Title

Experimental Infection of Necrotizing Hepatopancreatitis Bacterium (NHPB) in Kona Stock Litopenaeus Vannamei (Pacific White Shrimp)

Date of Award

2006

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Coastal Sciences, Gulf Coast Research Laboratory

First Advisor

Jeffrey M. Lotz

Advisor Department

Coastal Sciences, Gulf Coast Research Laboratory

Abstract

Necrotizing hepatopancreatitis (NHP) is a severe disease of the Pacific white shrimp Litopenaeus vannamei that produces up to 95% mortality in shrimp ponds. The NHP-bacterium (NHPB) is a gram-negative, pleomorphic, obligate intracellular α-proteobacterium that infects hepatopancreatic epithelial cells. Although NHPB remains unculturable in vitro, in vivo cultivation in L. vannamei produced infective material for use in experimental infections. Mortality in NHPB-infected shrimp ranged from 16 to 56 d post-exposure with a median survival time of 34 d. Associated pathology increased with progressive stage of NHP disease and with increased pathogen load. Over the course of infection, bacterial load ranged from 10 3 to 107 copies mg-1 in hepatopancreas and from 101 to 105 mg-1 in feces; whereas, lethal NHPB load ranged from 107 to 108 copies mg-1 hepatopancreas and 106 to 107 copies mg -1 feces as determined through real-time PCR. Transmission of NHPB to susceptible shrimp occurred by cannibalism of dead infected shrimp (mean βd = 0.62) but not through cohabitation with an acutely infected shrimp. Additionally, salinity did not appear to hinder NHPB transmission although mean βd was significantly higher at moderate salinity of 20 and 30[per thousand] (βd ∼ 0.62) than at 10 or 40[per thousand] (βd ∼ 0.22). A mathematical epidemic model of NHPB dynamics in a theoretical, closed, shrimp population was developed. Host states of NHPB infection include susceptible, prepatent, acute, dead, and removed. Transitions between states are transmission from a dead host (β d = 0.62), patency (ν = 0.06), virulence (α = 0.08), and removal of infectious material. The NHPB epidemic model is similar to that of white spot syndrome virus (WSSV) due to an absence of a chronic or carrier state of infection that is observed in Taura syndrome virus (TSV) infection. The evolution of increased virulence is predicted for NHPB and other pathogens of cultured organisms where transmission occurs exclusively from a dead host, i.e. cannibalism. This suggests that future disease problems in aquaculture may not only be caused by emerging pathogens but by new variants or mutants of existing pathogens evolving increased virulence.