Many agents that cause reproductive failure in sows produce a broad spectrum of sequelae, including abortions and weak neonates, as well as stillbirth, mummification, embryonic death, and infertility. Mummification is seen more frequently in swine than in many other species because of the large litter size. If only a few fetuses die, abortion rarely occurs; instead, mummies are delivered at term, along with live piglets or stillbirths.
1. Non-infectious Causes
High ambient temperature (>32°C [>89.6°F]) is associated with increased returns to estrus, increased embryonic mortality, decreased farrowing rates, and small litters. The effect is greatest if heat stress occurs at the time of breeding or implantation. Increased embryonic mortality and increased irregular return to estrus are seen in pigs bred during the summer. High ambient temperature may play a role, but there is evidence that seasonal low progesterone levels are a major factor.
The estrogenic mycotoxins zearalenone and zearalenol interfere with conception and implantation, causing infertility, embryonic death, and reduced litter size, but rarely, if ever, abortion. Another class of mycotoxins, the fumonisins, causes acute pulmonary edema in swine; sows that recover from the acute disease often abort 2–3 days later.
Other toxic causes of abortions or stillborn pigs include cresol sprays (used for mange and louse control), dicumarol, and nitrates. Nutritional causes of reproductive failure are not well defined. Vitamin A deficiency can cause congenital anomalies and possibly abortions. Riboflavin deficiency can cause early premature births (14–16 days), and calcium, iron, manganese, and iodine deficiencies have been associated with stillbirths and weak pigs.
Carbon monoxide toxicity due to faulty propane heaters has been associated with increased numbers of stillbirths and autolyzed full-term fetuses. Fetal tissues are cherry red; the sows do not appear affected.
2. Infectious Causes
The major infectious causes of reproductive failure in pigs include porcine reproductive and respiratory syndrome virus, porcine parvovirus, pseudorabies virus, Japanese B encephalitis virus, classical swine fever virus, Leptospira spp, and Brucella suis.
3. Porcine Reproductive and Respiratory Syndrome (PRRS):
PRRS is caused by an arterivirus. It is of major importance in the USA and throughout most of the world. Most PRRS strains do not cross the placenta until after 90 days of gestation. Consequently, most abortions are near the end of gestation.
Affected litters contain fresh and autolyzed dead pigs, weak infected pigs, and healthy, uninfected pigs that often develop respiratory disease within a few days of birth. The sows are often anorectic and feverish a few days before aborting.
Concurrent respiratory disease and increased numbers of bacterial infections in the herd are common. Hemorrhage in the umbilical cord, when present, is the only gross lesion associated with PRRS abortions. Not all fetuses are infected, so multiple fetuses should be sampled.
Viral antigen is most consistently present in the fetal thymus and in fluid collected from the fetal thoracic cavity. PCR testing of pooled thoracic fluid from three to five fetuses is the most reliable means of diagnosis. Herd management is important in control and prevention. Inactivated and modified-live virus vaccines are available.