Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This build utalizes the bonus magic damage and passive attack speed Kog'maw gets from his skills, while building quickly by completing early items which are very cost effective. One of the biggest complaints with this build is the purchase of malady. Malady is 1890 total cost while a bf sword alone is 1850, the rest of the cost for a bloodthrister is 450+900 (1350) where a phage is only 1315. so for 3205 you get two items which boost attack speed, magic damage (malady), a slow, and a little damage or you could go blood thirster which has very good damage and only slightly more leech for (3200) only 5g less. This allows you to build up your items much more efficiently and what you lose in physical damage you make up in magic damage from the small malady damage+bio damage from attack speed.
Once you finish your razor, the last items really depend on game length and team match up. Generally I feel like divine is a good choice for its bonus magic damage and no dodging, but Phantom Dancer or Nashor's tooth are also good choices and in rare cases Wits End can ruin casters.