Google: “Zyklon B” and “Mexican farm workers” and you will see that the United States Department of Immigration was using Zyklon B to fumigate and delouse immigrant farm laborers back in the 1920’s and 1930’s.

Produced by the German company pest control DEGESCH, the infamous “Zyklon” (hydrocyanic acid) was sold in four versions with different power. Zyklon E was recommended to clear environments weeds die hard, like cockroaches. Zyklon D was prepared most widely used free environments (the holds of ships, concrete buildings with furniture in the rooms) with lice, mice and rats.
Apparently, it was enough for humans the less powerful version, the Zyklon B. But on humans was not used for the first time in Germany.
Zyklon B was used since 1929 in the United States by the health (U.S. Public Healt Service) on the border with Mexico, to comb and trim Mexican migrants transiting from Juarez to El Paso.