Diseases carried by mosquitoes may be more prevalent after flooding | TheFencePost.com

Diseases carried by mosquitoes may be more prevalent after flooding

Once a horse is bitten by an infected mosquito the incubation period for EEE and WEE varies from two to three days up to three weeks. Most horses develop inapparent infections.
Photo by Amy Johnson

Mosquitoes are more than a nasty nuisance; they spread many serious diseases to humans and animals. Eastern and Western Equine Encephalitis (EEE and WEE) and West Nile Virus (WNV) are arboviruses (transmitted by blood-feeding insects) that can cause fatal neurologic disease.

All of these viruses have public health implications because they are zoonotic — affecting many species, including humans. “WNV and EEE are of most concern because of seriousness of clinical outcome of infection in humans,” said Dr. Peter J. Timoney at Gluck Equine Research at the University of Kentucky, a leading authority on infectious diseases of horses. “People who become infected with EEE frequently die or are left severely brain damaged,” he said.


EEE occurs primarily in eastern parts of the U.S. and Canada but has been seen as far west as South Dakota and Texas. “The first case was identified in 1937 in Connecticut,” said Kathy Seino, DVM. She did her graduate work in infectious diseases, at the University of Florida, focusing on the immune response to West Nile Virus in horses, then served as faculty at Washington State University College of Veterinary Medicine in Equine Medicine before joining the DeClue Equine Clinic in Plymouth, Minn.

Birds are the primary host for EEE and WEE; mosquitoes suck blood from infected birds and then pass the virus to the next animal they bite. Horses, humans and other mammals are considered accidental or dead-end hosts, infected by mosquitoes that feed on both birds and mammals. “Horses don’t transmit EEE but serve as a marker to show it’s in a certain area,” Seino said.

WEE occurs primarily in the western and midwestern U.S. “It was first identified in outbreaks in the 1930s and 1940s primarily in California and surrounding western states and there were huge numbers of horses affected, and humans as well. Since that time there have been only a few cases, probably due to the fact that there is an effective vaccine for horses,” Seino said.

“EEE, WEE and West Nile are three of the five core vaccines for horses recommended by the American Association of Equine Practitioners. For all of these vaccines, after the primary vaccine series you need to annually vaccinate one month before the mosquito season starts.” In an area with a long mosquito season you may need to revaccinate at least once, or maybe twice, before the season ends — when using an inactivated or killed virus product. In Florida, many horses are vaccinated every four months.

Some horsemen have become complacent about vaccinating because they haven’t heard of any cases in their region for several years, but this would be a good year to vaccinate because of all the flooding this spring. There will likely be a huge population of mosquitoes all summer long, due to an increase in breeding sites — thus more risk for spread of mosquito-borne diseases.

Vaccines play a key role in preventing these diseases. “Dr. Maureen Long, at the Emerging Disease and Arbovirus Research and Test Lab at the University of Florida, a few years ago did a survey looking at horses that died from EEE. Most of them had an incomplete or no vaccination history,” Seino said. Vaccination is reliable protection, as long as you make sure the horse is boostered at proper intervals, to keep immunity strong.

Controlling mosquitoes and reducing mosquito exposure are also a key part of prevention against arboviruses. Eliminate standing water, if possible. Some people use products in ponds and tanks that inhibit the mosquito larvae, or use fish that will eat them. “Insect repellents are also helpful, for both horses and humans, but you may need to reapply them frequently, especially when you or your horse are sweating. You can also keep horses indoors during the times mosquitoes are most active. Provide fans for air movement in stalls to prevent mosquitoes from feeding on horses,” she said. Mosquitoes don’t like to fly against a breeze.

Prevention is very important because there is no effective (anti-viral) treatment for encephalitis. Once a horse is bitten by an infected mosquito the incubation period for EEE and WEE varies from two to three days to three weeks. Most horses develop inapparent infections. “Young and immunocompromised animals are most likely to develop clinical signs such as moderate to high fever (102.5 to 104.5 degrees F), depression, lack of appetite, cranial nerve deficits (facial nerve paralysis, weakness of the tongue, difficulty swallowing), behavior changes (aggression, drowsiness), gait abnormalities (partial paralysis, incoordination or muscle weakness), to more severe central nervous system signs (head pressing, circling, blindness, seizures),” she said. “Treatment consists of intensive nursing care and making sure the horse is well hydrated and has adequate nutrition, along with use of anti-inflammatories (NSAIDs, DMSO) to keep fever down, and medication to control seizures,” Seino said. The horse needs good bedding to prevent bed sores, and a sling if he is down and unable to get up.

“Mortality rate in horses infected with EEE is high (75 to 95%). The course of the disease with EEE may be swift, with death two to three days after onset of clinical signs — despite intensive care. Horses with WEE are more likely to survive (mortality rates at 20 to 40%). Horses that survive may have long-lasting deficits, however,” she said.

Duration of natural immunity (after a horse recovers from the disease itself) may be variable for the Alphaviruses such as EEE and WEE but for the Flaviruses like West Nile, natural infection seems to induce lifelong immunity.


When West Nile first came to this country in 1999 there were hundreds of cases because the horse population was very susceptible. But now, after vaccination and many instances of natural exposure, case numbers have dropped. Young horses, however, or horses that have never been exposed or vaccinated, are vulnerable. Vaccination plays a key role in maintaining the low number of cases. WNV has been in the U.S. for more than two decades, and has spread across the entire country. Like EEE and WEE, this is a disease of birds that is spread by mosquitoes and sometimes affects horses and humans as end-stage hosts. The virus first entered the U.S. with birds brought to a zoo in New York. It is a common disease in Africa, west Asia, Europe and the Middle East.

“The 1999 outbreak that started at the Bronx Zoo affected 62 humans, with seven deaths. There were 25 horse cases that first year,” said Dr. Rob Keene, equine technical manager with Boehringer Ingelheim. This disease quickly spread, reaching the West Coast by 2003.

Birds are the reservoir for this disease and mosquitoes are the vectors that spread it from one animal to another. Crows, ravens, blue jays and magpies are very sensitive to the disease and often die. Other species of birds seem to tolerate it better, without clinical signs.

According to Keene, the only way horses or humans get the virus is through a mosquito bite (a mosquito that has bitten a sick bird). The virus level in the blood of a sick horse or human is so low that is virtually impossible for a mosquito to transmit it from horse to horse or to humans.

The signs of WNV in horses are usually neurologic. “The horse may or may not have a fever. He may be depressed and lethargic. Any neurologic disease can have similar symptoms, whether it is WNV, EPM, rabies, the neurologic form of the herpes EHV-1, EEE, WEE, botulism, etc. Diagnosis, to determine the cause of a neurologic problem is simpler now, however, because tests have been developed to check for WNV. Horses with clinical signs can be tested for WNV,” Keene said.

“Treatment includes IV fluids, DMSO (given orally or intravenously), non-steroidal anti-inflammatories, mannitol (an alcohol given IV, to reduce cerebrospinal fluid pressure and volume), and dexamethasone. The horse should be supported in a sling, if it cannot stand,” he said.

The first vaccine for WNV was marketed under a conditional license (because it had not been fully tested for efficacy). Since then, several other vaccines have been created. Some require a two-dose series the first year, with an annual booster thereafter. One WNV vaccine produces adequate immunity with just one shot — and an annual booster. Your veterinarian can answer questions about vaccines and which ones might be appropriate for your horses.

Most horses are vaccinated ahead of mosquito season. Risk for WNV is low during early spring, but increases with warm weather. The disease must get established each year in the bird population and become amplified in the mosquito population. If there are warm days in late fall, horses could still be at risk. It only takes a week of warm weather to re-establish a mosquito population. If mosquito season is long you may want to give another booster later in the year.

Vaccination is fairly effective in preventing the disease. Some of the unvaccinated horses exposed to WNV develop severe illness and die (even with extensive treatment), or recover after being treated, while others have such mild illness that the owner may not know the horse was sick. “There is evidence that many horses (and humans) who have never shown signs of illness have encountered WNV; serology studies have shown that many of them test positive although they have never been sick,” Keene said.

In one study, many people were interviewed and tested, and for every diagnosed hospitalized human, there were 150 subclinical infections. Either those people didn’t get sick enough to go to the hospital or did not feel ill, but they did have antibodies for WNV. With horses, testing has shown that there are many positive but healthy horses in regions where there have been sick horses. Some of the horses brought to veterinary hospitals for things like colic, unidentified fever, lack of appetite, etc. have tested positive. Veterinarians have seen colic cases that they decided to run tests on, even though the horse had no neurologic signs, but the horse tested positive to WNV. The horse was just colicky for a few days. There may be a certain number of horses that show clinical signs of illness that aren’t recognized as this disease, because the signs are not typical for WNV.

Some medical entomologists feel that WNV is usually in an area about a year before it is recognized (before people are able to isolate it from mosquitoes, or see dead birds or clinical cases in horses and humans). “When we start finding dead birds and clinical cases, we have to assume that WNF been there for awhile,” Keene said.

The vaccine is cheap insurance and very effective in preventing this disease. Since it can be deadly, the wise thing to do is keep your horses vaccinated, giving an annual booster ahead of mosquito season. ❖

— Smith Thomas is a cattle rancher, horseman, freelance writer and book author, ranching with her husband near Salmon, Idaho. She can be reached at hsmiththomas@centurytel.net.


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