Pigeon Fever, Part 2

Pigeon fever is contagious but rarely deadly.

From The American Quarter Horse Journal

This is the last in a two-part series. Need to review Part 1?

The American Quarter Horse Foundation is funding an ongoing research project to learn more about pigeon fever. Conducted by Occidental College in Los Angeles, the ultimate goal of pigeon fever research is to develop a preventative vaccine. The research has been headed by Roberta Pollock, a professor at Occidental College who uses molecular and cell biology to study the immune system; and Drs. Sharon Spier and Nicola Pusterla, veterinarians and researchers at the University of California at Davis.

The usual treatment for external abscesses, which can be quite deep and severe, is to lance and drain them after they have been drawn to the surface. If abscesses are deep, they may require ultrasound to help locate them.

A lanced abscess will drain for several days. As a result, you’ll need to isolate the horse from others and use good sanitation and fly control. The pus coming from the abscesses is infectious to other horses via insects, or contaminated soil. The bacteria can survive for months in the soil, barn or common barnyard implements. Any stable equipment used around the horse should be cleaned and disinfected to prevent the spread of the bacteria, and fly control should be practiced to prevent them from spreading it.

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Once an abscess is drained, the wound should be flushed with dilute povidone iodine.

Some vets choose to use antibiotics on the horse, while others prefer not to, as it is thought the antibiotics might cause a delay in the development of the abscess. Generally, antibiotics should be reserved for horses with internal infection, limb infection or severe or prolonged infection, and their use should be directed by a veterinarian.

Because the bacteria that causes pigeon fever reside in the soil, it is difficult to prevent the disease from occurring.

The best course of prevention is to discourage transmission via flies. Use fly-control methods to discourage fly populations. This includes discouraging fly breeding by controlling manure, using fly predators, and manual prevention on the horse, such as fly spray, fly masks and fly sheets.

If a horse gets the disease, try to minimize contact with other horses and pay strict attention to wounds, dermatitis and fly sores on all horses. The disease can easily be spread to unaffected horses by flies, and can also be spread by humans via skin, clothes or stable equipment.

Insects had long been suspected as vectors in spreading pigeon fever, and the UC-Davis research has shown that horn flies, stable flies and house flies all can carry the bacteria. During an outbreak at a farm, as many as 20 percent of flies – 1 in 5 – could be carrying the bacteria, but flies do not serve as a permanent reservoir of the disease.

Horn flies, which prefer to breed in cattle manure, tend to bite on the midline of the horse’s belly, while stable flies bite around the head, neck, chest and legs. House flies are attracted to secretions and mucous membranes, which can spread the disease through contact with open wounds or sores, and this, Dr. Spier believes, is why the abscesses can show up in so many different parts of the horse’s body.

“We believe the (ultimate) reservoir is the soil,” Dr. Spier says. “We are able to grow the organism in soil and can see that it survives a long time and it survives very well when manure is added to the soil. The bacteria survive in a very wide pH range. That’s probably what’s allowed it to spread to different geographic locations.”

Future Findings
The ongoing foundation-funded research project at Occidental College is working to create a vaccine that can help prevent the disease. This year, researchers are conducting a study designed to better understand the role specific lymphocytes play in the immune response to the bacteria that causes pigeon fever. They have also developed an ELISA test for the bacteria, which tests for the presence of specific antibodies.

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“One of the questions we wanted to look at is, ‘Does the type of immune response determine the outcome of the disease – (whether the horse gets) external abscesses, internal infections or ulcerative lymphangitis?’” Pollock says. “There are different types of lymphocytes (or white blood cells) called T-helper cells, and is the balance between the different types of T-helper cells what determines whether a horse gets the external abscesses or internal infections?”

The research might also answer why some horses fall to the disease while others in the same environment do not. This year, the research is focusing on the role insects play in transmission and how a horse’s immune system responds to the bacteria.

“We believe it’s really the horse’s immune system that determines (whether he gets) the straight-forward infection or whether it’s going to linger,” Dr. Spier says. “It can linger for months, or very rarely, up to a year. Some horses just continue to develop more abscesses and need more aggressive treatment with antibiotics to kill the bacteria.”

It is possible there will be a vaccine for pigeon fever available in the next 5-10 years.

“Most of my focus has been on the need for the protective vaccine,” Dr. Spier says. “Because it is an organism that survives in soil, it’s (the bacteria) not going to go away. It’s not like we can just isolate a horse and control it by quarantine. And more geographic regions are becoming endemic. We need to find better ways to manage it.”