Polio virus causes an acute infection that infects the central nervous system in its serious form. It is a highly contagious viral disease. It results in the destruction of motor neurons in the spinal cord that results in a flaccid paralysis. However, mostly polioviruses are subclinical. Poliovirus is a major enteric virus spreading very frequently because of less awareness among people and less knowledge. Polioviruses served as a model for the study of major enteric viruses. Although many efforts have been made in eradicating this disease of poliovirus, understanding this virus still remains crucial.
Polioviruses are typically enteric viruses. They are immediately inactivated when heated at 55°C for 30 mins but Mg+ prevents this virus inactivation. Purified poliovirus is inactivated by chlorine concentration of 0.1 ppm.
Susceptibility and growth of virus:
Polioviruses exhibit a very restricted host range. Most strains of this virus will infect monkeys when inoculated in the brain or spinal cord.
Most strains of poliovirus can be grown in primary or continuous cell lines cultures. These strains can be obtained from a variety of human tissues or from monkey kidney, testis or muscle. These viruses require a primate specific membrane receptor for infection.
Historical point of view:
The history of polioviruses dates back centuries but this disease became a major topic of the town in the 20th century. People start knowing about this disease and forthcoming problems associated with this disease. Polio outbreaks start spreading and becoming epidemics. That was the major challenge at that time. It causes widespread fear and leads to the foundation of the “Global Polio Eradication Initiative” (1988). It was a collaborative effort by national government and non government organizations. The main goal of this initiative was to eradicate poliovirus worldwide.
Causative agent of Polio virus:
The causative agent of polio is the poliovirus. It is a member of the Enterovirus genus and belongs to the family Picornaviridae. The poliovirus is a single-stranded RNA virus containing a non-enveloped icosahedral capsid structure. It is one of the smallest viruses known. It usually infects humans.
There are three distinct serotypes of the poliovirus:
Each serotype represents a different strain of the virus. Each strain has a unique genetic characteristic. The serotypes are really important in the context of vaccine development and immunity. Immunity to any one serotype does not confer immunity to the others. In this way, vaccines must target all three serotypes to provide comprehensive protection in the prevention of disease.
Poliovirus primary target:
The main target of this virus is the gastrointestinal tract. It replicates in the mucous membranes of the pharynx and the intestines. It further enters the bloodstream and spreads to other parts of the body, including the nervous system. In those cases where the virus reaches the central nervous system, it can cause damage to motor neurons. It then leads to paralysis in some individuals.
Pathogenesis of Polio virus:
Once the poliovirus enters the body, it multiplies in the throat and intestinal tract. From there, it further invades the nervous system, causing paralysis. While the majority of the cases of polio infections are asymptomatic or result in mild illness. Approximately 1 in 200 cases leads to irreversible paralysis, typically affecting the lungs. It is called the severe form of poliovirus.
Step by step process of pathogenesis:
The pathogenesis of poliovirus involves the progression of the poliovirus through various stages in the human body. It ultimately leads to infection and potential damage to the nervous system. Here is an overview of the pathogenesis of the polio virus in human body:
Route of entry:
The primary route of transmission for the poliovirus is through the fecal-oral route. Contaminated food, water, or surfaces can propose the virus into the human body. This virus primarily targets the mucous membranes of the pharynx and the intestines.
Once the poliovirus enters the human body, it replicates in the mucous membranes of the pharynx and the intestines. During the progress of this stage, the virus is shed in the feces and, to a lesser extent, in the saliva.
Multiplication in bloodstream:
The virus then enters the bloodstream. It allows it to further spread to various organs and tissues throughout the body. During this stage, the majority of individuals experience no symptoms or only mild, flu-like symptoms.
Invasion of the Central Nervous System:
In a very small percentage of cases, the virus invades the central nervous system (CNS). The whole mechanism by which the virus crosses the blood-brain barrier is not fully understood. However, once in the CNS, the virus specifically targets motor neurons. It particularly infects the spinal cord and brainstem.
The poliovirus has a particular greater affinity towards motor neurons. It led to their destruction. The damage to motor neurons can result in muscle weakness and paralysis. In severe cases, it causes respiratory failure. The severity of symptoms depends on the level of extent and location of the neuronal damage.
Asymptomatic Infection (no symptoms):
Many individuals infected with this type of poliovirus do not exhibit any noticeable symptoms.
Despite being asymptomatic, these individuals can still shed this virus in their feces, contributing to the spread of the virus.
Non-Paralytic Polio (mild symptoms):
- Sore Throat
- Gastrointestinal Symptoms: Nausea, vomiting, abdominal pain, and diarrhea.
- Flu-Like Symptoms: General malaise, headache, and muscle stiffness.
Paralytic Polio (Severe Symptoms):
- Initial Flu-Like Symptoms
- Muscle Weakness
- Loss of Reflexes
- Severe Pain and Spasms
- Respiratory Distress: In cases of bulbar polio, which affects the brain stem. Respiratory muscles can be compromised, leading to difficulty in breathing.
Post-Polio Syndrome (Late Manifestation):
- Muscle Weakness
- Joint Pain
- Atrophy: the loss of muscle mass.
- Permanent disability
- Joint deformities
- Skeletal deformities
- Respiratory issues
Vaccines for poliovirus:
Here are the names of vaccines that can be used for poliovirus:
- Inactivated Poliovirus Vaccine (IPV)
- Oral Poliovirus Vaccine (OPV)
- Combination Vaccines (may include poliovirus protection along with other vaccines)
- Global Polio Eradication Initiative (GPEI) recommended vaccines.
- Polio Endgame Strategy vaccines.
Challenges in eradication of Poliovirus:
Certainly, there are many challenges in the eradication of poliovirus. Some are listed below:
Security and Access problems:
Vaccines reaching each and every place is a major problem.
Reluctance by communities to accept vaccination, often due to misconceptions related to religion and cultural differences.
Weak Health Care Systems:
Inadequate healthcare infrastructure, including insufficient resources, trained personnel can hinder vaccination.
Hard to Reach Populations:
Difficult to reach in isolated and geographically remote areas.
Vaccine-Derived Poliovirus (VDPV):
The live, attenuated virus in the oral poliovirus vaccine (OPV) can mutate. It leads to new cases of vaccine-derived poliovirus (VDPV) cases.
Sustaining financial support for comprehensive vaccination programs.
For complete global eradication of poliovirus, effective collaboration among international organizations, governments, and NGOs is needed.
Public Awareness and Education:
Promoting understanding of the importance of vaccination and educating communities is important.
While the road to poliovirus eradication has been challenging, significant progress has been made. Its transmission, and the importance of vaccination are crucial for the understanding of the importance of this virus and its consequences on human health. Continued commitment to vaccination programs, research, and public awareness will contribute to our ultimate goal of a polio-free world.
It is important to study emerging viruses as well. If you wanna read about new emerging virus Monkey pox, give it a read “New Virus Outbreak: Is Monkeypox Dangerous?”
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