There are two types of coccidiosis, cecal and intestinal. There are usually affected at a young age, so I don't know if yours are affected because I don't know the age. My book says that bleeding in the vent Is a early stage for coccidiosis. The treatment should be, 1 teaspoon of amprolium (20%) per gallon drinking water for five days, anabiotic treatment guards against secondary infection. For treatment with a vitamin supplement (especially A and K); so survivors are immune but may never be as productive as an infected birds
The prevention- treat chicks (if you have any) to 16 weeks of age with medicated starter or coccidiostat in drinking water, according to directions on the label.
I'm still not sure if it coccidiosis, but I'm just throwing it out there.