Spanish slang, prob. Mexican in origin. Literally, "the junkie worm." Closest in meaning to the English "monkey on your back," but more insidious, with specific heroin overtones. In the words of David Foster Wallace, in Infinite Jest, the tecato gusano connotes "some kind of interior psychic worm that cannot be sated or killed."
The worm is what makes your withdrawal a personal hell; it makes the tecato feel like he's going to vomit if he so much as moves in any direction other than towards the heroin. This worm is closer to the English "wyrm" in serious, not-fucking-around-ness than the scrawny frat boy "worm" at the bottom of a tequila bottle. This is a worm that lives in your veins, in your stomach, in your spine, and in your psyche.
The metaphor goes a long way towards explaining what life is like for one who no longer uses heroin. Just as Alcoholics Anonymous insists that there are no "former" alcoholics, only "recovering alcoholics", when one kicks the habit, the worm is not said to be dead--you are simply "keeping the worm asleep."