Most viruses are transmitted in a density-dependent fashion and so rely on interactions between hosts for transmission and persistence. This becomes a problem in low-density populations due to a shortage of new hosts to infect. Consequently, viruses have evolved a number of mechanisms to ensure persistence in the face of low or variable host densities. For the insect viruses, these include long-term survival in the environment, alternative hosts and vertical transmission.
While it has long been thought that horizontal transmission and persistence in the environment (outside of the host) are the main means of baculovirus survival, it is becoming increasingly evident that many species harbour ‘covert’ infections that are passed from adults to their offspring. Our understanding of this process, even in a group as widely employed as insecticides as the baculoviruses, is poor and we have little idea of how covert infections contribute to the maintenance of virus populations in the field.
There are two types of covert virus infection; ‘persistent’ infections and ‘latent’ infections. They are distinguished by the degree to which virus-encoded gene products are expressed and whether or not infectious virus particles are present.
Persistent virus infections are characterised by a constant low-level production of virus particles within an infected cell. These infections represent a balance between the host and the virus, which may be maintained through the interaction of the cells and the virus alone, interaction with the host immune system, the production of defective interfering virus, or a combination of all three. Persistent virus infections still express the full range of viral genes, although this expression may be down-regulated.
In a latent virus infection, the viral genome, and possibly some virus-encoded products, are present, but infectious virus particles are not formed. Latent virus infections involve a shut-down in viral gene transcription with only those genes involved in maintaining the latent state being expressed. Latent infections do not represent a dead-end for the virus as, with an appropriate triggering stimulus, the infection can revert to a fully reproductive overt infection.
For the majority of these ‘covert’ infections in insects, it has yet to be determined whether they represent persistent or latent infections, but clearly their prevalence indicates that they may play an important role in the survival and persistence of the virus in host populations.
Indeed, if our understanding of the S. exempta-SpexNPV interaction is correct, it is the vertically-transmitted virus, and not virus in environmental reservoirs, that is important in initiating epizootics.