of viability dropped rapidly over an initial 5 7 day period and then gradually

Of viability dropped rapidly over an initial 5 7 day

This preview shows page 300 - 302 out of 594 pages.

of viability) dropped rapidly over an initial 5-7 day period, and then gradually decHned over 3-4 weeks. Some cells became small and spherical, corresponding to the notion of ultramicrobacteria (see Austin, 1988), whereas others elongated to short spirals. Protein synthesis, as measured by incorporation of [^^S]-methionine, declined during the first 6h of starvation, and increased to >70% of the rate in exponentially growing cells by 5 days into the starvation regime (Nelson et al, 1997). The precise origin of an isolate has importance for epizootiology. In this respect, Olsen and Larsen (1990) detailed a seemingly useful method, namely restriction fragment length polymorphism of the 65-70 kb (kilobase) plasmid. This method should have value for epizootiological investigations. The exact mode of infection is unclear, but undoubtedly involves colonisation of (attachment to) the host, and thence penetration of the tissues. Ransom (1978) postulated that infection probably begins with colonisation of the posterior gas- tro-intestinal tract and rectum. This conclusion resulted from the observation that V. anguillarum was seen initially in these sites. Home and Baxendale (1983) reported adhesion of V. anguillarum to intestinal sections derived from rainbow trout. All regions of the intestine were colonised (approximately 10^ cells/cm^), with maximum attachment occuring within 100 min. The skin appears to become colonised within 12h of immersion in a virulent culture (Kanno et ai, 1990). Then, invasion of the liver, spleen, muscle, gills and intestine follows (Muroga and De La Cruz, 1987). It has been well documented that epizootics occur in the warm Summer months when water temperatures exceed 10°C, the water is depleted of dissolved oxygen, and the fish stressed by overcrowding and poor hygiene (Anderson and Conroy, 1970). There are exceptions to the norm insofar as outbreaks have been documented in freshwater (e.g. Rucker, 1959) and at low temperature, i.e. 1-4°C (Olafsen et ai, 1981). It is perhaps ironic that isolates recovered from rainbow trout in freshwater have an obvious salt requirement for growth (Rucker, 1959). Perhaps, the organisms were contained in a protected ecological niche, such as within the fish body, prior to the manifestation of the disease. However, it should be remembered that the patho- gen has been recovered sporadically from freshwater (West and Lee, 1982). The determination of plasmid profiles may have value for epizootiological investigations (Wiik et aL, 1989). The presence of heavy metals, notably copper and iron, contributes to an exacerbation of vibriosis. Yet, sublethal concentrations of chlorine do not appear to promote the development of infections (Hetrick et ai, 1984). Levels of only
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Epizootiology: Gram-negative bacteria 281 30-60 |ig copper/ml and 10 |ig of iron/ml have caused severe problems (Rodsaether et ai, 1977; Nakai et ai, 1987). Further investigation demonstrated the susceptibility to vibriosis was dependent upon concentration and time of exposure to copper (Baker et al, 1983). The debilitating effect has been attributed to coagulation in the mucus
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