What is it?
Necrotising hepatopancreatitis (NHP), also known as infection with necrotising hepatobacterium, is caused by infection with a Gram-negative, intracytoplasmic species of alphaproteobacterium that infects the hepatopancreas of prawns, also referred to as NHP bacterium. The NHP bacterium exists in two morphological forms: a rod-shaped, nonflagellated, rickettsia-like organism; and a helical, flagellated form.
Various varieties of shrimp are known to be susceptible to the disease and mortality can be 90 to 95 percent within 30 days of an outbreak.
Where and when might it occur?
NHP outbreaks are often preceded by lengthy periods of high water temperatures (29˚C to 31˚C) and elevated salinity (up to 40 parts per thousand). NHP appears to be transmitted by direct ingestion of carrier prawns (survivors of NHP bacterial infection may carry the bacteria for life) and through contaminated water. NHP bacteria may also be shed in faeces and contribute to disease transmission. Mortalities usually occur midway through the grow-out phase.
Signs of the disease often include lethargy, emaciation, heavy protozoan or bacterial fouling and a reduced growth rate.
Gross pathological signs are:
- soft shell
- flaccid body
- black gills
- empty intestinal tract
- degenerated or atrophied digestive gland (hepatopancreas), which appears pale to white
- black (melanised) streaks in the hepatopancreas
Microscopic pathological signs are:
- multifocal granulomatous lesions in hepatopancreatic tubules, with atrophy of adjacent hepatopancreatic tubule epithelial cells
- tubular cells within the granulomatous lesions that can be hypertrophied and contain basophilic organisms within the cytoplasm
- sloughing of tubule epithelial cells
- severe haemocytic inflammation of the intratubular spaces.
Four distinct phases of infection have been described: initial, acute, transition and chronic. Acute and transition phases are identifiable by the presence of pathognomonic lesions in the hepatopancreas. Molecular techniques are required for positive diagnosis of NHP bacterium infected individuals in the initial or chronic phase of infection.
Source: Australian Government, Department of Agriculture, Fisheries and Forestry