Outbreaks of Edwardsiella ictaluri infection among natural riverine fish are driven by summer heat wave

Edwardsiella ictaluri, the causative agent of E. ictaluri infection, is one of the most well-known bacterial fish pathogen in worldwide. In Japan, outbreaks of E. ictaluri infection were recorded in some rivers in the summer... [ view full abstract ]

Edwardsiella ictaluri, the causative agent of E. ictaluri infection, is one of the most well-known bacterial fish pathogen in worldwide. In Japan, outbreaks of E. ictaluri infection were recorded in some rivers in the summer of 2007, and caused mass mortality of riverine ayu, a representative freshwater fish species in Japan. In this study, we carried out epidemiological survey in Tama river basin within Tokyo Metropolis, Japan between July and August of 2011-2015, and determined the driving factor(s) of E. ictaluri infection in natural rivers. Ayu were collected by angling, and examined for E. ictaluri by cultivation and selective enrichment cultivation using trunk kidney. The water temperature in each year was recorded by a temperature logger, and the diurnal water temperature range was calculated for each day. Additionally, the water level data for each year was obtained from the web site of Ministry of Land, Infrastructure, and Transport, Japan. The prevalence of E. ictaluri among ayu in 2011 was generally low, while the prevalence in August 2012-2015 was high (37.5-86.4%). However, it was only in the August of 2012 and 2013 that an outbreak of E. ictaluri infection occurred with mortality of ayu, although the prevalence in 2014 and 2015 were high (53.2-74.4%). In August, the average water temperature and diurnal water temperature range during the outbreak years (2012-2013) was 1.7-6.3 ℃ and 1.4-2.4℃ higher than during the non-outbreak years (2011, 2014-2015), respectively. Additionally, flooding (rising water level over 0.5m) occurred several times during the non-outbreak years, while no flooding was observed during the outbreak years. These results suggest that the outbreak of E. ictaluri infection is associated with environmental changes in natural rivers caused by summer heat waves, such as high water temperature and water shortage.