A Rough Start

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Several pathogens can cause pneumonia in foals, but one of the most devastating is Rhodococcus equi, a type of bacteria often present in the environment. Lais Costa, DMV, MS, PhD, Diplomate ACVIM (large animal) and Diplomate ABVP (Equine), assistant professor in large animal medicine at Tufts University, says that even though most of these foals don’t develop signs of pneumonia until they are three or four months old, they actually encounter the bacteria soon after birth, often within the first month.

R. equi can become endemic on a farm. If you send a mare to be bred on a farm that has a problem, there’s a high likelihood she’ll pick up bacteria from the environment. Adult horses rarely become ill, but can shed bacteria into the environment, resulting in exposure and possible infection of foals.

“The foal becomes exposed early on by ingesting bacteria, and for some reason the foal’s gut is a good place for R. equi to multiply,” says Costa. “Multiplication of R. equi is 10-fold greater in the foal’s gut compared to the mare’s gut.”

Worse, foals are more susceptible to infection. “Researchers are trying to figure out what’s involved, whether it’s the inability of foals to produce certain mediators that make the immune system effective in killing these bacteria. Even a low dose of bacteria seems to overwhelm the foal’s ability to clear it,” she says.

Mode of Infection

With R. equi, the bacteria can be ingested or inhaled. “They can be inhaled while the foal is eating or rummaging on the ground or in the stall eating the dam’s manure. He might inhale bacteria from soil, via dust from dry manure.

“When inhaled, bacteria can go directly to the lungs. Most cases of infection in foals are in the lungs, but if foals are breathing bacteria they are also eating them,” Costa says.

But, not all R. equi bacteria are virulent. “Only the ones that contain virulence-associated plasmid (VAP) can cause disease. This plasmid facilitates survival of bacteria within the alveolar microphages—cells that defend the lungs,” she says. “Merely culturing and finding them without identifying whether they carry the plasmid will not tell you whether the foal is at risk. If your vet cultures a sample from soil or from feces, the bacteria found might not be the type that can cause disease. But if you do a tracheal wash on a foal and find R. equi in the lungs, this is the virulent form.

“When virulent R. equi is ingested and absorbed through the bowel, it results in infection and abscesses in the intestine (often in the mesenteric lymph nodes). The bacteria may also get into the circulation and spread to other organs or the lungs. About 40 percent of foals with lung lesions have evidence they were exposed through the oral route; they have the bacteria in lymph nodes in the intestine and might have abscesses in the abdomen,” she says. Even after you clear up the pneumonia, the foal may still have intestinal abscesses.

“Foals may also have abscesses in bones, such as the vertebrae. In these instances the foal starts to show fever and lameness. These are more obscure cases, and harder to diagnose. The foal shows lameness but it’s not in the leg; it’s pain in the spinal column. The foal can have these bacteria anywhere in the body,” she explains.

Detecting the Infection

Foals often mount an antibody (humoral) response to R. equi bacteria, which can be detected with serology. “But positive serology just shows the foal was exposed to the bacteria and made antibodies. So a foal with antibodies may not actually have the disease,” she says.

“Some foals do mount an effective immune response, which involves a cellular immune response like adult horses do, and can fight off the disease and not get sick. Some foals have the ability to clear the bacteria,” says Costa.

“Like many other diseases, the antibodies just tell you this animal has been exposed to the bacteria and the immune system recognizes it. It doesn’t tell you if the immune response was effective in clearing the bacteria, and doesn’t tell you if the bacteria are still there,” she says.

Fighting the Infection

“There are some antibodies that probably do protect the animal at some point in the infection, and that’s the idea behind giving foals hyperimmune plasma—to give a booster of antibodies that will find the bacteria and prevent them from getting into the cells,” says Costa.

If an antibody test for Rhodococcus is negative, it’s unlikely the animal has the disease, because the immune system has not responded—unless the foal is in very early stages of disease and has not produced antibodies yet. “It takes a few weeks for antibodies to show up,” she says.

“These bacteria are intercellular pathogens, hiding within cells of the immune system, so they can’t be killed. They hide within the cells that are supposed to clean up the lungs. They’re like the Trojan horse, invading without the body being able to kill it. The immune system can sense it’s there, and makes antibodies, but often can’t clear the infection,” she says.

On some farms there are more problems, maybe because of dynamics between the virulence of the bacteria and susceptibility of the foal population. “There are endemic areas—some farms have many foals affected. There are also yearly variations in numbers of cases. Environmental factors make a difference (moisture, pH of the soil, etc.); either virulence of the bacteria changes or amount of bacteria in the environment increases in certain conditions. You may have a really bad year for foal pneumonia and then a couple years that aren’t as bad.” This makes the disease challenging to diagnose.


Effectiveness of hyperimmune plasma to prevent R.equi pneumonia is controversial. “Some studies show it decreased incidence of cases, but since it’s given preventively, you don’t know if it really worked; you don’t know if it was going to be a bad year or not. You might use it and since no foals got sick, you think the plasma worked. But it may have been just a better year,” says Costa.

“If people have a problem on their farm, they may prefer to use a liter of plasma than treat a foal after it gets sick. It’s often cheaper to give two transfusions of plasma than to treat the foals that get sick,” she says.

“Vaccinating the dam ahead of foaling to give the foal antibodies via colostrum has not proven to be successful,” she explains.


Several antimicrobials are fairly effective in treating the disease. “They have to be products that can penetrate cells and abscesses, since many foals develop abscesses. Many antibiotics will kill these bacteria, but since the bacteria hide within the cells, some of these antibiotics can’t get to them. R. equi can also be resistant to some of the drugs,” Costa says.

“You generally treat with more than one, because the bacteria are notorious for being resistant to at least one of the drugs. If we use two or three drugs that are all effective, chances of actually clearing the bacteria are better,” she says.

“Treatment tends to be several weeks,” she adds. This can cost several thousand dollars. Some of the drugs also have side effects including diarrhea, fever, etc.

“This is why using hyper-immune plasma to try to prevent infection is attractive. If you have valuable foals, you could spend $200 to $300 per foal to try to prevent it, and perhaps not be faced with thousands of dollars for treatment. It should be discussed with their veterinarian,” she says.

Signs Of R. Equi Infection

Foals may not show signs until a large portion of the lung is affected. It’s been called “walking foal pneumonia” because many foals don’t seem sick, and seldom show signs until 30 days of age or older. Some die suddenly, with post-mortem examination showing large abscesses in lung tissue or rupture of an abdominal abscess. The first sign may be a poor-doing foal, elevated temperature, or harsh lung sounds. By the time a diagnosis is made, there are already abscesses.

“Some infected foals are poor doers (stunted, rough hair coat), but some look healthy, then suddenly have a hard time breathing and die quickly,” says veterinarian Lais Costa.

“They may also have diarrhea if the bacteria affect the gut,” says Dr. Philip Johnson, professor of equine medicine and surgery, University of Missouri. “They may have signs of colic, and sometimes inflammation inside the eyes, or joint enlargement.”

Just because a foal isn’t coughing doesn’t mean he’s not sick. The attitude of the foal—bright and lively or dull���can tell you more. You should also monitor temperatures.

“Some of the signs are not very clear,” says Costa. “Horsemen should sharpen their skills of observation and notice slight changes. If you feel a foal isn’t quite right, have your vet check it. The person who sees the foal every day is best able to tell if there’s something wrong,” she says.

To detect the disease as early as possible, Dr. Johnson encourages clients to monitor foals with a stethoscope, listening to breathing sounds. “If you listen to the chest, you can usually hear abnormal sounds,” he says. The sooner this disease can be detected and treated, the better, before it becomes life threatening.