Changing Patterns in Mosquito-Borne Arboviruses
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Dncnrvrnnn.1986 Mosqurro-BoRNE ARBovTRUSES 437 CHANGING PATTERNS IN MOSQUITO-BORNE ARBOVIRUSES G. R. DEFOLIARTT. D. M. WATTS' eNo P. R. GRIMSTAD3 ABSTRACT. Researchleading to the current state of knowledge on the epidemiology of La Crossevirus (LACV),JamestownCanyon virus (JCV) and dengue(DEN) virusesis summarizedin relationto the generally recognizCdcriteria for incriminating vectors.The importance of vector biology and local ecologicalconditions is eriphasized as is the necessityofi good balanceb6tween laboratory and fiIld-based studies.The influence of huinan activity in shaping th'e epidemiologicalpatterns of all three'of these arbovirusesis readily apparent. INTRODUCTION such,the suddenlyvoluminous research on this virus and its major vector, Aedzstriserintus (Say), Four basiccriteria must be met in incriminat- is relatively condensedin time compared to any ing the mosquito vector(s)of an arbovirus: l) other arbovirus. It illusrates the types of isolation of virus from naturally infected questions raised in investigating a virus known mosquitoes;2) laboratory demonsration of the to be transovarially transmitted in nature. ability of the mosquitoesto become infected by Jamestown Canyon virus appears to be the feeding on a viremic host; 3) laboratory most rapidly emerging "new" arboviral prob- demonstration of the ability of these infected lem of public health importance in North mosquitoes to transmit the virus during America. Although serologicallyclose to LACV, bloodfeeding,and ; 4) evidenceof bloodfeeding its ecological support system (i.e., vectors, contact between the suspectedmosquito vector vertebratehosts, landscape) is completelydif- and suspectedvertebrate hosts under natural ferent. Despite this, these two viruses interface conditions. In a companion review (DeFoliart in intricate ways that carry potentially impor- et al. 1987),the focuswas on the importanceof tant public health implications. Current ques- a host of variables that must be considered in tions relative to JCV typify the formulation of attempting to satisfy these basic criteria and to hypotheses necessaryduring the early phases assesstrue vector comPetence.It was empha- of vector incrimination. Dengue viruses, a sized that a good balance of field- and group of 4 closely related serotypes laboratory-basedstudies must be maintained to (DEN-1,2,3,4),warrant inclusion becauseden- ascertainthe unique combination of ecological gue has greater world-wide public health and behavioral features essentialto survival of importance than any other mosquito-borne a given arbovirus in a given situation. With the arboviral entity. Regardeduntil recently as an input of field-basedstudies, it becomesappar- "there anthroponosis (i.e., transmitted by vectors ent as statedby the above authors that is solelyfrom human to human), it is probably a no aspect of vector ecology or behavior that zoonosiswith an ecologicalvariety of transmis- may not be found to require clarification sion cyclesincluding sylvan,rural, and urban, during the epidemiological investigation of an The DEN virusesvividly portray the difficulties arbovirus." encountered with a public health problem The implications of the preceding statements intimately adapted to the cultural and socio- can best be illustrated by the changing ques- economic environment of human populations tions that arise at progressive stages in the in the tropics. investigationof a specificarbovirus. As exam- ples, we discusshere the 3 viruseswith which we have had our most extensive research LA CROSSEVIRUS experience, La Crosse (LACV), Jamestown Canyon (JCV) and dengue (DEN). Each virus La Crossevirus is a Bunyavirusbelonging to has unique characteristicsthat make its inclu- (CAL) serogroupwhich contains sion appropriate. Lacrosse virus is the most the California virus and variet- recently recognized of the major mosquito- 14 recognized types, subtypes (Calisher 1983). At least 6 of the viruses borne arboviruses in North America, and, as ies cause human illness. La Crosse virus was originally isolated from the brain of a child who died of encephalitisin 1960 in La Crosse, I Department of Entomology, University of Wis- Wisconsin(Thompson et al. 1965).The major- consin,Madison, WI 53706. ity of virus isolations and human clinical cases 2 of Pathogenesisand Immunology, Department have occurred in Ohio, Wisconsin, Minnesota, Divisionof DiseaseAssessment, United StatesArmy MedicalResearch Institute of InfectiousDiseases, Ft. Iowa, Illinois, Indiana and New York, but Detrick. Frederick,MD 21701. there have been serologically confirmed hu- 3 Department of Biology, University of Notre man infections in Nebraskaand the majority of Dame,Notre Dame, IN 46556. statescontiguous with or eastof the Mississippi 438 Jounuel oF THEArunnrceN Mosgurro coNrnor. AssocrerroN vor.. 2, No. 4 River (Calisher 1983, Calisher and Thompson maternal vertical transmission rates (MVTRs) 1983). of up to 70Vo(the MVTR is the product of the It required 7 years after the identification of percentageof infected females passingvirus to LACV before the basic criteria for vector at least some of their progeny and the fitial incrimination had been satisfied. Chipmunks infection rate [FIR], i.e., ttre mean percentaqe and gray squirrels were suspectedas veitebrate of infected progeny from transovarialiy- hosts based on their high antibody prevalence transmitting females). These results led to the in nature (Moulton and Thompson l97l). view that LACV maintenance relies primarily Aedzstriseri,atus, a tree hole breeding mosquito, on this vertical transmission component with becamesuspected as the primary vettor on the horizontal amplification via vertebiates viewed basisof virus isolations from naturally infected as a processfor recruiting enough new vertical mosquitoes (Thompson et al. lli72), and transmitters to maintain a relatively stablevirus demonstration under simulated field condi- prevalencein the vector population. Increasing tions of bloodfeeding contact with the sus- evidence,however, that LA-CV undergoes high p€.J.{ vertebrare amplifying hosts (Wright and attrition at several points during vinebra"te DeFoliart 1970).Also, seasonaland geo[raphic amplification has riised a queition as to compatibility.was shown between peik fopula- whether this horizontal proiess can fully tions of Aa. triseriahts(Loor and Oefolait i9Z0; negate the erosion of virus prevalence that and human clinical cases (Thompson and occurs during maternal vertical transmission Inhorn 1967). This early era of vector incrimi- (DeFoliart 1983). The quesrion was first raised nation^wascapped by thi laboratory demonstra- by Miller et al. (1979) who reported that orallv tion of high ransmission ntes (93% or higher) infected Ac. trkerianu did noi become vertical by Ae. tri.seriatruorally infected on virus doiages transmitters until the second oviposition after as low as 1.8 median tissue culture infectlive the infectious blood meal. Female dailv life (TCIDso)/0.I dose ml (Waus et al. lg72), and expectancies of 80-95Vo have been reported demonstration that chipmunks and gray squir- for Ae. triseriatus(Beier et al. 1982, Hiramis rels developed viremias sufficient to"ini^eciac. and Foster 1983, Sinsko and Craig l9Z9), and (Pantuwatana triseriatus et al. lg72). Other it can be calculated that the orillv infected mosquito speciestested were relatively ineffi- cohort^probably experiencesa mortaiity rate of cient oral ransmitters of the virus, in'dicating about 90% prior to deposition of infected eggs. there were no important secondary vectors ii I hls assumptton was strengthened by the the upper Midwest (Watts et al. lg73a). finding at one endemic site that onlv ?.lVo of Additionally, the early conclusion that chip- bloodseeking Ac. triseriaus females were bipar- munks-and squirrels were amplifying hosis, ous (Porter and DeFoliarr 1985). originally somewhattentative as it was blsed on Overall, there appear to be at least 4 presence of.antibody to undifferentiated CAL sequential points and 7 or 8 factors during the serogroup virus, was substantiatedby isolations vertebrate amplification process that wiii, or of LACV from both of these mammalian may under certain condilions, result in high species(Gauld et al. 1975, Ksiazekand yuill virus attrition: 1977), and by evidenceof high LACV transmis- l) Virus atrrition resulting from bloodfeeding sion activity in areas with good-subsequent chipmunk on non-amplifier species of vertebrates: AeI habitat (Gauld et al. lg74). re- trtseriahr is catholic in its feeding behavior search (discussedbelow) showed 3 additional (Burkot and DeFoliart 1982, Naici l9g2a, types of transmissionand additional vertebrate 1985; Wright and DeFoliart 1970). White- and potential vector species.Also, it became tailed deer are non-amplifiers of LACV (Issel evident that, to explain the transmissiondynam- et al. 1972a, 1972b),buia 3-year studv of blood ics of LACV, information was needed on meal sources at an endemii site in i{isconsin 4T"r! every aspect of vector biology and revealed that 65Voof Ae. triserialru blood meals behavior. were from this vertebrate while only 24Vowete In the early 1970s, Watts et al. (lg?3b) from chipmunks and gray squirrils (Burkot experimentelly demonstratedtransovarial trans- and DeFoliart 1982). A blood meal ratio such mission of LACV by Ae. triseriatusand this as this from deer and amplifier speciesresults breakthrough was followed quickly by the in a nearly l6-fold decrCasein implification finding that the virus overwinteis in the potenrial, because only