On the morning of April 12, 2016, the incomparable racing mare Zenyatta gave birth to her fourth foal. Though described as a “textbook” delivery, the foal was stained with meconium and died a day later of meconium aspiration syndrome (MAS).
Meconium—the dark, sticky feces foals first pass—is composed of gastrointestinal secretions such as bile, pancreatic enzymes and mucous, desquamated cells, lanugo, vernix caseosa, water, amniotic fluid and other cellular debris. Although sterile, meconium is locally irritative and obstructive and is a medium for bacterial culture.1,2
Fortunately, aspiration caused by the release of meconium in utero is rare in horses. “The literature is limited on the number of cases per annum, and if you asked veterinarians to guess the number of foals affected, their answers would vary based on their type of caseload,” says Holly Mason, MS, DVM, clinical assistant professor at Utah State University School of Veterinary Medicine. “A neonatologist working in a neonatal intensive care unit may give you a higher number because they’re given the most challenging cases, but having foaled out several hundred mares, I can recall only two or three cases of MAS.”
A vicious cycle
According to Kent Scarratt, DVM, DACVIM, associate professor at Virginia-Maryland College of Veterinary Medicine, anything that would cause a hypoxic event in a foal can cause the release of meconium in utero. Potential triggers include include compression of the umbilical blood vessels, placentitis and placental insufficiency.
When a foal becomes hypoxic in utero, a vicious cycle can ensue. Hypoxemia can stimulate gastrointestinal peristalsis that passes meconium in utero. It can also trigger the foal to reflexively gasp for air and aspirate meconium and amniotic fluid into its lungs.
When aspirated, meconium is considered a toxin due to its damaging effect on surfactant and lung tissue, Mason says. Meconium also occludes the airways, from the upper airway to the alveoli. “Meconium can act like a one-way valve,” Mason explains. “During inspiration, air may be able to pass through a partial obstruction and fill the deeper portion of the lungs. But in the expiratory phase, air can become trapped behind the meconium obstruction and accumulate to the point where it ruptures lung tissue, causing a pneumothorax.
“Neonatal foals are delicate creatures and are unlikely to fend off the effects of meconium aspiration without—and sometimes even with—veterinary intervention,” Mason continues. “Even if we consider meconium a sterile substance, there is likely to be a bacterial infection associated with aspiration.”
Proactive treatment of MAS starts with good broodmare management and the ability to recognize and treat problems as they arise during late gestation and through parturition, Mason says. “It’s not normal for a foal to pass meconium while still in utero,” she explains. “Hence, when it does occur it’s a strong indicator of periparturient fetal distress.”
For this reason Mason recommends that equine veterinarians attend each foaling. If the mare needs assistance during delivery, the veterinarian can act quickly to mitigate fetal stressors. Even if parturition appears uneventful, it’s critical to identify signs of meconium exposure immediately postpartum, she says—signs typically include a mustard-colored hue to the amnion, to nasal discharge or to any nonpigmented hair.
Though meconium staining is not a guarantee that meconium was aspirated, only that a foal was exposed, veterinarians who observe it should consider diagnostics and treatment to prevent MAS, Mason continues. Prophylactic antimicrobial therapy and careful monitoring typically ensure good health. In those rare cases in which aspiration occurs and severe pneumonia develops—as indicated by abnormal vital signs, a leukogram or diagnostic imaging studies—a much more guarded prognosis is appropriate.
MAS cases may present a particular challenge for mobile equine veterinarians. “Once meconium aspiration is suspected or confirmed, appropriate aggressive treatment is often difficult for ambulatory practitioners because of the supportive care and diagnostics the foal requires,” Mason explains. “Ambulatory practitioners may wish to refer these cases to a well-staffed and better-equipped veterinary facility.”
Scarratt notes that treatment will depend on how quickly a veterinarian recognizes signs of MAS after the foal’s birth. “If a veterinarian observes the foaling or is foaling the mare out and notices meconium staining as the foal exits the birth canal, suction can be used to aspirate the meconium from the nose and pharynx,” he says. “An affected foal would likely show signs of respiratory distress within hours of being born.”
Because MAS foals have pneumonia and difficulty absorbing oxygen into their bloodstream, oxygen supplementation is a mainstay of treatment, Scarratt says. Antibiotics are often administered to prevent and treat bacterial pneumonia resulting from aspiration of meconium, and anti-inflammatory drugs such as flunixin or firocoxib are indicated to minimize inflammation in the lungs. Scarratt also uses gastroprotectant drugs such as omeprazole or ranitidine to minimize the possibility of gastrointestinal ulceration, which is occasionally seen in a foal being treated with an anti-inflammatory agent.
“Supportive care is also critical,” Scarratt concludes. “It’s important to make sure the foal has absorbed an adequate amount of antibodies from the colostrum and that the newborn continues to receive adequate nutrition from its mother. If the foal is not adequately hydrated, it may require intravenous fluids.”
MAS in humans
MAS occurs in human infants much more frequently than in foals. One research team notes that meconium-stained amniotic fluid is present in about 15 percent of deliveries and leads to MAS in 10 to 15 percent of those cases, typically in term and post-term infants.3 Other investigators estimate the fatality rate to be between 5 and 40 percent.4
In human medicine it is fairly common to induce labor or to deliver a meconium-affected fetus via cesarean section to try to get the fetus out as soon as possible—a strategy that does not translate well to equine care.
“Unlike human neonatology, induction of parturition in the mare is likely to result in dystocia, retention of fetal membranes and a foal that does very poorly as a neonate,” Mason says. “For this reason, I can only recall a few cases in my two decades of foaling experience in which induction was undertaken.”
In Mason’s experience, most practitioners choose to “wait out” an overdue mare rather than create a likely problem with an intervention.
According to human literature, physicians have tried amnioinfusion, in which 800 ml to 1 liter of sterile saline is infused in utero in an attempt to dilute the meconium, without much success. But lucinactant (Surfaxin—Discovery Laboratories) has been found to be beneficial in replacing damaged lung surfactant.5
MAS in other animal species
Meconium aspiration is also seen in other animal species, including piglets, calves and dolphin calves. In piglets, meconium staining is associated with fetal hypoxia, stillbirths, weak piglets and neonatal mortality.6 Researchers also note that hypoxia leads to visceral redistribution of fetal blood, increased peristalsis and relaxation of the anal sphincter that eventually prompts defecation in utero, along with violent inspiratory movements with an opened glottis causing aspiration of meconium-containing amniotic fluid.6 These piglets either die in utero or survive only to die days later after succumbing to postnatal MAS.
In one study of 52 calves that died from infectious and non-infectious disease within the first two weeks of life, 42.5 percent of calves had meconium, squamous cells or keratin in the lungs. The researchers also note that intrauterine aspiration of meconium is one of the most common events preceding abortion in bovine fetuses and that severe and prolonged intrauterine hypoxia can lead to aspiration of meconium and amniotic fluid by these fetuses.7
Remarkably, there has been a case of MAS “due to fetal hypoxia possibly associated with true knot of the umbilical cord in a bottlenose dolphin calf”—the first reported case. The researchers write that MAS due to “perinatal asphyxia should be taken into account as a possible cause of neonatal mortality and stillbirths of (dolphin) calves.”8
A sobering story and testament
The passing of Zenyatta’s foal is a sad reminder of the ups and downs experienced by those with a passion for the thoroughbred industry, Mason says. “Unfortunately, the loss of a foal, a mare unable to conceive or carry to term, or a foal that does not live up to expectations are all common events in the broodmare industry,” she says. “These misfortunes serve as a testament to the dedication and drive of farm managers, owners and veterinarians that love what they do.”
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2. van Ierland Y, de Beaufort AJ. Why does meconium cause meconium aspiration syndrome? Current concepts of MAS pathophysiology. Early Hum Dev 2009;85(10):617–620.
3. Hermansen CL, Lorah KN. Respiratory distress in the newborn. Am Fam Physician 2007;76(7):987-994.
4. Xu H, Wei S, Fraser WD. Obstetric approaches to the prevention of meconium aspiration syndrome. J Perinatol 2008;28(Suppl 3):S14-S18.
5. Wiswell TE, Knight GR, Finer NN, et al. A multicenter, randomized, controlled trial comparing Surfaxin (Lucinactant) lavage with standard care for treatment of meconium aspiration syndrome. Pediatrics 2002;109(6):1081-1087.
6. Castro-Nájera JA, Martínez-Burnes J, Mota-Rojas D, et al. Morphological changes in the lungs of meconium-stained piglets. J Vet Diagn Invest 2006;18(6):622-627.
7. Lopez A, Bildfell R. Pulmonary inflammation associated with aspirated meconium and epithelial cells in calves. Vet Pathol 1992;29(2):104-111.
8. Tanaka M, Izawa T, Kuwamura M, et al. A case of meconium aspiration syndrome in a bottlenose dolphin (Tursiops truncatus) calf. J Vet Med Sci 2014;76(1): 81–84.