04/20/2022
Five Deadly Diseases
Brian S. Burks, DVM
Diplomate, ABVP
Board Certified in Equine Practice
Sick horses can be difficult to see, especially when it is your own. They may be depressed and move slowly. They are given medications by veterinarians and there is hope for a speedy recovery. These are mild to moderate diseases. Fatal diseases, however, are a much larger problem.
Potentially fatal diseases bring the possibility of heartbreak for their humans. Horses with West Nile virus (WNV), Eastern equine encephalomyelitis (EEE), or Western equine encephalomyelitis (WEE) can spike high fevers, tremble, smash their heads against walls, and fall and struggle to get back up. Horses that contract tetanus experience painful muscle spasms and respiratory failure, with few recovering. In the case of rabies—which is 100% fatal in equids—a sick horse endangers the lives of humans, as well. These horses suffer immensely.
The good news is that these terrible diseases is that they are easily prevented, or at least attenuated by vaccines given once to three times per year, depending upon location. This is all it takes to keep your horses safe from deadly diseases.
There are five ‘core’ vaccines that every horse in the United States should receive at least annually, regardless of competition, location, housing, movement, or exposure to other horses. This is due to the high mortality rate, human risk, and the fact they are nearly impossible to prevent without vaccination. These are very dangerous diseases, but there are effective and affordable vaccines for each.
Rabies in horses is contracted via the bite of an infected (rabid) wild animal such as raccoons, skunks, bats, or foxes. The incubation period varies from several weeks to six months. The virus multiplies at the bite wound and travels via the peripheral nerves to the central nervous system. Once in the brain, the virus can spread to other organs, particularly salivary glands and possibly nasal secretions. Clinical signs in horses include lameness, poor athletic performance, lethargy, depression, aggressiveness, convulsions, head pressing and mindless wandering and recumbency. If you see a wild animal exhibiting unusual behavior, contact your local animal control agency immediately to remove the animal. Due to the potential risk of virus transmission from horses and other infected animals to humans, animals suspected of having rabies should be handled by individuals who have been appropriately vaccinated against rabies.
West Nile virus is an infection spread by mosquitoes. It affects birds, humans, horses, and other mammals such as dogs. The virus enters the blood stream after an infected mosquito bites the horse and then travels to the central nervous system (CNS) where is causes brain and spinal cord inflammation. Clinical signs begin in three to 15 days. These include ataxia (incoordination), muscle trembling, depression, lethargy, weakness, and sometimes fever. Disease severity varies, and not all horses develop clinical signs. Among those that do, one-third die from the disease. Among the survivors, 40% have lasting neurologic effects, mainly weakness and ataxia. Thanks to an aggressive vaccination program, the number of WNV cases has dropped in the U.S. An additional way to reduce risk of all mosquito-borne diseases is to control mosquito populations on the farm, by eliminating or reducing mosquito breeding habitats, and in conjunction with your local mosquito control authority.
Eastern equine encephalomyelitis is a viral disease transmitted via mosquito bites. Clinical signs of EEE include moderate to high fever, depression, lack of appetite, cranial nerve deficits (facial paralysis, tongue weakness, difficulty swallowing), behavioral changes (aggression, self-mutilation, or drowsiness), gait abnormalities, or severe central nervous system signs, such as head-pressing, circling, blindness, and seizures. The course of EEE can be swift, with death occurring 2-3 days after onset of clinical signs despite intensive care; fatality rates reach 75-80 per cent among horses. Horses that survive might have long-lasting impairments and neurologic problems. In the United States EEE occurs mainly in the East, especially in the South, but it has occurred as far west as Texas and as far north as Wisconsin in the Midwest. Birds harbor the virus silently—showing no signs—and act as reservoirs. Mosquitoes that feed on infected birds and mammals transmit the virus from birds to horses and humans.
Western equine encephalitis is transmitted by mosquitos and causes clinical signs like EEE. Western Equine Encephalitis, as the name implies, occurs mainly in western states of the US, generally west of the Mississippi river, but a subtype of the virus- Highland J- can be found in the eastern USA. The case fatality rate approaches 50%. One particular species of mosquito--Culex tarsalis--is implicated as being the vector that is responsible most of the spread of WEE. C. tarsalis feeds primarily on house sparrows and house finches. Many horses with WEE will become depressed, but some become agitated and excitable. Horses that become recumbent are likely to die. If the horse recovers from the initial disease, there is gradual improvement over many weeks or months.
Tetanus is caused by a potent neurotoxin produced by Clostridium tetani, an anaerobic bacterium present in soil and f***s. Bacterial spores enter the body through injuries to the skin; simple cuts and scrapes and deeper wounds can be involved. Tetanospasm toxin travels to the central nervous system via blood and lymphatics and peripheral nerves. It inhibits inhibition of muscular contraction, causing spasm. Tetanolysin increases tissue necrosis (death) furthering the proliferation of bacteria in the wound.
Clinical signs include muscle spasms, stiffness, extended neck and head, sawhorse stance, third eyelid prolapse, retracted lips, and an elevated tail. Some horses have difficulty breathing and can become recumbent. Horses are confined to a dark stall away from noise and confusion. Ear plugs are used to control auditory stimulation. A padded helmet may be useful for some horses to prevent cranial trauma. Recumbent horses need to be turned every 2-4 hours, and a sling is used to help them stand several times per day. F***s and urine may need to be manually evacuated. Last, dysphagic horses may need to be fed via nasogastric tube or intravenous catheter.
Treatment includes muscle relaxants, padding stalls for animals that have difficulty rising, removing infection (surgically and with antibiotics), neutralizing unbound toxin with antitoxin, and ensuring horses stay hydrated and receive appropriate nutrients. Despite treatment, up to 80% of affected horses die.
Fox Run Equine Center
www.foxrunequine.com
(724) 727-3481
Your horse's health is always our top priority.