Effects of rain events on Cryptosporidium spp. levels in commercial shellfish zones in the Hillsborough River, Prince Edward Island, Canada

• Cryptosporidium oocyst levels in seawater were associated with rain events.
• Restricted zones, defined by coliform counts, had the highest oocyst counts.
• Oocyst numbers in oysters depend on neither rain intensity nor harvest zone.
• Harvest zones for Cryptosporidium spp. should not be classified by coliform counts.

A longitudinal study was conducted during the 2014 spring eastern oyster harvesting season in Prince Edward Island. The concentrations of Cryptosporidium oocysts were assessed in seawater and oyster samples taken from three commercial shellfish zones (Approved, Restricted, and Prohibited) one day before a rain event (of at least 15 mm), and on days 1, 3, and 7 after the rain event. Two rain events were recorded during the study period. The first event (rain event 1) was characterized by precipitation of 15 mm over 14 h (low intensity) and the second event had 40 mm in 14 h (high intensity). Oocysts in 20 L of seawater ranged from 0 to10, 0 to 7, and 1 to 15 in Approved, Restricted, and Prohibited zones, respectively, and in oysters, ranged from 0 to 30, 0 to 48, and 0 to 25 in Approved, Restricted, and Prohibited zones, respectively. A significant increase in Cryptosporidium spp. oocyst counts was observed in seawater samples after an intense rain event. Oocyst counts in seawater were almost two times higher after a high intensity rain event than after a low intensity event, but no effect was observed in oysters. However, positive samples (seawater and oysters) were still present seven days after a rain event. These findings suggest that authorities should consider monitoring for Cryptosporidium spp. in shellfish and their associated waters, especially after heavy rain events.