suspected virus

[logcabin1]

I am a small producer, breeding five or six gilts and sows twice a year.For the past couple of years I having a problem with one or more sows breeding, never coming back into heat but not farrowing or farrowing and having but one piglet.The sows that have only one pig never did come into heat after they were weaned.I am not having a problem with gilts coming into heat and after breeding they farrow normally,I have had some, after weaning not come into heat normally(3 to 5 days) and some have a small litter the second farrowing.I have had my herd tested for prrs and parvo with the tests coming back negative.I have done some research on these smedi viruses and see that there are 4 that can cause reproductive problems.I also understand that there is no cure for them, but gilts and sows should develop an immunity for them if given 5 to 6 weeks exposure before breeding.This takes place here as all females are born here, I have a closed herd, and use parvo-shield l5e for vaccinating my breeding herd.At the time of testing we did not test for any other viruses as the vet nor I didn't know they existed.Presently I have four gilts bred and showing pig, due to farrow the end or march. I have one 4th litter sow bred due at the same time of the gilts and not showing pig.I'm assuming my problems are caused by these viruses, but don't understand why the sows aren't developing immunity or maintaining the immunity after their first breeding. Does anyone have any suggestions or ideas on what is going on.

[Piggy]

Have your feed tested for mold and mycotoxins.