At the present time there are several properties in the Woodside area that have recently had cases of “Pigeon Fever”. This article is a response to those asking for more information about the disease and what they can do.
Corynebacterium Pseudotuberculosis
Corynebacterium Pseudotuberculosis infection is also known as “Pigeon Fever” because of the large size of pectoral abscesses and “Dryland Distemper” because of its prevalence in arid regions. It is a gram positive intracellular organism that causes external subcutaneous abscesses, ulcerative lymphangitis and internal infection in horses. It has an incubation period of 1 to 4 weeks. The portal of entry for this soil borne organism is thought to be abrasions or wounds in the skin or mucous membranes. Several flies have been implicated as vectors for the transmission of the disease. These flies are horn flies, stable flies, and houseflies. At farms where diseased horses were present, 20% of the houseflies were positive. Corynebacteria organisms can live in the soil at all times, but they become most pathogenic during drought conditions. The incidence of infection fluctuates considerably year to year with the highest number of cases occurring in late summer and fall. Horses of all ages are susceptible although foals less than 6 months old seem less likely to be infected perhaps due to passive maternal immunity. Young adult horses (less than five years old) and horses on summer pasture have an increased risk of infection. Horses housed outside or with access to an outside paddock appear to be at higher risk than stabled horses. In one study 91% of horses had a complete recovery with no recurrence of infection in subsequent years implying long lasting immunity. Even when a horse on a property contracts pigeon fever, it doesn't mean that all or any other horses in that location will develop the disease. The presence or extent of the infection seems to depend largely upon an individual horse's immune system and how well he can fight off this organism.
External Abscesses
A common complaint from horse owners is that the horse got kicked as there are commonly large swellings present. These abscesses can occur anywhere on the body but are most commonly found in the pectoral region and along ventral midline of the abdomen. Other less common sites include the sheath, mammary glands, axilla (armpit), limbs and head. Horses may have one or more abscesses. Affected horses commonly have swellings or non-healing wounds. Although most horses do not exhibit sign of systemic illness some may have a fever, lameness, ventral dermatitis, weight loss, anorexia and depression. As an abscess matures it become hard and painful. After drainage is established either by spontaneous rupture or my lancing, many recover within a couple of weeks without complication. Ultrasonography can be helpful in determining the best location to establish drainage.
Internal Infection
A small percentage of affected horses may develop internal infection. Organs most commonly affected are the liver and lungs with kidney and spleen being affected less often. Common signs seen in these horses are concurrent external abscesses, decreased appetite, fever, lethargy, weight loss, signs of respiratory disease or abdominal pain.
Ulcerative Lymphangitis
Is the least common form. Limb swelling, cellulitis and draining tracts are seen. Horses often develop lameness, fever, lethargy and anorexia.
Diagnosis Culture
The typical appearance of single or multiple maturing pectoral abscesses is highly suspicious of C. Pseudotuberculosis infection. Culture of tan exudate is diagnostic.
Serology and PCR
Without positive culture blood tests can be run but may not be helpful for external abscesses as they may be negative in the early course of disease even at the time of abscess drainage. Positive results must be interpreted carefully to distinguish active infection from exposure or convalescence.
Ultrasonography
Extremely useful to diagnosis of internal infections not only in identifying affected organs but also determining the extent and nature of involvement.
Therapy
Treatment regime must be tailored to individual horses. Treatment with antibiotics may or may not be appropriate as in some cases they may prolong the course of the disease, whereas other cases may require a long course of antibiotics. Establishing drainage of external abscess depends on maturity and depth of the abscess but remains one of the quickest routes to resolution of the infection. Some lesions may require poulticing or hot packing to bring things to a head. Some abscesses may have to be monitored with ultrasound to determine the best time to lance it. Abscess contents should be retrieved to prevent further contamination. Recovery can take as little as two weeks or as long as two to three months.
Prevention
At this time there is no vaccine developed for use in horses. The main source of infection and potential spread of the organism is via the rupture of affected lymph nodes and abscesses with the discharge of thick caseous pus containing millions of organisms into the environment. Therefore purulent material from an opened abscess should be collected into a container and disposed of properly. Horse owners must rely on good sanitation and fly control and avoid unnecessary environmental contamination from diseased horses to prevent C. Pseudotuberculosis infection in their horses. Although there is no evidence that diseased horses should be quarantined affected horses with an actively draining abscess should be isolated. Strict insect control should be implemented. Proper sanitation, disposal of contaminated bedding, and disinfection may reduce the number of new cases. Proper wound care is important to prevent infection from a contaminated environment.
