Pathological Findings in White-Beaked Dolphins (Lagenorhynchus albirostris) and Atlantic White-Sided Dolphins (Lagenorhynchus acutus) From the South-Eastern North Sea

Abstract

In the North Sea, white-beaked dolphins (Lagenorhynchus albirostris) occur regularly and are the second most common cetacean in the area, while their close relative, the Atlantic white-sided dolphin (Lagenorhynchus acutus), prefers the deeper waters of the northern North Sea and adjacent Atlantic Ocean. Though strandings of both species have occurred regularly in the past three decades, they have decreased in the southern North Sea during the last years. Studies describing necropsy findings in stranded Lagenorhynchus spp. are, to date, still scarce, while information gained through post-mortem examinations may reveal valuable information about underlying causes of this decline, including age structure and the reproduction status. Therefore, we retrospectively assessed and compared the necropsy results from fresh Lagenorhynchus spp. stranded along the southeastern North Sea between 1990 and 2019. A full necropsy was performed on 24 white-beaked dolphins and three Atlantic white-sided dolphins from the German and Dutch coast. Samples of selected organs were taken for histopathological, bacteriological, mycological, parasitological and virological examinations. The most common post-mortem findings were emaciation, gastritis and pneumonia. Gastritis and ulceration of the stomach was often associated with an anisakid nematode infection. Pneumonia was most likely caused by bacterial infections. Encephalitis was observed in three animals and morbillivirus antigen was detected immunohistochemically in one case. Although the animal also showed pneumonic lesions, virus antigen was only found in the brain. Parasitic infections mainly affected the gastro-intestinal tract. Lungworm infections were only detected in two cases and no associations with pathological alterations were observed. Stenurus spp. were identified in two of three cases of parasitic infections of the ears. Twelve of the 26 white-beaked dolphins stranded in Germany were found between 1993 and 1994, but there was no evidence of epizootic disease events or mass strandings during the monitored period.

Keywords: Germany; Lagenorhynchus acutus; Lagenorhynchus albirostris; North Sea; The Netherlands; pathology.

Figure 1

Stranding location of 40 white-beaked dolphins (triangles) and three Atlantic white-sided dolphins (circles) as well as one WBD calf (star) on the North Sea coast of Schleswig-Holstein and the Netherlands. Stranding location of one WBD from Germany was not possible to obtain. Stranding location of four WBDs and three AWSDs from the Netherlands is not pictured, as they only consisted of loose bones (Basemap provided by Bundesamt für Kartographie und Geodäsie, Frankfurt am Main, 2011).

Figure 2

Year distribution of all strandings of white-beaked dolphins and Atlantic white-sided dolphins along the German and Dutch coast of the North Sea from 1990 to 2019. Strandings in the Netherlands prior to 2008 are not included, as they were not examined further at UU.

Figure 3

Elevated white plaque (~5 x 8 cm) cranial to the blowhole on the head of LA11 (A). Depressed scar (~8 × 6 cm at its widest point) on the dorsal tailstock (B) of the same animal. (photo credit: Steve Geelhoed).

Figure 4

Skin lesions of AWSD1, showing severe discoloration on the right side of the abdomen, ~8 cm cranial to the fluke (A) and round shaped lesions with holes perforating into the blubber (B).

Figure 5

Parasite infection with nematodes (Crassicauda spp.) of the subcutaneous fascia in AWSD2 appeared on both sides of the caudo-lateral abdomen (A). The same animal also displayed several abscesses in the musculature and sub cutis on the left side of the caudo-lateral abdomen, which were also infected by Crassicauda spp. (B).

Figure 6

Gastric ulceration (arrow) in the 1st compartment of the stomach of WBD8.

Figure 7

Dorsal view of the right hemisphere of the cerebrum with multifocal hemorrhages (stars) and purulent areas (arrows) in LA10 with acute meningoencephalitis. (photo credit: Steve Geelhoed).