The purposes of this PhD thesis were to examine the determinants of the severity of acute poliomyelitis and examine the long-term consequences after exposure to poliovirus either in-utero or later as child or adult.
The studies concerning risk factors of the acute severity of poliomyelitis were designed as case studies based on information extracted from hospital records from all the polio patients admitted to the main infectious disease hospital in Copenhagen, the Blegdamshospital, in the first half of the 20th century (n~6000). Information on name, address, age, date and place of birth, degree of severity (paralytic, non-paralytic, death), pregnancy, sibship size and birth order were registered. According to family name and address multiple family polio cases were identified. The follow-up studies of long-term consequences of poliomyelitis were designed as cohort studies (n~5700). The CRS number of all the polio patients were identified through the Local Municipal Registers and the Civil Registration System. The incidence of cancer and multiple sclerosis (MS) among the polio patients were examined by a linkage procedure with the Danish Cancer Registry and the Danish Multiple Sclerosis Registry. The observed number of cancers and MS cases among the polio patients were compared with the number expected, calculated according to national cancer respective MS incidence rates. Long-term consequences after in-utero exposure to poliovirus was likewise examined in a cohort design but this time using exposed (n=62) and non-exposed (n=1062) individuals.
The severity of poliomyelitis was associated with age, but not as simple as predicted by the polio-model which claims that severity increases steady by age. We found a U-formed association between severity and age, as both adults and infants had a high risk of developing paralytic polio. Furthermore increasing family size and higher birth order seem to increase the acute severity
of poliomyelitis. In addition short incubation period and being infected by another family member also tended to increase the severity of polio. Individuals exposed to poliovirus in-utero, did not have a higher mortality nor a higher risk of multiple sclerosis and amyotrophic lateral sclerosis. They might have a slightly higher however insignificant risk of cancer compared to the non-exposed
group. However, a large follow-up of 5700 patients with a history of polio in childhood or adulthood revealed a significantly increased risk of cancer being 10% higher than expected. This increase was mainly due to an higher risk of developing skin-cancer and a 60% increased risk of breast cancer among women with a history of paralytic polio. Finally irrespective of gender and severity of the acute disease polio patients had a 70% increased risk of developing MS.
The polio model is not the only explanation of the observed association between severity and age. Risk factors being a part of the transmission model such as crowding, short incubations period, high intensity of exposure might be an alternative or supplementary explanation. Long-term onsequences among the individuals exposed in-utero to poliovirus were limited. The increased risk of cancer among the polio patients, is probably not cause by the poliovirus it self. Several diagnostical X-ray examination of the paralytic female polio patients might cause the observed increased risk of breast cancer. The increased risk of MS in polio patients suggests that poliovirus, as other infectious agents, might be involved in the initiation of the autoimmune process in MS. The mechanism underlying the association is however unknown and further studies are needed.