Pigeon Fever

A look at the forms and symptoms of an uncommon but serious infection

by Dave Sauter, DVM

Other than their name, pigeons have nothing to do with Pigeon Fever, a highly contagious disease of horses. Pigeon Fever has been considered endemic to California, but in recent years the incidence has been increasing in other western states such as Colorado, Wyoming, Montana, Idaho and Washington. It has also increased in southern states such as Kentucky.

Pigeon Fever is caused by bacteria that live in the soil and are found worldwide. The bacterium that causes the disease has an impressive name, especially useful for those who enjoy equine trivial pursuits: with two 7-syllable words, Corynebacterium pseudotuberculosis is a mouthful. 

The bacterium prefers dry, cooler environmental conditions. Although it can occur any time of the year, it is most prevalent in early fall. The bacteria gain access into the horse’s body through breaks in the skin, through small wounds on the body or through mucous membranes (e.g., the gums). Although not yet proven, many experts believe insects such as the housefly and horn fly play a role in transmission, literally carrying the bacteria from one horse to the next. There is no vaccine for the disease.

After invading through the skin or mucous membranes, it can take weeks for Pigeon Fever to incubate before clinical signs of disease develop. The bacteria spreads locally (i.e., wherever it gains access) and via lymphatics (vessels that connect lymph nodes together). Early in the course of the disease, symptoms can be vague and non-specific, such as fever, depression, weight loss and lameness. There are three principle forms the disease can take in the horse:

1. External Abscesses

This is the most common form of the infection in the United States. Abscesses can range from very small to deep, massive abscesses containing quarts of tan colored malodorous pus. They can occur anywhere in the body, but one of the most common areas is in the pectoral muscles. The resulting greatly enlarged pectoral area reminded someone of pigeon breasts, hence the name "Pigeon Fever." There are many other names for this form of the disease, including Pigeon Pox, Dryland Distemper, Dryland Strangles, and Breastbone Fever. 

Other common areas for the abscesses to form include the sheath, the mammary glands, under the belly and between the hind legs. When lesions develop on the ventral midline, they tend to form large, painful plaques of edema with multiple draining tracts (similar to what happens on the legs with Ulcerative Lymphangitis, described below) rather than the large pus abscesses seen in the pectoral muscles. Horses with the external abscess form have a good prognosis for full recovery.

Treatment centers on establishing good drainage. Poultices and hot packs can encourage superficial abscesses to open. The deeper abscesses and many of the superficial ones require surgical lancing to establish drainage. Ultrasound can be helpful in locating and directing the veterinarian’s scalpel to the center of these deeper abscesses. Disinfectants can be used to flush out the opened abscesses. Non steroidal anti-inflammatory medication such as Bute, Banamine or Equioxx might be indicated to help manage pain and fever. Antibiotic therapy is controversial. 

Although humans generally don’t get the disease, we unfortunately can carry it in the form of pus and other discharge from infected to uninfected horses. Other animals, including insects, can also act as transmitters of the disease. Affected horses should be isolated to try to contain this highly contagious disease. Everything should be kept separate (clothing, feed tubs, water buckets, water hose, barn equipment, etc.) to maintain biosecurity. 

2. Internal Abscesses

In some cases, the bacteria can penetrate beyond the region of the body where they first gain access and spread deeper into the horse’s insides, reaching into the thoracic and/or abdominal cavity. Internal lymph nodes and organs can develop abscesses. This dissemination into the body poses far greater risk of loss of life. 

Because these abscesses are out of sight, diagnosis is more elusive. In addition to fever, weight loss, and depression, other symptoms might be present, reflecting the organ system affected (e.g., nasal discharge for pulmonary, colic for abdominal, bloody urine for kidney, ventral edema for liver). Since drainage might be impossible to establish, long term antibiotic therapy generally is indicated, from one to six months. Mortality rate from this rarer form of the disease is much higher, over 40% in some reports. 

3. Ulcerative Lymphangitis

In this form of the disease, the bacteria gain a foothold in the lower limbs and eventually make their way into the lymphatics of the legs, most often the hind limbs below the hocks. As the bacteria spreads up the lymphatic vessels, chains of nodules appear on the limbs that ultimately abscess, ulcerate and drain. New lesions develop as older ones heal. Commonly, the limb becomes swollen and painful. Damage to the limb can cause permanent thickening, disfigurement and debilitation. 

In Europe and other parts of the world, Ulcerative Lymphangitis is the most common form of Corynebacterium pseudotuberculosis infection. Treatment includes long term use of high levels of penicillin. Note that other bacteria are capable of causing similar symptoms. Culture is necessary to confirm that Corynebacterium pseudotuberculosis is the culprit.

Animals at Risk

It is important to mention that ruminants are also susceptible to Corynebacterium pseudotuberculosis infection. Small ruminants, such as sheep and goats, are affected by a different subtype that does not affect horses. In sheep and goats, it is referred to as Caseous Lymphandenitis, more commonly known as Cheesy Gland, Lympho or Thin Ewe Syndrome. This highly contagious disease can cause significant economic losses from culling, carcass condemnation, etc. 

Cattle are susceptible to both sub-types of Corynebacterium pseudotuberculosis infection, the equine form and the small ruminant form, but are more likely to have the small ruminant sub-type. Laboratory testing can be done to identify which sub-type is present.

Although fairly uncommon, Pigeon Fever is a disease of growing concern. An awareness of what it is and the symptoms of the various forms can help to expedite its identification and containment if and when it shows up.