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Blue Spot Disease of Northern Pike Esox lucius

Volume 3, Issue 5
April 2001
Updated November 2002

Illegally introduced populations of northern pike in the Belgrade Lakes region of Maine have been caught during the spring spawning season and found to be covered with pale bluish-white, granular skin lesions 3-10 mm in diameter and 0.25 mm thick. These spots occur mostly over the dorsal skin and fins of spawning age ( → 2 years old) adult fish.

Blue spot

This condition was first described in 1983 by Yamamoto in northern pike in several waters of central Canada. The cause of these white-blue spots is esocid herpesvirus-1 (EHV-1). The prevalence of EHV-1 in central Canada was 1-7% of the northern pike population. Margenau 1995 reported the disease in both northern pike and muskellunge Esox masquinongy in Wisconsin. The prevalence in Wisconsin was as high as 34%.

hypertrophied cells

Fig. 2. Light microscopy of an epidermal section from northern pike, showing numerous hypertrophied cells (arrows) interspersed in normal tissue.

The blue spots appear during the spring spawning season as water temperatures increase between 2° and 13°C. The lesions then disappear when water temperature increases above 14°C. The etiology of the virus is not known. The prevalence of the blue spot lesions differs among lakes, and is reported generally higher in female northern pike. It is hypothesized that EHV-1 may cause more severe morbidity and mortality in young pike and muskellunge, although this has not yet been proven. The observable blue skin spots appear to be less prevalent on older fish, however, these affected fish may remain lifelong carriers of EHV-1, similar to channel catfish herpesvirus that infects channel catfish Ictalurus punctatus.

EHV-1 is likely very contagious amongst northern pike populations. Individuals fishing for northern pike should disinfect angling equipment and personal gear between waters.

Special points of interest:


Fish with EHV-1 cannot transmit the disease to people.

EHV-1 is caused by a herpesvirus.

For more information read: Margenau et al. 1995. Blue spot disease in Wisconsin Esocids. J. of Aquatic Animal Health 7:29-33

Images were made possible, in part, by a grant from the Maine Outdoor Heritage Fund.