Problems in sow herds during autumn and winter are often not caused by a single disease, but by a chain reaction in which “environmental stress opens the door, and latent pathogens strike back all at once.” Low temperatures prolong pathogen survival, while sealed housing to retain heat increases ammonia and dust concentrations. Ongoing cold stress also depletes sows’ immune reserves. More importantly, any infection in a sow herd affects more than the infected sow itself: reproductive disorders determine litter size, diarrhea affects piglet survival, and repeated shedding after infection with a low-virulence ASF virus strain directly threatens the stability of the entire farm.
For autumn and winter disease prevention in sow herds, first identify “what is threatening them,” then address “how to build defenses around them.”
I. Three fronts facing sows in autumn and winter
Front one: Reproductive disorders—PRRS, pseudorabies, parvovirus, Japanese encephalitis
PRRS is the leading pathogen causing reproductive disorders in sows. In autumn and winter, deteriorating indoor air quality combined with cold stress can cause sows in herds with unstable positive status to experience abortions in late pregnancy, premature farrowing, stillbirths, and weak piglets. Piglets may show difficulty breathing. Pseudorabies also causes abortions, stillbirths, and mummified fetuses in sows, but distinguishing signs include neurological symptoms in piglets and white necrotic lesions in the liver and spleen. Parvovirus mainly affects gilts, primarily causing mummified fetuses while the sow herself shows no obvious symptoms. Japanese encephalitis, by contrast, has a clear seasonal association with mosquitoes.
These diseases overlap substantially in the outcomes of “abortion and stillbirth,” and are difficult to distinguish by appearance alone. When reproductive disorders recur in a sow herd, the right course of action is to collect aborted fetuses, placentas, and sow serum for testing, rather than blindly increasing immunization against a particular disease first.
Front two: Diarrhea—the source of viral diarrhea is in the sow
A common mistake in preventing and controlling diarrhea in autumn and winter is focusing on piglets with diarrhea while overlooking the actual sources of infection. The E. coli that cause yellow and white scours in piglets mainly come from the sow’s body surface and birth canal. The key to preventing and controlling viral diarrhea is to ensure that sows develop sufficiently high levels of maternal antibodies before farrowing and pass this protection to piglets through colostrum.
Porcine epidemic diarrhea virus, transmissible gastroenteritis virus, and rotavirus survive and spread significantly more effectively in the low-temperature, humid conditions of autumn and winter. Infected sows may show only mild loss of appetite or diarrhea, or even no obvious symptoms, yet continue shedding the virus and contaminating the farrowing environment. If newborn piglets are exposed to the virus during their first few days of life, mortality is extremely high. The sow’s immune status directly determines whether her litter can get through its first week after birth.
Front three: ASFV low-virulence strains—the sow is a “silent sentinel”
The biggest challenge in ASF prevention and control has shifted from highly lethal virulent strains to low-virulence variants. Infection with low-virulence strains in sows presents very atypically: pregnant sows may show only recurrent low-grade fever and loss of appetite, followed by abortion or a high proportion of stillbirths, while newborn piglets have low vitality. Fattening pigs show almost no visible abnormalities under non-stress conditions.
This means that the sow herd is the most important “outpost” to monitor for low-virulence ASF strains. If the sow herd shows repeated unexplained reductions in feed intake or a rising abortion rate, even without typical high fever or skin hemorrhages, the issue should not simply be attributed to “seasonal stress.” Reproductive data from sows can reveal problems earlier than a thermometer. Where possible, farms should record abnormal feed intake and temperature fluctuations in pregnant sows. If clustered changes occur, collect samples for testing immediately rather than administering medication and waiting to see what happens.
II. Disease prevention strategy: Build the defense “around the sows”
Strategy one: Combine “whole-herd vaccination” with “gestation-based vaccination”; do not take a one-size-fits-all approach
The key to immunization against viral diarrhea is to ensure that sow antibody levels peak at farrowing. The recommended approach is “whole-herd vaccination as a foundation, with gestation-based boosters”: in 9 each year, vaccinate all sows on the farm once with a bivalent or trivalent porcine epidemic diarrhea vaccine to lay the foundation for herd antibody levels ahead of the high-incidence period in autumn and winter. In addition, administer gestation-based vaccinations to multiparous sows, giving a booster 40–50 days before farrowing and another 10–20 days before farrowing. Farms with high incidence can increase this to three gestation-based vaccinations, at 60, 40, and 20 days before farrowing. Replacement gilts should complete live-virus acclimation and basic immunization before breeding to ensure an immune barrier is established before they enter the herd.
For PRRS immunization, the infection status of the farm must first be determined. On positive, unstable farms, a live attenuated vaccine may be selected for immunization based on the circulating strain on that farm. Sows may receive pulse immunization in spring and autumn. On positive, stable farms, the use of live attenuated vaccines should be gradually reduced or even stopped. Negative farms and boar stations should not use live attenuated vaccines. It is strictly prohibited to use two different live PRRS vaccines at the same time on the same farm. Before immunization, testing should be carried out to identify the strain type on the farm, rather than “using whatever someone else uses.”
Basic vaccines, such as those for foot-and-mouth disease, classical swine fever, and pseudorabies, should be checked and any required booster immunizations completed before winter. Use a new needle for each pig during vaccination. Monitor antibody levels after immunization, and administer booster shots promptly if the herd's compliance rate is below the required level. Do not assume that “once vaccinated means protected.”
Strategy two: Environmental management is an “amplifier” of immunization effectiveness
Whether vaccination produces sufficient antibodies depends on whether environmental stress has overwhelmed the sow’s immune system.
The most typical mistake in autumn and winter is “sealing things up too tightly.” Closing the doors and windows of the pig house to retain heat causes ammonia and hydrogen sulfide concentrations to rise rapidly. This continuously irritates the respiratory mucosa, leaving sows in a state of chronic stress and reducing the quality of their immune response. The correct approach is to follow the “minimum ventilation” principle: maintain the temperature while using the warmer midday period for concentrated ventilation and air exchange, ensuring that people entering the building do not find the air choking or irritating to the eyes.
The suitable temperature range for sows needs to be considered separately from that for piglets. Maintain the general environment in the farrowing room at 18—22°C, and create a locally warmer area for piglets with a creep box rather than heating the entire farrowing room to the temperature piglets need and subjecting sows to heat stress. The direct consequences of heat stress in sows are reduced feed intake, insufficient lactation, and delayed estrus after weaning. The deterioration of these production indicators often has more lasting effects than a clinically apparent infection.
Strategy three: Focus biosecurity on “the places sows can come into contact with”
In the era of low-virulence ASF strains, precision in biosecurity matters more than intensity. Staff moving between sow barns, shared tools, and inadequate cleaning and disinfection of farrowing rooms can all serve as routes for low-virulence strains to move between barns.
The farrowing house follows an all-in/all-out system. Before entering the farrowing house, sows are washed and disinfected all over. The farrowing area is disinfected 2 times per week, with 2—3 types of disinfectants used in rotation. These measures do more than kill diarrheal viruses; they also reduce the environmental load of low-virulence ASF strains. For herd introductions, the farm follows a self-breeding system. When introducing breeding pigs is necessary, they are quarantined, observed, and tested for negative nucleic acid results before being mixed with the herd, to avoid introducing pigs that appear healthy but carry the virus into the core herd.
Low temperatures can significantly reduce the effectiveness of disinfectants. In winter, prepare disinfectants with warm water. If the temperature is too low, switch to a low-temperature disinfectant. Allow supplies to return to room temperature before treatment to avoid “disinfecting in name only.”
Strategy four: Time medication and health management to coincide with stress events; avoid “routine medication”
The risk of secondary bacterial infections in sows does rise in autumn and winter. Glaesserella parasuis, Actinobacillus pleuropneumoniae, streptococci, and other bacteria are more likely to take advantage when cold stress and deteriorating air quality occur. However, the right approach to medication and health management is to “start in advance of stress events,” not to “mix medication into feed all winter.”
During clearly defined high-stress periods, such as before a cold spell, around group transfers, and before or after vaccination, sensitive medications may be added to feed or drinking water for a limited period as a preventive measure. On farms with unstable PRRS, medications such as tylvalosin may be used under veterinary guidance to suppress viral replication and secondary infections. However, medication should be rotated and withdrawal periods observed, avoiding the premature use of the treatment window for indiscriminate preventive medication.
Summary
The core of sow disease prevention in autumn and winter can be summed up in three points: for reproductive disorders, “use testing to determine the direction”; for diarrhea control, rely on “maternal antibodies from sows”; and for African swine fever virus strains of low virulence, “keep a close watch on sows as sentinels.”
Vaccination should be carried out before stress sets in, ventilation should remain part of the same system as temperature control, and biosecurity should be applied to the farrowing crates, tools, and personnel that sows actually come into contact with. When the sow herd is stable, piglets in the farrowing rooms can rely on colostrum, and the production rhythm across the farm will not be disrupted in autumn and winter.