Prevalence and development of intraleucocytic haemogregarines from Northwest and Northeast Atlantic mackerel, Scomber scombrus L.

Abstract

Abstract. Scomber scombrus on both sides of the North Atlantic were infected with similar intraleucocytic haemogregarines. Detection of parasites in blood smears alone was found to be an unreliable indicator of prevalence. In the Northwest Atlantic, prevalence of blood infections varied annually and seasonally, with an average of 18.4%, whereas tissue imprints showed up to 98.3% prevalence. Northeast Atlantic fish had a 4.0% prevalence in blood smears and 100% in spleen imprints. Age 1 mackerel were the principal hosts of haemogregarines in the Northwest Atlantic. Scanty infections of single and paired parasites were observed primarily in lymphocytes and neutrophils of peripheral blood and tissue imprints. Also present in the blood and tissues of Northwest Atlantic mackerel were intracellular schizonts with up to 20 merozoites, and fish from the same region showed haemogregarines within histological lesions in kidney and spleen. Ultrastructural studies on Northeast Atlantic mackerel indicated the parasite is apicomplexan and morphologically dissimilar to Haematractidium scombri. Its relationship with Goussia clupearum was not determined.