Desexing of rabbits and guinea pigs is a commonly requested procedure in exotic animal practice. Both the anaesthesia and the surgical procedures in these species are complicated by differences in anatomy and response to anaesthesia and surgery. This paper reviews the surgical anatomy of these species, anaesthetic considerations, surgical procedures and common complications.

Reasons and Age for Desexing

Desexing in rabbits in guinea pigs is performed for three main reasons:

• To control reproduction
• Rabbits reach sexual maturity at 4–8 months of age (depending on their size: small breeds mature at 4–5 months, medium breeds at 5–6 months and large breeds at 5–8 months. Each doe can have up to 10 litters a year, each litter with 4–10 kits.
• Guinea pigs reach sexual maturity at 2–3 months of age. Each sow can have numerous litters each year, each with anywhere between 1–13 offspring.
• To prevent disease
• Entire rabbit does have a high incidence of uterine adenocarcinoma, the most common neoplasm in rabbits. The study by Greene (1959) showed that 4% of does had uterine cancer at age of 2–3 years, rising to 60–80% at 5–6 years. Whether the rabbit has had a litter does not appear to affect this incidence; the incidence increases with age.
• Entire guinea pig sows have a high incidence of cystic ovaries. Although the majority of these cysts are non-functional rete ovarii cysts, they can have a spacy-occupying mass effect. Functional cysts frequently cause hormonal alopecia and uterine changes.
• Male rabbits and guinea pigs do not appear to have any significant sex-related medical problems.
• To prevent or manage behavioural reasons
• Entire rabbits of both sexes often become territorial as they mature and display aggression towards other rabbits, other animals and even their owners.
• Entire male rabbits housed alone may have marked sexual behaviours, mating with other animals and even inanimate objects.
• Entire guinea pigs of either sex may develop similar behavioural issues, although not as marked as rabbits.
• Desexed rabbits and guinea pigs of both sexes are usually more docile, less destructive, and are better ‘pet quality’ animals.

Rabbits are generally desexed at 3–6 months of age. It can be difficult to determine gender if the testicles have not descended, which occurs around 12 weeks of age - it is, therefore, worth waiting until the animal can be accurately sexed. Guinea pigs are easier to sex at an earlier age, and are often desexed at around 6–8 weeks of age.

Surgical Anatomy

The ovaries of rabbits are not located in a true ovarian bursa but are usually surrounded by fat within the mesovarium and the mesosalpinx. The uterus of young rabbits is found just dorsal to the bladder, coiled in the caudal abdomen. In older rabbits, the cervix is dorsal to the bladder, but the horns extend laterally. The uterus and ovaries are generally easy to exteriorize; however, they are more fragile than those of dogs and cats. The uterus is bicornuate and each horn has its own cervix. There is no distinct uterine body. The mesometrium of rabbits is a site of fat storage. In obese rabbits this can make surgery more challenging as it is often difficult to definitively identify the ovarian and uterine vessels for ligation. The vaginal body is a long, large, and flaccid unpaired organ, and the urethra opens into the ventral aspect of the vaginal body. This marks the division between the vestibulum, which is caudal to the urethral opening, and the larger true vaginal body, which is cranial to the urethral opening.

The testicles of rabbits are elongated, not round, and they move freely between the abdomen and the scrotum through the function of a well-developed cremaster muscle. The epididymis is located at the caudal pole of the testicle, but it is not as developed as in guinea pigs. The inguinal canal is open in rabbits; however, the intestine does not usually herniate because of the large epididymal fat pad which fills the inguinal canal when the testes are within the scrotum and the inguinal fat pads within the abdomen². The proper ligament of the testis which attaches the tunica vaginalis to the scrotum is quite strong. The position of the testicles at any given time depends on many factors including body position, body temperature, breeding activity, gastrointestinal tract filling, and the amount of abdominal fat. The position of the penis caudal to testicles makes a pre-scrotal approach with a single incision on the midline, if possible.³

Guinea pig sows possess paired ovaries and a bicornuate uterus, consisting of paired uterine horns, a short uterine body, and a single cervical canal which opens into the vagina^2,3. The ovaries are located in a craniodorsal position, immediately caudal to the kidneys.

Mature male guinea pigs possess paired, large, ovoid testes with a well-developed epididymis, located on either side of the perineum in distinct, shallow scrotal sacs, often with a large amount of surrounding fat. The testicles are usually fully descended into the scrotum by 3 months. If the testes have not descended by 4 months of age, the guinea pig should be considered cryptorchid. The wide inguinal canals remain open for life. Reproductive glands, located within the abdomen, include the seminal vesicles (vesicular glands), coagulating glands, bulbourethral glands, and the prostate.

Analgesia and Anaesthesia

Both rabbits and guinea pigs have low pain tolerance thresholds, often responding to pain by inactivity and inappetence. Both of these may, if untreated, result in intestinal ileus which may in turn lead to the death of the patient. It is therefore important that a sound analgesic plan is developed to manage both surgical and post-operative pain. The use of opioids, non-steroidal anti-inflammatory drugs and local anaesthesia can be combined to produce multimodal analgesia. It is important that analgesia be continued for several days to ensure the patient is comfortable and eating well.

There are several factors to consider when formulating an anaesthetic plan:

• Their small body size
• Large surface area to body mass ratio can lead to rapid temperature drops.
• Rapid metabolic rate means that there is a high demand for oxygen and energy.
• This metabolic rate affects the dose rate and frequency of many drugs.
• They are obligate nasal breathers.
• Intubation can be difficult.
• A rapid recovery with minimal ‘hangover’ effects is needed to ensure a rapid return to eating, minimising the risk of ileus.
• Rabbits, in particular, can have a catastrophic re- lease of catecholamines when stressed, leading to cardiac arrhythmias and arrest.
• They cannot vomit.

Anaesthetic plans should, therefore, include the following:

1.  Minimal/no fasting

2.  Washing the mouth out before anaesthetic induction

3.  Pre-medication should include analgesia and, if possible, be reversible.

4.  Induction can be given IM, IV or via mask. In rabbits, mask induction should only be used in a well-sedated and calm rabbit.

5.  Ventilation support can be given via a well-sealed face mask (guinea pigs, small rabbits), endotracheal tube (rabbits), or laryngeal mask (rabbits).

6.  Pre- and post-oxygenation of the patient is essential.

7.  Patients should be encouraged to eat as soon as possible after recovery.

Orchidectomy

Male rabbits and guinea pigs can be castrated via either an open or closed technique. The initial incision may either be scrotal (a 1–1.5 cm incision through the scrotum longitudinally on each side of the midline about midway along the length of the scrotum) or a single pre-scrotal incision.

Closed: The tunic is grasped and the testicle is removed from the scrotum with the tunic intact. The tunic is tightly adhered to the end of the scrotum by the proper ligament of the testis. This ligament must be broken down to allow exteriorization of the testicle. Caudal traction is applied to the testicle and dry gauze is used to strip the facial attachments allowing the narrow portion of the cord to be exteriorized. Once the testicle has been exteriorized adequately the cord is ligated using a 2 or 3 clamp technique.²

Open: The vaginal tunic is incised to allow exteriorization of the testicle, spermatic cord, and vascular supply. The tail of the epididymis will still be attached to the tunic. This attachment must be broken down freeing the testicle for removal. The spermatic cord is double ligated and the testicle is removed. The vascular pedicle is traced cranially and the inguinal canal is identified. A single interrupted suture is placed across the inguinal canal being careful not to compress the blood vessels passing through the canal. The vascular pedicle is ligated prior to transection. (It is not necessary to pull the testicle out far from the body, risking accidentally tearing the vessels. The surgeon only needs the entire testicle exposed and the vessels can be ligated close to the testicle.) Once transected (or torn) the vascular pedicle retracts into the retroperitoneal space as the testicular vessels are branches off the renal vessels. Haemorrhage from these vessels, therefore, occurs in the retroperitoneal space and does not cause haemoabdomen. Inadequate control of subcutaneous vessels and vessels within the tunics are the likely causes of scrotal hematomas, not haemorrhage from the testicular vessels.²

A modification of this technique in rabbits involves an open castration being careful to remove only the testicle and leaving the epididymal fat pad intact. The fat pad will then prevent herniation of intestine through the inguinal ring.²

With any of these techniques the scrotal incision may be left open to heal by second intention or it may be sutured closed using either an intradermal pattern, tissue adhesive, or skin staples.

Ovariohysterectomy

Rabbits: A 2–3 cm incision is made starting midway between the umbilicus and pubis extending caudally. The cecum and bladder may be directly under the linea alba and it is recommended that the body wall be elevated from the abdominal structures prior to making the initial incision in the linea alba. Once the peritoneal cavity is opened the viscera will drop away as air enters the peritoneal cavity. The uterus is usually visible dorsal to (under) the cranial pole of the bladder. The uterine horn is usually redder in colour than surrounding viscera, making it easily identifiable. One uterine horn may be lifted through the incision using atraumatic forceps. (It is best to avoid using a spay hook as such an instrument may perforate the cecum leading to disastrous consequences.) Once the uterine horn has been elevated through the incision it is traced to the ovary which is loosely attached to the dorsal body wall by a long, fat-filled mesovarium. The oviduct is usually visualized as a fine tubular structure which literally encircles the ovary. A clamp may be placed between the ovary and the uterine horn to allow traction to be applied to the ovary. The ovarian ligament does not usually need to be broken down. There are many vessels that supply the ovary within the fat of the mesovarium. An opening is created by blunt dissection through the fat of the mesovarium and a ligature is passed around the portion of the mesovarium containing the vessels supplying the ovary. As the suture is tightened it will cut through the fat, but will ligate the blood vessels. This procedure is repeated on the contralateral side and the fat-filled broad ligament of the uterus may be broken down by gentle blunt dissection. Any large vessels or any haemorrhage from vessels within the broken ligament may be controlled by ligation or haemostatic clips. Following dissection of both uterine horns the uterus may be ligated on either the cranial or the caudal side of the cervix. The uterine vessels lay on each side of the uterus several millimetres lateral to the uterus. It is best to ligate these vessels individually and place a transfixation ligature around the uterus prior to transection. Closure is routine with body wall, subcutaneous tissue, and skin being closed as separate layers.²

Guinea pigs: Two approaches can be used: a ventral midline approach as described above, or a bilateral flank approach. The latter approach is particularly useful in young sows as it minimises the risk of handling the intestinal tract. The 1 cm skin incision starts where the last rib passes under the lumbar muscles, and is directed at a 45° angle caudoventrally (the author directs it towards the stifle). The subcutaneous fat is separated (or removed) and the muscle wall is incised along the same line as the skin incision. Peritoneal fat comes into the incision, and when exteriorised gently with atraumatic forceps, it contains the ovary and/or fallopian tube. The ovarian blood vessels, between the ovary and the kidney, are ligated and transected and the fallopian tube followed caudally to the uterine horn. As much horn as possible is exteriorised, ligated and transected. The muscle, fat and skin are closed in separate layers and the procedure repeated on the other flank. If needed, the remnants of the uterine horn and the uterine body can be removed via a small ventral midline incision.

Post-operative Care

Post-operative complications include:

• Pain (see the earlier discussion on analgesia)
• Ileus occurs when the animal is inappetent, dehydrated, in pain, or all of these. Animals should be well hydrated before and after the surgery, fasting should be minimised, analgesia provided and, if the patient in not eating, it should be assist fed with a suitable diet (e.g., Critical Care®, Oxbow), Animals should not be discharged after surgery until they have been seen to eat and defecate.
• Infection is most common in guinea pig castrations, with abscess formation under the skin incision occurring several days after the procedure. Post-operative antibiotic therapy should be considered, and the owner advised to watch for swelling at the surgery site
• Scrotal herniation can occur in both rabbit and guinea pig castrations, and the owners should be advised to monitor the site for swelling.