Pigeon Fever is Back This Fall

Pigeon Fever, the disease that causes swollen chests and pus filled abscesses is prominent this fall throughout the bay area. I have seen more than 10 cases spread among multiple ranches. Pigeon Fever is caused by a bacteria, Corynebacteria Pseudotuberculosis. The name is not a reference to actual pigeons but came about as a nickname because in the most common form of the disease there is a swelling in a horse’s chest puffing it out like the big chest of a pigeon.

That swelling is a developing abscess and with time it will soften and feel like a water balloon. This is frequently accompanied by a small area of hair loss. Once the hard swelling turns into feeling like a water balloon, it is time to call the veterinarian out to lance the abscess and put your horse on antibiotics. This allows the pus to drain and the infection to be resolved.

Abscesses can occur from head to tail. The most common place for abscess formation is the chest (58%), but other areas can have abscess develop as well. Common sites include the bottom of the belly (27%) , prepuce (13%), mammary gland (11%), under the elbow (10%) , limbs (10%), and internal abscesses (7%).

Most horses have a single abscess which after being opened, drained and treated with antibiotics resolves the problem. However, about one in five can develop multiple abscesses requiring repeat and often prolonged treatment extending from weeks to months.

Does an infected horse need to be Isolated?

Pigeon Fever is spread by flies carrying the bacteria on them and then landing on a horse on any small wound to deposit and seed the infection.

The Pigeon Fever bacteria is present all over Northern California often living in the dirt of horse farms. So when the first horse on a farm comes down with Pigeon Fever it is more a signal that bacteria is present in the area and all horses at the farm are at risk, not so much from the infected horse, as from the ground around. The pus from a draining horse certainly represents a concentrated source of bacteria and as much as possible drainage should be cleaned, and bedding should be bagged and thrown away. At the same time, acting as if a single infected horse is putting other horses at the ranch at risk is misguided. If one horse has it, then the farm has it and putting that first infected horse in a sealed bubble would not eliminate the risk to the other horses because it must be in the dirt of the ranch itself. Fortunately outbreaks typically affect no more than 5 percent of horses at a given ranch.

Prevention

For the past few years a vaccine has been available that has proved effective. Deciding to use the vaccine is a risk versus benefit issue. The vaccine requires two shots given about four weeks apart to be effective and like most vaccines, its effectiveness kicks in 3 to 4 weeks after the second vaccination. So, looking to vaccinate your horse today for a problem at your barn today would not help until 7 to 8 weeks down the road.

The vaccine has shown to have a fairly significant reaction rate. About half the horses get a local reaction to the vaccines which will resolve whether treated or not. About 6-7% of horse receiving the vaccine can experience temporary colic symptoms. These symptoms typically occur 2-4 hours after the vaccine has been given and resolve with or without treatment. I also had one barn where some of the horses developed hives following vaccination. To try and mitigate the risk of these reactions, in most cases I will give the horse a shot of flunixin (Banamine) at the time of vaccination. That seems to help most horses.

Diagnosis

For external abscesses the appearance is a big clue. To confirm the diagnosis taking a culture to grow the bacteria or doing a PCR test specific to the Pigeon Fever bacteria is the way to go.

For suspected cases of internal abscesses, running a titer, a measure of the antibody levels to the Corynebacteria Pseudotuberculosis, is the way to determine the diagnosis.

Internal Abscesses

About 7-9% of cases can develop internal abscesses. This is a life threatening condition with a mortality rate of 40% when treated and 100% when not treated. Many of these cases also have external abscesses as well making diagnosis confusing. Regular blood work along with running a titer, a measure of the antibody levels to the Corynebacteria Pseudotuberculosis, helps make the diagnosis. Sometimes ultrasounds can be useful as well.

Once the diagnosis is made, patients are put on antibiotics for 6 to 8 weeks or more.

Immunity

There is no great research on how long immunity lasts for a single horse. One review of over 500 cases found that 90% of horses only had a single episode in a lifetime. For the other 10% re-infections occurred.

If you would like to have your horse vaccinated or more information, please contact my practice.