Cocoa Swollen Shoot Virus Transmitted Mainly Insect Vectors Biology Essay

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Cocoa Swollen Shoot Virus (also known as CSSV, Cacao Swollen Shoot Virus, theobroma virus 1, or cacao mottled leaf virus (Brunt et al, 1996)) is a Baltimore Class VII virus of the genus Badnavirus, and is transmitted mainly via insect vectors. It has great financial ramifications, as one of its hosts is Theobroma cacao, also known as the Cocoa tree, which produces cocoa beans, vital for the production of chocolate worldwide. Research into CSSV seems to be relatively rarely published, with booms in research being noted in the 1930s to the 1970s, but then a significant lapse in research (that will be addressed later) before the mid 2000s, when technology increased to the point that the genome of the virus itself could be read. Most of the research is performed in CSSV's native country of Ghana, West Africa. It was first officially described by Posnette (1940) and later classified by Brunt and Kenten (1960).

Brunt and Kenten (1963) also went on to design the basis of the currently accepted mode of purification of CSSV. This included macerating infected cocoa leaves in a proteinaceous fluid that extracted the virus, which could then be separated from the rest of the fluid by centrifugation. In the same experiment, they also discovered that egg albumin, blood albumin, casein and hide powder were all effective at keeping the virus alive and infective so that it could be studied. This method went on to be used and altered slightly by Adomako et al (1983) who simplified the process by creating an antiserum to the virus that was usable in an ELISA assay, isolating CSSV particles using multiple washes. Lot et al (1993) used a different method involving "a combination of celite filtration, polyethylene glycol concentration, and sucrose density gradient centrifugation", using the isolates to examine the CSSV genome, identifying it as a double stranded DNA virus.

Figure 1 - An icosohedral virion capsid. One such as this is the capsid that CSSV uses, giving it a bacilliform appearance under an electron microscope. Isolates of CSSV have been examined under electron microscopes and described as non-enveloped bacilliform viruses (Figure 2). This is unusual, as most icosohedral viruses have a roughly spherical shape, while CSSV is more rod-like, implying that it may be an enveloped helical virus. However it was discovered that it couldn't be a helical virus as it has a circular double stranded DNA genome, as opposed to the RNA genome that helical viruses contain. It has also been demonstrated that all Badnaviruses have a similar elongated icosohedral rod-like structure, and all are plant viruses.

The double stranded nature of CSSV was discovered by Lot et al (1991) using nuclease sensitivity tests. They also discovered the fact that there are 2 regions in the 7.4 kbp length of its genome that are made up of single stranded DNA. This study also determined that CSSV belongs to a particular type of plant virus identified by Lockhart (1990) that contain circular double stranded DNA.

HISTORY

CSSV has been found to affect cocoa trees in Western Africa (Figure 3) though it has been reported in other areas, such as Trinidad, in Northern South America and Sri Lanka. In Trinidad, however, there are only 2 currently identified strains, known as A and B (Baker and Dale, 1947). In Ghana particularly CSSV has been a major problem for cocoa farmers since the 1936, when the first reported case was discovered by W.F. Steven (F.K Danquah, 2003). In those times the only way to control a plant virus was to destroy the affected plants. Unfortunately for the cocoa farmers in Ghana, about 50% of their trees were destroyed in the interests of controlling the virus.

By 1938 the first lab was set up to study CSSV in Ghana, though the Second World War severely limited its research. In 1944 it was transformed into the West African Cocoa Research Institute (WACRI), which showed that the distribution of CSSV was larger than previously thought, and was threatening surrounding countries as well. By 1947, it had spread to the surrounding countries of Le Cote D'Ivore (known in English as the Ivory Coast), Sierra Leone, Nigeria and Togo and reduced Ghana's output of cocoa by about 50%, decimating their crops.

Figure 2 - Scientists with the CSSVDC studying samples of CSSV infected plantsIn the early 1950s, a UN delegation decided that the best response to the problem of CSSV was to produce and plant CSSV resistant hybrid trees, though it wasn't until 1962-3 that this plan actually began to make progress. The responsibility for the spread of the virus was removed from the government, who had been burning the trees until this time, and passed to the farmers themselves. The farmers were supplied with CSSV resistant plants produced by scientists who bred cocoa plants with a plant resistant to many diseases known as Amazonia, or F-2, to produce CSSV resistant cocoa plants. These plants were distributed to the cocoa farmers under the label of Tafo Hybrid.

Farmers implemented the UN's plan by cutting down their own cocoa trees and burning them themselves, replacing them with the new Tafo Hybrid trees. An added advantage was that these new trees had a higher yield, and a faster growing time. Since then, farmers have been burning off their infected trees, killing the virus, and replacing them with the new resistant trees.

Because of these measures that have been put in place to control the spread of CSSV, it is largely no longer a problem for cocoa farmers, and could help to explain why there was little research being done into it from the 1970s to the mid-2000s. That isn't to say that it has been completely eradicated; CSSV is still around in Ghana, though the amount of trees that are infected has been greatly reduced from the 1930s to 1960s, when at times up to 70% of the crops may have been infected at any given time. The continuing existence of CSSV is being combated by various boards and committees, most notably the Cocoa Swollen Shoot Virus Disease Control Unit (CSSVDC) that was set up in 1994 by the Ghana Cocoa Board to stop further spread of the virus. Their website, which gives a small amount of history on the creation of the unit, can be found at http://www.cocobod.gh/control_history.php.

EPIDEMIOLOGY

Figure 4 - Several trees that are vulnerable to CSSV infection. From left to right, they are: Adasonia digitata, Bombax brevicuspe, Ceiba chlamydantha, Cola gigantea and Theobroma cacao, the tree that produces cocoa beans used in making chocolate.CSSV has been reported in several different species of cocoa trees (Brunt et al, 1996), including Adansonia digitata, Bombax brevicuspe, Ceiba chlamydantha, C. Pentandra, Cola chlamydantha, C. gigantea, the Corchorus family, Sterculia tragacantha and most importantly Theobroma cacao, the tree from which cocoa is farmed (Figure 3).T. cacao is the plant from which cocoa is farmed. The financial repercussions of the infection of this tree being why it is the most commonly studied plant when research is being conducted into CSSV. Cocoa is one of Ghana's main exports, and an important part of their economy. Almost all of the cases have been reported in West Africa, originating in Ghana, and being found in Le Cote D'Ivore (The Ivory Coast), Sierra Leone, and Togo, all surrounding Ghana. There are no reported cases in the other countries surrounding Ghana. That isn't to say that the virus doesn't exist there (though it is theoretically possible that is the case), only that no trees have been symptomatically affected. The distribution of the virus seems to indicate that it is affected by the temperature, as all of the reported cases were found around the equator. There have also been trees found to be immune to the virus, including those of the families Amarantheceae, Chenopodiaceae, Cucurbitaceae (that is made up of modern watermelons, squashes, cucumbers and other such melons, gourds or curcurbits), Euphorbiaceae, Malvaceae (the family that the susceptible Theobroma cacao belongs to) and Solanaceae.

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