The clinical diagnosis of disease resulting from immune system dysfunction is challenging in any species, but is particularly difficult in free ranging marine mammals. Nevertheless, because of the high degree of public interest, the veterinarian and biologist are often asked to assess the health of individuals or entire populations. In the absence of a better answer, the public often is quick to assume that immunodeficiency is the underlying cause of disease or population decline.

The diagnosis of immunodeficiency encompasses the awareness that certain clinical conditions might be related to a dysfunctioning immune system. The diagnosis of immunosuppression begins with the realization of the necessity to confirm the occurrence of unusual or higher than normal rates of infectious or neoplastic diseases are a consequence rather than the cause of reduced immune function. Finally, assessing immune function is the science of determining the pathophysiologic basis and thus, the cause of the immunodeficiency state.

A systematic multidisciplinary approach was developed to accomplish these tasks in wild California sea lions. The results demonstrate sophisticated immunologic, pathologic, and infectious disease research methods can be applied under field conditions to determine the health and immunologic status of wild seals. The application of these techniques provides a workable basis to assess the impact various environmental factors have on the immunologic status and health of marine mammals.