Epizootiology of Hyalinocysta chapmani (Microsporidia: Thelohaniidae) infections in field populations of Culiseta melanura (Diptera: Culicidae) and Orthocyclops modestus (Copepoda: Cyclopidae): a three-year investigation Theodore G. Andreadis * The Connecticut Agricultural Experiment Station, 123 Huntington Street, P.O. Box 1106, New Haven, CT 06504, USA Received 11 July 2002; accepted 23 September 2002 Abstract The epizootiology, transmission dynamics and survival strategies employed by the microsporidium Hyalinocysta chapmani were examined in field populations of its primary mosquito host, Culiseta melanura and its intermediate copepod host, Orthocyclops modestus overathree-yearperiodinanaquaticsubterraneanhabitat. H. chapmani wasenzooticandwasmaintainedinacontinuous cycle of horizontal transmission between each host. There were three distinct periods during the summer and fall when developing mosquito larvae acquired infections; each was preceded by or coincident with the detection of infected copepods. Results were corroboratedinlaboratorybioassays,whereintransmissionwasachievedinmosquitolarvaethatwererearedinwaterandsediment samples taken from the site during the same time periods. The highest infection rates, ranging from 60% to 48%, were repeatedly observed during the first six weeks of larval development. These were coincident with the most sustained collections of infected copepods obtained during the year and highest levels of infection achieved in the laboratory transmission studies. The high prev- alence rates of lethal infection observed in larval populations of C. melanura at this site are among the highest recorded for any mosquito-parasiticmicrosporidiumandclearlysuggestthat H.chapmani isanimportantnaturalenemyof C.melanura. H.chapmani appears to overwinter in diapausing mosquito larvae but may also persist in copepods. The absence of vertical transmission in the lifecycleof H. chapmani andthesolerelianceonhorizontaltransmissionviaanintermediatehostareuniquesurvivalstrategiesnot seenamongothermosquito-parasiticmicrosporidia.Theepizootiologicaldatasuggestthatthistransmissionstrategyisafunctionof the biological attributes of the hosts and the comparatively stable environment in which they inhabit. The subterranean habitat is inundated with water throughout the year; copepods are omnipresent and C. melanura has overlapping broods. The spatial and temporal overlap of both hosts affords abundant opportunity for continuous horizontal transmission and increases the likelihood that H. chapmani willfindatargethost.Itishypothesizedthatnaturalselectionhasfavoredtheproductionofmeiosporesinfemale host mosquitoes rather than congenital transfer of infection to progeny via ovarian infection as a strategy for achieving greater transmission success. Ó 2002 Elsevier Science (USA). All rights reserved. Keywords: Hyalinocysta chapmani; Microsporidia; Culiseta melanura; Mosquito; Orthocyclops modestus; Copepod; Epizootiology; Horizontal transmission 1. Introduction Hyalinocysta chapmani HazardandOldacre,1975isa polymorphic microsporidian parasite of the mosquito Culiseta melanura Coquillett and the copepod Orthocy- clops modestus (Herrick). It represents one of the four microsporidian genera (Amblyospora, Duboscqia, and Parathelohania) that infect mosquitoes (primary host) and utilize copepods as intermediate hosts. Its life cycle hasrecentlybeenelucidated(AndreadisandVossbrinck, 2002) and is described below. Infections arise in C. melanura larvae following the oral ingestion of uninu- * Fax: 203-974-8502. E-mail address: theodore.andreadis@po.state.ct.us. Journal of Invertebrate Pathology 81 (2002) 114–121 www.academicpress.com 0022-2011/02/$ - see front matter Ó 2002 Elsevier Science (USA). All rights reserved. PII:S0022-2011(02)00154-4