All conditions and events associated with extreme levels of pain
that are not normally tolerated even if only for a few seconds. In humans, it would
mark the threshold of pain under which many people choose to take their life
rather than endure the pain. This is the case, for example, of scalding and severe
burning events. Behavioral patterns associated with experiences in this category
may include loud screaming, involuntary shaking, extreme muscle tension or extreme restlessness. Another criteria is the manifestation of behaviors that
individuals would strongly refrain from displaying under normal circumstances,
as they threaten body integrity (e.g. running into hazardous areas or exposing
oneself to sources of danger, such as predators, as a result of pain or of attempts
to alleviate it). The attribution of conditions to this level must therefore be done
cautiously. Concealment of pain is not possible.

Pain at this level takes priority over most bids for behavioral execution,
and prevents all forms of enjoyment or positive welfare. Pain is continuously
distressing. Individuals affected by harms in this category often change their
activity levels drastically (the degree of disruption in the ability of an organism
to function optimally should not be confused with the overt expression of pain
behaviors, which is less likely in prey species). Inattention and unresponsiveness
to ongoing stimuli and surroundings is likely to be observed. Relief often requires
higher drug dosages or more powerful drugs.

Experiences in this category disrupt the ability of individuals to function
optimally. Different from Annoying pain, the ability to draw attention away from
the sensation of pain is reduced: awareness of pain is likely to be present most
of the time, interspersed by brief periods during which pain can be ignored
depending on the level of distraction provided by other activities. Individuals can
still conduct routine activities that are important in the short-term (e.g. eating,
foraging) and perform cognitively demanding tasks, but an impairment in their
ability or motivation to do so is likely to be observed. Although animals may still
engage in behaviors they are strongly motivated to perform (i.e., exploratory,
comfort, sexual, and maintenance behaviors), their frequency or duration is likely
to be reduced. Engagement in positive activities with no immediate benefits
(e.g., play in piglets, dustbathing in chickens) is not expected. Reduced alertness
and inattention to ongoing stimuli may be present. The effect of (effective) drugs
(e.g., analgesics if pain is physical, psychotropic drugs in the case of
psychological pain) in the alleviation of symptoms is expected.

Experiences of pain perceived as aversive, but not intense enough to
disrupt the animal’s routine in a way that alters adaptive functioning or affects
the behaviours that animals are motivated to perform [54]. Similarly, Annoying
pain should not deter individuals from enjoying pleasant experiences with no
short-term function (e.g., play) and positive social interactions. Sufferers can
ignore this sensation most of the time. Performance of cognitive tasks
demanding attention are either not affected or only mildly affected.
Physiological departures from expected baseline values are not expected to be
present. Vocalizations and other overt expressions of pain should not be
observed.