The mammalian and avian immune systems function similarly; both incorporate humoral and cell-mediated cytotoxic mechanisms^ and are thought to share a 160m year old relationship with the reptilian immune system.55 The immune system of mammals shows sexual dimorphism^ a greater immune response is normally observed in females, which has been attributed to differences in steroid hormone concentration. In the toad Bufo regularis, sexual dimorphism of the immune system is also apparent. 5 7
Mammals. Many studies have shown that synthetic and natural oestrogens suppress the immune system^^ and that, during pregnancy, the female immune system is naturally suppressed, accompanied by a decrease in thymulin levels and an increase in circulating oestrogen levels. Oestrogen receptors in the thymus have been located in thymulin-producing cells, indicating a possible mechanism by which the T-cell mediated immune response is co-ordinated by these two hormones and manifested as a lowered maternal immunity to infections.6° Increased concentration of testosterone has a positive, enhancing effect on the immune system, whereas increase or decrease in thyroid hormones results in a negative effect.
High body levels of several EDs, in particular PCBs and organochlorine compounds, have been associated with suppression of the immune system. 13 Impaired immune function has been associated with mass mortality among sea mammals following infection with a strain of the phocine distemper virus, morbillivirus. Observations have been made in harbour seals, grey seals and harbour porpoises in north eastern Europe, in striped dolphins in the Mediterranean, and in baikal seals in Siberia.61 The same virus has been identified in polar bears (A. maritimus) in North America and Siberia, though the mortality noted in marine mammals has not occurred.62 Bottle-nosed dolphin (Tursiops truncatus) mortalities have occurred along the eastern seaboard and the Gulf of Mexico, USA. Pathological examination showed opportunistic infections associated with immunosuppression, and effects were found to correlate with high PCB and other organochlorine levels in adipose tissue. T-cell response was also suppressed in
54 J. M. Sharma, Vet. Immunol. Immunopathol., 1991, 30, 199.
55 C. R. Pope, Vet. Immunol. Immunopathol., 1991, 28, 173.
56 A. H. Saad and B. Plytycz, Folia Biol. (Krakow), 1994, 42, 63.
58 B. Milholjcic, L. Radic and Z. Aleckovic, Acta Vet. (Beograd), 1990, 7, 269.
59 A. G. Rijhsinghani, K. Thompson, S. K. Bahtia and T. J. Waldschmidt, Am. J. Reprod. Immunol., 1996, 36, 269.
6° M.D. Kendall, B. Safieh, J. Harwood and P.P. Pomroy, Sci. Total Environ., 1992, 115, 133.
61 A. D. M. E. Osterhaus, R. E. de Swart, H. W. Voss, P. S. Ross, M. J. H. Kenter and T. Barrett, Vet. Microbiol, 1995, 44, 219.
62 E.H. Follman, G.W. Garner, J.F. Everman and A.J. McKeirnan, Vet. Rec, 1996, 138, 615.
live pelagic dolphins from the same geographical area." A possible mechanism for the reduced T-cell activity is suppression of the circulating thymulin level.^ Chemical-mediated immune suppression has been identified from the experimental study of several wildlife species. Harbour seals fed either chemically contaminated fish from the Wadden Sea or uncontaminated fish were found to have differing immune responses, with the exposed group showing lowered immune response to microbial infections and certain types of cancer.64 Mink fed fish taken from below a discharge point for bleached Kraft pulp mill effluent have also shown impaired immune function^5 showing that the non-accumulative chemicals in this effluent can actively disrupt endocrine associated functions.
Birds. In avian populations, organochlorine-associated suppression of T-cell mediated immune response has been found in herring gull and caspian tern colonies adjacent to the Great Lakes in Northern America.66 In the most severe cases, immune response was suppressed by up to 50%. Similarly, seabirds exposed to petrochemicals from oil spills have been found to have suppressed immune function.67
Amphibians. Immunosuppression has been shown to occur during metamorphosis in several amphibians, and appears to be hormonally regulated. The disappearance of 11 populations of toad from Colorado was associated with immune suppression, and frog mortalities have been associated with infection by a commonly occurring bacteria, Aeromonas hydrophila; the cause of the im-munosuppression in these animals has not been elucidated.68
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