b'TINY TERRORSEastern Equine Encephalomyelitis favors a robust mosquito popula-neurologic disease similar to EEE. Fatal-Culiseta melanura mosquitoes trans- tion capable of transmitting the virusity rate among infected horses is 40-50%. mit EEE, causing serious neurologicamong birds and from birds to humansRecent years have seen a dramatic drop disease in horses, with a 90% fatalityand horses. The northern Midwest ex- in equine cases, with none reported in the rate. These mosquitoes reside east of theperienced a very wet 2019 beginning inwestern U.S. since 2004. However, birds Mississippi, localizing EEE to states inearly spring and extending into the fall.and mosquitoes in this area still harbor and around that region. Besides having aA new variant of the EEE virusthe virus, so vets recommend vaccinating fever, infected horses develop an uncoor- might be present. In Florida the virushorses annually as asafeguard.dinated gait (ataxia) and often experiencecirculates year-round, providing op-involuntary muscle twitching. Progressiveportunities for it to undergo geneticWest Nile Virusencephalitic signs develop, such as head- changes that could impact susceptibleAs the primary reservoirs for WNV, pressing, aimless wandering, seizures,bird populations. More birds becomemany avian species (corvids, such as hyperexcitability, and coma. Once a horseinfected and might carry the new vari- crows, ravens, and jays; house finches; goes down, hes unable to rise.ant north during normal migration. Forand owls and hawks) often succumb to Through 2019, most equine cases ofexample, in Massachusetts two of thethe infection. Some birds retain viremia EEE were seen in Michigan (29), Floridalast outbreak cycles of EEE involved(virus in the blood) for three months, (28), Louisiana (18), and near the Michi- new variants of the virus.possibly resulting in overwintering of the gan border in Indiana (11). The earliestHealth officials often document EEEvirus; in other words, even if mosquitoes cases appeared in March and continuedcyclical trends. After a year with par- have died off for the winter, new genera-throughout the year. ticularly high levels of viral activity, sur- tions can pick up virus from those birds Wendy Vaala, VMD, Dipl. ACVIM,viving bird populations develop long- in which its been lying in wait. Mosquito-director of life-cycle management equinelived immunity to that strain, resultinginfecting WNV titers have been found in and companion animal at Merck Animalin a transient decrease in EEE casesthe blood of infected cottontail rabbits, Health and a member of the Americanfor a number of years. Once that birdsome squirrels, and chipmunks; however, Association of Equine Practitionerspopulation dies off, another period ofscientists dont yet know their importance vaccination guideline review group, saysincreased susceptibility to EEE infec- as reservoirs for the virus.while officials reported far more humantion and carrier status in younger birdsA horse exposed to WNV through the EEE cases in 2019 than in any previouscontributes to further EEE activity. bite of an infected mosquito does not year, they did not see a similar spike inUsually, the number of WNV casesalways develop clinical signs, but when equine cases. As of mid-December, anin horses is double the number of EEEhe does they can be serious. Along with unprecedented number of 38 humancases. But in 2019 the number of EEEfever, many horses experience problems cases with 15 fatalities had been reportedcases far outnumbered the 88 WNV caseswith their cranial nerves, which are in nine states, and 182 equine cases ofreported by early December. Most wereresponsible for functions of the head, EEE had been reported in 24 states. (Forhorses that were unvaccinated, undervac- such as chewing, swallowing, blinking, comparison, EEE reportedly infected 712cinated, or had an unknown vaccinationand facial muscle tone. Infection with horses in the high-level year of 2003.)history, says Vaala.WNV causes other neurologic problems, Increased spikes in EEE cases haveincluding hind-limb weakness, ataxia, an been associated with a variety of factors,Western Equine Encephalomyelitis inability to stand, and paralysis. Encepha-says Vaala, including:The Culex tarsalis mosquito, whichlitic signs such as seen with EEE are also Warm, wet weather early in the yearlives primarily in the western part ofpossible. About 33% of horses with WNV and/or extending longer into the fallthe United States, transmits WEE, adie. Of those that recover, about 40% 2019 EEE Cases in the U.S. 2019 WNV cases in the U.S.18April 2020The Horse|TheHorse.com'