AAEP 2002: Prevention and Control of Pneumonia Caused by Rhodococcus equi

Pneumonia caused by the bacteria Rhodococcus equi is an often-fatal disease that can be difficult to eradicate from affected farms. “The impact of this disease can be large because prevalence and case-fatality rates are often high; treatment is prolonged, expensive, associated with adverse effects, and not uniformly successful; the disease may diminish future performance; and farms reputed to have the disease can suffer loss of clients,” said Noah Cohen, VMD, PhD, associate professor at Texas A&M University. Cohen presented “How to Prevent and Control Pneumonia Caused by Rhodococcus equi at Affected Farms” at the 2002 American Association of Equine Practitioners’ convention.

“For most infectious diseases, control and prevention has a greater impact on the burden of disease than does treatment,” he said. When developing a program to reduce or prevent R. equi infections on an endemic farm or a farm with sporadic problems, resources must be considered. How much money does the farm have to spend on a prevention and control program? Are they able to change many management factors? Cohen reminded attendees that programs for control and prevention should not be expected to be 100% effective.

The first incidence of disease could be the start of a recurring problem. Infected foals should be regarded as not only an individual in need of care, but as a source of virulent organisms and a signal that the farm has a problem. Three areas must be addressed: treat infected foals, perform screening tests on all other foals, and prevent any other foals from getting sick.

Cohen did not discuss treatment in-depth since the focus of his talk was on control and prevention. However, he did mention that erythromycin and rifampin are common drugs that are used. However, these drugs can come with side effects such as foal diarrhea and hyperthermia, and mares might experience severe acute colitis. The drug azithromycin is being studied to determine its effectiveness and incidence of side effects.

One thing that farm managers must decide is whether to isolate infected foals. The answer to this question remains unknown. One thought is that once one foal gets sick, the rest of the foals have probably been exposed, so isolation might not be necessary. However, Cohen points out that foals amplify the organism in their gastrointestinal tracts and excrete it from their respiratory tract, thus increasing the risk of exposure to other foals and their environments, where the organism can persist. Therefore, it might be advisable to isolate infected foals. He said that it is a good idea to minimize interaction among resident horses with those coming in and going off of the farm.

Screening for Early Detection
There are a variety of screening measures that can help early detection of infection and disease. Which screening measures are done and when treatment might begin versus additional screening tests is up to the farm’s budget and other resources, including veterinary resources. Cohen believes that the sooner treatment is begun, the better the prognosis. It is important to remember that screening tests are not a definitive diagnosis of the disease, but they can help determine if it is likely that treatment should be started.

The easiest screening method involves visually inspecting all foals. Many times clinical signs might not be present; however, if the foal is having a hard time breathing or is lethargic, this might be an indicator of a problem. Many times the first signs of disease do not relate to breathing. Therefore, twice daily monitoring of rectal temperature and watching for other signs, such as non-painful joint swelling are also a good methods of screening at farms with R. equi problems.

For endemic farms, a twice-weekly physical exam that includes listening to the chest might allow for early detection. While this is not always sensitive to identify R. equi pneumonia, Cohen said it does no harm. In addition, measuring complete blood counts (CBCs) can be useful. If a foal has an elevated white blood cell count or increased concentration of fibrinogen, then treatment might be indicated.

Radiography of foals up to one to three months of age can be performed in the field (i.e., at farms or clinics) by veterinarians and can help reveal pulmonary (lung) lesions whose appearance is strongly suggestive R. equi pneumonia. However, the need for special equipment, adequate personnel, exposure to radiation, cost, and the time needed for evaluating the findings can be a deterrent.

Ultrasonography can be more sensitive than radiography, but disadvantages include the potential to miss deeper pulmonary lesions surrounded by normal aerated lung, the need for expertise in diagnostic ultrasonography, the need for special equipment and adequate personnel, costs, and the time to perform the procedure and evaluate results.

Cohen does not recommend the use of serologic testing for screening. For more information on why these tests are not deemed effective, see article #4138. A definitive diagnosis can be achieved with a microbiologic culture or a polymerase chain reaction (PCR) test of fluid obtained by transtracheal wash. Sometimes if not often, an accurate diagnosis early in the course of the disease can remain elusive, said Cohen, and there is the possibility of a false negative result.
Preventing Disease
Cohen mentioned that the only proven method for preventing the disease is transfusion of hyperimmune plasma, but even this is not 100% effective. Plasma is costly, labor-intensive, and can result in transfusion reactions. “The amount of plasma that should be administered and the time(s) of administration for optimal protection are unknown,” he said. “We recommend administration of one liter of hyperimmune plasma to a foal during the first few days of life and again during the third week of life.”

He recommended that owners (with help from their veterinarians) conduct a cost-benefit analysis and estimate the average value of their foals, the expected prevalence of the disease, the case-fatality rate, and the average cost of treating R. equi pneumonia, and compare this to the cost of the plasma. Endemic farms will generally find it well worth the price. Cohen said that vaccination of mares or other oral immunoglobulin products are not effective.

He recommended the following management techniques:
• Remove manure frequently from stalls and paddocks.
• While replacement of top soil down to 12 inches in stalls and paddocks might not be practical, this method (employed with others) worked for one Texas farm.
• Do not spread manure from infected horses on pastures. Composting the manure or removing it from the farm are the best alternatives.
• Avoid dirt-floored foaling stalls since the bacteria reside in the soil.
• Reduce the density of horses on the farm if possible (this measure is particularly important for some farms).
• Keep resident and transient horses separate.
• Improving ventilation might reduce the incidence of exposure through inhalation.
• Watering pens and small paddocks can reduce dust, which has been anecdotally linked with R. equi pneumonia for the past 40 years.
• Rotate paddocks, avoid overgrazing, and reseed pastures and paddocks to reduce dust.

With proper prevention and control strategies in place, hopefully the incidence of R. equi can be reduced and the lives of foals saved.

About the Author

Sarah Evers Conrad

Sarah Evers Conrad has a bachelor’s of arts in journalism and equine science from Western Kentucky University. As a lifelong horse lover and equestrian, Conrad started her career at The Horse: Your Guide to Equine Health Care magazine. She has also worked for the United States Equestrian Federation as the managing editor of Equestrian magazine and director of e-communications and served as content manager/travel writer for a Caribbean travel agency. When she isn’t freelancing, Conrad spends her free time enjoying her family, reading, practicing photography, traveling, crocheting, and being around animals in her Lexington, Kentucky, home.

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