Stade had won 23 out of 26 Challenge Cup home games at the pool stage and Worcester arrived in Paris as huge underdogs, having lost their last 11 away matches in the competition as well as being without a win in France for a decade.
The Premiership side upset the Top 14 high-fliers with a brilliant Pool 2 performance on the road, though, Howe crossing for a second time just before the break after Matt Cox scored the opening try.
Ted Hill and Jamie Shillcock also went over in the second half and Jono Lance took his tally with the boot to 13 points, ensuring tries from Kylan Hamdaoui, Laurent Panis and Piet van Zyl were in vain for the 2016-17 Challenge Cup winners.
La Rochelle showed no mercy on Enisei in Russia, scoring six tries in each half of a 82-21 Pool 4 rout.
Jean Victor Goillot and Pierre Aguillon claimed two five-pointers apiece and Maxime Lafage finished with a 25-point haul in a ruthless mauling.
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