Polio Epidemiology – EPI 390

Define Poliomyelitis:
“Grey Inflammation” — a infectious viral disease due to an RNA virus that survives the pH of the stomach, and then enters the digestive tract and plants itself in the small intestine. So that makes poliovirus an enteric virus. It is absorbed into the Small intestine and gets into the bloodstream. Travels to the brainstem, then continues to destroy nerves.
– AKA: infantile paralysis
How is the polio virus transmitted:
Fecal Oral — less commonly through saliva
There are 3 strains of the poliovirus!
1.) Brunhilde (PV1)(The Ape)
2.) Lansing (PV2)(Discovered in Lansing)
3.) Leon (PV3)
What is the history of polio contradict ability:
Can be contacted by humans, apes, and the number of cases has dropped to zero in the US
– Vaccine introduced in 1955
– Last (wild-type) case in 1979

– Incubation time = 6-20 days
95% of people with Polio are asymptomatic

What are the various serotypes:
– what are the symptoms of each
1.) Abortive Poliomyelitis:
– Experience minor flu-like symptoms — upper respiratory infection,nausea, fever, sore throat, general fatigue (no CNS involvement)

2.) Nonparalytic Poliomyelitis:
– Associated with aseptic meningitis — 1-5% have neurological symptoms such as neck stiffness, headache, lethargy, light sensitivity

3.) Paralytic Poliomyelitis (0.1%-2% of cases) Severe form with muscle paralysis, (spinal 79%, bulbospinal 19%, and bulbar 2%)

Who is most affected by Polio:
Infants and young children — paralysis becomes more common and severe in older patients
Symptomatic Serotypes:
– Neuroanatomy
– Symptoms
A.) Neuroanatomy: spinal, bulbospinal, bulbar
B.) Spinal has genernal muscle paralysis, bulbospinal has respiratory problems along with general muscle paralysis, and bulbar has respiratory, speech, and swallowing problems along with general muscle paralysis
Illness duration of Polio:
The acute phase often lasts about two weeks or less with many cases going unreported as many patients simply suspected their children had the fyl. In long term, if there is a significant nerve involvement paralysis could last a lifetime
What is PPS:
Post Polio Syndrome:
– No known cause or treatment, affects up to 50%, some up to 10-40 years after initial polio infection
What are some major risk factors of PPS:
1.) Severity of Initial Infection
2.) Age of Onset of Initial infection
– Adults more susceptible
3.) Recovery
– greater recover after acute puts stress on neurons
4.) Physical Activity
– to the point of exhaustion/fatigue
5.) Gender – females
What are some competitive theories:
Theory 1: most widely-accepted theory holds that there is (continued?) degeneration or damage to nerve cells…especially of motor neurons in the spinal cord…disrupting the electrical impulses between brain and muscle…you have seen this “mechanism”

Compensatory Mechanism for Theory 1:
1.) (Axiom) poliovirus leaves neurons destroyed/damaged
2.) (Corollary) To compensate, surviving neurons grow new fibers, enlarging the surviving neurons.
3.) (Corollary) Use of the muscle is restored but at a price as the nerve cell body is stressed to nourish the additional fiber growth.
4.) (Corollary) Over many years the stress may overwhelm the neuron, leading to the (again) destruction of the additional fibers, or even the neurons itself/themselves.

Theory 2: Initial illness prompts an autoimmune response, causing the body’s immune system to attack or ‘recognize’ normal nerve cells (neurons) as foreign entities (Jonas Salk believed this one)

What are some complications they faced when addressing Polio:
– Falls: due to muscle weakness; lead to broken bones
– Malnutrition, dehydration (aspiration) pneumonia: if nerves innervating muscles responsible for chewing and swallowing are involved
– Acute Respiratory Failure: weakening of muscles involved with respiration, fluid in the lungs accumulated, sharp drop in blood O2 conc., ventilation therapy
– Osteoporosis: prolonged inactivity/immobility (
What were some preventative methods used in attempt to limit Polio exposure:
– Possible steroid experimentaion
– Common preventative measures included “maintaining a healthy lifestyle”
– Focus on cardiopulmonary over muscle strengthening, since physical activity could cause PPS
What was used as “treatment” for Polio:
Iron Lung Machines
– Negative pressure ventilators
– They produce rhythmic, oscillating alterations in air pressure to expand and contract the lungs — mimic physiological breathing dynamic
What was the basic historical context leading up to the Polio Vaccine Trials:
– Noticed a rise in cases during the summer in children mainly under 5 y/o
– The popularity of the disease made it easy to accumulate financial support from the community
– March of Dimes in 1938 (FDR died in 1945)
– Dr. Salk used a dead virus to provoke an immune response without actually infecting the host
– Pittsburg was the first town to test run the vaccine 1,800,000 children volunteered (some were placebo) but it was found to be 80-90% effective
– Mass production in 1955
Which of the two vaccines was more popular and where was it commonly used:
– Salk’s vaccine with a dead virus was commonly used on the public of well developed countries where physicians could administer and monitor the patients over time
– Sabin’s vaccine with the live virus was introduced in 1963 and given through a single drop in the mouth and was commonly used on a global prevention scale because the access to a single drop was easier than a series of vaccines
How were the two vaccines similar? Different? Effective?
– Dead Virus
– Given through injection
– Required 3 doses to be effective
– Live Attenuated Virus
– Once through the mouth
– Thought to be better for “herd immunity”
– Both mounted an immune response and were effect against all three strains of Polio
Vaccine Field Trials:
– major players
– important dates
A.) Salk and Sabin
B.) 1952 – Peak Epidemic
1955 – Salk Vaccine
CH: 2, 4, 6-18mnths + 4-6 yrs
AD: any age, 1-2mnths, ~ 1yr
1963 – Sabin Vaccine
What are some potential confounders when it came to Polio and the Vaccine Trails:
– Ethical issues of randomized controls
– Asymptomatic participants
– Clinical course of disease (seasonal)
How did Hygienic Background play into the design of the study and field trials:
– Kids in lower social classes had less hygienic living areas which allowed them to develop stronger immune systems at a younger age while still protected by moms immunity
How did Socio Economic Status’ play into the design of the study and field trials:
– If children developed polio earlier on when they were still protected by their mothers Ab they would develop the immunity to the disease
How did Volunteers play into the design of the study and field trials:
– Would usually have higher rates of polio and not be representative of the general population
– Tended to be of higher SES, and thus more prone to Polio; consequently bias against the vaccine
– Higher SES = Better Health Conditions = Weaker Immune System
How did defining cases and their matched controls play into the design of the study and field trials:
How would “caseness” be determined?
– Different forms of polio can be difficult to diagnose
– At examination physicians would ask if the child was or was not vaccinated
– Could be determined by knowing grade level and knowing treatment/non-treatment

– Asymptomatic; probably will volunteer to get their kid vaccinated because they don’t know they have polio

How did the considerations of power/underpowered play into the design of the study and field trials:
– Initial trial was only 10,000 controls and 10,000 kids in the treatment group; results could be due change and trial was therefore underpowered because the rate of appearance of disease was only .05%
– This led to a need for a massive – scale trail (>1.8 million kids; 600,000 in tx group) = 1.8 million kids.
– This was a large enough power for the results to be statistically significant
How did biases for and against he effectiveness of the vaccine play into the design of the study and field trials:
– Volunteers (against)
– Observed control (against)
– Those placed in treatment groups got vaccinated but were more likely to have the disease
– Those placed in the control group didn’t get vaccinated but were prone to lower rates of disease
Where globally are most of the current polio cases found?
– what type
Pakistan & Afganistan (wild-type)
– Less that 20 cases
Nigeria (possible wild-type)
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