There have been several news reports in the past years about polio cases in the United States (New York State), Ukraine, and Africa. In most of these countries, poliomyelitis was thought eradicated, with no cases reported for years, mainly due to successful vaccination campaigns. So why is it now being diagnosed again in both children and grown-ups?
Pathophysiology of polio
Poliovirus is an enterovirus. It is a single-stranded RNA virus with a very short genome (only about 7500 nucleotides long). It is considered as one of the simplest viruses that exist. There are three different wild-type serotypes of poliovirus, generally named types 1-3. Poliovirus type 1 (PV-1) is the most common serotype.
Humans are the only known reservoir for the virus. It is transmitted mainly through the faecal-oral or oral-oral route and is extremely infectious (even in asymptomatic cases). It most commonly affects children under the age of five years, although a severe course of the disease is more common in adolescents and adults. In this cohort, the poliovirus can spread from the tonsils and the intestinal tract to the central nervous system, infecting motor neurons in the spinal cord and the medulla oblongata.
Other non-polio enteroviruses can cause severe neurological symptoms, such as encephalitis and acute flaccid myelitis. There have been major outbreaks involving enterovirus A71 and D68 in the last couple of years.
What are the signs and symptoms of polio?
95% of infections are either asymptomatic or cause very mild disease. Patients may have influenza-like symptoms, with vomiting, headache, and/or myalgia. This is called abortive poliomyelitis (note: it’s often called poliomyelitis” even if there is no neurological involvement). In a minority of cases, polio can be more severe: in approximately 1:1000 infants or 1:100 adolescents. Patients may develop meningitis, encephalitis, or paralytic polio.
In most cases, the disease is limited to aseptic meningitis. Patients are sick with a high fever and lethargy and may be nauseated. Patients who have aseptic meningitis do not tend to experience paralytic symptoms.
In some cases, the disease progresses to acute flaccid myelitis. This is a sudden-onset weakness of the muscles without cognitive loss and sensory changes. Symptoms vary according to the level of paralysis — it may be spinal, bulbar, or both (bulbospinal). Spinal poliomyelitis is, by far, the most common. Patients may have decreased or absent tendon reflexes. They may also develop difficulties in breathing or swallowing. This explains why some children with polio died when respiratory support was not readily available. Paralytic polio is more common in older patients and more severe. Quadriplegia is more common in adults.
Encephalitis is rare. These patients present with a change of mental status and fever. Seizures may also occur, as may paralysis. This is usually of the spastic kind, characterised by hypertonia, unlike classic paralytic polio.
Neurological symptoms (such as muscle pain, new weakness or even paralysis) may return decades after recovery from the initial disease. This is known as post-poliomyelitis syndrome or post-polio syndrome. This occurs in up to 40% of survivors of paralytic disease.
How do we diagnose polio?
Suspect poliomyelitis in cases of acute flaccid paralysis without sensory changes, pyramidal tract signs, or cognitive loss. It is diagnosed by polymerase chain reaction (PCR) performed on stool samples. Sensitivity decreases when symptoms occur (more than a few weeks), and poliovirus serology blood testing is the next step. PCR testing of pharyngeal swabs is also an option. Poliovirus often causes pleocytosis in the cerebral spinal fluid, but the virus is very rarely recovered.
Even when caused by vaccine-derived strains (see below), poliomyelitis is rare and should be low on your differential diagnosis. Acute flaccid paralysis is more likely caused by other infections such as other enterovirus serotypes, West Nile virus, Guillain-Barré syndrome, transverse myelitis, or traumatic neuritis.
Vaccination and treatment of polio
Vaccination against polio has been around since the 1950s. Generally, it is given in either the oral form, consisting of a weakened virus strain (live attenuated) or as an intramuscular injection of an inactivated form of the virus. Vaccination is very effective – both against the disease itself and against transmission. It is so effective that polio has been virtually eradicated in many places due to herd immunity.
Only two countries (Afghanistan and Pakistan) remain where wild-type polio circulates. However, the strain contained in the oral vaccine can (very rarely) revert to a disease-causing variant that can infect others. This so-called vaccine-derived poliovirus (VDPV) can potentially cause paralysis. This is why there have been news reports about polio in places where exposure was previously deemed impossible. Over the past five years, more than 1,800 cases of circulating VDPV have been reported worldwide. The greatest risk factor for circulating VDPV is a local vaccination coverage below 80%.
One solution to this problem is switching to an inactivated virus-type vaccine, using a combined vaccination schedule, or using new oral vaccines that are less likely to cause VDPV. Understandably, this is more difficult in developing countries. The standard oral vaccine is cheap and easy to administer, and there is no need for syringes, making logistics easier. It is important to ensure that an adequate proportion of the population is vaccinated to ensure herd immunity. Oral-vaccine-derived strains mutating to a disease-causing strain would not be much of a problem if sufficient people were vaccinated, as re-emergence is highest when population immunity levels decrease.
Once infected, treatment is generally supportive and depends on the symptoms and complications of the disease. Analgesia and antibiotics for secondary bacterial infections are often necessary for the acute phase. Some patients need respiratory support in the acute phase. In the past, this required external negative pressure devices like the iron lung. These helped support ventilation long enough for their respiratory muscles to regain strength. Some patients required such support for years.
Much of what we know about respiratory support comes from work with poliomyelitis patients. In 1952, a severe poliomyelitis epidemic struck Copenhagen, Denmark. Hundreds of patients, mostly children, suffered from respiratory insufficiency. Anaesthesiologists from the hospital recruited hundreds of medical students to provide constant manual ventilation, as mechanical ventilation was uncommon. Some patients needed manual ventilation for weeks before regaining strength, and at the height of the epidemic, 70 patients received manual ventilation simultaneously. Students provided this by hand in 8-hour shifts. Mortality in these patients with respiratory insufficiency dropped from 90% to 25%. These heroic measures also led to a much better understanding of acid-base physiology and the first use of base excess to describe acid-base disturbances (but that’s another story).
Orthopaedic operations and supportive measures such as braces or corrective shoes may be needed in the long term as many patients develop skeletal deformities and asymmetric growth. Intensive occupational and physical therapy is often necessary. Some poliomyelitis survivors still need respiratory support decades after the onset of the disease, either via a tracheotomy or intermittently (e.g. at night). Many patients with post-polio syndrome suffer from debilitating fatigue.
Poliovirus is rare, but the incidence may increase in the future.
It is important to inform parents about the importance of vaccination, especially in areas where vaccine coverage is low.
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