| Category |
Criteria |
| Level 1 |
Study quickly terminated by euthanasia, prior to any clinical signs of disease or distress. |
| Level 2 |
Comparative study; experimental groups given novel drugs or vaccines compared with positive ‘‘gold-standard’’ controls. Non-lethal infection with only transient mild symptomology. Probably lethal infections terminated before the onset of the most debilitating symptoms. |
| Level 3 |
Studies resulting in lasting deleterious effects on animal health and welfare, not alleviated by means of refinement. |
| Level 4 |
All studies having spontaneous death or ‘‘moribund’’ state as experimental endpoints and/or resulting in severe distress non-alleviated by means of refinement. |
* It is adapted from Ref. 14.