A water filter may be over kill a bit. But some type of "emergancy" water sterilizer might be in order. There are a few to try, and all three I have used as a regular means of purification when out back packing.
1: Aqua Mira: comes in a two bottle liquid and has to be mixed with each other at the time of water treatment. I have survied cow pie water using this. Good stuff.
2: Katydine Micro Pure Tablets: almost the same as Aqua Mira but in a tablet form, and no mixing.
3: Grapefruit seed extract: My favourite. Can be bought at a organic health food store or in the health/ogranic section at FredMeyers (at least thats where I get it). The most bang for you buck and totally organic (the others are a chlorine dioxide mixture) Leaves no wierd taste and work just as well as the other above mentioned products. I have not used it in cow pie water but, I have not been in the that situation again and don't want to be either
Use about 15 drops per liter, and let cure for about 30 min..good to go.
Hope this helps