Companion small mammals, ferrets, rabbits, and rodents, face similar reproductive challenges and disease processes as other companion pets (dogs and cats) including cessation of breeding and reproductive tract neoplasia. Veterinarians are often called upon to surgically spay and neuter these species, which in many cases, does not necessarily require specialty training. Avian species present several additional challenges as removal of the gonads is often very challenging without life threatening consequences; therefore, alternative surgical approaches, such as vasectomy or salpingohysterectomy (removal of the oviduct and uterus) can be performed. Unlike in companion mammals, these procedures often do require additional specialty training and/or equipment such as endoscopy or laparoscopy.
This article reviews the reproductive tract anatomy and offers surgical techniques in several pet exotic species, including ferrets, rabbits, guinea pigs, and birds. In addition, several recent pertinent publications are reviewed, which provide novel information on these procedures.
A vast majority of the pet ferrets in the United States are spayed and neutered at a single large breeding facility at eight weeks of age or earlier; therefore, this procedure is rarely performed in ferrets in private practice within this country. If not bred, intact female ferrets will remain in estrus for most of the year; this puts them at risk for hyperestrogenism and subsequent life-threatening bone marrow suppression.
Ovariohysterectomy in a ferret is very similar to a cat. The ferret’s uterus is bicornuate, and it is located just dorsal to the bladder (Figure 1). Ovarian vessels may be difficult to locate due to the large amount of intrabdominal fat in older intact female ferrets. Castration via a scrotal approach in a male ferret is also very similar to domestic cat.
Rabbits have a duplex uterus, two cervices, no uterine body, and a long vagina. The mesometrium has abundant fat stores, especially in older females (also called does). In males, the penis is caudal to the testes, and the inguinal canals remain open throughout their life.
An ovariohysterectomy should be performed in females less than 12 months of age, ideally between six to nine months of age, depending on the size of the rabbit. With age, fat is deposited within the mesometrium making it more challenging to visualize the vasculature.
Ovariohysterectomy in rabbits is similar to a cat. The rabbit uterus is in the caudo-ventral abdomen; therefore, visualization is straightforward upon entering the abdomen. Rabbit soft tissues are more delicate than domestic dogs and cats, and, therefore, gentle tissue handling is imperative.
Transection of the uterus is recommended at the level of the cervices, to avoid the vagina, which is much longer in rabbits than domestic dogs/cats. If neoplasia is suspected to involve the cervices, surgical excision is recommended, which would require transection of the uterus at the level of the vagina.
Appropriate post-operative analgesia and monitoring for signs of gastrointestinal ileus are of utmost importance in all hind-gut fermenters, such as rabbits. In addition, rabbits are prone to development of post-surgical intra-abdominal adhesions, which can lead to colonic stricture and other complications.
This procedure is highly recommended in female rabbits to not only prevent pregnancy, but also to reduce the incidence of uterine disease. Rabbits greater than four years of age have a 50 to 80 percent incidence (depending on breed) of uterine disease, the most common of which is uterine adenocarcinoma (Figure 2). In addition, endometrial hyperplasia and endometrial venous aneurysms are also commonly reported, both of which can lead to life-threatening hemorrhage.
In a recent study of 1,928 pet rabbits, 54 percent had uterine adenocarcinoma, and 44 percent had endometrial hyperplasia. The most common reported signs of these diseases were hematuria or serosanguinous vaginal discharge. Those authors also found the incidence of uterine adenocarcinoma was positively correlated with increasing age of the rabbit.
Unlike other domestic species, prostatic and testicular diseases are uncommon in male rabbits. Therefore, the most common reason to neuter a male is to prevent reproduction and occasionally, behavioral issues (urine spraying, aggression, etc.). Castration can be performed as soon as the testicles are palpable, which could be as early as 10 weeks of age.
Prescrotal and scrotal approaches for a rabbit neuter have been described and are similar to other domestic mammals. The most common castration method is a scrotal approach and a closed technique. If an open technique is pursued, closure of the inguinal rings is imperative as they remain open throughout life.
A recent prospective study compared prescrotal to scrotal approaches for rabbit neuters, and those authors found the scrotal approach had a higher rate of scrotal edema compared to those with a prescrotal approach. However, both techniques were considered safe and effective.
Female guinea pigs have a bicornuate uterus with a short uterine body and single cervix. They have a long gestation length, compared with other small rodents, of 65 to 71 days. Their pubic symphysis is thought to close if they are not bred before seven to eight months of age, which could cause a dystocia; however, there is little published evidence to support this fact.
Ovariohysterectomies in guinea pigs are more challenging than in rabbits due to small patient size, a short mesovarium, which precludes extensive exteriorization of the ovaries, and they are often higher anesthetic risk patients. The use of cautery or hemostatic clips is highly recommended for this procedure, especially with a standard midline approach.
A flank approach has also been described for both ovariectomies as well as ovariohysterectomies in guinea pigs and other rodents (rats). The incision is made caudal to the last rib at a 45-degree angle ventral to the transverse processes of the vertebrae. The abdominal musculature is bluntly dissected, then the peritoneum is incised or bluntly dissected. The ovary is isolated and the ovarian pedicle is subsequently ligated. This process can be repeated bilaterally, or in some cases, a complete ovariohysterectomy can be performed from a single flank incision (Figure 3).
Ovarian cysts are the most common genital tract disease in guinea pigs, and the incidence increases with advanced age with a reported incidence of 75 percent in animals greater than six years of age. The most common type are cystic rete ovarii, which typically are nonfunctional and do not produce hormones. They can become quite large relative to body size, which may lead to abdominal distension and signs of abdominal pain (Figure 4).
Surprisingly, these large cystic structures are often not visible radiographically due to superimposition of the cecum; however, they are readily visible with an abdominal ultrasound. Functional ovarian cysts (follicular cysts) have also been described less commonly in this species, and these often lead to bilateral flank alopecia and hyperpigmentation/crusting of the nipples.
Castration in male guinea pigs can be performed via scrotal, prescrotal, or abdominal approaches—the scrotal approach is the most common in clinical practice in the author’s experience. The scrotum of guinea pigs is close to the anus and often touches the ground when they ambulate, which increases the risk of post-operative infections. The scrotal approach is similar to a rabbit, but there is a large epididymal fat pad in guinea pigs, which makes the testicles a bit more challenging to exteriorize (Figure 5).
Also, like rabbits, a closed technique is recommended due to open inguinal rings. A published study that compared scrotal to abdominal approaches for guinea pig castration found that the abdominal technique was faster and had significantly lower post-operative infection rates. Regardless of the technique used, it is important to keep the castrated male guinea pig separate from intact females for at least six to eight weeks as viable sperm can remain in the ductus deferens for this timeframe.
Most female avian species have reproductive organs only on the left side, but rudimentary right-sided ovaries or oviducts may persist into adulthood in certain species. The ovary is located at the cranial aspect of the left kidney and is suspended by the mesovarium. The ovary comprises multiple yolk-filled follicles, which resembles a bunch of grapes.
The arterial blood supply is derived from the short cranial renal artery, which has multiple branches within the mass of follicles. The oviduct is located in the left dorsocaudal coelom and is suspended by the mesosalpinx. It is divided into five sections: the infundibulum, magnum, isthmus, shell gland (uterus), and vagina. Shell formation occurs in the uterus, and the egg spends the majority of its time in this location. The oviduct terminates at the urodeum, one of the three sections of the cloaca, which the egg passes through during oviposition (egg laying).
The paired testes are located near the cranial pole of the kidneys, and each is covered with a fibrous tunic (tunica albuginea). The testes dramatically increase in size during breeding season secondary to increased diameter and length of the seminiferous tubules, as well as increased numbers of interstitial cells.
In some passerine species, the testicle can increase 300 to 500 times the size of a nonbreeding teste! The epididymis continues distally as the ductus deferens, which is intimately associated with the ureter. Male birds lack accessory sex glands. Some species possess a phallus located within the cloaca. It can be intromittent (as in ratites and Anseriformes) or non-intromittent (as in Galliformes). Male psittacines and passerines lack a phallus.
Unlike in domestic dogs and cats, birds are not routinely castrated or spayed due to the technical challenges presented by these two surgeries. Removal of the avian ovary is rarely performed due to significant risks for fatal hemorrhage. The ovary is connected to the cranial renal artery and the common iliac vein by several short vessels. In addition, there is significant risk of damaging the left adrenal gland and the kidney, both of which are intimately associated with the avian ovary.
Alternatively, a salpingohysterectomy can be performed if there is oviductal disease. The ovary usually left in place, which does pose a risk for internal ovulation and subsequent yolk coelomitis post-operatively. In certain species, such as psittacines, there is thought to be a negative feedback loop to the ovary, thus after the oviduct has been removed, the ovary becomes quiescent.
Orchidectomy is also rarely performed in clinical practice due to risks of fatal hemorrhage as the testes are closely associated with large arteries, the adrenal glands, and the cranial portion of the kidneys. Incomplete removal and subsequent testicular regrowth are common complications.
Alternatively, a vasectomy could be performed to halt reproduction, which is often performed via minimally invasive techniques, such as coelioscopy or laparoscopy. This can be performed with either a celiotomy or laparoscopically in larger birds. The phallus can be removed (phallectomy) if it becomes damaged or prolapsed as it serves no urinary tract function (Figure 6).
Olivia A. Petritz, DVM, DACZM, graduated from Purdue University, and then completed several internships and a residency in the field of zoo and exotic animal medicine. She became a diplomate in the American College of Zoological Medicine in 2013 and specializes in zoological companion animals (exotic pets). She started an exotics service at a specialty hospital in Los Angeles following her residency, and currently is an assistant professor of avian and exotic animal medicine at North Carolina State University.
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