Poliomyelitis, commonly known as Polio, is a disease caused by a virus, called poliovirus. Polio primarily affects children under the age of 5. It causes paralysis of muscles, often in the limbs. The virus affects the nerves that supply the muscles, and so the muscles cannot be moved. They then waste away as the child grows. For example, if one leg is affected (and the other is not), the affected leg may be shorter and weaker than the unaffected leg. This can cause significant problems with gait (walking) which persist into adulthood. In rare circumstances, polio can be fatal if it affects the muscles involved with breathing. In less severe cases, polio can cause flu-like symptoms and stomach pains or may be completely asymptomatic.  

The polio virus is spread via the faecal-oral route. This most commonly happens via contaminated food or water. Tropical climates, overcrowded living conditions and lack of sanitation (e.g. lack of sewage systems, lack of clean water for hygiene) exacerbate transmission. The virus enters the mouth and replicates within the intestines. Viruses enter the host’s cells and replicate within them. When the cells die, they burst, releasing many more viruses within the host. From the intestines, along with other nutrients from food, the polioviruses can be absorbed into the bloodstream where it can spread all over the body. Viruses can also be excreted in the faeces, and it can then be transmitted via the faecal-oral route. The immune system may deal with the infection at this point, and it may lead to only mild symptoms. If however, it reaches the central nervous system, (either the brain or spinal cord) this is when it can start to cause paralysis 

There is also evidence that the virus can spread via droplet infection (breathing in the air of a person who has been infected), although this is not as common via faecal-oral transmission. 

Poliovirus can survive for long periods of time outside the body. Evidence has shown that it can survive on hands, on surfaces, or in food for days to weeks. In contaminated soil or water (i.e containing human faecal matter), the virus can survive for up to months. Other ways of coming into contact are by changing clothes that are contaminated, or nappies of babies. Moisture and warmth enable the virus to survive for longer, hence why it is more prevalent in warmer climates. Therefore, having the infrastructure to provide clean water and separate sewage from main water supplies is essential for the prevention of not only polio but other water-borne diseases too. Humans however are the only form of life that are affected by polio or can be reservoirs of the virus (i.e it does not live in animals, and animals cannot be affected by polio either)

There are 3 strains of the wild poliovirus - types 1, 2 and 3. Types 1 and 2 have been eradicated. Type 3 remains endemic in two countries in the world- Afghanistan and Pakistan. Both of these countries have areas of low vaccination coverage, significant movement between the two nations (spreading the virus) and areas of inadequate sanitation. Evidence has reported cases of one case in Malawi and one in Mozambique which have originated from people moving out of Pakistan. The term “wild-type” polioviruses, is the term used for strains which are like the “original” polioviruses, which can be vaccinated against. 

Vaccination 

In 1988, there was a target made to eradicate the disease globally by the year 2000. (Though the vaccine was actually made in the 1950s, it was used mainly in richer countries, where polio was eradicated first.) The polio vaccine is part of childhood vaccine schedules in almost every country, and in many of the countries in the world, polio has been eradicated (eradication is defined as a 3-year period of no cases following the last case of the disease). 

There are two types of vaccines against poliovirus- both types of vaccine are capable of giving lifelong immunity.. There is an inactivated type which contains a dead polio virus so that when injected, it does not cause polio, but can induce an immune response. This is given as an intramuscular injection in the arm or leg. The vaccine requires multiple doses in order to give full immunity. The ages exactly when they are given varies from country to country, but for example, in the UK there are 5 doses given at 8, 12 and 16 weeks of age, a preschool booster at 3 years old, and a second booster dose at 14 years old. 

Another type of vaccine is the oral polio vaccine which is absorbed from the inside of the cheek and enters the bloodstream. It contains a live attenuated form of the virus, meaning it is a live virus which has been weakened in some way to prevent it from causing polio  It is cheap and easy to administer (because it is an oral medication, given as two drops, there is no need for a needle) and has been used in countries where administering the inactivated vaccine has been more difficult (i.e most developing countries). Again, exact timings vary but generally, the first dose is after birth, followed by 6, 10 and 14 weeks of age, then a booster around 18 months. A very rare complication of the oral vaccine is that it can cause vaccine-associated polio, though this is very uncommon. However, without the oral polio vaccine, the reduction in polio cases throughout the world would not have been possible.  

Vaccine derived poliovirus

 In areas where vaccine coverage has been low, the virus can mutate, and a different strain can emerge. These are usually places where the oral polio vaccine has been used- because it contains a live virus, there is a risk that it can mutate into a disease-causing form of itself (aka “wild type”). These are known as vaccine-derived poliovirus (VDPV). So even though Afghanistan and Pakistan are the only sources currently that have the wild type poliovirus, there are many more countries in the world which have reported cases of VDPV (of which there are different types). It is worth noting here that adults (or even children) who have had the full doses of either type of vaccine are generally immune to both wild types and vaccine-derived strains of poliovirus

However, if adults are not vaccinated, they can become susceptible to acquiring polio as well. Even for adults who are immune, if they acquire a mutated wild type, they can become carriers of the virus and then pass it on to babies and children who are not immune. There have been instances where adults who have been travelling from country to country have passed on the virus to other people. In this scenario, the virus that adults can pass on is acting like a “wild type” virus, where it will have mutated in order to be transmissible. 

Therefore, the only way that vaccination can be fully effective is for each and every person to be vaccinated and immune. If there is even a small proportion of the population who are not vaccinated, a vaccine-derived form of the virus can affect them and result in polio cases. 

Summary

Polio is a disease caused by a virus (poliovirus) that can cause paralysis of muscles. It mainly affects children under the age of 5, and as they grow they can have problems with their muscle growth and development, especially in the limbs. One classic presenting feature in those that have been affected is one leg that is shorter than the other with atrophied muscle, which can cause problems in walking. 

Poliovirus spreads between people via the faecal-oral route - usually from contaminated food or water. Inadequate sanitation, such as lack of clean water for hygiene, or lack of sewage systems, tropical climates and overcrowding of people all exacerbate the spread of the virus. Less commonly, poliovirus can spread via droplet infections or by direct contact. Poliovirus can survive for weeks up to months outside the body, so it is necessary for robust sewage systems and sanitation to be part of the infrastructure. 

One type of polio vaccine contains a live virus, which is weakened so that it cannot cause disease. However, it can mutate into disease-causing strains and result in polio cases. These are known as vaccine-derived poliovirus cases and account for most of the polio cases seen in the world today. The only countries in the world which still have the original (known as wild types) of the poliovirus are Afghanistan and Pakistan. Vaccination prevents disease of both the wild types and the mutated types. The cases which are happening now will be in people who have not been vaccinated and are therefore not immune. Therefore, vaccination is the single most important factor in ensuring that polio cases are reduced in number.