July 2000

Foal Pneumonia Caused by Rhodococcus Equi

by Heather Smith Thomas

Foal pneumonia can be caused by several organisms and is most common in foals two to six months old, even though cases can occur anytime between birth and 12 months. Streptococcus bacteria are the most common cause, but in young foals the most serious pneumonia is caused by Rhodococcus equi, formerly called Corynebacterium equi and still sometimes referred to as "Coryne" (kor-rin-ee) pneumonia.

This bacteria lives and thrives in soils contaminated with manure. On farms where horses have been for a long time, there may be several thousand R. equi bacteria per gram of pasture soil. The disease can become a problem if high numbers of bacteria are present in areas where foals are pastured or stabled. Climate conditions are also a factor; if spring rain provides moisture for bacterial growth and then hot weather creates dry and dusty conditions, the foals may be exposed to the disease when breathing the bacteria-laden dust particles.

R. equi can cause severe pneumonia in foals from one to six months old. Sometimes only a few foals in a group are affected, but some farms have a high rate of infection and mortality in foal crops every year, due to a combination of high bacteria levels, deficiency or immaturity of immune systems, and intensive management practices where foals are too closely confined. Pneumonia may be the only sign of infection, or may be combined with diarrhea which can be either acute or chronic. The foals with pneumonia usually show depression, reluctance to nurse, abnormal lung sounds, difficult breathing, and fever, but each individual case can present a different picture. The foal may have increased pulse rate, increased respiration rate (rapid and shallow), and a cough.

But not all present a typical picture. Some continue to nurse and seem fairly bright, maintaining good body weight until just before they die. Others stop nursing, appear depressed, and become progressively more dull and weak. The foal may stand with front feet wide apart in an attempt to breathe easier. Some have a high fever, while others exhibit widely fluctuating temperatures. There is usually a nasal discharge of some kind, but not always. Ultrasound views or x-rays of the chest may reveal a diffuse pneumonia, with obvious lung abscesses.

R. equi can localize in many organs, or in the joints. Foals that survive acute stages of the disease may be affected the rest of their lives with damaged or crippled lungs. These infections resemble tuberculosis, forming lung abscesses that are difficult to treat. The disease has become a major problem on some breeding farms; foals younger than four months seem to be most susceptible. R. equi can sneak in without acute or noticed symptoms at first, escaping detection until a large portion of the lung is affected.

Most infections start soon after birth, but clinical signs may not be obvious until later (R. equi is often called "walking foal pneumonia"), as the foal doesn't seem sick. Foals seldom show signs until 30 days after birth or older. The disease has slow onset, generally forming small abscesses in the lungs or abdominal cavity. The first sign of trouble may be death of a foal, with diagnosis made at post mortem examination; the foal may die from enlargement of abscesses in the lung tissue, or rupture of an abdominal abscess. Alternatively, the first sign of illness may be a dry cough or a poor-doing foal. An infected foal may look healthy, but a closer examination will reveal an elevated temperature and harsh lung sounds. Small abscesses have already formed by the time diagnosis is made.

Infection can enter the navel of a newborn foal and travel through the bloodstream to lungs and other organs, or be picked up by the respiratory tract, or even ingested into the gut. Symptoms may be acute, with fever, loss of appetite, and sometimes arthritic pain in joints, or subacute (especially in older foals), with pneumonia developing slowly, characterized by a progressive cough, "rattling" and difficult breathing in the later stages. In some chronic cases the foal has no fever and continues to nurse, but becomes thin. By contrast, serious cases may die within a few days.

Infections involving more than one organism are not uncommon, and treatment aimed at one invader may not have any effect on the other. It can be hard to determine the cause of trouble from the foal's outward appearance (whether the problem is truly pneumonia, which needs immediate attention, or just an upper respiratory tract infection like a cold), so get veterinary help for diagnosis. A tentative early diagnosis can be made by noting the clinical signs, having a history of the disease on the farm, and doing a white cell count. An aid in diagnosis is the tracheal wash, in which material from the foal's trachea (windpipe) is cultured. Other helpful aids are chest x-rays or ultrasound, which help to show the abcesses.

Because infection with R. equi may be sneaky, and the foal may fight the disease for several weeks before he has a respiratory crisis, researchers in the 1980's came up with a way to recognize the infection by measuring serum antibodies. Detection of these antibodies indicates that a foal is (or was) infected with R. equi. With funds donated by the Florida Thoroughbred Breeders' Association, the test was first performed in 1987 on hundreds of Thoroughbred foals in Marion County, Florida. Results showed that R. equi infections are very common and often subclinical; infected foals do not always show obvious disease, and some overcome the infection on their own. But some develop serious illness and die.

The disease is hard to detect, but early diagnosis is a must. This type of pneumonia should always be suspected when a foal is coughing or doing poorly. The disease must be differentiated from strep pneumonia (which usually produces more nasal discharge), and from more rare pneumonias caused by viruses or other bacteria.

Foals are almost always negative for R. equi antibodies, when they are initially tested during the first three weeks of life, and when mares have no antibodies to pass to their foals in colostrum. Since exposure usually occurs early, foals can be tested for antibodies, starting at about 30 days of age and then checked periodically. Any antibody-positive foals should be monitored, by taking daily rectal temperatures. Then if fever develops and blood tests reveal high white cell counts, antibiotic treatment can be started. Routine blood counts monthly, or every other month, may be helpful on some farms.

Early detection and treatment can halt the infection before it becomes serious. If you suspect foal pneumonia, consult your vet as soon as possible. Don't wait to see if it gets better or worse; death rate from this disease can be as high as 20% of a foal crop. Pneumonia can quickly become a life or death matter, so prompt and accurate diagnosis is crucial. Your vet can advise and assist you with treatment, and with supportive treatment such as additional fluids (by stomach tube or IV, if necessary). Vigorous treatment must be started early and it is often needed for several weeks or even months. A number of antibiotics may be necessary.

If treated early and diligently, the foal has a good chance of satisfactory recovery. But if neglected, or treated too late, or only half treated (or treatment halted too soon), the infection may get a stranglehold on the foal. Lung tissue is destroyed, the foal becomes weaker, and chances of recovery are poor. The foals that do recover may become respiratory cripples.

Veterinarians have become increasingly concerned about the effects of foal pneumonia on the animal's later ability as an athlete. The lungs may never completely regain normal capacity and elasticity after severe pneumonia, hindering the horse's future exertion and endurance abilities. Many vets feel that foalhood pneumonia can leave a horse with scar tissue and other problems in the lungs, that may create tissue stresses when the horse exerts in the future.

There is no vaccine at this time for R. equi pneumonia. The best preventative measures are good sanitation, close observation of all foals, and use of hyper-immune plasma soon after birth. Foal pneumonia is primarily a management disease rather than a contagious disease. In horses that have room to roam, with less crowding and stress, and uncontaminated surroundings, pneumonia is rare. But the domesticated horse is usually confined in small areas with large numbers of horses. This can produce stress, contamination, and a concentration of disease organisms. To reduce stress and contamination, mares should be allowed to foal outdoors whenever possible, and mares and foals should be isolated from other horses as much as possible. Because some pneumonia-causing organisms like R. equi flourish in manure-rich soils, removal of manure from the foal's environment may also help prevent the build-up of bacteria. Control of dust by sprinkling, pasture rotation or grass planting may also help. Thus, good management of the foal's environment can usually prevent the conditions that lead to respiratory disease.

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