- The pubic symphysis is a fibrocartilaginous bridge in breeding female guinea pigs. After about 6 months of age in males and unbred females, the symphysis calcifies and becomes permanently fused.
- Dystocia is most common in sows bred and having their first litter after approximately 6 months of age because the pelvic symphysis is fused.
- The most common pup-related cause of dystocia is large fetal size.
- Parturition proceeds quite rapidly in the normal guinea pig. A sow that has been in active labor for more than 10-20 minutes or intermittently for greater than 2 hours is typically in dystocia.
- The prognosis is poor to grave for guinea pigs that require caesarean section.
- Prevention of dystocia is much more effective than treatment. Prevent obesity and encourage exercise in the pregnant sow. Closely monitor the sow at Day 65 of gestation or later for pregnancy-related problems.
Dystocia is defined as the inability of a sow to deliver her litter normally. In breeding colonies, maternal mortality and loss of the pup is an important and common problem in the guinea pig (Cavia porcellus) (Fig 1) (Williams 2012, Brower 2006).
Pathogenesis of disease
Many variables can increase the risk of dystocia (Box 1). The most important maternal reason for dystocia is when the sow is bred too late. Female guinea pigs must be ideally mated for the first by 5-6 months, because the pubic symphysis must be open to allow normal delivery of guinea pig pups (DeCubellis 2016, Evans et al 2013, Shomer et al 2013, Gresham & Haines 2012, Paul-Murphy 2011, Brower 2006). Sows can be bred as early as 2-3 months or 350-450 grams body weight (Gresham & Haines 2012).
|*Pup size increases as litter size decreases|
The birth canal of the sow is narrow at the level of the os coxae (Williams 2012). The guinea pig pubic symphysis is a fibrocartilaginous bridge that binds the left and right pubic bones that sit on the floor of the pelvic canal (Brower 2006). As the guinea pig grows, this bridge calcifies and becomes permanently fused in males and unbred females (Brower 2006). In the pregnant female, the symphysis relaxes and stretches in response to secretion of relaxin by the pituitary gland and endometrium, allowing separation of the pubic bones (Williams 2012, Brower 2006, Martinho 2006). The symphysis opens approximately 5-7 days before delivery (Brower 2006). The pubic symphysis normally spreads to approximately 1.5 cm at 48 hours pre-partum and then increases to 2-3 cm just prior to delivery (Hawkins & Bishop 2012, Quesenberry & Donnelly 2012, Brower 2006). The symphysis returns to normal within 24 hours post-partum (Brower 2006). If the sow is not bred for the first time before 6-7 months of age (Shomer et al 2013, Gresham & Haines 2012), and certainly after 10 months, the pelvic symphysis fuses permanently in close apposition preventing passage of fetuses (Brower 2006).
Obesity is another important maternal factor as intrapelvic fat can impede delivery (Kondert & Mayer 2017, Hawkins & Bishop 2012, Blaes 2010, Brower 2006). Uterine inertia can develop secondary to muscle fatigue with large fetal size or positional dystocia (Kondert & Mayer 2017, Minarikova et al 2015, Shomer et al 2013, Hawkins & Bishop 2012, Brower 2006). Uterine inertia can also occur secondary to pregnancy toxemia, hypocalcemia, or infection, as toxins produced with infection or pregnancy ketosis can weaken uterine musculature (Shomer et al 2013, Brower 2006). The large gravid uterus is prone to torsion (Percy & Barthold 2009) and vitamin C deficiency has also been theorized to play a role in dystocia (Kondert & Mayer 2017, Minarikova et al 2015, Hawkins & Bishop 2012).
The most common pup-related cause of dystocia relates to fetal size (Kondert & Mayer 2017, Minarikova et al 2015, Shomer et al 2013, Brower 2006). The normal neonate is approximately 10% of sow body weight (Blaes 2010). There are normally two to four young in a litter and birth weights can reach up to 115 grams (Quesenberry & Donnelly 2012). Pup size increases as the number born decreases, therefore the risk of dystocia is higher when only one or two pups are born (Blaes 2010). The head or pelvis of these pups can be extremely large, which also enhances the odds of a difficult delivery (Blaes 2010).
Although a breech presentation can be normal, fetal malposition is another possible cause of dystocia (Blaes 2010). Problems can also occur when only one rear leg is presented in the birth canal, when one or both forelegs point backwards, and when the head presents facing backwards, sideways (ears first) or tipping downwards (Blaes 2010). Dystocia can also develop when the back or shoulders present first at the birth opening (Blaes 2010).
Fetal malformations are a rare cause of dystocia, however conditions such as conjoined twinning have been reported (Blaes 2010). A “bull-backed” abnormality has been associated with lethal gene of roans, which leads to a massive, short-backed, arched fetus with an enlarged head (Blaes 2010).
Gestation can range between 59-72 days in the guinea pig (Minarikova et al 2015, Quesenberry & Donnelly 2012, Brower 2006). The sow does not build a nest (Gresham & Haines 2012, Brower 2006). Once parturition has begun, pups are delivered rapidly with the entire birthing process lasting a total of 30-45 minutes (Brower 2006). Once strong abdominal contractions have begun, a pup appears within 5 minutes or less. The interval between delivered pups, or the rest period, is 3-10 minutes. All pups are delivered within 10-30 minutes (Brower 2006).
Dystocia can be challenging to detect early in the guinea pig because of this variable gestation length paired with the sow’s lack of nest building and the abrupt onset of labor (Brower 2006). A sow that has been continuously and unsuccessfully straining for more than 10-20 minutes or intermittently for greater than 2 hours is typically experiencing dystocia (Kondert & Mayer 2017, Evans et al 2013, Brower 2006).
Differentials for dystocia in the guinea pig include pregnancy toxemia and pseudopregnancy (Paul-Murphy 2011, Johnson-Delaney 2011, Hawkins & Bishop 2012). There are also rare reports of ectopic pregnancy presenting with the classic signs of dystocia (Martinho 2006, Hong & Armstrong 1978).
- A sow with the metabolic form of pregnancy toxemia usually shows no evidence of contractions, but she will often appear anorectic and depressed. Laboratory results include ketonuria (Johnson-Delaney 2011, Paul-Murphy 2011).
- The circulatory form of pregnancy toxemia, also known as preeclampsia, is caused by ischemia of the placenta and uterus due to compression of the blood supply by the large, gravid uterus. The affected sow can present in a hypotensive state, or even in shock (Johnson-Delaney 2011, Hawkins & Bishop 2012). Laboratory results can include proteinuria, elevated creatinine levels, and a hemogram suggestive of fetal death and decomposition (see blood work below).
- If fertilization fails, pseudopregnancy can develop and last 72 days. This condition can be differentiated from pregnancy by palpation as well as imaging 6 weeks after mating (Paul-Murphy 2011).
If dystocia is suspected, timely determination and prompt treatment are required (Brower 2006). Treatment will vary with the underlying cause, so the first hurdle will be to identify the underlying cause of dystocia.
On physical examination, the sow is often restless and irritable (Paul-Murphy 2011). Signs of pain, such as hunched posture or even biting at the lateral body wall, can be observed (Paul-Murphy 2011). The sow may also appear weak and depressed due to exhaustion when labor has been prolonged (Shomer et al 2013, Paul-Murphy 2011, Brower 2006). In severe cases, there may be bloody vulvar discharge or the green-brown, ink-like discharge caused by placental separation (Kondert & Mayer 2017, Shomer et al 2013, Hawkins & Bishop 2012, Paul-Murphy 2011, Brower 2006). Sows with preeclampsia or hypocalcemia may exhibit muscle spasms and possibly convulsions (Brower 2006).
Gently palpate the abdomen to determine if uterine contractions are present. As dystocia advances, contractions are often mild or even absent (Paul-Murphy 2011). Fetal masses may be palpated in the caudal abdomen and a fetus may also be visible or palpable within the birth canal. Visually examine the birth canal, then insert a digit covered with sterile, water-soluble lubricant to determine the presence and position of the pups. Also evaluate the degree of separation of the pubic symphysis by measuring the distance between the two pubic bones (DeCubellis 2016, Evans et al 2013, Paul-Murphy 2011, Brower 2006).
If no signs of physical obstruction are identified during the exam, obtain survey radiographs to evaluate the birth canal and the pubis in relation to pup size and to exclude other possible mechanical obstructions (Kondert & Mayer 2017, Paul-Murphy 2011). Examine the pelvic symphysis and confirm it has not fused. Determine the number, size, and position of the fetuses (Paul-Murphy 2011).
Abdominal ultrasound is useful for confirming the number of fetuses and determining the viability of pups (Kondert & Mayer 2017, DeCubellis 2016, Paul-Murphy 2011). Ultrasonography can also be used to determine if uterine inertia is present (Paul-Murphy 2011).
Perform laboratory assessment to look for evidence of toxemia, metabolic complications, or infection (DeCubellis 2016). Prolonged dystocia can result in hypoglycemia and dehydration, and hypocalcemia can be observed with uterine inertia therefore ionized calcium levels should be measured (Paul-Murphy 2011). If fetal death has occurred, toxic heterophils, thrombocytosis, and other changes suggestive of acute inflammation can be observed (Paul-Murphy 2011).
If the sow shows signs of exhaustion, provide energy in the form of Karo® syrup or 50% dextrose on the gums, and fluid therapy (Kondert & Mayer 2017, DeCubellis 2016, Brower 2006). Glucose can also be added to intravenous fluids (Kondert & Mayer 2017, Paul-Murphy 2011). Intravenous access can be difficult in the guinea pig, therefore consider intraosseous catheter placement in select patients (Paul-Murphy 2011). Additional supportive care can include supplemental heat, nutritional support, analgesia, and measures to minimize stress (DeCubellis 2016).
If the pubic symphysis has separated, lubricate the birth canal with water-soluble lubricant to assist parturition (DeCubellis 2016, Evans et al 2013, Hawkins & Bishop 2012).
If uterine inertia is suspected and the pubic symphysis has separated, consider oxytocin (0.2-0.3 IU/kg IM) to stimulate contractions (Box 2) (DeCubellis 2016, Hawkins & Bishop 2012, Paul-Murphy 2011, Brower 2006). Oxytocin will induce parturition if the pig is over 66 days of pregnancy (Brower 2006). Oxytocin should induce delivery within 30 minutes, but can be repeated in 15-20 minutes (Paul-Murphy 2011). Doses up to 1-3 units/kg IM, SC have also been described (Kondert & Mayer 2017, Shomer et al 2013) If no pups are produced within 15-30 minutes, consider surgical intervention (Brower 2006).
Remember that oxytocin will be ineffective if the muscles are too fatigued to respond. In cases of hypocalcemia, 5-10 ml of calcium gluconate can first be delivered as a 10% solution per os (Kondert & Mayer 2017, Paul-Murphy 2011).
Manual removal of the pup is sometimes possible for malpositioned or large pups using careful redirection and gentle traction; however there is risk to both the pup and the sow (Shomer et al 2013, Blaes 2010). Potential complications include vaginal or uterine tears and/or prolapse (Blaes 2010). If manual extraction is successful, remove fetal membranes promptly.
If these efforts fail to resolve dystocia or if separation of the pelvic symphysis is less than 2-2.5 cm, then surgical intervention is indicated (Box 3) (DeCubellis 2016, Paul-Murphy 2011, Brower 2006). Unfortunately, guinea pigs in dystocia do not handle the stress of anesthesia and surgery well. Provide preemptive and postoperative analgesia and balanced anesthesia (Paul-Murphy 2011, Brower 2006). Multimodal analgesia that includes opioids and non-steroidal anti-inflammatory drugs is recommended, however NSAIDs are contraindicated in the dehydrated and/or azotemic patient (Paul-Murphy 2011).
Preoperatively, tilt the surgical table so the patient’s chest is higher than the abdomen to minimize compression of the chest by the large, gravid uterus. During surgery, open the uterus close to the bifurcation of the horns (Evans et al 2013). Minimize blood loss and avoid placing excess sutures or tying sutures too tightly (Brower 2006). Also avoid excessive handling of the gastrointestinal tract to reduce the incidence of postoperative ileus or fibrous adhesions (Paul-Murphy 2011). It is advisable to surgically sterilize the sow to prevent future pregnancies (Paul-Murphy 2011). As viable young are removed, an assistant should clean, then immediately stimulate pups to breathe.
Postoperatively, encourage the sow to eat as soon as possible and continue to provide pain control (Brower 2006). Monitor the patient’s appetite and eliminations closely and ensure the sow is well hydrated (Paul-Murphy 2011). Antibiotics may be indicated if vaginitis is present or if the surgical incision was not maintained as aseptically as would be ideal (Paul-Murphy 2011). Following delivery of pups, metritis or pyometra can also develop from ascending bacterial infection (Paul-Murphy 2011). Obturator nerve paralysis is also a potential sequela of dystocia (Percy & Barthold 2009).
Uncomplicated dystocia is associated with a good prognosis for recovery (Evans et al 2013), however the prognosis is guarded to poor for guinea pigs that require surgery (Brower 2006). Since guinea pig young are relatively well developed, they have a high demand for oxygen and a low tolerance for carbon dioxide, and will quickly deteriorate if they are still within the uterus as placental separation occurs (Brower 2006).
Guinea pig young are precocial (Fig 2). They are born with teeth and hair, and the eyes and ears are open (Brower 2006). Pups may not nurse immediately, but when they do so they sit in a prone position (Brower 2006). Weigh pups daily. Young that lose weight or that weigh less than 60 grams carry a poor prognosis (Quesenberry & Donnelly 2012).
Orphaned guinea pigs will often refuse food for the first 12 hours postpartum (Brower 2006). Fortunately, the neonate obtains all maternal antibodies from the placenta, not the milk (Brower 2006). Weaning is generally complete by 21-28 days of age (Brower 2006). Separate male and female guinea pigs by the time they are 21 days old as the females will begin cycling (Brower 2006).
Prevention of dystocia is much more successful than treatment in the guinea pig. Educate breeders about the risk factors of disease. Ensure the sow is first bred before 6-7 months of age to make sure the pubic symphysis remains open (Shomer et al 2013, Paul-Murphy 2011, Brower 2006). Prevent obesity in sows by providing adequate space for exercise and monitoring food intake to avoid overfeeding (Shomer et al 2013, Paul-Murphy 2011, Brower 2006). Exercise is particularly important during pregnancy (Brower 2006). A gravid sow can more than double her weight and her food consumption triples (Williams 2012, Brower 2006). Be sure to offer a balanced diet, that includes sufficient vitamin C (up to 30 mg/kg/day) (Brower 2006).
Teach owners to recognize early signs of pregnancy-related problems (Kondert & Mayer 2017, Brower 2006). Sows that are at risk must be monitored closely at day 65 or later for non-specific signs of illness, such as anorexia and lethargy, and for signs of premature labor, such as vulvar discharge (Hawkins & Bishop 2012, Brower 2006). Delivery usually occurs at night (Brower 2006). It is possible for at-risk sows to deliver normally; however an assisted or caesarean delivery may be required (Hawkins & Bishop 2012).
Note: Marcia Brower is an attending laboratory animal veterinarian at Boston Scientific in Saint Paul, Minnesota.
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