Herd Immunity and Protection
Cells from the infected intestinal cells produce a type of antibody, IgA, found in the sow’s colostrum. This lactogenic immunity is critical to provide piglets with immediate protection against TGEV and PEDV infection. Based on clinical experience, herds previously affected with TGEV were typically protected for 2-7 years based on the stability of IgA antibodies transferred from the exposed sows to piglets. However, results from University of Minnesota Swine Health Monitoring Project –PEDV (713 breeding herds), suggests that approximately 30 percent of affected PEDV positive herds either continued with chronic disease and high piglet mortality or re-broke with the PED virus within 4 months. Recent studies have shown that herd immunity for PEDV is suspected to wane in 9-12 weeks post infection (Murtagh, 2014).
Another little known advantage of TGEV is the cross protection provided through natural exposure to Porcine Respiratory Coronavirus (PRCV). PRCV infection of pigs is mostly subclinical with self-limiting respiratory infection. PRCV -exposed sows that are re-infected during pregnancy secreted IgA TGEV antibodies. (PRCV) has silently boosted immunity in exposed herds by providing cross protection from TGEV.
What we know
PEDv produces, in each infected piglet, billions of virus in the intestinal cells or enterocytes. When the environmental load is high, the likelihood for re-infection or re-break is high. Ongoing disease protection in piglets is dependent on the continuous flow of IgA in colostrum and milk through the intestine and the absence of an environmental dose large enough to re-infect the piglets. Present research suggests that the sow herd does not retain high enough immunity longer than 12 weeks in the face of viral exposure and therefore piglets of sows with low antibodies are susceptible to viral infection and re-infection. The amount of virus shed in the feces of PEDV affected piglet is so much higher than with TGEV.
What we don’t know
Presently, there is not a diagnostic test to determine the level of PEDV generated IgA in sow’s milk. Therefore we do not know with certainty the outcome for subsequent virus infected piglet litters. While a vaccine is commercially available, it is unlikely to reduce the risk of infection. It has been shown successful in eliminating clinical signs in endemically infected herds.
Some colleagues and producers have discussed purposefully introduction of PEDV into a naïve herd with the intent of “establishing immunity”. I caution that we should choose our protocols based on the science and not on experience with a different agent. Understanding biosecurity protocols and following guidelines set out by pork.org is science based.
There are billions of particles released as the cells of the small intestine die. In the event of a PEDV infection, scrupulous attention to sanitation and disinfection of an affected entire sow farm becomes high priority.
These details include:
- Boot sanitation
- Wash cleaning and liming of hallways
- Reduced cross fostering and handling of piglets and reduced pig movement where possible.
- Follow up surveillance using swabbing techniques.