Figure 2 The arrangement of genes in the CCV genome predicted from the complete DNA sequence. The thinner and thicker portions of the genome denote the unique region and direct repeats respectively. The scale is in kilobase pairs. Protein-coding regions are shown as open arrows and numbered below the genome. Thinner arrows are used to reveal regions of overlap. Protein-coding regions 62, 69 and 71 are probably expressed by splicing; regions 57 and 58 may also be spliced. Potential polyadenylation sites are indicated by vertical arrows above and below the genome for rightward and leftward oriented genes respectively. Locations of tandem direct reiterations of short elements are denoted by filled rectangles above the genome. (Modified from Davison AJ (1992) Virology 186: 9-14, with permission of Academic Press.)

The primary route for virus shedding is probably via the urine.

Given the virulence of CCV in young catfish, it is quite reasonable to suppose that the virus might persist in an inapparent form in adult fish. This hypothesis is made all the more attractive by the existence of a characteristic latent phase of infection in the natural growth cycles of higher vertebrate herpesviruses. Viral mRNA and antigens have been detected in adult fish and, more recently, virus was recovered by cocultivation of tissues from wintering adult fish, some of which had been immunosup-pressed. The site of latency has not, however, been identified. Reactivated CCV from adult fish could, of course, be transmitted horizontally, but there is good circumstantial evidence that vertical transmission may also occur.

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