They are just as filthy, disease ridden, ferocious, and fleet of foot as their smaller cousins.
I suppose you could argue either way for the diseased bite. The standard rat doesn't have it, but perhaps the dire rat only does because it's bigger. And these guys are bigger yet.
Here's a dire rat scaled up to Medium and accounting for some of the other minor differences from the original text. I left the filth fever bite in, but we can drop it if all of you want to do so.
Giant Zakharan Rat
Medium Animal
Hit Dice: 4d8+8 (26 hp)
Initiative: +2
Speed: 40 ft. (8 squares), climb 20 ft.
Armor Class: 14 (+2 Dex, +2 natural), touch 12, flat-footed 12
Base Attack/Grapple: +3/+5
Attack: Bite +5 melee (1d6+3 plus disease)
Full Attack: Bite +5 melee (1d6+3 plus disease)
Space/Reach: 5 ft./5 ft.
Special Attacks: Disease
Special Qualities: Low-light vision, scent
Saves: Fort +3, Ref +5, Will +3
Abilities: Str 14, Dex 15, Con 14, Int 1, Wis 12, Cha 4
Skills: Climb +10, Hide +3, Listen +4, Move Silently +3, Spot +4, Swim +10 (3 more ranks to distribute)
Feats: Alertness, 1 more
Environment: Any underground
Organization: Solitary or pack (2–12)
Challenge Rating: x
Treasure: None
Alignment: Always neutral
Advancement: 5–8 HD (Medium); 6–12 HD (Medium)
Level Adjustment: --
A dire rat can grow to be up to 4 feet long and weigh over 50 pounds.
Combat
Disease (Ex): Filth fever—bite, Fortitude DC 14, incubation period 1d3 days, damage 1d3 Dex and 1d3 Con. The save DC is Constitution-based.
Skills: Dire rats have a +8 racial bonus on Swim checks. Dire rats have a +8 racial bonus on Climb checks and can always choose to take 10 on Climb checks, even if rushed or threatened.
Dire rats use their Dexterity modifier for Climb and Swim checks.