Many agents that causereproductive failure in sows produce a broad spectrum of sequelae, includingabortions and weak neonates, as well as stillbirth, mummification, embryonicdeath, and infertility. Mummification is seen more frequently in swine than in manyother species because of the large litter size. If only a few fetuses die,abortion rarely occurs; instead, mummies are delivered at term, along with livepiglets 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 orimplantation. Increased embryonic mortality and increased irregular return toestrus are seen in pigs bred during the summer. High ambient temperature mayplay a role, but there is evidence that seasonal low progesterone levels are amajor factor.
The estrogenic mycotoxinszearalenone and zearalenol interfere with conception and implantation, causinginfertility, embryonic death, and reduced litter size, but rarely, if ever,abortion. Another class of mycotoxins, the fumonisins, causes acute pulmonaryedema in swine; sows that recover from the acute disease often abort 2–3 dayslater.
Other toxic causes ofabortions or stillborn pigs include cresol sprays (used for mange and lousecontrol), dicumarol, and nitrates. Nutritional causes of reproductive failureare not well defined. Vitamin A deficiency can cause congenital anomalies andpossibly abortions. Riboflavin deficiency can cause early premature births(14–16 days), and calcium, iron, manganese, and iodine deficiencies have beenassociated with stillbirths and weak pigs.
Carbon monoxide toxicity dueto faulty propane heaters has been associated with increased numbers ofstillbirths and autolyzed full-term fetuses. Fetal tissues are cherry red; thesows 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 anarterivirus. It is of major importance in the USA and throughout most of theworld. Most PRRS strains do not cross the placenta until after 90 days ofgestation. Consequently, most abortions are near the end of gestation.
Affected litters contain freshand autolyzed dead pigs, weak infected pigs, and healthy, uninfected pigs thatoften develop respiratory disease within a few days of birth. The sows areoften 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 mostconsistently present in the fetal thymus and in fluid collected from the fetalthoracic cavity. PCR testing of pooled thoracic fluid from three to fivefetuses is the most reliable means of diagnosis. Herd management is importantin control and prevention. Inactivated and modified-live virus vaccines areavailable.