Conclusions While a wide range of mosquito-borne flaviviruses circulate in SE-Queensland and horses in the region over time may acquire immunity to these viruses, the current survey suggests that horses under the age of 10 years could function as sentinels for medically important flaviviruses including WNV, MVEV and JEV. antibodies JW74 to viruses in the Kokobera computer virus (KOKV) complex or antibodies to unknown/untested flaviviruses. Amongst eight JW74 yearlings one presented JW74 with clinical MVEV-encephalomyelitis, while another, clinically JW74 normal, experienced MVEV-neutralizing antibodies. The remaining six yearlings were flavivirus antibody unfavorable. Of 19 foals given birth to between August and November 2011 all were flavivirus antibody unfavorable in January 2012. This suggests that horses in the area acquire over time active immunity to a range of flaviviruses. Nevertheless, the relatively infrequent seropositivity to MVEV, WNVKUN and ALFV (15%) suggests that factors other than pre-existing immunity may have contributed to the low incidence of arboviral disease in SE-Queensland horses during the 2011 epidemic. Keywords: flavivirus, equine, antibody response, mosquito-borne, encephalitis 1. Introduction More TMSB4X than 70 arthropod-borne viruses (arboviruses) have been isolated in Australia, but only a few have been implicated in human and livestock diseases [1]. The family comprises three genera, the flaviviruses, pestiviruses and hepatitis C-like viruses. Only the flavivirus genus contains viruses that are transmitted by arthropods. Flaviviruses are small, 40C55 nm in diameter, enveloped, single-stranded, positive sense RNA-viruses with a genome of ~11 kb which encodes three structural and seven non-structural proteins. The flaviviruses have been placed in nine serologically defined groups, with an additional few still to be classified. The type species of the flavivirus genus is usually yellow fever computer virus, with other major members being the dengue viruses, Japanese encephalitis computer virus (JEV) and the tick-borne encephalitis viruses [1]. Flaviviruses cause a wide variety of symptoms in humans and other mammals ranging from subclinical contamination to moderate fever, fatigue, myalgia, arthralgia, severe hemorrhagic syndromes or hepatitis and meningo-encephalitis [2]. In Australia, Murray Valley encephalitis computer virus (MVEV) and a strain of West Nile computer virus (WNV) named Kunjin computer virus (WNVKUN) are the main aetiologic brokers of viral encephalitis in humans, occurring as annual isolated cases or sporadic outbreaks, mainly in the north-western part of the continent [1]. MVEV is the more virulent pathogen, with morbidity and case fatality rates of 0.1C1.0% and 15C20%, respectively, and the only proven cause of fatal human cases [1]. In contrast, WNVKUN infections are less frequent and less severe [3]. Horses have been described as a susceptible species and have been involved in WNV outbreaks overseas, most notably the outbreak that commenced in the USA in 1999 [4]. In Australia, contamination with WNVKUN has been observed in horses, but the disease has rarely been confirmed [5], while cases of equine MVEV-encephalitis are mostly, but not exclusively, seen under the same environmental conditions as those precipitating human MVEV cases (Barton in the plates and subjected to an ELISA protocol [21] using anti-flaviviral E protein antibody, Mab 4G2, and pre-absorbed secondary antibody-conjugate to ascertain viral replication. 3. Results and Conversation Of 62 adult horses (age range 3C35 years) and eight yearlings tested 41 were positive in the flavivirus-specific blocking ELISA (63%), and of these 31 experienced neutralizing antibodies to one or more of the following flaviviruses: MVEV, KUNV, ALFV, STRV and KOKV (Table 1 and Table 2). Blocking ELISAs using JW74 antibodies specific for MVEV (10C6) and KUNV (3.1112G) were also used to further analyse positive serum samples in the flavivirus-specific blocking ELISA. Horses positive for MVEV by blocking ELISA also experienced neutralizing antibodies against MVEV (T141, T143). However, some horses positive in the KUN blocking ELISA did not have detectable neutralizing antibodies against KUNV (T131, R10, T105 and T129). Seven horses were judged to have either given a false positive reaction in ELISA or, more likely, were positive for an unknown/untested flavivirus, such as EHV. Of the eight yearlings, all of whom were born at the UQ Gatton Campus, two experienced neutralizing antibodies to both MVEV and KOKV. One of the MVEV-positive yearlings (T143) presented with clinical encephalomyelitis and was eventually euthanatized due to prolonged neurological sequela (Barton spp.), herons (systemic replication of computer virus [26,28,29,30]. It has been suggested that equines might be useful sentinels for flavivirus activity [31,32,33,34,35]. Thus, Nielsen [32] concluded that immunologically naive horses are more sensitive for detection of WNV transmission than mosquito sampling alone. Similarly, Phoutrides [34] concluded that horses constitute an.