Warwick is a very easy jungle, great sustain in the jungle. Though he suffer's from terrible ganks pre-six. He's great for his ganks once he hits that level though, he gives the attack speed bonus to the others you're helping, got a good chase with his Blood Scent, and his Ultimate is a great suppress that with proper teamwork, they'll die before they get out.

Team Fight stage: You're jungle is to assassinate their carries, the squishiest of them.

My item build is slightly different then others, where I believe that you can get by with just two thicker items (eg. Frozen Heart and the Frozen Mallet). My reasoning behind this is for the extra damage off you're ultimate, and if you can all those auto attacks you should get off.
Explaination behind each item.

Merc Threads: This isn't a set in stone item, it all depends on what their team has more of. If they have very little cc and lots of AD characters, by all means Ninja Tabi. Please never get Bezerker Grieves.

Wit's End: This item proc's off you ultimate, so you get the extra 252 magic damage without reduction and as a side bonus, you get full stacks from the magic resistance which gives 20 more MR.

Malady: Now this is one of the biggest things most people argue with, Wit's End is a better item, and instead of getting both, they take another tankier item. I prefer to get this as well, the 25 ap this item gives does feel like a waste but it does go off with your Q. The damage from the passive compared to Wit's End is less then half. Also this shred's their MR by 24.

Madrid's Bloodrazor: This item is just great, easier Baron, stacks off Infinite Duress. It also give you a little extra damage, attack speed, and armor. So off one Infinite Duress you will do 24% of their maximum health not including their MR.

Frozen Heart: Why I suggest this tank item over the many other options, it gives you insane amounts of defense, more mana which is always a nice buff so you AP carry can have the blue. It's aura give's a great amount of help in a teamfight by lowering all the enemies attack speeds by 20%.
Thornmail: Yes I realize this wasn't in the above item's but I'm mentioning it as a replacement for Frozen Heart, if your tank gets it. Thornmail allows you to 1v1 a melee fighter a lot easier. Especially with the help of a hp item like Frozen mallet.

Frozen Mallet: The last item on my list, I love this item, it'll also be more useful if Riot does the Jungle remake they talked about a while back. 700 hp, is great for survivability, 20 attack damage is pretty much just a little added bonus. The main reason is for the slow on every physical attack you do, and with this much attack speed, it's pretty hard to ever get away from Warwick with this,