How Do We Know When an Animal Is Suffering?
The Question We Must Answer
Every year, millions of animals are used in biomedical research worldwide. While this research has led to countless medical breakthroughs, it comes with an ethical responsibility: to ensure that animal suffering is minimized and scientifically justified. But how do we actually know how much an animal is suffering? And how can we compare the burden of different experimental procedures?
These are not just philosophical questions—they have real consequences for how we design experiments, set humane endpoints, and make regulatory decisions about which procedures are acceptable.
The Problem with “Expert Judgment”
Traditionally, assessing animal welfare has relied heavily on experienced researchers observing their animals. They look at body weight changes, activity levels, facial expressions, nest building, and clinical signs. While this expertise is invaluable, it has limitations: different observers may reach different conclusions, and comparing severity across different experiments or laboratories becomes nearly impossible.
Imagine trying to answer: “Is a mouse with colitis suffering more or less than a mouse with a tumor?” Without a common measuring stick, this question remains unanswerable—and yet it is exactly the kind of question that ethics committees and regulatory authorities need to address.
A New Approach: Making Severity Measurable
The RELSA (Relative Severity Assessment) algorithm was developed to address this challenge. Instead of relying on subjective impressions, RELSA combines multiple welfare indicators—body weight, activity, clinical scores, and other parameters—into a single, quantitative severity score.
Think of it like a weather index that combines temperature, humidity, and wind speed into a single “feels like” temperature. RELSA does the same for animal welfare: it takes complex, multidimensional information and translates it into a number that can be tracked over time and compared across different animals and models.
Why This Matters for the 3Rs
The 3Rs—Replacement, Reduction, and Refinement—are the guiding principles for ethical animal research. RELSA contributes to all three:
Refinement: By providing objective severity data, researchers can identify which aspects of a procedure cause the most distress and target them for improvement. If RELSA shows that animals suffer most on day 1 after surgery, that is when additional pain management might be most beneficial.
Reduction: When severity can be measured precisely, researchers can design more efficient experiments. Better endpoint criteria mean fewer animals need to be used to achieve statistically meaningful results.
Replacement: Quantitative severity data helps validate alternative methods. If a cell culture model produces results comparable to an animal model with high RELSA scores, the case for replacement becomes stronger.
From the Lab to the Clinic
One of the most exciting developments has been the translation of RELSA principles to human medicine. The same algorithm that monitors mouse welfare has been adapted as the Patient Vital Status (PVS) score for pediatric intensive care units. This demonstrates that the underlying concept—fusing multiple parameters into a meaningful composite score—has value far beyond animal research.
This kind of translational success validates the approach: if RELSA can help monitor critically ill children, it reinforces that the method captures something real and clinically meaningful.
What Does a RELSA Score Actually Tell Us?
A RELSA score of zero means an animal is at baseline—behaving normally, maintaining weight, and showing no signs of distress. As the score increases, it indicates growing deviation from this healthy state. Importantly, RELSA doesn’t just flag that “something is wrong”—it quantifies how wrong and allows tracking over time.
This temporal dimension is crucial. Some procedures cause brief, intense stress followed by recovery. Others cause chronic, low-level burden. RELSA captures these patterns, enabling researchers to make informed decisions about humane endpoints and intervention timing.
A Common Language for Severity
Perhaps RELSA’s greatest contribution is providing a common language for discussing severity. When a researcher in Hannover and a researcher in Munich both report RELSA scores, they can meaningfully compare their findings. This standardization is essential for:
- Multi-center studies that need consistent welfare monitoring
- Regulatory submissions that require objective severity documentation
- Scientific publications that should report animal welfare outcomes
- Ethics committees that must evaluate and compare different protocols
The Road Ahead
RELSA is not the final answer to severity assessment—it is a step toward more objective, data-driven animal welfare science. As more laboratories adopt quantitative approaches, our collective understanding of what animals experience during experiments will improve.
The goal is not to make animal research “acceptable” through better measurement. The goal is to ensure that when animal research is deemed necessary, we do everything possible to minimize suffering—and that we have the tools to prove we are succeeding.
For the 3Rs to be more than just principles on paper, we need methods that translate ethics into action. RELSA is one such method: a bridge between our moral obligations and our scientific practice.
Want to learn more? The RELSA package is freely available, and a web-based version allows researchers to explore the method without programming knowledge.