Researchers studied 200 young foals on a farm where a specific bacteria commonly causes pneumonia (lung infection). They collected blood samples from birth through 20 weeks of age to measure immune system markers and hormones that might predict which foals would get sick. While they found some differences in immune markers between healthy foals and sick foals, the blood tests weren’t reliable enough to predict which foals would develop pneumonia. The study suggests that one immune marker called interferon-gamma might be worth studying more, but overall, simple blood tests alone can’t yet predict lung disease in young foals.

The Quick Take

  • What they studied: Whether measuring immune system chemicals and stress hormones in foal blood could predict which foals would develop pneumonia caused by a bacteria called Rhodococcus equi
  • Who participated: 200 healthy newborn foals on a farm where this type of pneumonia is common, with 30 foals selected for detailed blood analysis (10 that stayed healthy, 10 with mild self-healing pneumonia, and 10 with serious pneumonia needing antibiotics)
  • Key finding: One immune marker called interferon-gamma showed differences between healthy and sick foals at certain ages, but overall, the blood tests measured were not good predictors of which foals would get pneumonia or how severe it would be
  • What it means for you: If you raise foals, blood tests measuring these specific immune markers cannot yet reliably tell you which foals will develop pneumonia. Veterinarians still need to watch foals carefully for signs of illness rather than relying on these blood tests alone. More research is needed before these tests could be useful in practice.

The Research Details

This was a prospective cohort study, which means researchers followed a group of foals over time from birth to 20 weeks of age and tracked what happened to them. All 200 foals started out healthy, and researchers collected blood samples at seven different time points (at birth, then at 1, 2, 4, 8, 12, and 20 weeks). They measured five different immune system chemicals (called cytokines), a stress hormone called cortisol, and vitamin D levels in the blood. Throughout the study, veterinarians checked on the foals weekly to see if they developed pneumonia.

At the end of the study at 20 weeks, the researchers divided the foals into three groups based on their health: those that never got sick, those that developed mild pneumonia that went away on its own, and those that got serious pneumonia requiring antibiotic treatment. They then compared the blood test results from the 30 foals selected for detailed analysis (10 from each health group) to see if the immune markers and hormones were different between groups.

This research approach is important because it follows foals from birth through a critical period when pneumonia commonly develops. By measuring immune markers before foals get sick, researchers can potentially identify which foals are at risk. Understanding how the immune system changes with age and how it differs in sick versus healthy foals could eventually lead to better ways to predict and prevent disease.

The study has some strengths: it followed a large number of foals (200) over time, used validated laboratory tests to measure immune markers, and tracked foals’ health status carefully. However, there are limitations: only 30 of the 200 foals were selected for detailed blood analysis, the researchers didn’t identify the exact bacteria causing pneumonia in all cases, and they only collected blood samples at specific times, which might have missed important changes between sampling dates.

What the Results Show

The main finding was that periodic blood tests measuring immune markers and hormones were not reliable predictors of which foals would develop pneumonia or how severe it would be. This was surprising because researchers expected these blood markers to be different in foals that got sick compared to those that stayed healthy.

However, one immune marker called interferon-gamma (IFN-γ) did show statistically significant differences between the three groups at two time points: at 4 weeks of age and at 20 weeks of age. This suggests that interferon-gamma might be worth studying further, though it wasn’t strong enough to predict disease on its own.

The researchers also found that cortisol, vitamin D, and other immune markers changed naturally as foals aged from birth to 20 weeks. These age-related changes were significant and consistent across all foals, regardless of whether they developed pneumonia.

The study revealed that immune system development is a normal, expected process in young foals, with hormone and immune marker levels changing predictably as foals mature. The fact that these changes occurred in all foals—sick and healthy alike—suggests that normal immune development doesn’t necessarily protect against or predict pneumonia. Additionally, the researchers noted that measuring immune markers in the blood may not accurately reflect what’s happening in the lungs and other tissues where the infection actually occurs.

Previous research suggested that cortisol and vitamin D affect how the immune system responds to bacterial infections and that abnormal levels of these hormones are associated with serious infections in newborn foals. This study expected to find that foals with pneumonia would have different cortisol and vitamin D levels compared to healthy foals. However, unlike some previous research in other animal populations, these markers were not reliable predictors in this group of foals with endemic pneumonia.

The study has several important limitations. First, researchers didn’t identify the exact bacteria causing pneumonia in all sick foals, so they couldn’t confirm all cases were caused by Rhodococcus equi. Second, only 30 of the 200 foals were selected for detailed blood analysis, which is a small percentage. Third, blood samples were collected only at specific times (birth, 1, 2, 4, 8, 12, and 20 weeks), so important immune changes between these dates might have been missed. Fourth, measuring immune markers in blood may not accurately reflect what’s happening in the lungs where the infection occurs. Finally, the study was conducted on one specific farm with endemic pneumonia, so results might not apply to other farms or regions.

The Bottom Line

Based on this research, blood tests measuring immune markers and hormones cannot yet be recommended as a reliable way to predict which foals will develop pneumonia or how severe it will be. Veterinarians should continue using traditional methods: careful observation of foals for signs of illness (cough, fever, difficulty breathing), regular physical exams, and when necessary, chest ultrasound or other imaging. If a foal shows signs of pneumonia, prompt veterinary treatment is important. This is a low-confidence recommendation for using these blood tests in practice at this time.

Horse breeders and farm managers on properties where Rhodococcus equi pneumonia is common should care about this research because it addresses a real problem they face. Veterinarians working with foals should be aware that these specific blood tests are not yet reliable for predicting disease. However, this research doesn’t apply to foals on farms where this type of pneumonia is rare. Foal owners should continue working with their veterinarians on proven prevention strategies like good hygiene, proper nutrition, and monitoring for early signs of illness.

This research doesn’t provide recommendations for foal owners to implement, so there’s no timeline for seeing benefits. However, the findings suggest that researchers need more time—possibly years—to develop better predictive blood tests if they’re possible at all. In the meantime, traditional monitoring and early treatment remain the best approaches.

Want to Apply This Research?

  • For foal owners using a health tracking app, record weekly observations of foal behavior and health signs: appetite, energy level, presence or absence of cough, respiratory rate (breaths per minute), and any nasal discharge. Note the date and any veterinary visits or treatments. This creates a health timeline that’s more useful than blood tests for early disease detection.
  • Rather than waiting for blood test results, implement a daily health check routine: observe your foal’s breathing at rest, listen for cough, check for fever by taking temperature if trained to do so, and note appetite and energy level. Record these observations in the app weekly. This active monitoring approach is more practical and effective than relying on periodic blood tests.
  • Create a long-term health log in the app that tracks foal wellness from birth through weaning. Include weekly notes on general health, any signs of respiratory illness, growth and development milestones, and veterinary visits. This comprehensive record helps you and your veterinarian identify patterns and catch problems early, which is more valuable than any single blood test.

This research applies specifically to foals on farms where Rhodococcus equi pneumonia is endemic (common). The findings suggest that blood tests measuring immune markers are not currently reliable for predicting pneumonia in foals. This does not mean foals don’t need monitoring—rather, traditional methods of observation and veterinary care remain important. Always consult with an equine veterinarian about your foal’s health, disease prevention strategies, and any signs of illness. This summary is for educational purposes and should not replace professional veterinary advice. The study was conducted on one farm and may not apply to all foals or all farms.