Florida EEE cases up sharply with mosquitoes on rise

GAINESVILLE, FLA. — It's a big year for mosquitoes in Florida, and one result is a much higher incidence of Eastern equine encephalitis (EEE).

"We want to get the word out that EEE is a serious problem here this year, so anyone bringing horses down from the North should make sure the animals are vaccinated," Maureen Long, DVM, MS, PhD, Dipl. ACVIM, assistant professor of equine and large-animal medicine at the University of Florida's College of Veterinary Medicine, tells DVM Newsmagazine.

About 75 horses, mostly in north and north-central Florida counties, which have the state's highest equine populations, were infected with EEE by the middle of August, Long says, adding that "we're only at the beginning of the peak season for EEE. We haven't seen numbers like that at this point in the season for a long time."

While most horses are vaccinated against EEE, younger horses that haven't had time to build up sufficient immunity from vaccines are most at risk, she explains.

Emu farms, which raise the large birds for meat and for pets, also present a risk. "They're very pathogenic for EEE," Long says. The birds shed the virus, putting producers at risk while butchering, and veterinarians while examining the pet birds.

There can be spillover risk to dogs and other animals, too. "Any mosquito-borne virus can be seen in many animals; mosquitoes may seek a blood meal from anything that walks. When you start seeing it (virus) in species in which you don't normally see it, that's a strong indication of a serious problem. So the warning is out there that it's time to adjust behaviors as necessary to deal with it," Long says.

In addition to the nearly 75 equine cases of EEE, three emus, one alpaca and one dog have been infected, but so far no human cases have been reported.

Populations of mosquitoes that breed in salt marshes near Florida's ocean and Gulf shores, where most people live, are surging in concentrations not seen in more than a decade, entomologists say, and they're tough to control because of built-up resistance to several pesticides and the fact that water levels in the marshes are low.

Because salt-marsh mosquitoes don't lay their eggs in free-standing water, many counties, including Brevard County on the eastern coast around Cape Canaveral, have built mosquito impoundments, or earthen dikes, around marshes and mangrove swamps to flood out the pests, but this year water levels in them are much lower because of insufficient rain, allowing the mosquitoes to lay their eggs in moist soil above the water mark.

The same is true on the state's Gulf coast.

"This is indeed one of our worst years for salt-marsh mosquitoes," James Burgess, entomologist with the Lee County Mosquito Control District near Fort Myers, tells DVM Newsmagazine. "I don't believe we've seen them in these numbers in the last four or five years, following some good years when there was enough rain to flood them out."

While salt-marsh mosquitoes are a huge annoyance for humans, they aren't responsible for spreading EEE or West Nile virus. They can, however, spread heartworm in dogs and cats, prompting veterinary clinics to place even greater emphasis on preventive medication.

"There probably are 60 different mosquito species in Florida," Burgess explains. "The salt-marsh variety live about 30 days, feed at night and can fly up to 20 miles on the wind. Fresh-water mosquitoes live about two weeks and feed in the daytime. The ones that carry encephalitis lay their eggs in standing water. The particular one responsible for EEE, culiseta melanura, breeds mostly in swampy areas in central and northern Florida."

That mosquito typically doesn't bite mammals but does amplify the virus, after which a "bridge vector" mosquito such as Cq perturbans, passes the virus on to horses and humans, according to James Clauson, environmental administrator for the state's Bureau of Entomology and Pest Control, Department of Agriculture and Consumer Services, in Tallahassee.

"EEE season normally peaks in late summer to early fall, and West Nile is more prevalent in the later part of the season," through late September and early October, Clauson tells DVM Newsmagazine.

Only one case of West Nile virus in a horse was reported in the state by mid-August, in Madison County near the Georgia border. The mosquito Culex pipiens carries West Nile.

"Human cases of WNV are most frequent between July and September, although we have not had any human cases yet this year. WNV activity currently is low to average in our animal sentinels," says Danielle Stanek, DVM, medical epidemiologist in the Bureau of Environmental Public Health Medicine, Florida Department of Health. "With that said, it only takes the bite of one infected mosquito for transmission to a person, and there are certainly mosquitoes carrying WNV out there, so good control and bite prevention are essential."

"The state experienced a drought the past two years, and only now are we seeing normal rain patterns. The eggs that have laid dormant for the past couple of years may be hatching off with the recent rains, and that may explain some of our unusually high numbers of mosquitoes," Clauson says.