Epizootiology of Ichthyophonus hoferi in herring populations off the Swedish west coast

Abstract

During August-September 1991, Ichthyophonus hoferi caused a mass mortality among the herring Clupea harengus population of the Skagerrak-Kattegat. This was the first record of a mass mortality of herring due to I. hoferi along the eastern coast of the North Atlantic and the first record of I. hoferi from the Skagerrak and Kattegat area. From August 1991 to December 1994 herring were sampled and examined both macro-and microscopically for signs of ichthyophonosis. Macroscopically visible signs of ichthyophonosis were found in 269 of the 25 156 herring examined. Microscopic examination of different organs showed more than 4 times the prevalence than did macroscopic examinations. The ratio between the results of the micro-and macroscopical examinations, i.e. signs of passive/signs of active phase of infection, varied with the age of the fish as well as with the season. The prevalence of infection fluctuated seasonally, probably due in part to the migration pattern of herring; the spring spawning population was assumed to be the most heavily infected population of the area. The number of herring lulled as a result of infection is estimated to be more than 300 million, i.e. more than 10% of the population. Differences between the recent epizootic among herring stocks due to I. hoferi and those reported from the western North Atlantic are also discussed in the 'Discussion' section.

An Ichthyophonus hoferi epizootic in herring in the North Sea, the Skagerrak, the Kattegat and the Baltic Sea

“…This corresponds with our observations in a preliminary study (unpubl.). In contrast, Rahimian & Thulin (1996) detected an approximately 4 times higher infection level in herring when using microscopic examination of heart tissue compared to macroscopic examination. During the present study, the detection of I. hoferi infection was carried out entirely by macroscopic observation as we regarded the 10% underestimation as being within an acceptable level of accuracy.…”

Failure of population recovery in relation to disease in Pacific herring

“…Within a year, background mortality is 25% of the population; fishery mortality is the sum of the spring harvest and the previous fall's harvest; and disease mortality is estimated based on the spring prevalence of (1) Ichthyophonus hoferi for all ages (Table 1) and (2) (Marty et al 2003). Although this prevalence is high compared with Atlantic herring Clupea harengus populations in Europe (Rahimian & Thulin 1996, Mellergaard & Spanggaard 1997, multiyear study confirmed that this prevalence adjusted for age was within normal limits for Prince William Sound (since 1994) and Sitka Sound (1994. The model estimates that mortality from I. hoferi in 1998 was significant, but more of the population decline first noticed in 1999 in Prince William Sound was associated with the ulcer/VHSV outbreak of 1998 (Fig.…”

Assessing the accuracy of a polymerase chain reaction test for Ichthyophonus hoferi in Yukon River Chinook salmon Oncorhynchus tshawytscha

“…The latter has been considered the most sensitive of these methods (Rahimian & Thulin 1996, Kocan et al 1999, 2004. A method which has become a routine diagnostic tool for many fish pathogens (Cunningham 2002), and is potentially much more sensitive than explant culture, is the polymerase chain reaction (PCR), which targets the DNA of the pathogen.…”