Yellow Head Virus in shrimp & prawn

1. A SEMINAR ON :
SUBMITTED TO:
PROF. T. J. Abraham
Dept. of AAH
SUBMITTED BY:
Abhijit Das
M.F.Sc 1st Year

2. Yellow head disease (YHD) is a viral infection caused by
the Yellow head virus (YHV) of shrimp and prawn, in
particular of the giant tiger prawn (Penaeus monodon), one of the
two major species of farmed shrimp. The disease is highly lethal
and contagious, killing shrimp quickly. Outbreaks of this disease
have wiped out in a matter of days the entire populations of many
shrimp farms that cultivated P. monodon, i.e. particularly South-
East Asian farms. In Thai, the disease is called Hua Leung.
YHD had been reported first from Thailand in 1990, the
closely related GAV has been discovered in 1995 during a yellow
head-like disease in Australian shrimp farms.

3. The disease is caused by the Yellow head virus (YHV), a
positive-sense single-stranded RNA virus related to
coronaviruses. A closely related virus is the Gill-associated
virus (GAV), which is the type species of the genus Okavirus,
Family Ronaviridae and order Nidovirales.
YHV forms enveloped,
rod-shaped, size of 40-50 nm
× 150-180nm, cytoplasmic virus.
Envelopes are studded with prominent peplomers projecting
approximately 11 nm from the surface. Nucleocapsids appear
as rods (diameter 20-30 nm) and possess a helical symmetry
with a periodicity of 5-7 nm.

4. YHV is highly infectious for most known species of
cultivated penaeid prawns.
Crustaceans known to be susceptible to yellow head disease:
 Black tiger prawn (Penaeus monodon) — primarily
 Gulf banana prawn (Penaeus merguiensis)
 Prawn (Palaemon styliferus)
Pacific white shrimp (Penaeus vannamei)
 P. monodon are susceptible to YHV infection beyond
PL15.

5. YHV infection can be transmitted horizontally by injection,
ingestion of infected tissue. Infection of shrimp has also been
established by injection of extracts of paste prawns collected
from infected ponds. The dynamics of how YHV infection
spreads within aquaculture ponds have not been studied.
However, the rapid accumulation of mortalities during disease
outbreaks suggests that horizontal transmission occurs very
effectively.
YHV has been reported in Chinese Taipei, Indonesia,
Malaysia, the Philippines, Sri Lanka, Thailand and Vietnam.
YHV has also been detected in P. vannamei in Mexico.

6. With P. monodon being farmed in ponds, disease caused
by YHV can cause up to 100% mortality within 3–5 days of the
first appearance of clinical signs. Whilst mortalities can easily
be induced by experimental exposure of P. monodon to YHV or
GAV, bioassays have identified YHV to be far more virulent.
YHV virus infection levels accompanied by disease can be
precipitated by physiological stress induced by sudden changes
in pH or dissolved oxygen levels, or other environmental
factors

7.  Moribund prawns aggregate near surface at pond edges.
 Infected 5–15 gram prawns begin feeding at abnormally
high rate for several days and then cease feeding entirely.
 Mass mortality 3 days after cessation of feeding.
 White, yellow or brown gills.
 Yellowing of the cephalothorax and
general bleaching of body.
 Yellow, swollen digestive gland
makes head appear yellow.

8. Injection of shrimp with double-stranded(ds) RNA
homologous to ORF1a/1b gene regions of YHV or GAV can
inhibit viral replication and prevent mortalities following
experimental challenge. The antiviral action of the dsRNA
appears to involve the RNA interference (RNAi) pathway.
Specific pathogen free (SPF) or PCR-negative seedstock
and biosecure water and culture systems may be used to
reduce the risk of disease.

9. Shrimp can get infected with YHV from the late post-
larval stage onwards. The clinical signs are most commonly
observed and the mortality rate is the highest during the early
to late juvenile stages. In hatcheries, shrimp can get infected
from other shrimp when the virus is transmitted through the
food materials ,water or when they ingest tissues that are
contaminated by dead shrimp. Sometimes crustaceans and
other animals may also act as disease vectors.
In haemolymph Smear method shrimp that are infected
with YHV, the nucleus of dead cells can be seen as pycnotic
nucleus. However, there shall be no evidence of bacterial
infection in the same specimen because it may cause similar
changes in haemocyt. This may cause misinterpretation

10. The smear preparations and the RT - PCR analysis are the
best diagnostic methods to detect the infection. In
histopathological method, the positive samples showed as the
necrotic cells in the smear, but sometimes it can be a
misidentification. In haemolymph smear method pycnotic
nucleus or karyorrhectic nucleus can be observed, but the same
results appear with the bacterial infection. Therefore, it also can
be misidentified. When the same samples were subjected to RT
- PCR, results showed variations among the samples which
were not positive for the haemolymph smear method or
histological method. In RT - PCR it always analyze with the
positive control and the negative control. Therefore RT-PCR
method can be used as the confirmation method as well as the
most accurate, appropriate and highly sensitive detection
method by comparing to other two methods of detection.

11. Google
Wikipedia
Oie.int