Pain management in small mammal patients (Proceedings)

What is meant by small mammals? Small mammals in this document refer to those species commonly kept as pets, not including dogs and cats, for example, rodents (rats, mice, chinchillas, guinea pigs), rabbits, and ferrets. Even less is known about hedgehogs (rodent) and sugar gliders (marsupial), but they are included as well.

Not only are small mammals used in laboratory medicine (ferrets are used in flu vaccine development every year), but they are also kept as pets, taken to shows (yes, even mice have shows!), are exhibited at schools or zoos, and even in the case of rabbits, provide meat and fur (production animals). The needs of the client may dictate the approach taken to resolving a problem in a particular animal.

What research has been done in small mammals and what is known?

Because of the advanced field of laboratory animal medicine, there is more scientific information available regarding analgesia in small mammals than what is known about birds and reptiles combined.

Remember that there is no generic small mammal. Some known differences in drug metabolism are known and include: after tramadol IV or PO rabbits make over 5 different metabolites, rabbits can seizure and die after being given fipronyl (Frontline®), and rabbits' GI flora is disrupted by certain antibiotics including oral penicillins, macrolides and cephalosporins. From the above description, it seems rabbits are the different ones, hmmmmm...... well there are probably some unknown differences in metabolism within each species and, as some researchers are finding out, there are subtle differences between strains.

The most is known about the rat, including published studies of pharmacokinetics, pharmacodynamics (using the tail flick test) and established, standard methods of assessing pain. A study was performed determining the magnitude and duration of analgesia using the standard hot plate test and the tail flick test, showing (n=61 ea. spp.):

Table 1. Study results

Using information gained from the study above, the following was recommended:

• use morphine for severe pain and redose every 2-3 h

• use butorphanol for mild pain of short duration and redose every 1-2 h

• use buprenorphine for mild to moderate pain of increased duration and redose

• rats every 6-8 h and mice every 3-5 h.

How do you assess pain in small mammals and when do you give analgesics?

It is very difficult to assess pain in small mammals. They seems very stoic and do not cry out in pain unless they are in extreme pain (I assume this is probably like a 10 out of 10 on the human scale we see when we go to the hospital). Small mammals are prey animals, so they do not want to be conspicuous if they are in pain or injured. So, try to observe your patient before they are aware you are observing them.

Dr. Flecknell has available a training video with an objective scoring system to evaluate pain in rats. By counting the number of times a rat "flinches" the skin on its back, stretches like a cat or falters in its gait during a set period of time, a numerical score is assigned as to how much pain that rat is experiencing. It is my opinion that all practitioners should learn how to assess pain in their rat patients from this video. There is much laboratory animal medicine information available in the literature and from colleagues. Small mammal practitioners would do well to consult and use this large body of information. Less is known about objectively assessing pain in the mouse and rabbit, but Dr. Flecknell and his colleagues are currently attempting to develop a scoring system for these species as well.

TABLE 2: Formulary doses and references (from Carpenter's formulary and Quesenberry book):

References

Heard DJ. Anesthesia, Analgesia, and Sedation of Small Mammals. In: Quesenberry KE and Carpenter JW (eds). Ferrets, Rabbits and Rodents – Clinical Medicine and Surgery, 2nd ed., 2004, pp.356-369.

Gamble C and Morrisey JK. Ferrets. In: Carpenter JW (ed). Exotic Animal Formulary, 3rd ed., Elsevier, St. Louis, 2005, pp.447-478.

Hernandez-Divers SJ. Rabbits. In: Carpenter JW (ed). Exotic Animal Formulary, 3rd ed., Elsevier, St. Louis, 2005, pp.410-446.

Ness RD. Rodents. In: Carpenter JW (ed). Exotic Animal Formulary, 3rd ed., Elsevier, St. Louis, 2005, pp.377-410.

Martin LB+, Thompson AC, Martin T, Kristal MB. Analgesic efficacy of orally administered buprenorphine in rats. Comp Med, 2001, 51(1):43-8.

Gades NM, Danneman PJ, Wixson SK, Tolley EA. The magnitude and duration of the analgesic effect of morphine, butorphanol, and buprenorphine in rats and mice. Contemp Top Lab Anim Sci, 2000, 39(2):8-13.