Rushing water, stagnant ponds, or even sudden dry stages after wet periods can lead to outbreaks of livestock disease. Mosquitoes and biting flies, capable of carrying and transmitting diseases, thrive in the damp weather. Prolonged wet periods can also bring anthrax spores to the surface, making them a threat to livestock and wildlife after pastures dry.

Horse owners should take precautions against mosquito-transmitted diseases by having their animals vaccinated against West Nile virus (WNV) and the reportable diseases Eastern and Western equine encephalitis (EEE and WEE). "If you wait until cases of 'sleeping sickness' occur in your area, you may have waited too long to vaccinate," said Bob Hillman, DVM, Texas' state veterinarian and head of the Texas Animal Health Commission (TAHC), the state's livestock and poultry health regulatory agency.

"You need time for the vaccine to do its job, which is to build the animal's immunity to a particular disease," he said. "Vaccines can provide the best disease protection possible, but it's also crucial to keep up with booster shots, as recommended by the vaccine manufacturer or your private veterinary practitioner. In 2006, 111 horses were confirmed to have West Nile virus in Texas. So far this year, the disease has been confirmed in one horse, located in Willacy County."

Three cases of Potomac horse fever (which is not a regulatory disease) have been confirmed in Kerr County by the Texas Veterinary Medical Diagnostic Laboratory. Potomac horse fever, first detected in Maryland in l979, is not normally found in Texas. Clinical cases usually are found near rivers, streams, ponds, or canals. The infection involves tiny flukes that are parasites of water snails. The flukes hatch their offspring into the water, and these are then picked up by aquatic insects that molt into flying insects, including caddis flies and mayflies. Horses can become exposed to Potomac horse fever when they eat or drink anything contaminated with the insects.

Kerr County equine veterinarians are urging all owners to call their practitioner if equine animals exhibit signs of Potomac horse fever, which can include a reluctance to eat, fever, diarrhea, colic, or inflammation around the hoof. Treatment includes antibiotics and supportive care.

Biting flies can mechanically transmit equine infectious anemia (EIA), sometimes called "swamp fever." This incurable disease, for which no approved vaccine exists in the U.S., is a regulatory disease. TAHC regulations require horses and other equine animals to have a negative test for EIA within the previous 12 months before entering the state, or going to events, assemblies, trail rides, undergoing change of ownership, or entering a breeding farm. The most common test for EIA is the "Coggins" blood test. The incidence of EIA has dropped dramatically since l997, when 750 equine animals in Texas were found to be infected. Since January 2007, 29 infected animals have been detected.

"EIA-infected horses can develop severe anemia, fever and swelling. In severe cases, EIA can kill the animals, but many times, the infected animal has few signs of disease," Hillman noted. "Prevent EIA transmission by avoiding blood-to-blood contact between infected and 'clean' equine animals. Disinfect medical instruments and tack, and control flies. Biting flies carry blood from one animal to the next on their mouthparts, and they play a key role in the mechanical transmission of EIA."

In Texas, infected equids must be maintained in quarantine at least 200 yards away from other equine animals, or euthanatized.

"In the past, untested equine animals could be sold for slaughter through a livestock market. At the slaughter plant, blood samples were collected and sent for laboratory testing. Now that Texas equine slaughter plants have been closed, we can not ensure that horses moving to plants in other states or countries are tested. Therefore, we are requiring all horses to have a current EIA test, even when being sold for slaughter," Hillman said.

Rain followed by hot weather can coax the invisible bacteria Bacillus anthracis to the surface, a situation that has occurred on a ranch in Tom Green County, where 17 head of cattle and a number of white-tailed deer have died.

"Anthrax in Texas occurs nearly every year, and it is a reportable disease to the TAHC. If we know an outbreak is occurring, we can let ranchers in the area know that it's time to vaccinate their livestock. There is no preventive treatment for wildlife," said Hillman.

Anthrax naturally occurs worldwide and in many areas of the United States. Disease outbreaks have been reported in Minnesota, South Dakota, and Canada this year. When an anthrax-infected animal dies and isn't properly burned, the bacteria will infiltrate the soil and lie dormant (but not spread) for many years. Under ideal weather and soil conditions--a cool, wet spring, followed by a hot, dry spell--the bacteria will resurface on grass and forage. The disease cycle starts again, when another animal ingests the resurfaced anthrax bacteria.

TAHC regulations require that an infected animal's carcass, manure, and bedding be incinerated. This prevents wild animals from being exposed to the disease, and it kills the bacteria, preventing another site where the anthrax bacteria can resurface. Hillman urged ranchers to wear gloves and long sleeves when preparing the burn site and to avoid moving the carcasses, if possible. Exposure to anthrax carcasses could cause a handler to develop black skin lesions that require prompt medical treatment.

"In Texas, it is often feast or famine, drought or flood," Hillman said. "With each of the conditions, there are concerns about livestock health. So, while ranchers enjoy the lush grasses the rains have produced, they may have to control pests to lower disease risks. However, I don't know many ranchers who would choose last year's drought over some extra vigilance this year."

For more information on horse management following wet conditions, see  

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