1. Question Generated
1. What is the polio virus and its pathophysiology?
2. How do vaccinations lead to immunity?
3. What are the different types of polio vaccine and their advantages and disadvantages?
4. Discuss the global effort to eradicate polio
5. What challenges are presented to health workers in delivering the polio vaccine?
2. Written Report
Polio or poliomyelitis is a highly infectious virus with three serotypes that results in inflammation of the grey matter (1, 2). It is an enterovirus which is a virus that “enters the body through the gastrointestinal tract, multiplies there and invades the central nervous system” (3).
Polio only infects humans as it uses the human CD155 gene to aid entry into human cells (4). It gains entry to the body via the oropharynx and is secreted into saliva in the mouth, swallowed and reaches the intestinal mucosa and lymph nodes where it multiplies (4). Polio can withstand the highly acidic pH present in the stomach as it is a member of the Picornaviridae family of viruses which are acid resistant, contain single stranded RNA and possess no envelope (5). Table 1 examines the different kinds of polio and their clinical characteristics.
Types Transmission Individual Characteristics Potentially fatal Age
Spinal
Faecal-oral route or contaminated food or water • Anterior horn cells in spinal cord damaged, particularly in lumbar region Yes, with mortality rate of 5%-25% due to respiratory failure
Most common in children under 5
Bulbar • Respiratory muscle
• Paralysis
• Pneumonia
• Soft palate palsies
• Cranial nerve involvement Yes
Abortive • Fever
• Myalgia
• Sore throat No
Non-paralytic • Similar to abortive but with meningeal irritation No
Paralytic • Paraplegia or quadriplegia Yes
Post-polio syndrome Second attack from initial infection • Progressive weakened muscle mass
• Decreased power No 25-35 years after initial infection
Table 1: The various types of poliomyelitis and their associated characteristics. Self- made from (2, 4, 6, 7, 8)
90% of people infected usually present as asymptomatic or with very mild symptoms therefore it has an almost “silent transmission” (1, 9). Figure 1 illustrates the variety of symptoms associated with polio.
Figure 1: The common and extreme symptoms of polio. Self-made from (2, 9, 10)
Once excreted, the virus can persist for 2-3 weeks in an infected persons faeces, therefore it has a high incidence rate where there is poor hygiene standards and control (5). There is currently no cure for polio which is why health workers seek to eradicate polio globally via vaccination (9).
The goal of vaccination is to familiarise the adaptive immune system with the antigens of a disease in order for it to elicit a secondary response during the first infection (11). Vaccines are altered pathogens that have not lost their antigenicity, thus B and T cells still react with the viral epitope by cell-surface receptors, inducing B-cell and T-cell immunity (12). Once the B lymphocytes are sensitised they may survive for long periods of time as memory cells that will respond more rapidly to subsequent infection, promoting clonal proliferation of the required antibodies (5). This results in an active and acquired immune response which is much faster to materialise than a primary response.
Vaccinations can be inactivated/killed or live/attenuated. Inactivated vaccines consist of viruses which have been treated so that they are unable to replicate in human tissue (13). Attenuated vaccines are viruses that have been grown in cultured nonhuman cells so they are weakened but not destroyed. They replicate very slowly and poorly in human hosts, thus generally do not cause disease in the host and will induce immunity. Live vaccines induce production of secretory IgA (sIgA) alongside IgM and IgG in viral infections which yield protection at the point of entry of many viruses (like polio), the gastrointestinal mucosa. This is because sIgA is the immunoglobulin of the “mucosa-associated-lymphoid-tissue” (12). Comparatively, killed vaccines do not stimulate any duodenal or nasal IgA.
Polio has both an inactivated and an attenuated vaccine. Inactivated polio vaccine (IPV), devised by Salk, protects against all three serotypes of polio. Modern IPV uses Vero cells derived from monkey kidney cells for a substrate (5, 14). The live attenuated vaccine, devised by Sabin, is given orally hence its name oral polio vaccine (OPV). Table 2 compares the advantages and disadvantages of both vaccines.
Advantages Disadvantages
OPV • Lower cost
• Easy to administer
• Mucosal immunity provided
• Provides secondary immunity
• Provides people in close contact who are not immunised with immunity
• Highly successful in developing countries
• Savings in terms of hospitalisation and treatment • Attenuated = chance of mutation and reversion to virulence e.g type 3 OPV which differs by 10 nucleotides out of 7429 from progenitor strain
• Cannot be used in immuno-compromised patients
• Three doses required, with each dose only giving immunity to one serotype
IPV • Killed virus = no risk of mutation of vaccine
• Highly effective
• Does not cause paralysis
• Savings in terms of hospitalisation and treatment
• Induces primary immune response in first dose
• Reduce risk of outbreak after type 2 OPV vaccine withdrawal
• Can be added to other childhood vaccines • Contaminated with animal viruses in past
• More expensive
• Requires trained health workers
• Potential for needle stick injuries
• Disposal of sharps
• Fear of needles in some patients
• Does not mimic usual route of entry of polio
• Doesn’t produce secondary immunity
• Three doses required at monthly intervals
Table 2: Comparison of the advantages and disadvantages of OPV and IPV. Self-made from (5, 12, 13, 15, 16, 17)
The risk of vaccine derived viruses continues to increase the more OPV’s that are used, therefore the Global Polio Eradication Initiative’s developed a Polio Eradication and Endgame Strategic Plan (2013–2018) which has a second objective to “Strengthen immunization systems and withdraw oral polio vaccine” (18).
The full population does not have to be immunised in order to eradicate the polio virus. Herd immunity will ensure that the disease will run out of hosts and eventually die out. (16,19). Due to the polio immunisation drive initiated by the Global Polio Eradication Initiative in 1988, cases have decreased by over 99% from an estimated 350,000 in 1988 to 359 reported in 2014 (20). Wild type 2 polio has in fact been eradicated globally including in the endemic countries and Europe itself has been declared polio free since 2002 (1, 11).
The real challenge lies in eradicating polio from Afghanistan and Pakistan which are “polio-endemic” (20). Many children are missed during vaccination programmes with this being linked to a lack of trust in vaccinators with “Pakistan’s caregivers’ trust in vaccinators at 26% and 34% compared to Pakistan’s average of 61%” (21). Care is difficult to provide due to instability in the infrastructure and areas of insecurity, making transport and organisation highly problematic. But there has been an improvement, in 2013 approximately half a million children were inaccessible for vaccination, now that has decreased to approximately less than 35000 in September 2015, providing great ambition for the eradication of polio to be within the 21st century (21).
Word Count: 750
Reference List:
1) Philip D Minor. Polio vaccines and the eradication of poliomyelitis. The Lancet [Internet]. 4-10 August 2012 [cited 14-12-2015]: Volume 380 (Issue 9840): Pages 454-455 Available from: http://www.sciencedirect.com.ezproxy.lib.gla.ac.uk/science/article/pii/S0140673612609210?np=y
2) Rubin R, Strayer DS. Pathology. 6th ed. Philadelphia. Lippincott Williams & Wilkins. 2012. Pages: 1325-1326.
3) Oxford University Press. Concise Medical Dictionary. 9th ed. Oxford University Press. 2010. Pages: 256
4) Kumar. Abbas. DeLancey. Malone. Pathological Basis of Disease. 8th ed. Philadelphia: Saunders Elsevier: 2010. Pages: 1304
5) Collier L, Oxford J. Human Virology. 3rd ed. London: Oxford University Press; 2006. Pages: 44-45, 127-132 and 272-273
6) Walker BR et al (eds.). Davidson’s Principles & Practice of Medicine. 22nd ed. Edinburgh: Churchill Livingstone; 2014 Pages:148-149, 1207
7) WHO. Poliomyelitis (polio). http://www.who.int/topics/poliomyelitis/en/ [cited 14-12-2015]
8) Kumar P. Clark M. Kumar and Clark’s Clinical Medicine. 8th ed. Saunders Elsevier. 2012. Pages: 101-102
9) WHO. Poliomyelitis. http://www.who.int/immunization/diseases/poliomyelitis/en/ [cited 14-12-2015]
10) Thomas Abraham. Polio eradication: a complex end game. BMJ [Internet]. Published 2 August 2012 [cited 14-12-2015]:344:e2398. Available from: http://www.bmj.com/content/344/bmj.e2398
11) Mims C et al. Medical Microbiology. 3rd edition. Elsevier:2004. Pages: 513, 527, 530-531
12) Male D. Brostoff J. Roth DB. Roitt I. Immunology. 7th ed. Philadelphia: Mosby Elsevier; 2006. Pages: 14-15, 325-326, 331-333
13) Murphy K, Travers P, Walport M. Janeway’s Immunobiology. 7th ed. New York: Garland Science: 2008. Pages 688-698
14) Joint Formulary Committee. British National Formulary. 69th ed. London: BMJ Group and Pharmaceutical Press: 2015. Section 14.1 and 14.4. Pages: 842-844 and 861.
15) WHO. IPV Implementation. Module 1 – Introduction to Polio and IPV. http://www.who.int/immunization/diseases/poliomyelitis/endgame_objective2/inactivated_polio_vaccine/implementation/en/ [cited 14-12-2015]
16) Leon Gordis. Epidemiology. 4th ed. Philadelphia: Saunders Elsevier; 2009. Pages: 24-25.
17) NHS. Vaccination Ingredients. http://www.nhs.uk/Conditions/vaccinations/Pages/vaccine-ingredients.aspx [cited 14-12-2015]
18) Global Polio Eradication Initiative. Polio Eradication and Endgame Strategic Plan 2013–2018. http://www.polioeradication.org/resourcelibrary/strategyandwork.aspx [cited 14-12-2015]
19) Naidoo J. Wills J. Developing Practice for Public Health and Health Promotion. 3rd ed. Bailliére Tindall Elsevier. 2010. Page: 189
20) WHO. Poliomyelitis: Fact sheet. http://www.who.int/mediacentre/factsheets/fs114/en/. [cited 15-21-2015]
21) Independent Monitoring Board of the Global Polio Eradication Initiative. IMB Report – Now is the time for peak performance. Global Polio Eradication Initiative. Report Number 12; October 2015. Pages: 8 and 16