Pre-anaesthetic preparation   

There is no need to fast rodents, as they cannot vomit

and are prone to hypoglycaemia (and gut stasis in herbivores) if fasted. Prior to anaesthesia the animal should be clinically examined and any fluid deficits should be corrected. The animal should be weighed and pre-operative food and water intake assessed. It is useful to remove food an hour before surgery in guinea pigs and chinchillas, to ensure the pharynx is clear. 

Pre-anaesthetic medication/sedation 

Guinea-pigs and chinchillas can be easily stressed so sedation may be indicated, or to carry out minor procedures such as radiography. Fentanyl/fluanisone (‘Hypnorm’) is a good sedative for these species, although other drugs such as ACP, diazepam and midazolam may be used.

Injectable anaesthesia is not commonly used in veterinary practice for small rodents (although commonly used in laboratory rodents). 

Inhalation anaesthesia  

Inhalation anaesthesia – chamber induction with a volatile anaesthetic agent such as isoflurane or sevoflurane is the recommended induction method in small rodents. The chamber should be pre-oxygenated. Provide a non-slip surface on the floor and cover the chamber to deuce stress. Isoflurane may be irritant to the mucus membranes, so a suitable eye lubricant should be applied prior to induction. Following induction the animal is maintained using a face mask and Ayre's T-piece. Intubation is difficult and problematic due to the small tube size which is easily obstructed and is generally not used. 

Intra-operative care  

Hypothermia is common. Rats and mice loose much heat from the tail so during anaesthesia this should be wrapped in tin foil or bubble wrap. Minimise heat loss by clipping only small areas and avoiding overwetting with skin preparations. Place on a heat pad, heated table or water or air heat source and use an overhead heat lamp. Beware overheating; body temperature should be monitored throughout. Insulate the animal using bubble-wrap or tin foil. Pulse oximetry and ECG monitoring is useful. Restore fluid balance if there has been any significant blood loss during surgery. Analgesia should be provided where indicated. Buprenorphine is the drug of choice, as it is the longest lasting (8 - 12 hours). Sarmasenil can be used to reverse benzodiazepines. Buprenorphine and butorphanol will reverse the respiratory depressant effect of fentanyl while still providing analgesia. NSAID's can also be used for post-operative analgesia, e.g. meloxicam. As in other species, care should be taken if the animal is hypotensive/hypovolaemic, because of the risk of renal toxicity. 

Post-operative Care  

As for rabbits including (high fibre assisted feeding and gastrointestinal motility support for hystricomorphs). 

Prevent breeding; reduce territorial or sexual hormone associated behavior.
 In females prevent cystic ovarian disease, pyometra and reproductive neoplasia,

pituitary adenomas ( rat).
 In males prevent orchitis and reproductive neoplasia. 

•   rats- 6-8 weeks

•   mice- 6-7 weeks

•   Hamsters 6-8 weeks (Syrian & Russian), 7-14 weeks (Chinese)

•   Gerbils10-12 weeks

•   Guinea pigs male 9-10 weeks, female 4-6 weeks

•   Chinchillas 6-8 months 

0. Rodents have open inguinal canals and retract the testicles. If the testicles are not

   visible in the scrotum push gently on the abdomen caudally until they appear. 

0. Scrotal approach
   

0. A bilateral scrotal incision is the most common approach. Damage to the scrotal vessels can lead to haematoma formation. The closed technique leaving the tunica intact is preferred to prevent intestinal herniation into the scrotum. A transfixing or circumferential ligature is placed around the spermatic cord. If the tunica is incised (open castration) ensure the inguinal canal is closed after ligating the blood vessels of the spermatic cord. The fat pad is closely associated with testis and will need to be removed if an open castration is performed, as it is friable and difficult to reposition. 

0. Pre-scrotal approach 

•   This is preferred as it is easier to keep the surgical site sterile. A prescrotal incision

  is made just cranial to each scrotum, on either side. The tunic is also incised and the castration performed open with double ligatures again placed around the vas deferens and the vascular structures. Following removal of the testis the tunica is closed with a continuous suture pattern and the skin closed routinely.

•   Male rodents should be kept separate from entire females for 6 weeks post- operatively. 

0. Ovariohysterectomy  

0. A standard 3cm ventral midline incision is made caudal to the umbilicus and towards the pubis. Care must be taken not to damage the underlying caecum when cutting through the skin and linea alba. Haemostatic clips or encircling ligatures should be applied to the ovarian vessels. If the uterine vessels are large they should be ligated separately. 

0. In guinea pigs, hamsters, rats and mice an encircling or transfixion ligature on the uterus cranial to the cervix, to prevent spillage of urine from the vagina into the abdomen, when it is transected. In chinchillas and degus there is no uterine body (like the rabbit), so the encircling or transfixion ligature should be placed just caudal to the paired cervices on the vagina. Care must be taken not to place the ligature on the caudal region of the vagina, as the ureters are likely to also be ligated. 

If carried out before sexual maturity at 4-6 weeks of age this will prevent cystic ovarian disease and will reduce the incidence of uterine disease.
The guinea pig is positioned in dorsal recumbency. The fur is clipped and surgically prepped on the dorsolateral region just caudal to the caudal ribs. A 1cm horizontal skin incision is made 1cm caudal to the last rib and just ventral to the lumbar muscles. The abdominal muscle is incised or bluntly dissected to reveal the yellow coloured retroperitoneal fat. This is gently rolled out to exteriorize the ovary in the fatty mesovarium. Haemostatic clips or encircling ligatures should be applied proximal and distal to the ovary, ligating the ovarian ligament and vessels. 

If carried out before sexual maturity at 6-8 weeks of age this will reduce the incidence of mammary neoplasia and pituitary neoplasia. The same surgical technique is carried out as for the guinea pig. However as the skin over the dorsum is loose a single 1 cm longitudinal dorsal incision may be made and the skin, the incision is then moved over the abdominal muscles, just ventral to the lumbar muscles and 1cm caudal to the last ribs. Where surgical neutering is not practical (owner does not want surgery, rat too old at presentation) the implantation with a gonadotropin-releasing hormone (GnRH) agonist deslorelin may be appropriate. 

• In guinea pigs and chinchillas it can be difficult to visualise the ovarian pedicles due

  to fat and the ovaries have a very short mesovarium caudal to the kidneys making it

  difficult to exteriorise them.

•   Female guinea pigs have paired uterine horns, a short uterine body and a single

  cervix.

•   Female chinchillas have paired uterine horns, and two cervices. They should therefore

  have a ligature placed just caudal to the

•   Pyometra is common in hamsters and rats. Female hamsters normally have a white

  vaginal discharge postovulation every 5-7days, when secreate copious white vaginal discgarge. Diagnosis is based on cytology of the vaginal discharge, ultrasonography and ovariohysterectomy is the treatment of choice. 

•   Mammary gland neoplasia is common in both male and female rats. The majority tends to be benign (fibroadenomas) but they rapidly increase in size, becoming ulcerated and causing problems with locomotion.

•   The mammary tissue extends from the ventral and lateral surfaces of the neck to perineum.

•   In mice and gerbils they tend to malignant (adenocarcinoma and fibrosarcoma) and in the former may be predisposed by strain and mammary tumour viruses. Prognosis in these species is poor.

•   Treatment is surgical removal, with as wide margins as possible. Care must be taken to ensure good wound closure is possible.

•   Early neutering (ovariectomy or castration) will reduce the incidence.

•   Recurrence is common in uninvolved mammary tissue and repeated surgery may be required. Implantation with a gonadotropin-releasing hormone (GnRH) agonist

  deslorelin should reduce occurrence. 

0. Fracture management  

0. Fracture healing is rapid in rodents with callous forming in 7-10 days. External stabilisation with splints and slings can be attempted. External fixators (see avian surgery notes) are well tolerated. Rodents cope well with amputation and similar surgical principles are as for dogs and cats.