polio virus

Polio mycelitis

Polio mycelitis is an acute infectious disease that in its serious form affects the CNS. The destruction of motor neurons in the spinal cord results in flaccid paralysis. However, most poliovirus infections are subclinical.

Microbiology of polio virus

Classification

Group- group iv (+ss RNA)

Order- picornaviridae

Family-picornavirida e

Genus- Enterovirus.

Species- polio virus

Introduction

• Polio virus was first isolated in 1909 by Karl Landsteiner and Erwin popper.
• It is one of the most well-characterized viruses and has become a useful model system for understanding the biology of RNA viruses.
• Poliomyelitis is an acute communicable disease of humans caused by a human enterovirus of the Picornaviridae
• The virus is composed of a single-stranded, positive-sense RNA genome and a protein capsid.
• The 3 serotypes of poliovirus carry are antigenically distinct.
• They are inactivated when heated at 55C for 30minute but Mg ion 1mol/l prevents this inactivation.
• Purified polio virus is inactivated by chlorine a concentration of 0.1 ppm but much higher concentration of chlorine is required to disinfect sewage containing virus in faecal suspension and in the presence of other organic matter
• Most poliovirus infections cause asymptomatic viral replication that is limited to the alimentary tract.

Morphology

The virus particle is non- enveloped which is about in diameter with icosahedral symmetry. Poliovirus composed of an RNA genome and a protein capsid. The genome is a ss positive sense RNA genome i.e. about nucleotide long. Because of its short genome and its composition i.e. only RNA and a not- enveloped icosahedral protein coat, poliovirus is widely regarded as the significant virus.

Serotypes

Three serotypes are PV1, PV2 and PV3 each with a slightly different capsid protein. A capsid protein defines cellular receptors specificity and virus antigenecity. Specific strains of each serotype are used to prepare the vaccine against polio. PV1 is the most common from encountered in nature. However, all 3 forms are extremely infectious. As a report published in 2012, PV1 is highly localized to regions in Pakistan and Afghanistan in Asia and Nigeria, Niger and Ehod in Africa. PV2 has probably been eradicated. It was last detected in October 1999 in Uttar Pradesh, India PV3 is found in parts of only 2 countries: Nigeria and Pakistan.

Transmission

• Poliovirus is often transmitted from person-to-person through fecal matter.
• In addition, the virus can be spread by contaminated food or water or direct contact with another infected person.
• People living in areas with limited water supply or flush toilets often get the virus from the human waste.
• Pregnant women, people with weakened immune systems, such as HIV+ people, and young children are the most susceptible to the polio virus.
• If people have not been vaccinated, it increases risk of contracting polio by:

1. having your tonsils removed
2. traveling to an area that has had a recent polio outbreak
3. handling a laboratory specimen of the virus
4. taking care of or living with someone infected with polio
5. extreme stress, which can compromise immune system function

Recognizing Symptoms of Poliovirus

The earliest features associated with a phase of viremia consist of fever, malaria (fatigue), headache, drowsiness, constipation and sore throat and last for 1-5 days. Then one of the following types of response may occur:

• inapparent infection without symptoms
• Absorptive infection
• Non- paralytic poliomyelitis (aseptic meningitis)
• Paralytic poliomyelitis
• Post-polio syndrome

Sub-clinical polio may not trigger noticeable symptoms but the symptoms can range from mild to severe.

• Paralytic polio: this condition of polio that leads to paralysis and has more severe symptoms which can also be fatal.
• Non-paralytic polio: Patients experience mild flu-like symptoms.

Sub-Clinical Polio

In this case, patients with symptoms usually last for 72 hours or less and may include symptoms such as:

• slight fever
• headache
• vomiting
• sore, red throat
• general discomfort

Inapparent infection without symptoms

It is the most common form (90%) of the diseases recovery takes place in a few days.

Absorptive infections

Absorptive infections or minor illness are associated with nonspecific symptoms. It occurs in 4-8 hrs of infected people with fever, headache, malaise, sore throat constipation etc within 3-4 days of exposure.

Non-Paralytic Polio

Approximately 1% of patient develop stiffness most frequently.The symptoms of nonparalytic polio may last for a couple of days to a week or two and includes condition such as:

• A sore throat in the absence of upper respiratory infection
• fever
• abnormal reflexes
• headache
• vomiting
• problems swallowing and/or breathing
• fatigue
• back and neck pain and stiffness
• particularly neck stiffness with forward flexion of the neck
• muscle tenderness and spasms
• arm and leg pain or stiffness

Paralytic Polio

Paralytic poliomyelitis is the major illness that occurs in 1-2 hrs of infected patients. It appears 3-4 days after the minor illness has subsided. Flaccid paralytic occurs due to damage of lower motor neurons. Maximum recovery takes place within 6 months but residual paralysis lasts much longer. People with paralytic polio experience the symptoms associated with non-paralytic polio first. Soon after, the following symptoms appear:

• loss of reflexes
• sudden paralysis (temporary or permanent)
• muscle pain
• loose and floppy limbs, sometimes on just one side of the body, this is due to the weakness which results from the involvement of the spine
• deformed limbs

Post-Polio Syndrome

It is a sequel of poliomyelitis that may occur in 20-30% of the original victims much later in the life (25-30 years later). These individual suffer deterioration of the originally affected muscles. The symptoms of post-polio syndrome are:

• trouble breathing and/or swallowing
• continuing muscle and joint weakness
• exhausted or fatigued
• muscle pain
• muscle wasting also called muscle atrophy
• sleep related breathing problems

Pathogenesis

virus entry and multiplication

the virus enters the body through mouth and multiplies in the intestinal epithelium. the incubation period ranges from 9-12 days.

initial viraemia

from the intestinal epithelium, the virus spread to the adjacent lymphatic tissue of oropharynx and intestine. then they enter the bloodstream producing initial viraemia. the patient may from antibody at this preventing the infection from spreading (abortive infection).

SECOND VIRAEMIA

iN patient, who failed to control the initial viraemia, there is second major viraemia from where they are disseminated in spinal cord and brain by passage across the blood-CNS barrier. The virus multiplies in the neurons of CNS and destroys anterior horn cells of the spinal cord as well as nerve cells in the medulla oblongata. Children are most susceptible to polio mycelitis.

Treatment

There is no cure for polio. Doctors can only treat the symptoms while the infection runs its course. The most common treatments include:

• rest
• painkillers to relieve headaches, muscle aches, and muscle spasms
• portable ventilators to help with breathing
• antibiotics
• physical therapy to treat pain in the affected muscles
• heating pads or warm towels to ease muscle aches and spasms
• physical therapy to address breathing and pulmonary problems and

REFERENCE

Cheesbrough, M.Medical Laboratory Manual for Tropical Countries. Vol 2. ELBS London, 2007.

Tille, P.Diagnostic Microbiology.13th. Elsevier, 2014.

Grenwood D, Slack RCB, and Peutherer J.Medical Microbiology.Dunclude Livingstone: ELBS, 2001.