Biocontrol viruses need to be both readily transmissible and highly virulent, requirements that are complicated by a possible evolutionary trade-off between these two variables [
12]. It is therefore important to ask how might these traits evolve in the case of CyHV-3? Although predictions in this area are inherently difficult with, for example, different levels of virulence seemingly favoured in MYXV and RHDV [
10], we hypothesise that close contact between carp is required for efficient transmission of the virus and that this has a number of implications for transmissibility and virulence. The importance of close contact is supported by a number of observations: (i) skin is the major portal of viral entry in carp [
13], and both physiological and immunological changes in the skin and surface mucus following infection suggest that skin is also the major site of virus shedding [
14]; (ii) CyHV-3 only survives for about three days outside the host [
15]; (iii) virus is excreted at low titre and for long periods before the development of clinical disease [
16]; (iv) carp are highly sensitive to CyHV-3 infection implying that even transient direct host-to-host contact may allow transmission [
17]; and, (v) CyHV-3 likely results in latent infections in survivors [
18], thereby allowing repeated opportunities for virus excretion in the face of stress [
19]. Together, these observations are consistent with a virus-host relationship that has evolved to facilitate maximum transmissibility during periods of carp aggregation (such as breeding) when fish are also likely stressed and immunosuppressed [
20]. These observations then raise a key question: is the transmissibility of CyHV-3 optimized by the evolution of low virulence strains of virus that induce latency in fish? This would enable efficient transmission at those regular periods in the life-cycle when aggregations of infected and uninfected stressed fish lead to recrudescence of latent virus. If true, then it also seems reasonable to predict that the initial high virulence of this virus in
C. carpio in Australia will gradually decline, mirroring what has happened with MYXV [
21], although selection pressures may change as the density of carp declines.