Between July and August 2009, a total of two hundred crossbreed dogs in three contiguous littoral provinces of Guilan, Mazandaran and Golestan were sampled. Circulating antigens and/or microfilariae of Dirofilaria immitis were found in 25.5% of dogs in the whole studied area. The highest infection rate was found in Guilan (51.42%), followed by Golestan and Mazandaran provinces (15.38% and 7.69%, respectively). Prevalence values were not influenced by age and sex in the whole area; the values detected in low altitude regions were 4.6 times the values found in high altitude areas. This is the first detailed survey on canine dirofilariasis in these coastal provinces of Iran. Guilan province, with its certain hyper endemic counties, was shown to be the most endemic for canine heartworm infection in the country. Chemoprophylaxis should be considered for the dog population in these areas.

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