As pigeon fever spreads across the United States, so does information—and misinformation—about it.
As equine diseases spread geographically, fueled by changing climates and animal and vector movement, horse owners and veterinarians can no longer afford to ignore a disease, leaning on the excuse “we don’t see it around here.”
Corynebacterium pseudotuberculosis infection, commonly called “pigeon fever” or “dryland distemper,” is one of those conditions, once relegated to a handful of U.S. states and now making its way across North America. As a disease spreads, so does information—and misinformation—about it. In this article we will look at some common misconceptions about this condition.
Myth No. 1: Pigeons cause pigeon fever
Pigeon fever is not a valid excuse for mass bird extermination; the pigeon is falsely accused. Rather, the soil-inhabiting C. pseudotuberculosis produces a toxin called phospholipase D that causes the characteristic severe inflammation and local tissue damage infected horses exhibit. This can be evident as:
- External abscesses These abscesses can break out anywhere along the body, head, limbs, or trunk, says Sharon Spier, DVM, PhD, Dipl. ACVIM, a professor at the University of California, Davis (UC Davis), Department of Medicine and Epidemiology. However, the most common locations are the midline of the belly and beneath the chest muscles (the pectoral region). Large lumps in the latter location are the reason for the term pigeon fever; as the chest abscesses develop, the swollen tissue causes a pigeon breast appearance.
- Internal abscesses Fewer than 10% of affected horses develop internal abscesses, which Spier says can be challenging to diagnose, require prolonged treatment, and are potentially lethal. Common internal abscess locations include the liver, lungs, kidney, and spleen. Veterinarians can diagnose these abscesses using ultrasound or by considering a combination of clinical signs, lab work results, and the synergistic hemolysis inhibition (SHI) blood test outcome. Historically, high C. pseudotuberculosis titers on SHI have been associated with internal abscess formation. However, in a study published in 2013, Spier and colleagues demonstrated that while high SHI titers indicate C. pseudotuberculosis infection, they do not necessarily distinguish between the presence of internal and external abscesses. The blood test is most useful when veterinarians suspect internal infection and no external abscesses are present.
- Ulcerative lymphangitis Horses with this form develop swelling along their lower limbs, as well as oozing sores. This presentation is uncommon but extremely painful.
So, if pigeons don’t cause C. pseudotuberculosis infections, what does? While scientists have speculated about a number of infection routes, they have not yet identified one specific vector (carrier). There is, however, a reservoir where the bacterium can persist; Spier and others have proven that it can live in soil for extended periods of time. In another study, she and colleagues demonstrated a link between certain fly species and C.- pseudotuberculosis. In all likelihood, horses are infected by the bites of contaminated flies (that have collected the bacteria from soil and/or from an infected horse’s discharge) or via wound contamination with the organism.
Myth No. 2: “My horse got it from the neighbor’s sheep”
While sheep and goats can also contract disease related to C. pseudotuberculosis, horses cannot contract the unique bacterial strain causing disease in sheep and goats. Both cattle and horses, however, can be infected with the same strain. But it is uncommon for horses with no exposure to cattle to become infected.
Myth No. 3: Corynebacterium is a California thing
Veterinarians first isolated C. pseudotuberculosis in California, and cases have been commonly seen during the late summer to late fall months in arid regions of the state. They surmise the organism prefers hot, dry environments, giving rise to the term “dryland distemper.”
However, this landscape is changing. Owners and veterinarians are reporting C. pseudotuberculosis infections in Arizona, California, Colorado, Idaho, Kansas, Kentucky, Missouri, Montana, Nevada, New Mexico, Oklahoma, Oregon, Texas, Utah, Washington, and Wyoming, along with Alberta and British Columbia, Canada, says Spier.
Although she notes that C. pseudotuberculosis has been common in West Texas and the Texas panhandle for several decades, cases have begun cropping up in less arid Texas regions within the last few years. Benjamin Buchanan, DVM, Dipl. ACVIM, AVECC, of Brazos Valley Equine Hospital, in Navasota, Texas, says, “We went from one case every four to five years to a couple of cases a week for a few years.”
Both Spier and Buchanan suspect that the shifting incidence of C. pseudotuberculosis cases relates to regional weather changes, though Spier notes that researchers haven’t definitively linked weather events to case occurrence. “The record drought of the past couple of years may play a role in the spread,” she says.
Myth No. 4: Just let it drain
While external abscesses associated with pigeon fever typically mature and rupture spontaneously, given enough time, both Spier and Buchanan stress the importance of veterinary intervention.
Spier suggests owners make treatment decisions on a case-by-case basis in conjunction with the veterinarian and cautions against attempting to manage the disease alone.
Buchanan prefers to “lance and drain” external abscesses. To reduce the chances of soil contamination on a premises, Spier adds, “I try to encourage people to collect the pus and dispose of it. That’s a good reason to have the veterinarian lance and drain the abscesses rather than letting the horse wander around dripping pus.”
Another argument in favor of involving the veterinarian is proper diagnosis. Spier cites an overall lack of data as one of the barriers to C. pseudotuberculosis research. She encourages owners and veterinarians to submit pus samples to a diagnostic laboratory for bacterial culture and antibiotic sensitivity testing. This helps practitioners track case numbers, geographic spread, and any developing antibiotic resistance.
Not all cases of C. pseudotuberculosis infection result in “simple” pigeon fever or even the internal organ abscesses described previously. Some abscesses can develop very deep within the muscle, particularly in the groin region, causing severe pain, non-weight-bearing lameness, weight loss, and even infection of the bone and nearby joints. Very deep abscesses might take months to mature to the point they can be drained.
In a paper published in a 2012 issue of the Journal of the American Veterinary Medical Association, Spier and co-author Nora Nogradi, DVM, Dipl. ACVIM, examined the clinical profiles of equine musculoskeletal C. pseudotuberculosis infection cases over a 10-year period. Of the deep musculoskeletal abscesses evident, they concluded, “This long maturation process can compromise the horse’s systemic health by the ongoing inflammatory process … We recommend aggressive attempts to localize (find) the abscess and initiate treatment in cases of musculoskeletal infection.”
These diagnostic and treatment methods might include using ultrasound to locate an abscess pocket so the vet can access it surgically and begin administering appropriate anti-inflammatory or antibiotic drugs to reduce associated complications.
Myth No. 5: Antibiotics make the disease worse
Spier tries to dispel the mistaken belief that antibiotic treatment of pigeon fever causes horses to develop internal abscesses. In a 2005 retrospective study of 30 horses with internal C. pseudotuberculosis abscesses, Suzanne Pratt, DVM, and co-authors from UC Davis found that 63% of the affected horses also had a concurrent or recent history of external abscesses. This overlap of internal and external disease might have led some people to believe that treating the external disease causes internal abscess formation. However, no scientific evidence has been found to support this theory. Other immunologic factors specific to each horse likely dictate how the disease manifests, says Spier.
“We are researching to see if there are strain variations of the bacteria which cause more severe disease,” she adds. “However … during outbreaks of disease with large numbers of horses affected, veterinarians will see horses with abscesses in less common sites on the horse’s body. When we run the math, the majority of cases are still pectoral/ventral midline abscesses.”
Buchanan supports a case-by-case approach to treatment. “I will go to antibiotics on horses that are chronic cases (those persisting for months or with multiple, recurrent abscesses). Those tend to be older horses; I wonder if there is some immunosuppression in those horses.”
His impression that horses with internal abscesses tend to be older than average is borne out of what’s in the literature. In the 2005 study of internal abscess cases, affected horses’ median age was 8.5 years, considerably higher than the ages of 1-2 years and 3-5 years seen in a 1998 study of risk factors for C. pseudotuberculosis infection.
Ongoing research might help clarify some of the factors associated with the organism. “Genome sequencing is the next step,” says Spier, who cites the limited available data as a challenge for progress.
Myth No. 6: Affected horses should be locked away
Neither Spier nor Buchanan advocates a stringent quarantine of individually affected horses. Instead, both recommend using sensible judgment when dealing with C. pseudotuberculosis cases.
“In my opinion, it’s a fly control problem,” says Buchanan. Although his clinic does isolate hospitalized horses while their abscesses are actively draining, Buchanan does not believe the disease is highly contagious from horse to horse. “We rarely see multiple horses (with C. pseudotuberculosis infections) from the same facility. I don’t feel like horses are a big risk to the stable if you use common sense such as good fly control and cleaning up the pus.”
Spier points out that C. pseudotuberculosis is not as contagio—us as infectious respiratory diseases such as influenza. While she feels that there is a growing need for a vaccine, she says that such a development is, realistically, a few years away. In the meantime, like Buchanan, Spier recommends using sensible judgment to minimize soil contamination by collecting pus from lanced abscesses, providing proper wound care, and cleaning and disinfecting nonsoil surfaces that pus can contaminate, such as wash racks, stall doors, tack, or grooming equipment.
Owners and veterinarians can take steps to minimize pigeon fever’s impact on affected and susceptible horses (exposed via infected horses and/or the environment):
- Involve the veterinarian Besides being able to control pus spread by proper surgical lancing and drainage (rather than allowing the abscess to break in an uncontrolled fashion) a veterinarian can minimize the affected horse’s pain through medical treatment.
- Implement fly control measures These pests are still considered to be the main vector for disease spread.
- Run cultures on pus Not all equine abscesses are caused by C. pseudotuberculosis. Culture and sensitivity testing of pus can help confirm cases and document any changes in antibiotic sensitivity as the disease moves from one region to another.
“While it’s disturbing to see increasing numbers of cases, the renewed interest will stimulate more research,” Spier says. “Researchers are working to further understand the disease and improve treatment.” These research efforts will be best supported by the collection of data from those on the front lines: veterinarians and horse owners.