PARASITE CONTROL Something he should have outgrown?

Q:Just over a year ago, we purchased an almost-6-year-old gelding. When I dewormed him with an ivermectin/praziquantel paste, I found a large quantity of ascarids in his manure over the next three to five days. Everything I read online says that ascarids affect only young horses, who develop a natural resistance once they reach about 2 years of age. This horse clearly has not developed any such resistance, and I am curious if this is happening more and more.

I deworm my horse every two months, and I have tried both ivermectin and fenbendazole products. I continue to find ascarids in his manure after deworming, although the number has diminished from more than 100 adult worms the first time to 10 to 20. Is it likely that my horse will eventually outgrow this parasite? Am I doing the right thing to continue to deworm him every two months until there are no longer worms in his manure, and then start him on routine fecal tests? Cathy Johnson Roy, Washington