The polio virus is possibly the most innocuous virus that affects humans. The improvement of sanitation standards across the world has meant fewer people are afflicted by polio as infants, upsetting the historical balance between virus and host. But, since poorer sections still live in squalor and consume water and food contaminated by faeces, the virus remains a threat to the global community
Polio - alive and kicking
The polio virus (pv) is possibly the most innocuous virus that affects humans. The improvement of sanitation standards across the world has meant fewer people are afflicted by polio as infants, upsetting the historical balance between virus and host. But, since poorer sections still live in squalor and consume water and food contaminated by faeces, the virus remains a threat to the global community.
pv was first discovered in 1905. It contains rna (ribo-nucleic acid) as its main genetic component and belongs to enteroviruses, a group that invades the digestive tract and gut and manifests in other forms. pv, the prototype enterovirus, can be further classified into three serotypes called wild types 1, 2 and 3. The virus as an entity is rather innocuous and can be used to impart immunity to children.
The term poliomyelitis is derived from the Greek words 'polios', meaning grey, and 'myelos', meaning marrow, referring to the fact that the disease causes the destruction of neurons located in the grey matter of the anterior horn of the spinal cord. However, pv is primarily an enteric pathogen that most often causes only a silent or abortive infection. Poliomyelitis and central nervous system (cns) lesions occur only rarely; in about one per cent of infected cases.
pv infection is initiated by ingestion of the virus through tainted water or unhygienic food. The virus incubates in the human gut for a week or two. This is followed by the multiplication of the virus in the upper gut and intestine, which are excreted in the stool for several weeks. This contaminated faeces is capable of infecting other susceptible children. Extensive viral multiplication then occurs in the tonsils and the small intestine.
Within a few days, the virus invades other parts. pv has a predilection for the motor neurons of the spinal cord and most of the brain stem. These manifestations can be fatal as they cause respiratory or cardiac failure. In a few cases, many years following paralytic poliomyelitis, survivors may develop a new disease called the post-polio syndrome (pps). Ten to fifty per cent of patients develop pps after recovering from acute poliomyelitis, sometimes after 30 to 40 years of clinical stability. The symptoms for this affliction range from fatigue and muscle pains, to slowly progressive muscle weakness and paralysis. Treating poliomye-litis and vaccine-associated paralytic poliomyelitis is difficult aftersymptoms appear.
It took two decades to find a complete cure for polio. In 1949, after years of research, scientists finally succeeded in growing pv in cultured cells -- a turning point in polio research. In 1954, Jonas Salk introduced the first vaccine called inactivated polio vaccine (ipv), derived from a 'killed' virus. The success of mass trials that year caused an international sensation. Through immunisation using this vaccine, polio epidemics in North America were quickly brought to a halt. In 1957, another us researcher, Albert Sabin, introduced the oral polio vaccine (opv), based on a weakened live virus. Following successful trials in the Soviet Union, opv would go on to largely replace Salk's vaccine, first in industrialised countries, then worldwide.
opv is today the vaccine of choice for the global eradication campaign. Delivered in drops that are swallowed, it is easier to administer than the needle-injected ipv. It is also cheaper to manufacture. In addition, while ipv protects individuals from the effects of polio, the virus can continue to live inside them for some time. But opv completely denies the virus a home by inducing immunity in the intestines, where it usually breeds.
Even opv is not completely risk free. There is a remote chance (one in 2.5 million) that the vaccine could provoke paralysis. There is an even more remote possibility of the vaccine virus becoming virulent and causing an outbreak in unvaccinated populations. Three such incidents have been documented recently in Assam and Orissa where several cases of paralysis have been reported. However, when compared to the billions of doses delivered, the overwhelming benefits of opv far outweigh the rare risks. opv is usually administered in three spaced doses because it builds up immunity gradually. This also reduces the possibility of a vaccine shock. Despite all efforts we have not been able to eradicate this disease, a seemingly possible task. Polio has resurged again and again, sometimes even in industrialised nations, often overwhelming the public health system.
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