Pain Management in Small Mammals

Synopsis

This brief article was created to serve as a synopsis of LafeberVet’s longer, more detailed “Analgesia in Small Mammals” authored by veterinary anesthesiologist, Dr. Paul Flecknell.

Recognizing pain

Pain assessment is more challenging in small mammals. In an effort to assess the patient, carefully observe the animal while undisturbed.

Signs of pain in small mammals:

  • Anorexia
  • Immobility
  • Hunched posture
  • Press the ventral abdomen into the ground
  • Bruxism or a loud, crunching teeth grinding
  • Position themselves in the farthest corner of the cage
  • Hide under bedding
  • Separation from cage mates
  • Reduced grooming
  • Build up of reddish-brown or porphyrin tears in rats
  • A change in behavior or aggression
  • Abnormal vocalizations

May small mammals are prey species. Fear and pain can appear similarly, and fearfulness or stress will cause the animal to freeze. Immobility makes an assessment of behavior virtually impossible.

Preemptive analgesia

Administer pain medication before, rather than after injury, whenever possible. Doing so prevents “pain windup” by preventing noxious stimuli from reaching the central nervous system. Unfortunately pre-emptive analgesia is not always appropriate or practical in small, stressed patients. Analgesics must sometimes be administered post-induction or post-operatively making multimodal analgesia even more important in these species.

Multimodal analgesia

The mechanisms of pain involve multiple pathways and a variety of neurotransmitters. No one drug or class of drugs can be used to treat all pain. Multimodal or balanced analgesia relies on drug combinations to provide greater pain relief. Agents given in combination are often used at lower doses, which reduces the risk of undesirable side effects.

Administer a relatively short-acting opioid like butorphanol (Torbugesic®, Fort Dodge) concurrently with a non-steroidal anti-inflammatory agent (NSAID )to provide more prolonged analgesia. Many opioids act rapidly within 10 to 15 minutes of s injection whereas it can be an hour before significant analgesic effects are noted after NSAID administration.
Pain pie chart

  • Local anesthetics block ion channels preventing the creation and conduction of pain impulses. Local anesthetics are ideal for use in pre-emptive analgesic combinations as they reduce post-operative pain by avoiding central and peripheral changes.
  • Opioids have been used extensively for moderate to severe pain, such as with fractures, trauma, or surgical pain.
  • Non-steroidal anti-inflammatory drugs (NSAIDs) are commonly prescribed for mild to moderate pain because of their extended therapeutic activity in inflamed tissues.

Practical options

Opioids

  • Butorphanol and buprenorphine are widely popular agonist/antagonists that have been used to provide analgesia in a variety of species. The analgesic effects of butorphanol typically last 3-4 hours.
  • Buprenorphine has the advantage of having a prolonged duration of action of 6-12 hours in many species including rabbits and rodents.

Potential adverse effects with opioid analgesics:

  • Respiratory depression
  • Sedation or excitement
  • Vomiting, delayed gastric emptying

All opioids can produce some degree of respiratory depression, but when administered at clinically effective doses this is rarely a serious problem in small mammals. Other adverse effects are generally of minimal clinical significance although herbivores at risk for gastrointestinal ileus should be monitored particularly well.

Non-steroidal anti-inflammatory drugs

Select newer agents like ketoprofen (Ketofen®, Fort Dodge), carprofen (Rimadyl®, Pfizer), and meloxicam (Metacam®, Boehringer Ingelheim) (Table 1). The palatable oral preparation of meloxicam makes this drug particularly useful when additional doses are required.

The most significant problems associated with NSAID administration are gastrointestinal disturbances, notably ulceration and hemorrhage, nephrotoxicity, and interference with platelet function. Adverse effects are rarely significant when these drugs are used in young, relatively healthy animals. Exercise caution with NSAID use in animals that may have preexisting organ damage. Chronic renal disease is very common in older hamsters (Mesocricetus auratus), rats, and guinea pigs (Cavia porcellus).

Local anesthetics

Local anesthetics can be used to provide post-operative pain relief and as adjuncts to general anesthesia. A longer lasting local agent such as bupivicaine (Marcaine®, AstraZeneca) may be infiltrated into wound margins or used in a local or regional nerve block. Epidural or spinal administration of drugs has been described in ferrets (Mustela putorious furo), rabbits (Oryctolagus cuniculus) (lidocaine 0.4 ml/kg epidural), guinea pigs, and small rodents.

Table 1. Analgesics for use in small mammals
Analgesic Ferret Guinea Pig Hamster and Gerbil Mouse Rabbit Rat
Buprenorphine 0.01-0.03mg/kg IM, SC, IV q6-12h 0.05 mg/kg SC q6-12h 0.1 mg/kg SC q6-8h 0.1 mg/kg SC q6-12h 0.01-0.05 SC 6-12h 0.05 SC
Butorphanol 0.4 IM 4h 2 SC 4h 1-2 SC 4h 0.1-0.5 SC 2 SC
Carprofen 4 SC  24h 5 SC 12h 4 SC 24h
1.5 PO 12h
5 SC, PO 24h
Ketoprofen 3 IM 24h 5 IM, PO 24h
Meloxicam 0.2 SC
0.3 PO 24h
1-2 SC, PO
Morphine 0.5 IM, SC 4-6h 2-5 SC, IM 4h 2-5 SC, IM 4h 2-5 SC, IM 4h 2-5 SC, IM 4h
Meperidine 5-10 IM 2-4h 10-20 SC, IM 2-3h 10-20 SC, IM 2-3h 10 SC, IM 2-3h 10 SC, IM 2-3h
All doses listed are in mg/kg. IM: intramuscular, SC: subcutaneous, IV: intravenous, PO: per os. h: hours

References