Victor Sanchez struck a late consolation at San Mames Stadium, but the form book was never in danger as the visitors went a fifth match without victory.
It was Espanyol who were knocking on the door before the half-hour mark as Sanchez and Abraham tried their luck from long range.
Bilbao opened the scoring against the run of play on 29 minutes as Markel Susaeta crossed for Aduriz to nod home.
Aduriz turned provider to release Viguera, and the striker finished off a swift counter-attack on 44 minutes.
They could have been three up shortly after the break as Susaeta wasted a good chance created by Iker Muniain, but were dealt a warning on 68 minutes when Salva Sevilla hit the post from inside the box.
Espanyol did score when Sevilla squared for Sanchez to convert from close range on 84 minutes, but Athletic had already killed the game off six minutes earlier when Iturraspe placed the ball into the bottom corner.
The result takes Bilbao to ninth in the table, while Espanyol remain 12th, four points behind.