**9. Persistent infections**

It is known that some viruses are capable of persisting in their hosts without causing disease. This can be accomplished by producing either a latent or a persistent infection. The main difference between both is that during latent infection the virus is not replicating and keeping a minimal gene expression while in persistent infections all the genes are expressing, at low levels, without causing any symptom. Herpesviruses can establish latent infections in specific cell types [73]. This state is characterized by a unique transcriptional program that involves the expression of latency-associated transcripts (LATs) as the only viral products synthesized in large quantities. The virus is maintained as an independent quiescent genetic material within the host cell nucleus. An alternative mechanism is observed in measles virus by which the virus remains at low levels with the production of viral proteins. This is usually referred to as persistent infection [74].

The White Spot Syndrome Virus (WSSV) is a non-occluded virus pathogenic to shrimp, phylogenetically related to baculoviruses. It was found at very low levels in asymptomatic shrimps. The virus may reside within cells in a quiescent state as in a latent infection or causing a persistent infection [75]. Similarly, a nudivirus was found infecting persistently the cell line IMC-Hz1, derived from the corn earworm *Heliothis zea*.

Baculoviruses are highly lytic, causing a lethal disease in infected larvae. Epizootics caused by these viruses can reduce dramatically their host population [76]. Persistence of baculoviruses in the environment is mainly thought to be due to the OBs that protect virions from UV light and allow horizontal transmission. But there seems to exist another way for baculoviruses to persist in the environment at low host densities. Baculoviruses can cause sublethal infections, and so be vertically transmitted from adult to offspring [77-79] or may as well become persistent or latent [80]. A laboratory colony of *Mamestra brassicae* was found to harbour an occult infection by the baculovirus MbMNPV with expression of viral genes at a low level [81]. Later, Burden *et al*. demonstrated the persistence of this virus in naturally occurring field populations of *M. brassicae*. RT-PCR analysis showed the presence of *polyhedrin* transcripts in asymptomatic larvae, indicating a covert infection [80-81]. Similar results were obtained using *ie1* as a target [81]. Moreover, these studies revealed that covert infections could be induced to produce overt infections when infecting these larvae with another baculovirus. This means that the persistently infecting virus retains its ability to produce a lethal disease in the larva.

There is accumulating evidence of persistent baculoviral infections. Kemp et al [83] detected baculoviral presence (CfMNPV, CfDEFMNPV and a GV) in laboratory and field populations of *Choristoneura fumiferana*. Also, there were baculoviruses (SeMNPV and MbNPV) identified in *Spodoptera exigua* populations that could be reactivated to full lethal forms [84]. A study in

field populations of *Spodoptera exempta* showed that virtually all the insects collected in the field were positive for *S. exempta* nucleopolyhedrovirus (SpexNPV) DNA and 60% of these insects had transcriptionally active virus, suggesting that SpexNPV is transmitted vertically at extremely high levels in field populations of *S. exempta* and can maintain a persistent infection without obvious symptoms [85].

On the whole, baculoviruses seem to use different strategies to persist in nature: on one hand OBs permit their subsistence outside the host for horizontal transmission while, on the other hand, they can persist as covert infections in the host, allowing vertical transmission too. Moreover, these covert infections can be triggered to overt infections producing the typical lethal disease in the host. Nevertheless, the mechanisms of reactivation of these sublethal infections remain to be elucidated.
