Introduction

Guinea pigs are popular pets and frequent patients in exotic pet practices. They present unique challenges when it comes to obtaining diagnostic samples and performing routine procedures because of their small size, stocky bodies, short limbs, and low tolerance to prolonged restraints and invasive procedures.

While guinea pigs are easy to restrain, sedation and “flash” inhalant anesthesia considerably facilitate a wide range of common procedures such as diagnostic sampling and imaging. Common sedation protocol typically uses a combination of midazolam 1–2 mg/kg and buprenorphine 0.2 mg/kg IM. Ketamine at 3–5 mg/kg may be added to the protocol depending on the patient’s and procedural needs. Fasting for only a few hours is sufficient in guinea pigs prior to a procedure. As guinea pigs frequently have food in their oral cavity, even after fasting, their oral cavities should be swabbed clean prior to sedation.

Guinea pigs are small, so excessive shaving and use of alcohol may promote hypothermia, especially during surgical procedures and abdominal ultrasound examination.

Diagnostic

Venipuncture

Guinea pigs are hard to take blood from as their peripheral vessels are small and their limbs are short. The lateral saphenous is an easily accessible site that can be used without sedation, but blood volume collected from that site may be small. The tarsal vein is more distal than the lateral saphenous and may also be used for small samples. Small blood volumes may also be harvested from the cephalic veins. An insulin or tuberculin syringe may facilitate blood sampling from these locations and several sites may be concurrently used to obtain enough blood for clinical pathologic tests. The femoral vein may also be sampled. The cranial vena cava site is also used routinely in guinea pigs. Guinea pigs are anesthetized with isoflurane or sevoflurane as a “flash anesthesia” and placed in dorsal recumbency. A 25-g or 26-g needle is inserted laterally to the sternal manubrium, cranial to the first rib in caudomedial direction inside this notch at a 60–70 degrees angle. The vein is typically midline. It is not uncommon to get arterial blood from this location, meaning that major arteries at that site can be inadvertently sampled instead of the cranial vena cava. The procedure is considered relatively safe but large hematomas or laceration of the vein are possible. The jugular veins are another venipuncture site in guinea pigs but is hard to palpate and sample due to the species short neck. Guinea pigs should be sedated for this as they do not tolerate prolonged restraint in this position. Finally, blood may be harvested from the gingival vein, which is located subgingivally, caudally to the lower incisors. Anesthesia or sedation is required for sampling from this site as well.

Deep Nasal Swab

Deep nasal swabs are required to obtain a representative sample of the nasal cavity for culture and sensitivity as nasal discharge is prone to bacterial contamination. Guinea pigs frequently present with upper respiratory signs, nasal discharge, and middle ear disease. Bordetella bronchiseptica is a common culprit for upper respiratory disease, middle ear disease, and pneumonia in guinea pigs. While the bacteria may be acquired from other species such as rabbits, a significant proportion of healthy guinea pigs are also asymptomatic carriers with the nasal cavity being the primary site of carriage. Deep nasal culture may be useful in the management of respiratory diseases in guinea pig colonies. A small tip swab must be used, and the guinea pig should be sedated as this can be uncomfortable. The swab is advanced in the dorsolateral nasal meatus for about 1 cm. It is frequently contaminated with blood. Lidocaine has antimicrobial activity and should thus not be used when sampling the nasal cavity for bacterial culture.

Point-of-Care Ultrasound

Point-of-care ultrasound (POCUS) can easily and quickly be performed in guinea pigs presented for emergencies, prior to obtaining more complete diagnostic imaging such as whole-body X-ray, CT-scan, or a complete abdominal ultrasound. A high-frequency probe should be used (8–10 MHz) in guinea pigs as examined structures are not deep. An A-fast POCUS examination should focus on bladder, stomach, ovaries, and uterus in females, liver, and scanning for abdominal fluid. A small amount of abdominal fluid is normal in guinea pigs. T-fast is a bit harder to do in guinea pigs as their thorax is very small. However, the chest can be screened for common conditions in dyspneic guinea pigs such as pleural effusion, pericardial effusion, and consolidated lungs.

Guinea pigs frequently present for urolithiasis and most commonly have a single large calculus that is readily seen by ultrasonography with strong shadowing. Cystocentesis can be performed, but guinea pigs need to be heavily sedated or anesthetized as they tend to move suddenly.

Ovarian cysts are common in female guinea pigs and an A-fast ultrasound can quickly detect them as well as give an idea of their size. Cysts are anechoic and can be quite large. Ultrasound-guided drainage of large cysts is possible to relieve pressure on internal organs. Finally, hepatic lipidosis is common in anorexic guinea pigs and a hyperechoic liver may raise the suspicion for this disorder.

Therapeutic Techniques

Urethral Catheters

Urethral catheterization is relatively easy in guinea pigs in both males and females because of their wide urethra. Urolithiasis is extremely common in guinea pigs. Catheterization is often used to try to retropulse calculi into the bladder, especially in males. Urethral catheterization is also recommended in males prior to cystotomy to prevent migration of calculi into the urethra preoperatively, which is not uncommon and may lead to urethral obstruction and remain undetected prior to starting the surgery. In males, the penis has an os penis and also an intromittent sac caudoventrally that should not be confused with the urethral opening. Two horny styles (projections) are visible on the intromittent sac when it is everted. A small 5–8 Fr tube is placed in the urethral opening and can be secured by sutures to the prepuce and skin. In females, the urethral opening is located outside of the vagina on the urinary papilla, cranial to the vaginal opening, as in other rodents. It is easily catheterized and advanced into the bladder.

Nasogastric Tubes

Nasogastric tubes are harder to place in guinea pigs than in rabbits because of their smaller size. The food that is administered through these tubes should be more dilute and using a product with a finer powder. Oxbow fine grind® contains smaller particle size and may be used for this purpose. They are also obligate nasal breathers and placement is contraindicated with concurrent respiratory disease. Guinea pigs should be sedated, and a local anesthetic may be instilled locally into the nostril. A <5Fr tube should be used and pre-measured from the nose to the stomach (last rib). The tube is lubricated and inserted into the dorsolateral nasal meatus and advanced slowly. Verification of tube placement may be determined by aspiration of stomach contents or radiography. The tube is sutured or glued to the perinasal skin and on top of the head.

Anesthetic Techniques

IV and IO Catheterization

Intravenous catheters are difficult to place in guinea pigs because of their small and short legs, but also their tough skin, especially in males. A small cut-down with a 22-g needle should be performed first and guinea pigs should be sedated. Veins that can be used include the cephalic vein, the lateral saphenous vein, and the tarsal vein. Using a 26-g catheter is best. A flash back of blood into the catheter is not necessarily seen prior to advancing the catheter, especially in smaller or debilitated animals.

The tibia is the most easily accessible site for intraosseous placement. A 22-g spinal needle or 25-22 hypodermic needles can be used.

Intubation

Intubation is difficult in guinea pigs due to their pharyngeal anatomy. Guinea pigs have a palatal ostium at the back of their pharynx that is made by the fusion of the soft palate to the base of the tongue. There are several techniques to guinea pig intubation. The author favors using an endoscope to visualize the back of the throat. The classic 2.7-mm 30-degrees rigid endoscope with its operating sheath works well. As guinea pigs may salivate excessively and have food debris, cotton-tip applicators should be readily available to clean the throat and oral cavity prior to intubation attempts. The guinea pig is placed in ventral recumbency on a dental table with elevation of the head and body at about 60–70 degrees in relation to the table and the mouth open with the mouth gag. Once the soft palate is visualized, gentle pressure with the endoscope will allow to see the epiglottis and sometimes the glottis. A small stylet (small plastic stylet or soft guidewire) should be placed in the trachea (just dorsal to the epiglottis if the glottis is not visible). The endoscope is then removed while holding the stylet making sure not to pull it out. A 2-mm endotracheal tube or 14–16-g catheter is fed around the guidewire into the trachea. Placement is confirmed using a capnograph. A similar technique using an otoscope is also possible.

Dentistry Techniques

Guinea pigs have an aradicular (no roots), elodont (continuously growing), hypsodont (high clinical crown) dentition with the following dental formula 2 × (1/1 0/0 1/1 3/3). Their dental arcades are anisognathic with the maxillary arcade narrower than the mandibular arcade. The dental arcades also converge rostrally and are not on a straight line. Cheek teeth are bilophodont (2 subunits). Dental disease in guinea pigs consists mainly of clinical and reserve crown elongation. Clinical crown elongations tend to be homogeneous and involve the whole arcade and dental spurs and steps are less common. As the dental arcades converge rostrally, severe clinical crown elongation of the rostral cheek teeth coupled to malocclusion will lead to the mandibular teeth colliding and “bridging,” thereby entrapping the tongue.

Guinea pigs should be fully anesthetized for dental procedures. They are typically not intubated for dentistry as they are hard to intubate (see above) and the oral cavity is already small. They are placed on a dental table with a mouth gag and a cheek spreader and clinical crown can be reduced and occlusal plane adjusted with a diamond-coated rotatory burr under stomatoscopic guidance. The back molars specifically deserve special attention as even mild elongation may lead to a fulcrum effect. Periodontal disease is not uncommon in them with pocketing of the gingiva around the cheek teeth that can be filled with hair and debris. This should be cleaned during the procedure. Incisors should also be trimmed in proper occlusion.

Surgeries

Castration

Orchidectomy is recommended in guinea pigs to decrease aggression and prevent reproduction. There are several techniques for castration in guinea pigs. Guinea pigs have a poorly developed scrotum and have an open inguinal canal. Because scrotal incisions are easily soiled in guinea pigs, an abdominal approach for orchidectomy is preferred to reduce complications. A small laparotomy incision is made caudally to the umbilicus, the testis are pushed back inside the abdomen, and then the abdomen is gently squeezed until the epididymal fat is observed through the abdominal incision. The fat is gently grasped and exteriorized, which will also exteriorize the testicles. The spermatic cord is then sutured and the procedure is repeated for the contralateral testis. The abdomen and skin are closed routinely.

A scrotal approach with either a closed or open technique is also feasible but will lead to more postoperative complications such as scrotal infection and wound dehiscence. The technique is similar as in other rodents and in rabbits.

Ovariectomy

Ovariectomy is recommended in guinea pigs to prevent reproduction in colonies but also to prevent the occurrence of ovarian cysts, which are extremely common in this species. The ovaries are hard to exteriorize through a midline approach in guinea pigs due to the large size of the gastrointestinal system and the short length of ovarian ligament. The midline approach is still recommended with large ovarian cysts or uterine pathology. The incision may be larger than for other rodent species and there may be more manipulation of the internal organs. Sutures should be placed more in depth or the ovarian ligament may be severed prior to further exteriorization.

Ovariectomy through the flank is relatively easy in guinea pigs and is less painful than a midline incision and does not require manipulating the gastrointestinal system. The guinea pig is placed in ventral or lateral recumbency. On the right or left side, a skin incision is made immediately ventral to the epaxial muscles and about 1 cm caudal to the last rib. Once the skin incision is made, subcutaneous tissue and the muscle layer are bluntly dissected with fine hemostat until the abdominal cavity is seen. If the peritoneal membrane is not open, it can be grasped and opened with tenotomy scissors. Then, the visible fatty tissue can be grabbed and pulled through the incision, which will bring the ovary with it. In some cases, pressing on the abdomen will readily exteriorize this fat. The ovarian pedicle is sutured and transected as well as the periovarian oviductal tissue.

Ovarian cysts may be drained prior to removal through either technique, but large cysts are more easily removed through a midline ventral approach.

Cystotomy

Cystotomy is not technically difficult in guinea pigs. However, there is a high rate of mortality postoperatively that may be linked to perianesthetic death or comorbid diseases (sepsis, hepatic lipidosis, chronic anorexia, stress, and pain). There is typically a single calculus, therefore postoperative radiographs may not be necessary. It is recommended to catheterize the urethra prior to going to surgery, as it is not uncommon for smaller calculi to migrate to the urethra following sedation and during presurgical preparations. In case of a smaller calculus, the enclosure and litter box should be checked as some guinea pigs will spontaneously passed smaller calculi. In doubt, radiographs may be repeated prior to surgery.

Cystotomy is routine and as described in other species. Calcium carbonates are the most common types of calculi. Concomitant bacterial cystitis is common, particularly with Corynebacterium kutscheri.

References

1.  Brabb R, Newsome D, Burick A, Hanes M. Infectious diseases. In: Suckow MA, Stevens KA, Wilson RP, eds. The Laboratory Rabbit, Guinea Pig, Hamster, and Other Rodents. San Diego, CA: Academic Press; 2012:637–683.

2.  Guilmette J, Langlois I, Helie P, de Oliveira El Warrak A. Comparative study of 2 surgical techniques for castration of guinea pigs (Cavia porcellus). Can J Vet Res. 2015;79(4):323–328.

3.  Huneke RB. Basic experimental methods. In: Suckow MA, Stevens KA, Wilson RP, eds. The Laboratory Rabbit, Guinea Pig, Hamster, and Other Rodents. San Diego, CA: Academic Press; 2012:621–637.

4.  Pignon C, Mayer J. Guinea pigs. In: Quesenberry KE, et al, eds. Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery. St. Louis, MO: Elsevier; 2021:271–297.