One of the diseases covered in physical therapy education to a disproportionate extent from its current prevalence is polio. This is due to its role in the history of the profession—which itself has dubious clinical relevance. However, the story of polio is itself quite interesting, as we have documentation of its transition from an endemic to an epidemic disease, to one which no longer circulates in much of the world. There is a well-known story here about the triumphs of vaccination, and a less well-known story about how public health experts began to appreciate the burden of disease polio presented only as a society with less polio, or without polio, became imaginable. The following is a paper I wrote in 2019, as part of a writing portfolio requirement.
Polio is now a rare disease, nearing global eradication in the wild as a result of the work of the Global Polio Eradication Initiative. As of mid-2018, the Initiative reported that wild poliovirus was still endemic to only three countries, Nigeria, Afghanistan, and Pakistan, with Afghanistan and Pakistan treated as “a single epidemiological block” due to repeated transmission of the virus across their shared border (7). This is especially remarkable given that poliovirus is commonly believed to have been endemic in most human populations up until the late 19th and early 20th century, with most people being infected within the first few years of life (De Jesus; Nathanson and Kew 1214; Horstmann 80). Perhaps equally remarkable is the fact that polio became a prominent subject of medical concern only as it ceased to reliably infect members of the population at early ages, transitioning from an endemic to an epidemic disease (Nathanson and Kew 1214; Horstmann 80). How is it that a disease which reliably infected nearly an entire population early in life could be described as “not uncommon” (Horstmann 80), while that same disease, transmitted slightly less reliably, led to large-scale public concern and public health responses including vaccine trials involving over a million school-age children (Meldrum 1233)?
Polio’s unique pathophysiology played a key role in this transformation. One crucial aspect of the disease is that while its characteristic symptoms are neurological, most typically weakness or paralysis in one or more limbs, it is now known that around 96% of non-immune children infected with poliovirus exhibit either no manifestations of disease, or vague symptoms that could be attributed to any number of viral infections, and fewer than 1% experience lasting paralysis (U.S. Centers for Disease Control and Prevention, “Poliomyelitis”). Prior to the development of modern medical technology and modern models of illness, it was not possible to track the spread of a disease when so few infected individuals exhibited identifiable symptoms. Even now, the infrequency with which infection with poliovirus produces clinical symptoms leads to challenges in monitoring and tracking the spread of the disease—one case of characteristic paralysis may indicate hundreds of undetected infections, and workers for the Global Polio Eradication Initiative track the presence of the wild virus through environmental surveillance in addition to through individual reports of infection (Global Polio Eradication Initiative 7–12).
Another aspect of polio’s pathophysiology that decreased its visibility during the endemic period is that a small number of exposures can confer long-term immunity. Unlike some other viral illnesses such as influenza, polio only has a small number of antigenic types: exposure to a strain of polio from each of three groups is sufficient to produce immunity to all polioviruses (Horstmann 83). The duration of immunity produced either by exposure to wild poliovirus or by immunization is unclear, particularly with regard to effects on subclinical infection and transmission (Duintjer Tebbins et. al. 596). However, the pattern of disease leading to polio having been known as “infantile paralysis” prior to the epidemic period, with few or no cases being reported in older children and adults, suggests that typical levels of immunity in those populations under endemic conditions are enough to prevent development of characteristic symptoms (Nathanson and Kew 1214; Horstmann 80). Additionally, a study of an isolated Inuit population where polio was not endemic found that antibodies to specific strains persisted over spans of 40 or more years (Horstmann 83). Thus, an individual’s chances of developing characteristic symptoms of polio where it is endemic do not increase with their frequency of exposure to the disease, but are concentrated in the first exposure to each immunologically distinct strain.
The lasting immunity conferred by exposure to polio viruses means that as polio shifted from an endemic to epidemic disease, it began to affect more age groups. In addition to any corresponding differences in the perceived social impact of the disease, the immediate, physical effects of the disease while it is endemic (and thus primarily affects infants) are potentially less severe for two reasons. Firstly, infants who are breastfed, as was typical prior to the 20th century, receive some immune benefits. Therefore, if a first polio infection occurs while a child is mostly or exclusively breastfed, it may allow the child to develop their own lasting immunity, while still being partially controlled by an immune response mediated by antibodies received from the mother (Nathanson and Kew 1214). Secondly, while the evidence for age-specific effects of polio is limited, some evidence suggests that among individuals with paralytic polio, older individuals more frequently experience more severe or life-threatening forms of paralysis (Nathanson and Kew 1217-1218, Nielsen 181). For example, one study of case reports from Sweden found that 23.5% of cases in individuals over the age of 25 led to death, compared to only 4.5% of cases in individuals under 3 years of age (Nathanson and Kew 1217). Another study, looking at cases of polio leading to hospitalization in Denmark, found that for each analyzed age group with patients 30 years and older, over 10% of cases led to death, while no age group with patients under 25 years of age had over 5% of cases leading to fatality (Nielsen 183).
None of this, however, should be taken to mean that polio presented a low burden of disease in areas and times when it was endemic. Rather, its effects, although significant, were continual and expected, allowing them to exist in plain sight even while they were rarely noted as a coherent and potentially avoidable pattern. The effects of polio were continual in the sense that polio infections occurred relatively continuously over time, although possibly with some seasonality in temperate climates (Nathanson and Kew 1214, 1218; Horstmann 80). They were also continual in that paralysis induced by polio is frequently permanent, so that affected individuals continued to display the signs of the disease throughout their lives; as a result others in their community might come to expect that permanent paralysis was one potential outcome of a childhood illness. In the growing tradition of Western medicine, this came to be called infantile paralysis (Horstmann 80). We don’t have records of how common infantile paralysis was in Europe or North America prior to polio’s emergence as an epidemic disease, but during the 1970s and 1980s many surveys were conducted to try to establish the prevalence of lingering paralysis characteristic of polio in areas where polio was still endemic, in part in order to understand the benefits of vaccination in those areas (Bernier S371). These surveys looked specifically for gait abnormalities in children, therefore potentially missing other presentations of polio-related paralysis, such as flaccid paralysis of an upper extremity, or cases of polio that ended in death (Bernier S371). Still, they found prevalence rates ranging from under 1 child in 1000 affected, up to 25 children in 1000 affected; rates around 5-10 children per 1000 were common (Bernier S371-S373). For comparison, modern studies find that cerebral palsy, the current most common cause of childhood motor disability, affects under 5 children per 1000 (U.S. Centers for Disease Control and Prevention, “Data and Statistics for Cerebral Palsy”).
Despite these continual infections, and continual reminders of past infections, it is clear from the scientific record that researchers and policy-makers in the late 1970s and early 1980s were surprised by mounting formal evidence of the high burden of disease polio presented in areas where it was still endemic. (By this time, polio was no longer circulating as a wild virus in the United States, or in an increasing number of other countries (Nathanson and Kew 1220). ) Writing in 1980, LaForce et. al remarked that “Until recently poliomyelitis has not been considered as a major public health problem in developing countries,” then went on to note that recent studies had found incidence rates for polio in those countries comparable to those seen in the United States prior to the development of polio vaccines (609). Indeed, only two years earlier, and with access to some of the studies reported on by these authors, a World Health Organization publication on the benefits of polio vaccination discussed vaccination in areas where polio was endemic almost exclusively as a public health program seeking to “anticipate and forestall the epidemic phase of poliomyelitis” (Melnick 25). The publication included recommendations to test populations for prior exposure to polio viruses in order to determine the correct time to begin vaccination, rather than waiting for epidemics to occur, underscoring the perception that polio began to present an unacceptably high burden of disease only when it began to appear as an epidemic disease (Melnick 25-26).
The policy transition in response to the mounting evidence, however, was swift; by 1984, Bernier noted that results of surveys on infantile paralysis had influenced several countries’ decisions to begin vaccination programs (S371). And in 1988 the World Health Assembly announced a goal of eradicating wild poliovirus by the year 2000 (Nathanson and Kew 1220; DeJesus). This policy required a coordinated effort to address polio not only where it was (or, increasingly, had been) epidemic, but also in the endemic areas where it had previously been seen as a low priority. Indeed, work in these areas has been extensive, and as polio is eliminated in these areas, the Global Polio Eradication Initiative plans to transition the infrastructure built by responses to polio to address other public health concerns in “priority countries” where polio remained endemic longest and polio eradication infrastructure is most developed (18).
Due to its unique pathophysiology, polio was a challenging disease to understand before and during the early part of the 20th century. It became visible as a serious public health issue primarily as it began to operate as an epidemic disease, affecting larger numbers of individuals in a short span of time and affecting older individuals. This rise in visibility did not correspond neatly to an increasing burden of disease. However, it took until late in the 20th century, once polio had already been eradicated in many of the areas where it first rose to prominence, for scientists and policy makers to fully understand the burdens that polio causes in areas where it is endemic. This delay resulted from incomplete reporting, along with the challenges of tracking a disease in which the majority of cases do not include the symptoms thought of as characteristic of that disease. That careful studies eventually exposed the burden of polio on communities where it is endemic may be in part responsible for the current state of near-global eradication of wild poliovirus, a state made possible by intensive worldwide vaccination campaigns.
Works Cited
Bernier, Roger H. "Some Observations on Poliomyelitis Lameness Surveys." Reviews of Infectious Diseases, vol. 6, no. Supplement 2, 1984, pp. S371-S375. doi.org/10.1093/clinids/6.Supplement_2.S371.
De Jesus, Nidia H. “Epidemics to Eradication: The Modern History of Poliomyelitis.” Virology Journal, vol. 4, no. 70, 2007. doi.org/10.1186/1743-422X-4-70.
Duintjer Tebbens, Radboud J., et al. "Expert Review on Poliovirus Immunity and Transmission." Risk Analysis, vol. 33, no. 4, 2013, pp. 544-605. doi.org/10.1111/j.1539-6924.2012.01864.x.
Global Polio Eradication Initiative. “Semi-Annual Status Report January to June 2018.” World Health Organization, 2018. polioeradication.org/wp-content/uploads/2018/11/2018_January-June_SemiAnnualReport_POL_highresolution.pdf.
Horstmann, Dorothy M. "The Poliomyelitis Story: A Scientific Hegira." The Yale Journal of Biology and Medicine, vol. 58, no. 2, 1985, pp. 79-90. PubMed Central, www.ncbi.nlm.nih.gov/pmc/articles/PMC2589894/.
LaForce, F. M., et al. "Clinical Survey Techniques to Estimate Prevalence and Annual Incidence of Poliomyelitis in Developing Countries." Bulletin of the World Health Organization, vol. 58, no. 4, 1980, pp. 609-620. PubMed Central, www.ncbi.nlm.nih.gov/pmc/articles/PMC2395925/.
Meldrum, Marcia. "“A Calculated Risk”: The Salk Polio Vaccine Field Trials of 1954." BMJ, vol. 317, no. 7167, 1998, pp. 1233-1236. PubMed Central, www.ncbi.nlm.nih.gov/pmc/articles/PMC1114166/.
Melnick, Joseph L. "Advantages and Disadvantages of Killed and Live Poliomyelitis Vaccines." Bulletin of the World Health Organization, vol. 56, no.1, 1978, pp. 21-38. PubMed Central, www.ncbi.nlm.nih.gov/pmc/articles/PMC2395534/.
Nathanson, Neal, and Olen M. Kew. "From Emergence to Eradication: The Epidemiology of Poliomyelitis Deconstructed." American Journal of Epidemiology, vol. 172, no. 11, 2010, pp. 1213-1229. doi.org/10.1093/aje/kwq320.
Nielsen, Nete Munk, et al. "The Polio Model. Does it Apply to Polio?." International Journal of Epidemiology, vol. 31, no. 1, 2002, pp. 181-186. doi.org/10.1093/ije/31.1.181.
United States, Centers for Disease Control and Prevention. “Poliomyelitis.” Epidemiology and Prevention of Vaccine-Preventable Diseases. 13th ed. Edited by Jennifer Hamborsky, Andrew Kroger, and Charles (Skip) Wolfe. Washington D.C. Public Health Foundation, 2015. www.cdc.gov/vaccines/pubs/pinkbook/polio.html.
---. “Data and Statistics for Cerebral Palsy.” Centers For Disease Control and Prevention. 9 Mar. 2018. www.cdc.gov/ncbddd/cp/data.html. Accessed 15 July 2019.
