The loss due to the yellow head virus is second only to that of white spot syndrome virus (WSSV) in South East
Asia. Despite the continual search for its reservoir, since its discovery in the early 1990s, the reservoir for YHV-1
(the most virulent strain of YHV found in Thailand) has still not been identified. Outbreaks of the virus in cultivated,
exoticwhiteleg shrimp P. vannamei that originate fromSPF stocks known to be free of the virus, suggesting
that the outbreaks occur via horizontal transmission from an environmental source. Based on our results, exotic
red claw crayfish (Cherax quadricarinatus) that are cultured in fresh water areas where marine penaeid shrimp
are also frequently farmed should be added to the list of potential reservoirs. Red claw crayfish, fromour results,
is susceptible but highly tolerant to the virus. Infected red clawcrayfish could transmit the viruswithout showing
any signs of yellow head disease. These facts make them perfect carrier for the virus. YHV infection in red claw
crayfish could be detected by RT-PCR only not by routine histology or immunohistochemistry.