Last Updated on February 23, 2022 by Allison Price
American horsekeeping has a new reality: Pigeon fever can occur anywhere, at any time of the year.
Sometimes called dryland disease or Colorado strangles pigeon fever is caused by Corynebacterium pseudotuberculosis bacteria entering a horse’s system, most likely through insect bites or skin breaks. Infected horses often develop abscesses in their chests or other areas. This infection was first discovered in California’s San Mateo County (San Francisco Bay Area), in 1915. Pigeon fever was most common in California and the Southwest for much of the 20th Century.
Pigeon fever first appeared in areas outside its normal range around two decades ago. There were cases of pigeon fever in Colorado, Wyoming, Utah, and Utah in 2002 and 2003. These states are where it was previously rare. In Idaho and Oregon, there were outbreaks in 2005 and 2007. More than 60 cases were recorded in the Florida’s northwestern panhandle in 2012.
Although pigeon fever was once considered a California-specific disease by horsemen, it’s now found in all 50 states, as well as Hawaii. It is now recognized in Mexico, western Canada, and all across the United States,” Sharon J. Spier of the University of California Davis, DVM, PhD DACVIM says. She also states that it is not a seasonal condition. It used to be thought that it only appeared in the fall and summer, but it is now seen all year.
Although most horses recover completely, it can take several weeks for the disease’s course to fully develop. This can cause serious disruption in riding and training schedules. Spier says that although the disease isn’t as fatal as West Nile virus and tetanus but can be a bother, it can cause serious illness and even death. Internal infections can pose a serious threat to your health.
It’s important to be familiar with pigeon flu. This includes how it happens, what symptoms it causes, and when it is most likely to occur. Here are six facts that you might not have known, but may be important to know.
1. PIGEON FEVER USES THREE FORMS
Horses infected by C. pseudotuberculosis will develop a toxin from the bacteria that causes them to form thick-walled abscesses around the pathogens. Incubation is the time between the first infection and the formation or abscess. It can take anywhere from three to four weeks.
Three types of the infection can be seen:
* An external abscess is a condition that occurs under the skin or in the muscles close to the surface of the body. These can occur anywhere, but they are more common in the chest (pectoral muscle) and midline of the abdomen. This is the most common type of pigeon flu. Spier and his colleagues conducted a large study of 538 horses and found that 91 percent had an external abscess. Nearly 60% of these abscesses were located in the chest. The abscess is cleared and most horses heal quickly.
* An internal abscess can occur when bacteria is introduced into the body. This could infect the liver or kidneys, lung, or any other internal organs. According to Spier’s study, this type of disease is not common. It accounts for just 8% of all cases. Internal abscesses can be more difficult to treat and are responsible for 40% of all deaths. Horses with internal abscesses are more difficult to diagnose. The signs that do appear include lethargy and weight loss, fever, colic and coughing. Ultrasound may be required to find abscesses and assess their size.
Only 1% of cases involve ulcerative lymphangitis. This causes swelling and ulcerations in the lower legs. Abscesses can form in lymph channels and cause marked swelling on the legs. Ulcerative lymphangitis can cause loss of appetite, lameness, and lethargy.
2. In most cases of PIGEON FEVER, a FEVER may not be detected.
About 25% of horses suffering from pigeon fever will show symptoms. The location of the infection will determine the first symptoms of pigeon flu. A horse will usually develop one or more painful, hardening swellings. These are often located along the middle of the abdomen or on the chest. However, the swellings may appear anywhere on the horse’s body, particularly if bacteria has entered through cuts or abrasions. The swellings on the chest, legs, and hindquarters can often be confused with those from kicks from pasturemates.
3. DIAGNOSIS CAN HEAL FROM LABORATORY TESTS VERY Often
Even a classic case of pigeon disease, where the horse’s chest bulges like a pigeon’s, can require a bacterial culture to confirm diagnosis. Nathan Slovis, DVM (DACVIM), Hagyard Equine Medical Institute, Lexington, Kentucky, says, “I always tell my clients that an abscess will only be an abscess until it is cultured.” It’s impossible to know the exact cause of it. You have to culture it. Strangles could be the cause, but that could be more serious or a puncture wound infection.
For internal infection, blood tests like complete blood counts (CBCs), and serum biochemical profiles can show signs of chronic inflammation or infection. If there are no external abscesses, elevated serum antibodies to C. pseudotuberculosis may be useful in diagnosing internal infection.
Slovis states that internal abscesses could be caused by many bacteria. It’s difficult to diagnose an inner abscess using cultures. Most veterinarians won’t insert a needle in an abscess to collect a sample. Although they may not be 100% specific, blood tests can still prove to be useful. These tests are the best way to determine if the horse has a pigeon-fever internal abscess.
The synergistic hemolysis inhibitor (SHI) antibody test is used to test for C. pseudotuberculosis. This test needs to be interpreted carefully as horses may have been exposed and developed antibodies, but not become infected. Spier says that the SHI test does not provide a complete diagnostic picture for pigeon flu. “The titer alone isn’t enough. It is important to look for other signs such as weight loss and inflammatory changes in bloodwork. Although blood tests can be helpful in diagnosing internal abscesses, they must be combined with other diagnostic and clinical methods like CBC, biochemistry and ultrasound of the chest and abdomen.
4. FLIES PARTICIPATE IN THE TRANSMISSION of PIGEON FEVER
C. pseudotuberculosis is capable of surviving for up to two months in bedding or hay, and more than eighteen months in soil. Research is continuing to explore the mechanism by which the bacteria enters the horse’s system. C. pseudotuberculosis can be transmitted from horse to horse and through direct contact with soil contaminated with the bacteria. Based on research by Spier at UC-Davis, it is believed that insects bites are the primary source of disease transmission. “We visited a variety of farms that had been affected by outbreaks in different years and then revisited the same farms years later when they did not have any cases. Spier says that they wanted to know more about the transmission of these bacteria and how they persist in the environment. I worked with entomologists at UC-Davis and trapped flies using drift traps. These traps capture all insects that pass through. We also caught flies from horses with abscesses. We could find bacteria in three species of flies when there was an outbreak on farms with infected horses. These were the stable fly, horn fly, and housefly.
Spier’s team discovered that as high as 20% of houseflies in farms with horses were carriers of the disease. She says, “We returned to those farms where there had been no cases for years and found that the fly populations were positive for the bacteria.” This shows us that the soil is the reservoir, not the fly. Although insects can transmit the disease, they are not the reservoir. They do not spread bacteria from horse to horse.
5. EVEN IN ENDEMIC AREAS THE DISEASE RUNS INSIDE CYCLES
Although pigeon fever is widespread in California and the Southwest regions, it has a tendency to increase or decrease in severity over time.
Hector Gonzalez, DVM of Bakersfield Veterinary Hospital, California, says that “pigeon fever cases were scattered extensively in our region this year.” “We saw seven horses with pigeon fever in 2015, including one with ulcerative Lympangitis. 2014 saw fewer cases. It was a perfect year for it with the drought in the summer and just the right conditions. It seemed that more horses were being affected at the beginning of 2016, more similar to what we saw eighteen years ago.
The disease is more common in Texas. The number of cases rose by a thousandfold between 2005-2011, but they have now slowed down. The year before the big pigeon flu outbreak, there were cases that looked like strangles. However, when cultured, they turned out to be C. pseudotuberculosis. That was something we thought was very interesting because we don’t usually see pigeon disease here,” said Ben Buchanan DVM, DACVIM and DACVECC of Brazos Valley Equiline Hospital. There are multiple locations throughout Texas. “I haven’t seen a case at this location in about two years. It’s back to the way it was before the drought. Perhaps we’ll see more cases in Texas when we have another drought, but the new infections seem to have stopped since it began raining.
6. WARMING TRENDS ARE RISKING PIGEON FEVER RISK INCREASE
Climate change has brought more extreme and hotter conditions to many areas, possibly increasing the risk of pigeon flu. Spier says, “Based on the evidence of epidemics in Texas and Colorado, it seems that there is an increase in drought conditions.” We don’t know the exact environment that leads to epidemics. We know that bacteria thrives in soils that are dry and moist, which could allow them to spread faster.
She adds that horses may seek shade when it is hot and dry. These organisms thrive in soil that is mixed with manure. Dry conditions can cause the soil beneathfoot to become dusty and blow around. Horses are defecating by kicking flies and churning soil and manure to dust. This allows bacteria to flourish and spread more easily.
As climate change becomes more prevalent, dry and hot weather will increase in the United States. Your horses’ safety will depend on your vigilance as pigeon flu outbreaks are common. Gonzalez says that horse owners need to be vigilant about their horses’ welfare. “If your horse develops abscesses in their pectoral or inguinal0 regions or ventral edema in the lower abdomen, you should consult your veterinarian. They can take a culture.