Age-Related Susceptibility to R. equi Studied
The team concluded that foals are susceptible to developing R. equi infection for at least the first two weeks of life before that susceptibility starts waning. Their susceptibility to disease decreases by the time they reach a month old.
Photo: The Horse Staff
Rhodococcus equi infections are something no breeding farm manager wants to deal with, but they're something that demand attention. Researchers know this pathogen can cause life-threatening infections in foals, but they are still trying to learn at what age foals are no longer at risk and develop a better way to study R. equi in live horses.
At the 2014 University of Kentucky (UK) Equine Showcase, Macarena Sanz, DVM, MS, Dipl. ACVIM, a PhD candidate at UK's Maxwell H. Gluck Equine Research Center, presented results from a two-part study evaluating foals susceptibility to R. equi.
R. equi is found worldwide and is spread through soil. R. equi infection is rare in adult horses, typically only being seen in those with underlying immune system issues.
Foals, on the other hand, are very susceptible to R. equi infections and mainly develop disease between two and five months of age, Sanz said. Clinical signs of disease are similar to those of pneumonia, including fever, lethargy, increased respiratory effort and rate, and foals usually appear distressed. The bacteria cause characteristic abscesses (or lesions) in the lungs that can become severe enough to cause death.
Veterinarians diagnose R. equi infections using a combination of:
- Clinical signs in a foal of the appropriate age;
- Thoracic (chest) ultrasound or radiographs (X rays);
- Blood work; and
- Lung fluid culture.
Traditionally, veterinarians have treated R. equi with antibiotics several times per day for four to eight weeks; however, Sanz said in a recent study from Germany researchers found that many affected foals are able to recover without treatment. This is important, Sanz said, as antibiotic resistance has been reported for drugs used to treat R. equi.
Although some research has been done (including some by Sanz's Gluck Center research advisor David Horohov, PhD), R. equi is a difficult problem to study, Sanz said, because results from studies in mice aren't always applicable to foals.
"We need a foal model to better study this disease," she said.
To that end, Sanz and colleagues set out to a) develop an effective model of disease using live foals, and b) investigate foals' age-related susceptibility to R. equi.
For the first part of the study, the team employed newborn foals residing in a pasture with their dams. They separated the foals into groups and challenged each group with 1,000,000; 100,000; 10,000; 1,000; or 100 R. equi bacteria. The researchers then monitored the foals daily for any abnormalities. In addition, the performed a physical exam twice a week, along with a weekly thoracic ultrasound and weekly blood work.
At the end of the six-week study, the team determined that administering 1,000 R. equi bacteria to foals in the first week of life best mimics the natural disease progression veterinarians see in the field, Sanz said. Foals that received higher doses developed more severe clinical signs and lung scores than naturally infected foals generally do and sooner than foals do in the field. On the other hand, foals receiving lower bacteria doses did not develop clinical signs of disease.
"Now we have a model of disease using foals to better understand this serious problem," Sanz said.
In the next part of the study and using their model from the first part, Sanz and colleagues challenged foals of varying age groups—2 to 7 days old, 10-14 days old, and 18-21 days old—to determine which group is at highest risk of infection. They used the same techniques to monitor the foals over the six-week study as in the first part.
Sanz and colleagues determined that:
- About half of the youngest foals challenged developed lung lesions and clinical signs of pneumonia; however, the remaining half did not, similar to that observed on the field.
- None of the oldest foals challenged developed clinical signs, and none had more than 5% of their lungs impacted; and
- Half of the foals challenged at 10 to 14 days old developed clinical signs later than foals challenged at 1 week of age, and their lung scores were less severe.
Thus, the team concluded that foals are susceptible to developing R. equi infection for at least the first two weeks of life before that susceptibility starts waning. Their susceptibility to disease decreased greatly by the time they reach a month old in this experimental challenge.
Despite these advances, researchers and veterinarians still have much to learn about R. equi, Sanz said. In the future, she said, researchers should seek to:
- Evaluate the use of the recently developed model to better understand the foal’s immune response to R. equi;
- Determine why some foals develop disease while others clear R. equi from their systems without developing illness; and
- As many of the foals in this study cleared the disease without treatment, similar to that observed in the studies from Germany, a more critical evaluation of preventive measures, such as hyperimmune plasma or vaccination is needed.
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About the Author
Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in three-day eventing with her OTTB, Dorado.
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