Geographic variations among infectious hypodermal and hematopoietic necrosis virus (IHHNV) isolates and characteristics of their infection

Nucleotide sequence variations of a 2.9 kb fragment of infectious hypodermal and hematopoietic necrosis virus (IHHNV) isolated from samples of Penaeus monodon were determined and compared with an isolate from Hawaii. The infection characteristics of these isolates were examined by histology, in situ hybridization, and laboratory challenge studies with P. vannamei. Isolates of IHHNV were obtained from samples collected from the SE Asia region (the Philippines, Thailand, and Taiwan). Isolates of putative IHHNV were obtained from African samples (Tanzania, Madagascar, and Mauritius). The Philippine isolate had a very high nucleotide sequence identity (99.8%) to Hawaii IHHNV. The Thailand isolate showed a slightly lower identity (96.2%). The putative IHHNV sequences collected from Tanzania and Madagascar showed greater divergence from Hawaii IHHNV, 8.2% difference for Tanzania and 14.1% difference for Madagascar. A phylogenetic analysis showed that the Philippine IHHNV clustered with IHHNV found in the western hemisphere. This supports the theory that the Philippines was the origin of IHHNV that was first detected in Hawaii. In the laboratory infection study, both the Philippine and Thailand IHHNV were passed into P. vannamei, and the infected shrimp did not suffer any mortalities. In another laboratory infection, P. vannamei injected with a tissue homogenate of P. monodon from Madagascar, which tested positive for IHHNV by PCR, did not demonstrate IHHNV infection, suggesting that this putative IHHNV is not infectious to P. vannamei.