Infectious disease is a major problem in both marine and terrestrial mammals. In most instances, when pathogenic microbes invade a host, a number of non-specific inflammatory mechanisms serve to localize or remove the invaders from the body. This usually coincides with an adaptive, pathogen-specific immune response that clears the infection and establishes a state of protective resistance. On occasion there is a failure of host defenses against invading pathogens. This may be a result of the nature or extent of the infection, reflect the virulence of the organism, or less commonly, arise from a primary or secondary immunologic failure. Over the past several years a number of marine mammal populations have suffered epidemic outbreaks of infectious disease. In addition, infectious disease has been implicated as a significant cause of mortality in several populations that are in decline. It is difficult to know whether these patterns of infectious disease are normal in these populations, or whether, as some investigators suggest, some marine mammals have impaired immune function. In this paper we provide a brief overview of possible causes of immunodeficiency in marine mammals and use this information as a framework for a rational approach to diagnosing immunologic dysfunction in one or more individuals.
A number of immunodeficiency classification schemes have been developed in other species. Since there is little published information regarding clinical immunology in marine mammals, it is useful to borrow these classification criteria, at least until we gain a better understanding of factors and conditions that impair immune function in marine species. The classification scheme proposed by the World Health Organization (WHO) is broadly based upon which compartment of the immune system is involved in the deficiency. Primary immunodeficiencies are those caused by intrinsic defects (congenital or acquired) that involve either T or B lymphocytes or a combination of both. This category consists of a large number of inherited defects, but also includes intrinsic defects induced by environmental insults. In secondary immunodeficiencies there are no intrinsic abnormalities in the development or function of B or T cells but instead an external factor or condition interferes with immune function. These include viral-induced immunodeficiencies, and those arising from stress, malnutrition, or iatrogenic factors. Clinical investigations of suspected immunodeficiency should be directed at identifying which compartments of the immune system are affected. This is the first step in determining an underlying cause for the abnormality.
Immunodeficiency syndromes, by definition, are characterized by an unusual susceptibility to infection. The type and extent of the infection provides the first clue as to the nature of the immune dysfunction. For instance, recurrent infections with pyogenic bacteria, are likely caused by defects in humoral (antibody-mediated) immunity. In marine mammal species, specific information regarding the immune system is difficult to obtain by physical examination, because of the difficulty in palpating external lymph nodes. A large amount of information can be obtained from routine complete blood counts and clinical serum chemistry analyses. Absolute numbers of lymphocytes, and the change of these values over time, serum protein chemistry, and in particular globulin values, all reflect immunologic processes. For a number of species, the quantification of blood mononuclear cell populations by immunofluorescence using monoclonal antibodies, is becoming commercially available. In addition, a number of B and T cell function tests can be performed on blood samples. Using the information obtained from this data, a preliminary diagnosis as to the presence and nature of the immune deficiency should be possible. Following this initial diagnosis an expanded, but specific, assessment of immune function can be performed. This should help the clinician to identify possible causes of the immunodeficiency, and therefore implement appropriate therapeutic or prophylactic strategies.