And indeed of anthelminthic, the best-formed derivative from the Greek. The prefix anti- assimilates to a following h.
Conceivably, the change of one of the two <th>'s to <t> could be an authentic reflection of Greek phonetics: Grassmann's law. If the ancient Greeks themselves ever used this word, it would have dissimilated one of the <th>'s. But it's not in Liddell & Scott so I'm afraid that makes it a mere spelling mistake.
"Intestinal worms were a common ailment, especially among the young, many of whom died from them. To one girl, McCormick gave '3 antihelmintic powders,' hoping to kill the parasites." —Edward Ball, Slaves in the Family (NY: Ballantine Books, 1998), 247