November 01, 2010


 High prevalence of EEE in Michigan

Unvaccinated horses, weather were factors in outbreaks  

Posted Oct. 18, 2010 

The prolonged hot and wet summer in some Midwestern and Southeastern states this year created conditions favorable for outbreaks of eastern equine encephalitis. As of late September, the Department of Agriculture's National Animal Health Surveillance System reported 180 laboratory-confirmed cases of EEE in horses. 

Though the national total may come in below the typical number of cases seen each year—the mean number of equine EEE cases seen annually from 2003-2009 is 283—some states have been harder hit than others.
In all, Michigan has had 45 confirmed equine cases of EEE. The southwest portion of the state has long been an area associated with cases of arbovirus infection, particularly a nine-county area that accounted for about 90 percent of the EEE cases in Michigan this year. The area has a lot of surface water and a soil type that provides a favorable pH for mosquitoes.

Despite hundreds of cases seen each year throughout the
United States, some horse owners decline to vaccinate
their horses against eastern equine encephalitis.

Dr. Steven L. Halstead, Michigan state veterinarian, said he's been shocked by the number of cases the state has seen, noting that all reported cases have involved animals that had not been vaccinated or had only recently been vaccinated.

"The issue is people are making discretionary choices about how they're spending limited income, and sometimes the livestock—in this case, the horses—don't get first option of that money," Dr. Halstead said. "Alternatively, it's going to pay the mortgage or tuition bills or more basic things. You know, putting food on the table. We have great concerns about the unintended lower level of care the horses are getting in Michigan in general."

That cost cutting couldn't come at a worse time.

"An early start (to the vector season), warm temperatures, increasing mosquito activity, shortening of the incubation period for mosquito eggs and larva(e), increasing the number of reproductive cycles out there, combined with ample resources in the form of surface water for mosquito breeding ... and then you throw the birds in," which act as a reservoir for the virus that causes EEE, he said.

For the past few years, cases of arbovirus infection have decreased, but Dr. Halstead speculates that there has been a natural reduction in native immunity in birds. He said those out there this year have diminished antibody titers either because they haven't been exposed to the viruses in the past few years or because they have hatched since the last time the state had a substantial outbreak. That leads to a large pool of susceptible birds to amplify the virus, he said.

The situation has been exacerbated not only by less spending by horse owners on vaccines but also by states having less money for prevention efforts.

In previous years when the Michigan state government had more money, there was more mosquito surveillance. Local health departments and mosquito abatement districts would trap and test mosquito pools in every county. Little of this is done any more, something Dr. Halstead calls unfortunate.

"It would give us the advantage of being able to say, 'It's out there, ... horse owners, vaccinate your horses now if you haven't already.' We just don't have the money to put that effort together anymore," Dr. Halstead said. "If we had been able to say back in maybe June or July that this is happening out there, we might have been able to get more horses vaccinated and possibly save some lives."

Michigan isn't the only state struggling with an uptick in the number of EEE cases or earlier reports of infection. Florida averages 73 reported cases of equine EEE each year. In years when conditions favor the spread of EEE, the number of reported equine cases can exceed 200; however, this year the state is only slightly above average, with 90 confirmed cases.

Dr. Michael A. Short, the equine programs manager for the Florida Department of Agriculture & Consumer Services' Division of Animal Industry, estimates the actual number of horses affected is usually double the amount reported.

"The ones we see in Florida, a vast majority—maybe 90 percent, haven't been vaccinated in the past year. ... We tell people, but they are busy or doing things or are not educated, and they have other priorities. They just don't get (their horses) vaccinated for whatever reason," Dr. Short said.

Other states reporting more than one equine EEE case this year are Mississippi (18), Alabama (8), Georgia (7), Massachusetts (4), New York (2), and North Carolina (2).

Horses rarely survive once infected. The only means of control is through preventive vaccination.

The American Association of Equine Practitioners has core vaccination guidelines posted on its website here.