Corynebacterium pseudotuberculosis is a bacterial infection affecting horses and other species of animals such as cattle, goats, sheep, camels and buffalo. The disease has one biotype that horses can contract and another one that the other species can contract, thus the condition is not transferable from other species to the horse. There are numerous common names for the disease, but it is most often referred to as “Dryland Distemper” or “Pigeon Fever”. Regardless of the name, it should be noted three forms of the disease exist; external or cutaneous abscessation, “ulcerative lymphangitis”, and internal abscessation. The external or cutaneous abscess form is the most common form, and presents itself with hard edematous swelling and abscess formation in either the pectoral region of the chest, the inguinal region (groin) or ventrally in the midline (other sites are possible). These swellings become larger as the abscess matures and may cause fever, lameness or depression in the horse. This form of the disease has been shown to cause abscessation in virtually every region of the body, but is primarily seen in those regions listed above. The second form, ulcerative lymphangitis, is characterized by severe swelling in one or more legs, commonly referred to as a “stove-pipe” leg. The horses are usually very painful and have extreme stiffness or lameness with one or more ulcerative lesions (open sores) developing over the lymphatic ducts of the legs. The third and final form of the disease is internal abscesstion, which is the most serious and the most difficult form to treat. External abscessation comprises 91% of the total cases of corynebacterium, while internal abscessation is seen about 8 % of the time. Ulcerative lymphangitis is seen in only about 1% of total cases, but this form is found almost exclusively in California. Geographically the disease is seen primarily in the southwest, but there have been reported cases in most states.
Other interesting facts about the disease are that it is usually seen in the late summer and fall and outbreaks usually occur in cycles of 5-10 years. Studies have shown that the soil borne organism can survive in soil for nearly a full year and in shavings or straw for up to two months. The organism is thought to gain entry into the horse via abrasions or wounds in the skin or mucous membranes. Once a horse has been exposed to the organism, the incubation period can vary from 3 weeks to several months.
Treatment of the external form is directed at assisting the abscess to mature, usually done with hot packs and poultices such as Icthamol. Once the abscess has matured it can then be drained, with special care being taken to collect and properly dispose of the infectious exudates (pus). Following drainage, the pocket should be flushed thoroughly and packed if possible. Strict attention to fly control is essential throughout the treatment period and even more so following abscess drainage. Nonsteroidal anti-inflammatory drugs (NSAIDS) are used to aid with the edema and pain. Antimicrobial therapy is case dependent based on the location of the abscess, severity of the illness and the veterinarian’s discretion. Simply put, each case is handled on an individual basis.
Treatment for ulcerative lymphangitis should be aggressive and it’s important to note that these horses need to be treated for several weeks. They need to be on high levels of NSAIDs for the swelling and pain, and unlike the external abscess form, antibiotic therapy is essential for treating the ulcerative lymphangitis. There are several antibiotics that can be used for treatment, but selecting the correct one is sometimes difficult as each horse responds differently to the various antibiotics. Sodium iodide is also beneficial when treating ulcerative lymphangitis and is commonly used by our practice. Blood panels and serum titers for the organism also aid in determining the response to treatment and the longevity of treatment with antimicrobials.
Internal abscesses are the most difficult to diagnose and treat. Typically, horses diagnosed with internal abscesses had another form of the disease 4-6 weeks previously. Clinical signs of horses with internal coryne abscesses include fever, weight loss, anorexia, depression and sometimes death. If undiagnosed, 100% of horses with internal abscesses will die and even with treatment, it is estimated that between 30-40% will still die, although early detection and treatment can greatly reduce this number. A diagnosis of internal abscessation is confirmed with blood results, serum coryne titers and sometimes with ultrasound imaging. These horses are treated extremely aggressively with antibiotics, sodium iodide, NSAIDs and quality nutrition.
A new vaccine has just been released on a provisional basis. This means the efficacy of the vaccine has not yet been proven. But, the incidence of vaccine reaction is less than 2%. We recommend vaccination with the new vaccine.
We still recommend good horse husbandry practices. Prevention of the disease is centered around reducing exposure by strict fly control and management of wounds or sores on your horse. Limit your exposure by staying clear of areas contaminated with the disease, but if you are in a region where the disease is present, exercise fly control and constant removal of manure. Products such as “Stall Safe” are commercially available for treating the soil to protect against the condition. Frequent grooming of your horse is important and will help you to be more observant of open sores or wounds on your horse that you can treat and protect from flies with products such as “SWAT”.
Although corynebacterium pseudotuberculosis is a serious disease condition that requires early detection and treatment for best results, the overall prognosis for horses with the disease is good. Remember, less than 1% of horses with the external abscess form (the most common form) will die from this disease. Horses are treated on a case by case basis, based on their specific symptoms and the disease form or forms they exhibit. Good horse management and fly control are crucial for reducing your horse’s risk of exposure. If your horse exhibits any of the symptoms suggestive of the disease, contact your veterinarian immediately.