Natural, in ovo, vertical transmission of the RNA viruses, Chequa iflavirus and Athtab bunyavirus, but not Cherax reovirus in redclaw crayfish (Cherax quadricarinatus)

The first molecular evidence for natural, in ovo, vertical transmission of Chequa iflavirus and Athtab bunyavirus from farmed female redclaw crayfish (Cherax quadricarinatus) cultured in northern Queensland, Australia is presented. Cherax reovirus was also examined, but evidence that this is spread via in ovo vertically transmission is very limited. Amongst 57 broodstock-derived pools of fertilized eggs, 38 (66.7%), 34 (59.6%) and 6 (10.5%) were positive for Chequa iflavirus, Athtab bunyavirus and Cherax reovirus respectively with the respective average loads of 6.56 x 10(4), 2.33 x 10(3) and 1.03 x 10(1) copies/mu l of tested samples. Grouping samples by farm origins, all viruses in most farms were statistically similar (P > 0.05) for viral loads. As two viruses are transovarial transmitted, surface sterilizing practices would be ineffective and the load remains high as is seen in Chequa iflavirus and Athtab bunyavirus, but extremely low on surface-treated eggs as appears with Cherax reovirus. Our evidence, plus the literature, supports the general rule that if the virus is in the gut, most of the vertical transmission is faecal /oral and can be drastically reduced by efficient egg surface sterilization. If the virus is spread in ovo, then RT-qPCR testing coupled with therapeutic methods and selection of resistant crayfish families or detection of viral negative, geographically isolated populations of crayfish is the way forward for the removal of Chequa iflavirus and Athtab bunyavirus.

Automated summary

This study provides the first molecular evidence of natural, in ovo, vertical transmission of Chequa iflavirus and Athtab bunyavirus from farmed female redclaw crayfish in northern Queensland, Australia. Analysis of viral loads in eggs showed differences between viruses and farms, indicating varying effectiveness of surface sterilization methods. Evidence suggests that efficient egg surface sterilization can drastically reduce vertically transmission of fecal/oral viruses, while selection of resistant crayfish families or viral negative populations may be necessary for eliminating Chequa iflavirus and Athtab bunyavirus.