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Rhodococcal Pneumonia in Foals

By: Dr. Philip Johnson

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Preventing Rhodococcus equi pneumonia on farms where it has previously caused disease and fatalities in foals represents one of the important tasks with which veterinarians are faced. Education of horse owners with respect to this important disease is a crucial first step; it is important to emphasize that early recognition of the problem in infected foals will lead to a higher probability for successful treatment.

On specific farms where this disease has already been recognized as a problem, twice-weekly physical examinations of foals at risk is a useful strategy for the early identification of disease and has been shown to be an effective method to increase the success of treatment. The use of regular ultrasonographic examinations of the foals' lungs for abscess development is useful for identification of infected foals prior to development of any clinical signs.

Although blood tests for antibodies against Rhodococcus equi are available, interpretation of these tests does not differentiate infected foals from exposed foals and should not be relied on as the sole method of identifying diseased foals; positive blood test should certainly increase awareness of risk of disease.

Strategies that are directed at reducing the potential level of exposure of foals to Rhodococcus equi include relocating to premises on which the ground has not become severely contaminated with the organism. This option may not be practical, but it is important to recognize that foals raised on contaminated, dusty lots that are devoid of grass are at particular risk compared with foals raised on grassy paddocks. Advice should be sought from county agronomy specialists to encourage grass growth, establishment and maintenance on at-risk premises. Feces should be regarded as a source of Rhodococcus equi contamination in the environment, especially from infected foals, and should be quickly removed and composted.

Although a specific vaccine for Rhodococcus equi has not yet been approved, several immunological strategies are recommended by veterinarians in order to reduce the risk of infection.

The prevention of Rhodococcus equi pneumonia has been significantly impacted in recent years by the availability of a hyper-immune plasma product containing concentrated antibodies against the organism. This plasma product is not useful after infection has occurred, but there is much to support its use in the prevention of pneumonia when administered to at-risk foals during the first two weeks of life.

Your veterinarian must give this plasma product into the vein at one to two weeks of life. It may not be necessary to use the plasma in those foals born early in the season (prior to April) when the risk of infection is lower because of the weather; however, those early foals that are maintained in the same at-risk environment into the warmer weather should probably be protected by the plasma treatment at the beginning of the warmer weather period.

Although not currently available in the United States, a vaccine has been shown to be useful in Argentina; the mare is vaccinated prior to foaling with a view to stimulating antibody production against Rhodococcus equi for natural incorporation into the colostrum. The foal's blood is subsequently tested to ensure that the vaccine was effective at producing antibodies in the colostrum. Foals that failed to receive antibodies through this protocol then receive the hyper-immune plasma product described above.
There is some evidence that rhodococcal infection may be predisposed by prior damage caused by certain viruses. To that end, it is also recommended that mares should be thoroughly immunized against routine respiratory pathogens (influenza, herpes viruses, etc.) on a regular basis to ensure that the foal receives colostral antibodies against those diseases as well. It is important to emphasize that the adequacy of all foals' antibody complements received through colostrum should be routinely checked by a standard antibody blood test at 18 to 24 hours of age. Any foals receiving insufficient antibodies should be identified and treated with a plasma transfusion.

Prognosis

Foals treated early in the course of disease are more likely to respond favorably compared with foals in which the appropriate treatment is delayed. The prognosis also depends on the extent to which the lung fields have been "consumed." Foals presented in a state of extremely severe respiratory distress in which the radiographic appearance of the lung fields indicates that most of the lung has been infected often fail to respond to treatment. Those foals that do recover from the infection can grow up to be healthy, normal adults. In some studies, the fatality rate for Rhodococcus equi pneumonia exceeds 40 percent.

With respect to race horses, surviving Rhodococcus equi pneumonia has been shown to reduce the animal's chance of becoming a racehorse. Racehorses that were affected with Rhodococcus equi pneumonia as foals are less likely to perform successfully compared with non-infected peers.

Approximately two-thirds of foals infected with Rhodococcus equi pneumonia may develop other (extrapulmonary) complications that may affect both the method and the outcome of treatment. During treatment for Rhodococcus equi pneumonia, veterinarians are vigilant for these diverse and sometimes challenging complications. Approximately 50 percent of infected foals develop some degree of intestinal disease (enterocolitis) that may cause diarrhea. Other possible complications include: joint swelling, joint infection and lameness, bone infection, lymph node abscess development, inflammation of the eyes, hyperthermia, peritonitis, anemia, abnormal hair coat, and infection of the guttural pouches.

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