Equine study finds ocular thermography overestimates rectal temperature

A new study in Equine Veterinary Journal adds another caution flag to the push for non-contact temperature monitoring in horses: lacrimal caruncula thermography overestimated rectal temperature across devices and distances, and accuracy fell off as the camera moved farther away. The authors reported that one camera, the UTi165A, performed better at 0.5 meters, but neither system showed sufficient agreement to replace rectal thermometry. That matters because eye-region thermography has attracted interest as a faster, lower-stress option for monitoring equine health and welfare, especially in settings where repeated handling is difficult.

The appeal of infrared thermography in horses is easy to understand. It’s non-invasive, rapid, and already used in equine medicine and welfare research to assess surface heat patterns, stress responses, and possible inflammation. The lacrimal caruncula, in particular, has been studied because it is highly vascular and relatively accessible on thermal images. But the broader literature has also been warning for years that surface temperature is not the same thing as core temperature, and that readings can shift with equipment differences, operator technique, ambient conditions, and even mild airflow. (sciencedirect.com)

That context is important for interpreting the new findings. The study’s core result, as summarized in the journal listing, is that lacrimal caruncula thermography overestimates rectal temperature in horses regardless of which of the two infrared cameras was used, with poorer agreement at greater distances. The best-performing setup was the UTi165A at 0.5 meters, but even that was not accurate enough to replace rectal measurement. Those results are directionally consistent with a 2023 Journal of Equine Veterinary Science paper from Texas A&M, which found a large and inconsistent bias between a non-contact infrared thermometer and rectal digital thermometry in healthy horses and foals, concluding that the non-contact method was not a suitable substitute for estimating core temperature. (pubmed.ncbi.nlm.nih.gov)

Other recent and foundational studies help explain why. Research on thermal camera performance in equine thermography found that camera-dependent factors accounted for a large share of measurement variance, underscoring that device choice itself can materially affect results. Separate work has shown that even low levels of airflow can quickly alter thermographically measured temperatures in horses, enough to create false positives or false negatives in practice. And a recent Brazilian study on ocular thermography methods in horses reported that capture distance significantly influenced eye temperature, with 0.5 meters producing more consistent image acquisition and lower operational variability. Taken together, those findings reinforce the new paper’s message that methodology is not a side issue here; it is central to whether the numbers mean anything clinically. (sciencedirect.com)

Expert-style commentary from the published literature has been similarly cautious. A review of infrared thermography in animal emotional-response research noted that, in horses, maximal eye temperature has not consistently correlated with rectal temperature, and emphasized that environmental conditions can undermine reliability. Another equine heat-stress review described rectal temperature as the most commonly used method for monitoring body temperature in horses, while acknowledging ongoing interest in infrared alternatives because rectal measurement can be slower and carries handling risks. That balance, clinical practicality on one side and measurement validity on the other, is exactly where this new study lands. (pmc.ncbi.nlm.nih.gov)

Why it matters: For equine veterinarians and practice teams, this study is less about whether thermography is useful at all and more about where its limits are. If a tool systematically overestimates rectal temperature and loses agreement as distance increases, it’s not ready to replace a standard method used for fever detection, biosecurity decisions, pre-purchase assessments, hospitalization monitoring, or post-exercise evaluation. In ambulatory and field settings, where non-contact tools are especially attractive, the risk is that convenience could create false reassurance or unnecessary concern if clinicians treat thermographic values as interchangeable with rectal temperature. The safer interpretation is that ocular thermography may be useful as an adjunct, for trend monitoring, welfare research, or exploratory screening, but not as a diagnostic replacement for core temperature measurement. (pubmed.ncbi.nlm.nih.gov)

There’s also a communication angle for veterinary teams. Pet parents may increasingly encounter wearable, remote, or camera-based monitoring claims across animal health, and non-contact temperature tools can sound intuitive and lower stress. Studies like this one give clinicians evidence to explain why “less invasive” doesn’t automatically mean “clinically equivalent.” For hospitals, sports medicine programs, and equine field services, the practical next step may be to define where thermography fits in protocol, for example, as a supplemental observation tool under tightly controlled conditions rather than a stand-alone temperature measure.

What to watch: The next phase of this research will likely focus on standardization and narrower use cases: tighter control of distance and angle, better calibration across camera models, environmental controls, and possibly AI-assisted image analysis to improve region-of-interest selection. Newer work in other species suggests that image-processing advances may improve repeatability, but the current equine evidence still supports rectal thermometry as the reference standard when clinicians need an accurate estimate of body temperature. (link.springer.com)