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Necrotizing
hepatopancreatitis, NHP, is a severe bacterial
disease of penaeid shrimp aquaculture. NHP
was first reported in 1985 from shrimp ponds
in Texas, and resulted in significant mortality
and devastating losses to shrimp crops. Since,
NHP has been observed in penaeid shrimp ponds
in Central and South American countries.
NHP primarily affects Western Hemisphere
shrimp aquaculture with no documented reports
from the Eastern Hemisphere. Synonyms of
NHP include granulomatous hepatopancreatitis,
Texas necrotizing hepatopancreatitis (TNHP),
Texas pond mortality syndrome (TPMS), and
Peru necrotizing hepatopancreatitis (PNHP).
Reported hosts of NHP include Litopenaeus
vannamei, L. setiferus, L. stylirostris,
Farfantepenaeus aztecus and F. californiensis.
Gross signs of NHP include reduced feed intake,
empty gut, lethargy, anorexia, and discoloration
and atrophy of the hepatopancreas. Diagnostic
methods include wet mounts of the hepatopancreas
examining for characteristic melanized tubules,
histology, and molecular PCR and in situ
analysis specific for NHP. Treatment with
oxytetracycline-medicated feed has been effective
in reducing the spread of NHP in the early
stages of infection; however, oxytetracycline
is not FDA approved for use in United States
shrimp aquaculture. Environmental factors,
such as temperature and salinity, are thought
to greatly influence the occurrence of NHP
disease in penaeid shrimp aquaculture. Epizootics
associated with TNHP and PNHP followed periods
of prolonged elevated temperature (greater
than 29o C) and salinity (20 to 40‰).
In addition, physical conditions similar
to those encountered with TNHP and PNHP have
been observed in NHP outbreaks in Central
and South America shrimp ponds.
The agent responsible for NHP is a gram-negative,
pleomorphic, obligate intracellular bacterium.
The NHP organism is non-culturable; no methods
are currently available for culturing the
NHP agent in established cell lines or on
laboratory media. Therefore, laboratory research
of NHP is dependent on maintaining the disease
agent in live animals. The Crustacean Disease
Group at the Gulf Coast Research Laboratory
(GCRL) in Ocean Springs, MS has recently
developed a method for continuous maintenance
of NHP infections in SPF Kona stock (Consortium
line animals used for disease research) Litopenaeus
vannamei. A stock of NHP-infected L. vannamei
is maintained in 2-ton tanks filled to approximately
one-third depth with artificial seawater
at 30 ppt salinity and 30o C. Susceptible
individuals are placed in the tank with NHP-infected
individuals. As NHP-infected animals become
weak and moribund, susceptible animals feed
on those infected animals and acquire the
NHP organism through ingestion. This stock
provides infectious material for experimental
studies.
An experimental system has been developed
where susceptible hosts are exposed orally
to tissue from an infectious shrimp. Susceptible
animals are individually isolated in aerated
4-L Sterilite® storage containers at
30 ppt and 30o C. The relatively small volume
containers allow numerous replicates to be
included in bioassay challenges and isolation
of individual shrimp ensures that susceptible
hosts may become infected with NHP only through
initial oral exposure to the NHP-infected
tissue, and not from contact with other infected
individuals. Shrimp are exposed to an approximately
0.05 g piece of an NHP-infected hepatopancreas.
Shrimp feeding on this piece of infected
tissue may acquire an NHP infection. Typically,
symptoms of disease produced by the NHP agent
are visible two to three weeks post-infection.
NHP-infected shrimp exhibit reduced feed
intake and eventually stop responding to
the presence of food. Shrimp at this stage
may display an empty gut. During this time
of arrested feeding, the hepatopancreas may
turn from the normal brown-orange to an abnormal
pale-white coloration, which in some cases
can be viewed externally through the carapace
of infected animals ). In addition,
the hepatopancreas of infected animals is
atrophied, displaying a noticeable reduction
in size.
Experimental exposures of NHP to individuals
of L. vannamei have been attempted
to estimate several parameters, including NHP-induced
mortality
rate and mean survival time of infected shrimp.
NHP-induced mortality from feeding exposure
was observed 18 to 46 d post-exposure, with
no infected animals recovering from an NHP
infection. Some animals exposed orally to NHP-infected
material did not exhibit signs of disease and
were diagnosed NHP-negative. Mean survival
time for NHP infected animals was 33.4 d. The
NHP-induced mortality rate, which is the probability
of death due to disease, was estimated to be
0.05 per day. In comparison to viral diseases
of shrimp aquaculture, NHP incubation time
is considerably longer than that of White Spot
Syndrome Virus (WSSV) and Taura Syndrome Virus
(TSV) in SPF Kona stock L. vannamei. In addition,
mean survival time of NHP infected animals
is 33.4 d compared to 2.9
d for WSSV and 11.7 d for TSV. Estimates of
mortality
rate for WSSV (0.4) and TSV (0.3) are nearly
an order of magnitude larger than that observed
for NHP (0.05).
Future research of NHP at USM-GCRL will involve laboratory and field studies
to identify several aspects of NHP disease outbreaks in shrimp aquaculture ponds.
First, experimental infections of NHP in Kona Stock L. vannamei will be attempted
to estimate parameters of NHP spread in a shrimp population, which will be used
to generate perature to elucidate their role in NHP outbreaks. Third, selective
breeding of L. vannamei is currently being investigated to determine if some
groups of hosts are more or less susceptible to NHP infection. Fourth, the life
cycle of NHP likely involves a reservoir host and prospective laboratory research
aims to identify reservoir hosts for NHP infection in the shrimp pond environment.
Finally, field work involving an on-farm epidemiology study of NHP in affected
shrimp farms in Texas, USA
is proposed to assess NHP transmission and mortality observed during a grow-out
season.
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