Shrimp vaccination with insect-adapted yellow head virus (YHV) extends survival upon YHV challenge

This short paper on yellow head virus Type1 (YHV-1) describes preliminary research worthy of further study. YHV-1 disease outbreaks can cause severe mortality in the cultivated shrimp Penaeus (Penaeus) monodon and Penaeus (Litopenaeus) vannamei. No practical preventative treatment such as vaccination has been reported. However, it has been shown that C6/36 mosquito cell cultures can adapt to YHV-1 and become persistently immunopositive for the virus after 30 split-cell passages or more. Shrimp injection with homogenates from low passages caused yellow head disease (YHD) but from high passages did not, even though injection resulted in immunopositive hemocytes in the injected shrimp. This suggested YHV-1 attenuation during insect cell passaging and the possibility of using cell homogenates as a vaccine to protect shrimp against virulent YHV-1. To test this hypothesis, we injected shrimp with 30th passage homogenates to test for protection against YHD upon subsequent challenge with virulent YHV-1. Results confirmed earlier work that hemocytes of the infected shrimp became both reverse transcriptase PCR positive and immunopositive for YHV-1 but exhibited no mortality. Similarly, there was no mortality in the control group injected with homogenate from YHV-1 negative insect cells. When subsequently challenged with YHV-1, shrimp in the positive control group injected with homogenate from naive insect cells gave 100 percent mortality within 7 days post challenge while total mortality in the group injected with YHV-1 homogenate did not occur until day 9 post challenge. Kaplan-Meier log-rank survival analysis revealed that survival curves for the two groups were significantly different (p<0.001) and that the mean survival time for the test group (6.5 days) was significantly longer than that in the positive control group (5.4 days). The results confirmed that shrimp injection with YHV-1 immunopositive insect-cell homogenate gave transient resistance to YHV infection and that further research into the possible use of insect cell lines to produce shrimp antiviral vaccines is warranted.