector-Borne Infectious Diseases in Fort Collins. Pools of mosquitoes were processed by using the methods of Beaty et al. (19
) and Dogget et al. (20
) and were tested for the presence of live virus with vero cell culture plaque assay, as described by Gerhardt et al. (16
).
All data were entered and analyzed by using EpiInfo software, version 6.0 (CDC, Atlanta, Georgia) (21). Categorical data were compared for confirmed La Crosse cases versus confirmed noncases by using chi-square analysis. Continuous data were analyzed by use of analysis of variance. Data shown to be not normally distributed by using Bartlett's test for homogeneity of variance were further analyzed with the Kruskal-Wallis test for two groups.
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RESULTS |
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Table 1 summarizes the demographic and clinical characteristics of the 40 children for whom full data could be obtained. The mean age of children with La Crosse virus infection was 7.5 years, and 60 percent were male. Fever, headache, vomiting, and behavioral changes were the predominant symptoms, while loss of consciousness and seizures were observed less frequently. No clinical or cerebrospinal fluid findings were significantly different between confirmed La Crosse infection cases and noncases.
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DISCUSSION |
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In this study, no clinical or laboratory characteristics distinguished La Crosse encephalitis from the undiagnosed central nervous system infections in the enrolled children. The recent report from McJunkin et al. (22) on a case series of 127 children with La Crosse encephalitis also did not reveal any uniquely distinguishing clinical feature of the acute illness. In the present study, seizures occurred in 25 percent of La Crosse infection cases, a rate somewhat lower than the 40 percent noted in a study by Gundersen and Brown of 178 patients (23
), the 46 percent noted by McJunkin et al., and the 4260 percent reported by Rust et al. (24
). Although seven of the 10 La Crosse infection cases for whom we had medical records data were noted to have some symptoms at discharge, only one (with tremor, decreased manual dexterity and balance, and difficulty in thinking) could be characterized as having a true neurologic deficit. McJunkin et al. reported primarily on children who had true neurologic deficits at discharge, and they noted that 12 percent of their patients had such sequelae. Rust et al. described a broader array of outcomes, including seizures, migrainous headaches, and behavioral disturbances (such as emotional lability).
Although the appearance of encephalitis may truly be similar for a variety of etiologies, there are at least two possible explanations for the absence of any uniquely distinguishing clinical characteristics of La Crosse encephalitis. First, the vast majority of nonLa Crosse encephalitis cases might indeed be La Crosse encephalitis cases, not diagnosed either because they failed to develop antibodies or because the tests currently available might not be sufficiently sensitive. A second possibility is that the nonLa Crosse encephalitis cases of disease represent one or more illnesses identical to La Crosse encephalitis that are also arthropod-borne. Both of these areas are important for further research.
Children with La Crosse infection spent a greater total number of hours outdoorsand a greater number of daylight hours outdoorsthan children who were not infected, important since both Ae. albopictus and Oc. triseriatus are daytime feeders. Grimstad et al. (25) have shown that a laboratory population of Ae. albopictus was capable of transmitting La Crosse virus at rates equal to or greater than the rate observed for Oc. triseriatus. Anecdotal evidence exists that Ae. albopictus may be a more aggressive (human) biter than Oc. triseriatus (Reid Gerhardt, University of Tennessee, personal communication, 2000), further heightening the concern about this mosquito species.
La Crosse-infected children were much more likely to live in a residence with one or more tree holes nearby. This finding is consistent with that of a case-control study in West Virginia conducted by Woodruff et al. (26), which found an increase in risk for La Crosse infection in children who lived in households with one or more tree holes within 300 feet (90 m) of the house. The degree to which Ae. albopictus may competitively displace Oc. triseriatus for similar breeding habitat such as tree holes is not clear. Given the rapid spread of Ae. albopictus, this is an important area for future research.
The total Ae. albopictus burden was significantly higher around residences that included La Crosse-infected children versus noninfected children, but there were no such differences for Oc. triseriatus. In this study, we defined the total mosquito burden to be the number of female and male larvae and adults collected at the site, and we did not include eggs in this equation. Compared with egg collections, the presence of larvae and adults is a much better indicator of likely human exposure to biting mosquitoes at that time. Eggs may die before hatching, or they may remain dormant and not hatch until a rainfall floods the container or tree hole later in the season or during the following summer. No significant difference was found in Oc. triseriatus and Ae. albopictus egg density between case and noncase sites.
Since its 1995 arrival in a tire shipment in Houston, Texas, Ae. albopictus has spread rapidly throughout the United States and has been documented in 928 counties in 30 states (17, 18
). Ae. albopictus is now the most commonly encountered mosquito in Tennessee; during 19982000, 100 percent of more than 100 locations in which oviposition traps were placed were positive for eggs of Ae. albopictus (Reid Gerhardt, University of Tennessee, unpublished data), and this mosquito species has been collected in all Tennessee counties (27
). Ae. albopictus not only shares a similar ecologic niche with Oc. triseriatushistorically a forest-dwelling speciesbut also is now found in less-forested areas as well. This expanding range of Ae. albopictus has the potential to increase risks for La Crosse virus transmission, both in settings in which human habitation is spreading into more remote environs and in suburban sprawls.
Ae. albopictus can transmit other viruses. In its Old World habitat, Ae. albopictus has been shown to be a natural vector for dengue and may play a role in the transmission of Chikungunya, Japanese encephalitis, and other arboviruses (28). To date, seven viruses have been isolated from Ae. albopictus collected in the wild in the United States, including eastern equine encephalitis and Keystone, Tensaw, Potosi, Cache Valley (29
), Jamestown Canyon (30
), and La Crosse (16
) viruses. Most recently, a pool of Ae. albopictus collected in the northeastern United States showed evidence of West Nile virus infection (31
). Isolations of La Crosse virus from wild populations of Ae. albopictus in eastern Tennessee and in western North Carolina remain the only such incidences from mosquitoes collected from a location directly linked to known cases of disease in humans.
A potential limiting factor to this study is that it included only those children with overt illness, whereas the majority of La Crosse virus infections are likely to be asymptomatic or subclinical. It is not known whether risk factors, including exposure to multiple potential vectors, differ between those who become seriously ill and those who remain asymptomatic. Although laboratory strains of Ae. albopictus may be capable of transmitting La Crosse virus at rates equal to or greater than that observed for Oc. triseriatus (25), it is not clear whether these differences appear in nature. It is equally unclear whether differences in the infectious dose of La Crosse virus could account for the differences in clinical presentation.
Attempts to control or prevent La Crosse virus infection may be aimed at both mosquitoes and humans. Although it is not clear whether modifying the natural habitatsuch as filling tree holesaffects the mosquito burden, it seems reasonable for people to keep the number of man-made breeding containers at a minimum, especially when children are present. The use of mosquito repellent remains a logical preventive measure, although changing people's behavior so they use repellent before being bitten rather than as a response to mosquito bites, remains a challenge. Efforts to develop a La Crosse virus vaccine are ongoing; however, it may be exceedingly difficult to implement a cost-effective and appropriate vaccination program given the high numbers of asymptomatic cases.
Finally, it is unclear whether or how further spread of Ae. albopictus can be limited. As this nonnative species expands its range in the United States, concerns about its ability to transmit other virusesincluding West Nile viruswill likely increase. Turell et al. (32) recently showed Ae. albopictus to be highly susceptible to infection by West Nile virus. Given the now-proven capability of Ae. albopictus to transmit La Crosse virus, the possibilities of its transmitting viruses that cause much greater mortality remain real.
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ACKNOWLEDGMENTS |
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NOTES |
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REFERENCES |
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