The White Spot Syndrome Virus (WSSV) is
a pathogen that affects marine shrimp as well as a range of Crustaceans. The
virus belongs to the Nimaviridae family, and can be morphologically considered
either egg –shaped or rod-shaped about 80-20nm wide by 250-380nm long with a
small tail. The name ‘white spot syndrome’ is clearly based on the noticeable
white spots that are present on the infected shrimp’s cuticle. The other symptoms
exhibited by the shrimp are lethargy and anorexia; this causes mass mortalities
in 1-2 days. A number of shrimp farms have been affected by WSSV, the earliest
recognised outbreak was in the early 1990s in eastern and south-eastern Asia.
The risks of natural carriers such as wild Crustaceans are very important to
shrimp farmers. This paper examines the presence of WSSV in different Crustaceans
in India using PCR and evaluated different PCR primers for the detection of
WSSV in various asymptomatic carrier species.
A variety of species of Crustaceans were
collected from March to December 1999, these included wild Crustaceans, non-cultured
Crustaceans and cultured fresh water prawns. The DNA from each organism was
extracted repeatedly suspended in TESP buffers and centrifuged until the final
pellet containing the desired DNA was dissolved in a 25µl TESP buffer and used
for PCR analysis. Two sets of primers were used; the first primer pair was Lo
1-2 and Lo 5-6 and the second primer pair was IK1-2 and IK 3-4. The DNA from the
samples of P. monodon that were
naturally infected with WSSV was used as a positive control. Each PCR product
(20µl) was mixed with a gel loading buffer (5µl) and subjected to
electrophoresis. The gel was then observed on a UV transilluminator and photographed
using Kodak electrophoresis documentation and analysis system. Probe
Hybridisation was used to confirm that the PCR products obtained were from WSSV.
Many wild caught species were positive
in non-nested PCR using the Primer IK3-4, these results verify the wide spread occurrence
of WSSV in several wild crabs. The
organisms P.indicus and Acetes sp. did not show signs of the
disease and were positive only in a nested PCR indicating a low viral load. The
most unusual result was that the cultured
M.rosebergii had WSSV present even though it was cultured inland at least
250km from the coast; the author’s theory for this is that the larva was
contaminated in the hatcheries which are located near the coast. Previous
studies with M. rosebergii suggest
that it has a high tolerance for the virus hence the fact it does not
demonstrate any symptoms. To check this tolerance, in lab tests Hossain et al (2001)
applied several environmental stressors and the organisms still did not show
any symptoms. This tolerance means there is a risk that the disease can spread
to distant locations.
When the virus load is low, there are
better chances of getting a positive result with primers with smaller amplicon
sizes. Which explains why the highest numbers of non-nested positives were with
the primer IK3-4, since it has the smallest amplicon size of 310bg overall.
Consequently several organisms could have been carrying a low viral load but
showed negative results with the other primers. It is clear that when analysing
possible carriers a small amplicon primer should be used to reduce this
possibility.
In this paper the results section contained four tables that
simplified the results well, but only three organisms actually specify the
tissues that were sampled; gill, stomach or cuticle, it is not clear whether
this was to identify main areas of infection or just more samples from one
particular specimen. Overall this paper was successful in suggesting the main
asymptomatic carriers that may threaten the shrimp farms in India; however
since P.monodon and other
wild–caught Crustaceans have been recorded in Japan and Taiwan, there is a need
to compare data on the current and past infected species, because there could
be some unidentified asymptomatic carriers still threatening shrimp farms. This
could be crucial if more species develop the tolerance that M.rosebergii possesses, and
eventually leading to the virus spreading to distance locations or further
inland and becoming a threat to fresh water systems. It may be interesting to
find out twelve years on, if the abundance of carriers is distinct in specific
shrimp farming areas or has become diverse globally.
Hossain S, Chakraborty A, Joesph B, Otta
S.K, Karunasagar I, Karunasagar I, (2001) Detection
of new hosts for white spot syndrome virus of shrimp using nested polymerase
chain reaction, Aquaculture issue 198, pages 1-11
hey, really interesting post, just one thing to ask if possible, the first is you've briefly mentioned the optimum areas that this virus inhabits, and some of the papers i've reviewed and read have seen an increase in abundance out of the natural habitats that these viruses usually are restricted to, was there any mention of an expansion of this species, or is it just speculation that you have mentioned, i hope this makes sense, basically saying are their any studies over the past 11 years of an expansion in areas that this virus can inhabit. or even of new species, not just shrimp as perhaps it is a farming environment that it thrives in.
ReplyDeletethanks
Ollie
Hi Ollie
ReplyDeleteThank you for your comment and sorry about the late reply. At the end of the discussion the authors made a suggestion that the tolerance of some species would contribute to the spread of the virus to distant destination. This was based on the paper by Sahul Hameed et al, (2000) who reported the high WSSV tolerance present in Macrob rosenbergii. The tolerant species do not present the symptoms of the virus and may be imported along with other healthier species, however with increasing advances in technology it is likely that we can screen these animals and identify the virus depending on how thorough the scanning process is. I have found it hard to find recent examples of spreading WSSV which is probably due to our increasing knowledge and advanced technology since the previous outbreaks. Although the idea of a spread in this paper seems to be speculation by the authors, it is still possible. Especially since Sánchez-Paz (2010) mentions that there is still no treatment available to interfere with the spread of the disease. The rapid transmission of the virus means that it can spread quickly between and among species; it takes at least one infected individual to spread the virus to other broad stocks especially in densely populated farms. Since you have said there is an increase in abundance out of natural habitats in the papers you have reviewed do you agree with the possibility of a spread?
I hope this helps, let me know if it doesn't
Kathryn
Sánchez-Paz, A., (2010), White spot syndrome virus: an overview on an emergent concern, Veterinary Research - A Journal on Animal Infection, Volume 41, Number 6,
Sahul Hameed, A.S., Xavier Charles, M., Anilkumar, M., (2000), Tolerance of Macrobrachium rosenbergii to white spot syndrome virus, Aquaculture, Volume 183, Pages 207–213.