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Diseases Transmissible Between Horses and People

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Zoonoses, also called zoonotic diseases, are diseases agents of which are transmitted between vertebrate animals and people. Vertebrate animals are resevoirs (where the agent persists in nature) of zoonoses. Many organisms, such as bacteria and viruses that affect animals, can also cause disease in people. Numerous zoonotic diseases are passed from wild animals to people, or from wild animals to pets to people. A number of zoonoses are transmitted from horses to humans, which include:

Brucellosis, also called Bang's disease, occasionally infects horses where it is associated with fistula of the withers and "poll evil". Abortion in mares has been reported as a result of brucella infections. Brucellosis is caused by a bacteria, Brucella sp. which in horses usually localize in the muscles, tendons and joints, although abortion has been reported. When the organism invades the shoulder joint capsule, swelling occurs and the joint capsule may rupture. Once the joint opens, pus producing bacteria usually invade the joint to cause a suppurative condition called "poll evil" or "fistula of the withers". The drainage from the wound contains huge numbers of the Brucella organism which are infectious to persons who touch or inhale the bacteria.

Rabies. Among the large variety of viral diseases affecting horses, rabies is one of the major diseases that is particularly dangerous to human beings. Although the frequency of occurrence of rabies in horses is low (less than 1% of all diagnosed rabied animals), 45-50 cases are reported annually in the United States. Because of the vagueness of its early clinical signs, rabies is suspected only late in its progression. Thus, the number of persons exposed to the rabid horse is usually high. Clinical signs of the disease at the time of initial examination include weakness in the hindquarters, lameness, colic. Paralytic signs follow marked by difficult swallowing. Horses affected with rabies usually die within a week of the onset of the clinical signs.

Anthrax is a zoonosis that affects virtually all animal species. Humans can develop local lesions, such as carbuncles and pustules from contact with infected blood and tissue and acquire pneumonia from inhaling the infectious agent.

Other zoonoses of horses include glanders (a rare devastating bacterial infection), tetanus, tularemia, tuberculosis and leptospirosis. Even though they are not often seen in horses, they are still a threat to human health.

Glanders. Glanders is caused by the bacterium Burkholderia mallei. It is a notifiable disease of especially horses, but also donkeys and mules. Goats, cats, and dogs have also been known to acquire this disease. The disease is contracted from contaminated soil or water and is spread to animals and humans through direct contact. There two forms of the disease: pulmonary and cutaneous. In both cases, the disease is often fatal. No vaccine is available to prevent infection with Burkholderia mallei.

Leptospirosis. The disease is caused by organisms of the genus Leptospira. The disease is an occupational hazard for sewage, water and canal workets. Vets, aid workers, watersport enthusiasts are also at a higher than average risk of contracting the disease. The highly invasive spiral bacteria may be transmitted to humans by the urine of infected animals of all species. The organism may enter through minor skin lesions and probably via the conjunctiva. The pathogen is known to cause uveitis (a potentially devastating inflammation of the internal structures of the eye, also known as periodic opthalmia or moon blindness), corneal opacity, abortion, fever, and jaundice. It is also believed to be the cause of some cases of still births, hoof lesions, decreased milk production, kidney failure, and even death in horses. The disease in man ranges from unapparent infection to severe infection and death. Horses can contract leptospirosis by drinking water that has been infected or eating hay or grain that has been contaminated by the urine of infected horses, cattle, swine, or wildlife including skunks, rats, raccoons, foxes, opossums, and deer.

Tetanus is caused by a bacteria called Clostridium tetani. The organism does not affect most healthy animals and they tend to develop immunity rapidly, although there may be fatalities, especially in newborns. The disease usually commences after traumatic wounds are infected and leads to muscle spasms and difficulty breathing resulting in death. Many human cases have followed injuries considered too trivial for medical attention and in some cases there is absolutely no record of a physical injury or wound.

Ringworm, also called dermatomycosis, is a zoonosis caused by fungi. Because this infection is easily transmitted to humans, effective sanitary measures should be employed and all debris from fungus-infected lesions should be treated chemically or burned. The fungi most commonly causing ringworm in horses are Trichophyton equinum, Trichophyton mentagrophyes. Microsporum canis (the common ringworm fungus of dogs is also often isolated from horses. Ringworm lesions are circular and hairless, with the afected skin being thickened and covered with scales.

Salmonellosis is caused by the bacterium Salmonella enterica. It is one of the most common infectious causes of diarrhea in adult horses. Although most horses clear the infection in a few weeks, some can carry the bacteria for long periods and shed in the manure. Initially, there is fever followed by severe, watery diarrhea, sometimes bloody, and dehydration. In humans, the infection causes inflammation of the intestines and blood poisoning.

Other zoonotic diseases transmissible to people include ehrlichiosis, blastomycosis, coccidioidomycosis, dermatophilosis, rhinosporidiosis, sporotrichosis, hendra virus infection, Venezuelan equine encephalomyelitis, vesicular stomatitis, West Nile virus infection, and Western equine encephalomyelitis.

Whenever a zoonosis is suspected, contact your veterinarian; remember hat it is easier to use precautionary methods and prevent disease than fight it.

References
  1. Basic Horse Care. Eleanor F. Prince, Gaydell M. Collier
  2. The University of California, Davis Book of Horses. Mordecai Siegal, Jeffrey E. Barlough
  3. Occupational Health and Safety Issues. IACUC, University of Arizona

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