It also allowed the development of one of the early pain VAS scales, which
included both gait impairment and the pathology of the lesions (Ley et al. 1989 ).
Studies in other ruminant species have been less extensive and analgesic studies
are often limited to the pharmacokinetics of some of the more widely used analge-
sics (Ingvast-Larsson et al. 2007 ).
3.5 Birds
A major consideration in establishing and validating avian pain behaviours is the
wide range of species, ranging from farmed ostriches to caged songbirds and from
battery and egg laying hens to exotic parrots.
An important consideration influencing the study of pain behaviours in birds is
that, apart from gait impairment in lame animals, they have little in common with
the behaviours exhibited by mammals. This has led to the erroneous view that avian
species do not feel pain in a similar manner to mammals, but apart from the inability
to exhibit facial expressions in the same way as mammals, due to the presence of a
beak and limited facial musculature, birds with acute pain both vocalise and exhibit
strong escape behaviours (e.g. flapping of wings). Moreover, they show avoidance
and preference behaviours, while in response to chronic pain, lameness and reduced
food intake occur and both gait and food intake return to normal patterns when birds
are treated with analgesics (McGeown et al. 1999 ).
Relatively few studies have been carried out on bird pain responses and many of
these have focussed on the welfare of intensively farmed poultry (Duncan et al.
1989 ; Gentle et al. 1997 ). However, the nature of the welfare considerations has not
always led to analgesic therapy, but rather to husbandry changes and improved
genetics (Kuo et al. 1991 ). Flock medication with analgesic drugs is generally
impracticable and, if changes can be achieved through improved husbandry, then
medication may not be required.
Studies on cage birds and other species (Paul-Murphy et al. 1999 , 2004 ) have
shown that the analgesic agents used in mammals have similar effects in birds. This
suggests that the enzymatic and neurotransmitter receptor mechanisms associated
with pain inception and transmission have a broadly similar physiological basis,
although certain physiological responses in some species do reveal differences
(Machin 2005 ). The major difficulty in providing effective analgesic therapy in
birds remains the possible failure to recognise more subtle manifestations of pain,
particularly the lower grade pain states that are better recognised in mammalian
companion animals and have been used in composite pain scoring and VASs.
3.6 Other Non-mammalian Vertebrates
Compared with the difficulties in recognising avian pain, described in Sect. 3.5,
those associated with evaluating pain in reptiles, amphibians and fish are even
Pain and Analgesia in Domestic Animals 173