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Rhodococcus equi pneumonia: A Deadly Cough

by Steve Giguire, DVM, Dipl. ACVIM, and John F. Prescott, VetMB, PhD

Rhodococcus equi pneumonia in the foal can be deadly, but good management could save the day

When foals get sick, horse owners can sometimes face many sleepless nights, as well as weeks or months of intensive management, to get these babies through the rough spots. One of the most common problems in the ill foal is pneumonia, caused by a bacteria called Rhodococcus equi, which attacks foals between one and six months of age. Most foals will tend to show clinical signs, including coughing, abnormal breathing patterns and, less commonly, diarrhea, before the age of four months. Most farms are susceptible to the disease, but some farms ar considered enzootic or particularly prone to continually have the problem. On these enzootic farms, the disease can lead to significant financial loss because of the cost of therapy and the occasional death of foals.

While adult horses can carry R. equi in small numbers in their intestine and not suffer any ill effects, in the foal, the organism can actually multiply in the intestine for up to three months of age so that the presence of foals inevitably contributes to the development of the infection on the farm. Foals with R. equi pneumonia swallow the mucous that accompanies the illness, the bacteria ends up back in their intestines. Therefore, their manure is likely a major source of contamination. Under suitable conditions of high summer temperatures, R. equi can multiply in the environment by 10,000-fold in only two weeks. A single gram of soil contaminated with foal manure may therefore, under favorable conditions, contain millions of virulent R. equi. The inhalation of dust particles laden with virulent R. equi. is the major route of pneumonic infection in foals.

The successful control of R. equi infections on the enzootic farm depends on several factors: decreasing the size of the infective challenge, earlier recognition of the disease and passive immunization.

Decreasing the size of infective challenge
There is a progressive buildup of infection on horse farms whose business it is to handle broodmares and foals. Enzootic farms are therefore likely to be those used for breeding horses for many years, those with heavy concentrations of mares and foals, and those located where summer temperatures are high, where soil type is sandy and where dust is extensive. Enzootically affected farms are usually easily recognized as those where the loafing paddocks have been turned into grassless dusty sand pits. Keeping large numbers of foals on bare, dusty, manure-containing paddocks will result in heavy challenge, with clinical disease maintaining virulent bacteria.

It is important to house foals in well ventilated, dust-free areas and to avoid dirt paddocks and crowding. Pneumonic foals must be isolated because they are the major source of contamination of the environment with virulent organisms and their manure must be composted. Pastures must be rotated to decrease dust formation and consequent inhalation of R. equi. Any sandy or dirt areas should ideally be planted with grass and made off limits to foals, or alternatively, irrigation may be useful in decreasing dust formation and encouraging grass. Manure should be regularly removed from paddocks and composted. Because mares and foals tend to congregate around water sources and under shade in hot summers, a reduction in the size of mare and foal bands may reduce destruction of grass and exposure to barren soil.

The environmental growth of R. equi increases progressively with temperatures to 30oC (86oF) after which it plateaus. Immunization of mares and foals against common respiratory viral agents (influenza, equine herpesvirus types 1 and 4), an effective parasite control program, and adequate colostrum intake with detection and treatment of cases of failure of passive transfer may help by preventing potential predisposing illnesses.

Earlier recognition of the disease
R. equi pneumonia is often not recognized until it is well-advanced and, therefore, difficult to treat. Even severely affected foals may appear to a casual observer to suck and behave normally. Early recognition of the disease with isolation and appropriate treatment of infected foals will reduce losses, limit the spread of virulent organisms in the environment and decrease the cost of therapy.

Careful weekly physical examination and auscultation of the lungs by a veterinarian is successful in promoting early diagnosis of pneumonia, therefore reducing mortality. Another approach said to be helpful in early diagnosis is testing foals from birth to five months of age at two-week intervals with the agar-gel immunodiffision (AGID) assay. The AGID test mainly detects precipitating antibody against enzypes produced by R. equi. However, a few foals may develop R. equi pneumonia without developing enough antibody to be detected by AGID and many foals will produce antibody from exposure to R. equi in their environment without subsequently developing the disease. For this reason, a combination of careful daily observations and temperature recording, as well as AGID testing or measurement of fibrinogen concentrations at two week intervals, although labor intensive, may prove to be the best approach to early identification of pneumonic foals on enzootic farms.

On such farms, routine ultrasonographic examination of the lungs by a veterinarian is another sensitive method that may allow early detection of lung lesions before development of clinical signs. Ultrasound is also helpful to evaluate response to therapy. A combination of the diagnostic tests presented here will allow early identification of foals with pneumonia. However, the only definitive way to confirm R. equi, as opposed to other bacterial pathogens, really is the cause of the infection, is bacteriologic culture of tracheal or bronchial fluid. The distinction between pneumonia caused by R. equi and that caused by other bacterial pathogens in important because the treatment approaches are different.

Passive immunization
Passive immunization with the intravenous administration of hyperimmune (HI) plasma has become a popular and generally effective way to reduce the incidence of and deaths resulting from R. equi on enzootic farms. Although time consuming to administer and expensive to produce or buy, this is the single most cost-effective measure to prevent the disease on farms with a high incidence of R. equi pneumonia.

Active immunization
It would be considerably more convenient to control R. equi pneumonia on enzootically infected farms by the active immunization of mares and their foals with a protective antigen rather than by the intravenous administration of a large volume of plasma. Such experimental attempts have been generally unrewarding and vaccines against R. equi infections are not currently available.

Clearly, R. equi can be a devastating and demoralizing problem on some horse farms. Control is possible, but expensive. It is also crucial that pneumonic foals are not sent to spread infection from enzootic farms to those farms with little contamination. Improved and less expensive control approaches in the long term will depend on a better understanding of why this disease attacks the young foal.

Dr. Steve Giguere is a graduate research assistant in the Department of Pathobiology at the Ontario Veterinary College, University of Guelph, Ontario, Canada. He presented his findings during the 1997 AAEP Convention in Phoenix, Arizona.

From the American Association of Equine Practitioners

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