In fish, the first line of defense against infectious microorganisms is based on a broad range of nonspecific humoral and cellular immune mechanisms ('innate immunity') which without prior specific activation can act in forming a more static barrier (Fish Shellfish Immunol. 10 (2000) 243; Dev. Comp. Immunol. 25 (2001) 827). This natural resistance is normally effective enough to protect fish from infectious diseases until specific immune responses are being induced ( Fig. 1; Dev. Comp. Immunol. 25 (2001) 841). Healthy fish exhibit both nonspecific and specific immune responses depending directly on environmental temperature. Pollution of the natural aquatic environment with industrial or agricultural sewage is an important immunosuppressing factor resulting in higher susceptibility to infectious diseases. To date, the possible immunotoxicity of a substance is evaluated using quantification of humoral factors like lysozyme, complement, C-reactive protein or total immunoglobulins but less often using functional assays. Furthermore, most of the functional assays (phagocytosis, respiratory burst, proliferative response) are based on the measurement of the response of resting but not of specific activated immune cells. However, the physiological responses of the immune system to an infection are based on a complex, stepwise activation and proliferation, especially of the specific immune functions after first contact to the microorganisms. In this report we describe in vitro methods for the evaluation of cellular immune functions of different leukocyte populations after specific in vivo triggering of the immune system. Parameters to be evaluated are activation and proliferation of leukocyte populations, phagocytosis and respiratory burst, secretion of antigen-specific antibodies and specific cell-mediated cytotoxicity. Furthermore, challenge models with bacterial (Aeromonas salmonicida) and viral pathogens (Viral Haemorrhagic Septicemia Virus, VHSV) are presented.
In 2011, a severe outbreak of hemolytic-uremic syndrome was caused by an unusual, highly virulent enterohemorrhagic E. coli (EHEC) O104:H4 strain, which possessed EHEC virulence traits in the genetic background of human-adapted enteroaggregative E. coli. To determine magnitude of fecal shedding and site of colonization of EHEC O104:H4 in a livestock host, 30 (ten/strain) weaned calves were inoculated with 1010 CFU of EHEC O104:H4, EHEC O157:H7 (positive control) or E. coli strain 123 (negative control) and necropsied (4 or 28 d.p.i.). E. coli O157:H7 was recovered until 28 d.p.i. and O104:H4 until 24 d.p.i. At 4 d.p.i., EHEC O104:H4 was isolated from intestinal content and detected associated with the intestinal mucosa. These results are the first evidence that cattle, the most important EHEC reservoir, can also carry unusual EHEC strains at least transiently, questioning our current understanding of the molecular basis of host adaptation of this important E. coli pathovar.
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