Search for articles focusing entirely on pigs:
(pig[ti] OR pigs[ti] OR Piglet*[ti] OR sow[ti] OR sows[ti] OR boar[ti] OR boars[ti] OR swine[ti] OR porcine[ti] OR suidae[ti] OR hog[ti] OR hogs[ti] OR phacochoerus[ti] OR "Swine"[Majr]) NOT ("guinea pig"[ti] OR "guinea pigs"[ti])