Pigeon Fever: Abscesses Within and Without
- Apr 1, 2005
The first thought that might run through your mind as you approach your horse in the pasture and see his swollen chest or belly is that he has been kicked. As you await your vet, you think back over the past few days, and realize that perhaps he felt a little more sluggish than usual, moved with a little more stiffness than normal, and in general, he hasn't behaved like his usual, shiny self. If he had been kicked in recent hours, then how come he hasn't been quite right for a few days? Rather than your horse experiencing blunt trauma, what he might be doing, in fact, is growing an infection.
A bacterial infection that used to be confined primarily to California has recently made its way in epidemic proportions through the western United States, particularly Colorado, Wyoming, and Arizona, as well as showing up in Kentucky. This disease is referred to by many names, among them dryland distemper, pigeon breast, or pigeon fever. Although this disease appears worldwide, it has a predilection for the arid western United States, being at its worst in drought years. And although it occurs seasonally mostly in the fall months, it occasionally appears as early as summer.
Dryland distemper is caused by bacterial infection with Corynebacterium pseudotuberculosis, which localizes in deep abscesses in the breast (pectoral) muscles, along the abdomen, and/or in the groin (sheath or udder) region. It is dubbed "pigeon fever" because in profile the swelling on the chest resembles the rounded appearance of a pigeon's breast. Pigeons have nothing to do with causing or spreading the disease.
No doubt you'd love to see your veterinarian appear and immediately fix the problem, but this is not always the case. These abscesses might take weeks to months to grow to the point where they can be effectively lanced and drained. Some horses will spike a fever for a day or even many days. Most continue to eat, although they act lethargic. Many demonstrate discomfort or lameness due to pectoral muscle swelling, which usually gets worse before it gets better. Swelling expands around an abscess, forming plaques of edema (fluid swelling) along the belly and/or between the front legs. The sheath or udder enlarges if infection localizes there. Since these abscesses invade the lymph nodes and act like space-occupying masses, they block normal lymphatic drainage, adding to swelling, local edema, and discomfort.
Incidence and Forms of Infection
Corynebacteria organisms can live in the soil at all times, but they become most pathogenic during drought conditions. Dryland distemper is thought to be transmitted through an abrasion, wound, or mucus membranes. Since these infections appear to peak during the autumn months when flies are particularly abundant, it is probable that flies serve as vectors to carry organisms to the skin.
Sharon Spier, DVM, PhD, Dipl. ACVIM, associate professor in the department of medicine and epidemiology at the University of California, Davis, has researched insect vectors for pigeon fever. "We were able to identify the organism in three species of flies from farms that were experiencing outbreaks," says Spier. "The flies were horn flies, stable flies, and houseflies. At farms where diseased horses were present, 20% of the houseflies were positive. The flies became negative once the disease outbreaks halted, which suggests that the reservoir is not the flies, but likely the soil. This is an area we are now investigating."
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.
Spier comments that adaptations of the organism amplify infective capabilities. She explains that an exotoxin produced by Cornebacteria increases permeability of the blood vessels to facilitate deeper invasion into the tissues, surrounding muscle, and to regional lymph nodes. Protective mechanisms of the bacterial cell wall enable its survival and continued replication despite being scavenged by white blood cells (macrophages) that target foreign invaders. In addition, exotoxin produced by the bacteria might inactivate the ability of the macrophages to disable them.
An active immune system is a horse's best defense against infection, and that is what differentiates horses that don't develop abscesses from those that do.
Although not common, it is possible for infection to travel to internal organs, such as the liver, kidneys, or heart. There might be no other indications that a horse has a simmering infection other than continually declining health problems. He might demonstrate weight loss, fever, depressed appetite, lethargy, colic, and a variety of other symptoms depending on where in the body the abscess has started. A veterinarian should be asked to examine any horse that displays symptoms of poor health, or if a horse with known exposure stops eating or drinking.
Any swelling in the groin or difficulty in using one or both rear legs should also receive prompt veterinary attention. If infection lodges in the rear limbs, it can cause a chronic syndrome known as ulcerative lymphangitis that can be difficult to resolve.
In affected horses, swelling is usually restricted to one rear limb, which can swell to gargantuan proportions. Once lodged in the lymphatic system in that limb, some degree of leg swelling can remain indefinitely, with signs of systemic illness and limb swelling recurring intermittently over many years.
When the disease takes the superficial form, causing large abscesses on the chest or small ones along the ventral abdomen, it becomes a matter of patience until these can be resolved. With time, an abscess on the chest will finally "point," mature, and feel soft to the touch. At that point, it can be drained by your veterinarian. The abscesses located along the belly can open and drain on their own because there is less tissue to break through. Since these belly abscesses are pointing down, gravity brings them to a head more quickly.
In an effort to wall off an infection, the horse's body builds a thick capsule around the abscess to contain it, particularly when it's located deep in the chest muscles. This necessitates local anesthetic and a scalpel incision to make an opening through the skin and the capsule. Many times, this capsule is honeycombed with strong, fibrous strands that separate the abscess into separate pockets of pus. An ultrasound exam is helpful in locating the multitude of abscess pockets. These capsules are loaded with thick, creamy pus, as much as a quart at a time, which flows freely through the incision.
A container should be held beneath the opening to collect as much of the infectious drainage as possible for removal from the premises.
Once drained, the abscess pocket is irrigated daily with a dilute povidone iodine solution made by mixing 10 mL of povidone iodine in 1 liter of saline.
The use of antibiotics is controversial; unless there are extenuating circumstances such as internal abscesses or ulcerative lymphangitis, it is recommended that the horse affected by external or pectoral abscesses not be given antibiotics. If a horse is placed on antibiotics prematurely, in most cases the infection will simply simmer along and resurface when antibiotics are discontinued. Supportive care includes:
- Hot packing of the swollen area to help bring the abscess to a head, improve circulation, resolve edema, and to make the horse more comfortable;
- Administration of a low dose of a non-steroidal anti-inflammatory (NSAID) medication (phenylbutazone or flunixin meglumine) once a day if swelling or lameness is extreme or if the horse feels so poorly that he is not eating and drinking. Only administer such anti-inflammatory medications under advisement of your veterinarian.
Recovery can take as little as two weeks or as long as two to three months. It has been reported that 91% of infected horses recover with no subsequent relapse or re-infection; these individuals might develop a long-lasting immunity. In others, despite receiving medical attention and having an abscess lanced and drained, it is possible that a horse will form another or several abscesses near the original swelling. Usually these are of lesser consequence and some resolve spontaneously. Even when fly season has abated with freezing temperatures, it is still possible for an infection to show up due to lengthy incubation time (think months, not days or weeks).
The ulcerative lymphangitis form and disease disseminated into internal organs do not respond well to treatment. Internal abscessation can be identified with testing of a blood sample sent to UC Davis or Texas A&M for serologic testing for antibodies to C. pseudotuberculosis. Diagnostic ultrasound examination yields specific information about the organ location of internal abscesses. The prognosis for horses affected by internal abscesses is poor, mainly because the disease has persisted for a long time before there is recognition that the horse is sick.
It is estimated that 40% of horses with internal Corynebacteria abscesses will die from complications of internal organ infection.
Since the bacterial organisms persist in the soil, the primary means of control is through management practices; it is impossible to completely eliminate the bacteria. Ideally, management strategies reduce the possibility of exposure, and if a horse is exposed, then techniques are applied to minimize the degree of exposure and the risk of infection. The best means of prevention and control rely on common hygienic practices:
- Affected horses should be isolated, particularly if an abscess is actively draining;
- Purulent material from an opened abscess should be collected into a container and disposed of properly;
- Contaminated stalls, bedding, blankets, tack, tools, and equipment should be disinfected;
- People should be aware that they, too, can serve as vectors to transmit infectious material from horse to horse. Sick horses should be handled or fed only after healthy horses have been attended. Hands should be washed after handling sick horses. Care should be taken to change clothing and shoes that have been contaminated with pus or that have contacted a sick horse;
- Rakes, shovels, and manure carts should not be transferred from areas containing sick horses;
- Insect vectors should be controlled with ample use of fly spray and the use of protective fly sheets and fly face masks; and
- Preventive herd health strategies (such as deworming, vaccination, dental care, and good dietary management) should be used to maintain a horse in the best of health with a strong immune system competent to ward off infection.
The bacteria in the pus of draining abscesses can remain infective for almost two months in manure, hay, straw, and shavings. On surfaces of stalls, floors, and equipment, the bacteria can remain viable for at least a week, and it could persist longer in cold environmental temperatures. Horses in exposed areas should be monitored carefully, and rectal temperatures should be taken daily. Refrain from scheduling elective surgery, such as castration, on horses during an active outbreak.
Spier notes, "I have seen several infections post-castration; the surgery site heals well, then two weeks to one month later abscesses occur in the surgical area."
A similar strain of Corynebacteria occurs in small ruminants, for which there is a vaccine.
Current research efforts conducted by Spier at UC Davis are focused on development of an equine vaccine. Although the research shows promise, to date there is no reliable vaccine for horses.
PIGEON FEVER OUTBREAKS: 2002-2003 Pigeon Fever In Kentucky
An outbreak of Corynebacterium pseudotuberculosis (pigeon fever) affecting more than 50 horses was diagnosed in Central Kentucky in 2002-2003. C. pseudotuberculosis cases were identified based on bacterial cultures. One mare aborted with placentitis, and another mare had a fetus with liver abscesses similar to hepatic abscessation in C. pseudotuberculosis infection. These observations support the hypothesis that this bacterium can cross the placental barrier.
C. pseudotuberculosis infection is frequently diagnosed in California and Texas, as well as other southwestern states. The bacterium can live in the environment, and it can be spread by insect vectors, especially flies. Kentucky historically has had only one or two cases of pigeon fever per year. A change in weather pattern (increased rain and temperatures in fall and winter) might have led to the disease outbreak in 2002-2003 by encouraging insect vector reproduction.
Recommendations for disease prevention include improved stable hygiene and insect control measures, early identification and isolation of infected horses (to prevent/decrease contamination of the soil with the draining exudates), and proper disposal of contaminated bedding.--Kimberly S. Herbert
About the Author
Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.
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