The global shrimp aquaculture, with export values of billion dollars per year in the past decade, is a key economic sector of several countries in Asia and South America
. Despite overall shrimp production from aquaculture continuing to rise, shrimp susceptibility to viral pathogens is a constant threat to the shrimp production. Development of effective means that would reduce risks posed by viruses will be beneficial to the shrimp culture industry.
A shrimp parvovirus (densovirus) was first identified from high mortality disease outbreaks in cultured Penaeus stylirostris in the Americas
, and it has been classified as Penaeus stylirostris densovirus (PstDNV) in the family Parvoviridae. PstDNV is also known as infectious hypodermal and haematopoietic necrosis virus (IHHNV), and it will be referred to as IHHNV in the present work. Its genome contains a linear single-stranded DNA with an estimated size of 4.1 kb
[4, 5]. To date, 3 types of IHHNV have been reported based on their original found location and DNA sequences
[6, 7]. Target organs for IHHNV include gills, cuticular epithelium (or hypodermis), all connective tissues, haematopoietic tissues, lymphoid organ, antennal gland, ventral nerve cord - its branches and its ganglia
More recently, non-infectious inserts of IHHNV in the shrimp genome have been found in both captured and farmed P. monodon from East Africa, Australia, and Thailand
[9, 10]. Recently, the study by Saksmerprome et al. (2011) indicated that random insertion of IHHNV sequences may yield false-positive results by using the currently recommended detection methods. Thus, it is necessary to improve the detection method for diagnosis of a real IHHNV infection. Besides the diagnostic implications, it is interesting to investigate if viral insertion in shrimp genome is involved in natural, transmissible immunity in crustaceans as previously proposed
. In addition, previous reports demonstrated that P. vannamei and P. stylirostris with pre-infection of IHHNV were more resistant to white spot syndrome virus (WSSV) as compared to the IHHNV-free shrimp
[12, 13], although the mechanism remains unknown. WSSV is one of the most severe viruses that could lead to mass mortalities in ponds and heavy production losses, therefore effective control strategies against the virus would be highly desirable. To elucidate the effect of persistent IHHNV infection and non-infectious viral inserts on WSSV resistance in P. monodon, a multiplex-PCR method developed herein was applied to conveniently distinguish between IHHNV-infected and IHHNV-inserted types. Then, the diagnosed shrimp with real IHHNV infection and viral inserts were examined for WSSV resistance by considering cumulative mortality, time-to-death, and WSSV copy number. Application of the multiplex PCR for selection of shrimp with viral tolerance could be useful for future development of a program of specific resistant (SPR) shrimp.