Enrofloxacin or Baytril is the best treatment, although the FDA has prohibited it's use in chickens. However if you are not going to eat the chicken, it would be up to you to use it. Many vets quietly prescribe it for chickens, and in Europe, it is not prohibited. Here is a link where to find it online: http://www.backyardchickens.com/t/959119/baytril-enrofloxacin-sources
Procaine Penicillin G, which is refrigerated and sold in many feed stores in the US under different brand names, can be used instead. Dosage is 1/4 ml given 1/4 inch deep into the breast muscle once a day for 4 days.Use a 20 gauge needle, shake the vial, refrigerate it, and alternate different sides of injection daily.