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Investigations of Peritoneal and Intestinal Infections of Adult Hookworms in Northern Fur Seal and California Sea Lion Pups on San Miguel Island, California

February 24, 2011

A study of hookworms and their effects on northern fur seal and California sea lion health.

The major significance of this research is the unusual finding of adult hookworms in the peritoneal cavity of so many dead pups. No obvious explanation for this occurrence could be determined.

The peritoneal cavity and intestine of northern fur seal (Callorhinus ursinus) pups and California sea lion (Zalophus californianus) pups that died in late July and early August 2003 on San Miguel Island, California were examined for hookworms. Prevalence and morphometric studies were done with the hookworms in addition to molecular characterization. Based on this and previous molecular studies, hookworms from fur seals are designated as Uncinaria lucasi and the species from sea lions as Uncinaria species A. Adult hookworms were found in the peritoneal cavity of 35 of 57 (61.4%) fur seal pups and of 13 of 104 (12.5%) sea lion pups. The number of hookworms ranged from one to 33 for the infected fur seal pups and one to 16 for the infected sea lion pups. In addition to the peritoneal cavity, intestines of 43 fur seal and 32 sea lion pups were examined. All of these pups were positive for adult hookworms. The worms were counted from all but one of the sea lion pups. Numbers of these parasites in the intestine varied from three to 2,344 for the fur seal pups and 39 to 2,766 for the sea lion pups. Sea lion pups with peritoneal infections had higher intensity infections in the intestines than did pups without peritoneal infections. This lends some support to the hypothesis that peritoneal infections result from high-intensity infections of adult worms. There was no difference in intestinal infection intensities between fur seal pups with and without peritoneal infections. Female adult hookworms in the intestines of both host species were significantly larger than males. Sea lion hookworms were larger than those in fur seals. Worms in the intestine also were larger than worms found in the peritoneal cavity. Gene sequencing and analysis of amplified ribosomal DNA were used to diagnose the species of 172 hookworms recovered from the peritoneal cavity and intestine of 18 C. ursinus and seven Z. californianus hosts. These molecular data revealed that U. lucasi (hookworm of C. ursinus) and Uncinaria species A (of Z. californianus) infrequently mature in the intestine of the opposite host species in California rookeries. However, there is no support from molecular data for the hypothesis that cross-infection with “the wrong” Uncinaria species is a contributing factor in these cases of host peritonitis.

Further research should help understand and monitor the apparent ever-changing role of hookworm disease in the health of northern fur seal and California sea lion pups on SMI.


Eugene T. Lyons, R. L. DeLong, S. A. Nadler, J. L. Laake, A. J. Orr, B. L. DeLong, and C. Pagan. Published in Parasitol Res 109:581-589. 

Last updated by Alaska Regional Office on 09/23/2022

Northern Fur Seal Northern Fur Seal Research