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International Notes Update: Progress Toward Eradicating Poliomyelitis from the Americas
In May 1985, the Pan American Health Organization (PAHO) established a plan for eradicating the indigenous transmission of wild poliovirus from the Region of the Americas by the end of 1990 (1). In response to this initiative, PAHO's Expanded Program on Immunization (EPI) implemented a program strategy that included 1) achievement and maintenance of high poliomyelitis immunization levels through accelerated immunization efforts, including national immunization days held twice a year at least 4 weeks apart; 2) surveillance to detect all new cases of acute flaccid paralysis (AFP); and 3) a rapid, vigorous response, including containment measures, to all new cases of paralysis (2). This report updates efforts through 1989 toward the polio eradication initiative and provides preliminary laboratory surveillance data for 1990.
Through 1989, rates of reported paralytic poliomyelitis continued to decline substantially, coincident with a doubling in oral poliovirus vaccine (OPV) coverage in young children (Figure 1). In 1988, regional estimates of OPV coverage with three doses of vaccine in children by 1 year of age were greater than 70%; in 1989, this estimate reached an all-time high of 73%. Although polio vaccination levels should be interpreted with caution because of changes over time in the methodology for assessing coverage (3), results such as these are encouraging for the rest of the world.
The intensification of surveillance activities in 1986 resulted in a nearly twofold increase in the number of AFP cases that were investigated and reported, from 1100 in 1985 to 2094 in 1989 (Figure 2). Despite yearly increases since 1986 in reported AFP cases, however, the number of AFP cases confirmed* as poliomyelitis decreased to 130 in 1989, representing an 86% decline from the 930 cases confirmed in 1986 and a 62% decline from the 340 cases confirmed in 1988. These polio cases were located in 99 (0.7%) of the 14,372 counties in Latin America.
For 1989, of the 2094 reported AFP cases in the Region of the Americas, 1964 were determined not to be polio. For 703 of these cases determined not to be polio, a final diagnosis was submitted to the regional PAHO office and was available for this analysis. The most common known alternative diagnosis was Guillain-Barre syndrome (43%), followed by trauma (3%), transverse myelitis (2%), neoplasms (2%), and other diagnoses (50%).
Of the 130 confirmed cases, 24 were caused by culture-confirmed wild poliovirus, and eight were vaccine-related. Of the remaining 98 patients who either died (18 patients), had residual paralysis (61), or were lost to follow-up (19), 36 (37%) had no stool sample taken for virus isolation, and 15 (15%) with negative stools had their stool specimens obtained greater than 2 weeks after paralysis onset. (Because the likelihood of virus isolation diminishes with increasing duration between paralysis onset and collection of stool sample, patients for whom stool samples were not taken and patients for whom isolates were negative and stool specimens were taken greater than 2 weeks after paralysis onset both should be monitored.)
When the characteristics of cases caused by wild poliovirus were compared with those of cases in the other categories, patients with wild poliovirus were more likely than patients who died to be less than 5 years of age (82% vs. 27%; p less than 0.01).
Of the 24 wild poliovirus cases confirmed in 1989, 16 were type 3 and eight were type 1. These cases were limited to six countries in three geographic regions in the Americas: northwestern Mexico, northern Andean subregion, and northeastern Brazil. During 1989, 13 wild type 3 cases occurred in Mexico. In the northern Andean subregion, type 1 wild polioviruses were isolated in Colombia (two cases), Ecuador (two cases), Peru (one case), and Venezuela (one case); type 3 wild polioviruses were isolated in Colombia (three cases). In northeastern Brazil, type 1 wild polioviruses were isolated from two patients.
As of the first 32 weeks of 1990, wild polioviruses had been isolated from three patients with AFP, including type 3 virus from a patient from northwestern Mexico with paralysis onset on February 19, 1990, and type 1 virus from two patients in the northern Andean subregion (one in Ecuador and one in Peru) with respective dates of paralysis onset of March 26 and April 25, 1990. Reported by: Expanded Programme on Immunization, Pan American Health Organization, Washington, DC.**
Editorial Note: As efforts to eradicate polio from the Western Hemisphere proceed, the surveillance of paralytic poliomyelitis has shifted to focus on the surveillance of wild poliovirus. Accordingly, EPI has been using surveillance indicators, such as those assessing the quality of stool collection, to maximize detection of wild poliovirus in persons with suspected polio. Of cases that were confirmed as paralytic poliomyelitis (because of either loss to follow-up, presence of residual paralysis, or death), half were inadequately investigated because stool samples were not obtained or were negative but obtained greater than 2 weeks after paralysis onset. The difference in age distribution between persons with culture-confirmed wild poliovirus and fatal cases provides additional indirect evidence that polio may be overdiagnosed among patients from whom wild poliovirus is not isolated.
During the initial stages of the PAHO eradication effort, surveillance of paralytic poliomyelitis was designed to be highly sensitive; consequently, many reported AFP cases ultimately were determined not to be caused by wild poliovirus. This aggressive approach to case detection by a sensitive surveillance system, combined with immediate action to control outbreaks, has contributed to the containment of wild poliovirus within the two remaining areas of risk: northwestern Mexico and the northern Andean subregion.
A large number of suspected cases are ultimately classified as "confirmed" because adequate diagnostic specimens were not collected or tested or because the patients were lost to follow-up or died (98 (75%) of the 130 confirmed cases in 1989). Consequently, at PAHO's most recent Technical Advisory Group (TAG) Meeting on the EPI and Polio Eradication, held in March 1990 in Mexico City, TAG members recommended the following changes in classification of AFP in the Region of the Americas (4):1. Confirmed poliomyelitis. Acute paralytic illness associated with the isolation of wild poliovirus, irrespective of residual paralysis. 2. Vaccine-associated poliomyelitis. Acute paralytic illness in which vaccine-like poliovirus is isolated and is believed to be the cause of the disease. Vaccine-associated cases should be reported separately. They are considered as a category separate from confirmed polio with wild poliovirus isolates. 3. Polio compatible. Acute paralytic illness with compatible residual paralysis at 60 days or death or loss to follow-up in which at least two adequate stool specimens were not obtained within 2 weeks after onset of paralysis and examined in three different laboratories. These cases can neither be confirmed nor discarded. This should be a very small proportion of the cases. 4. Not poliomyelitis. Acute paralytic illness in which at least two adequate stool specimens were obtained within 2 weeks after onset of symptoms and were negative for poliovirus. Aliquots of the original samples should be held at the laboratory for possible future use. To ensure the accuracy of this categorization, any patient who dies, is lost to follow-up, or has residual paralysis at 60 days should have aliquots of the original specimens examined in two other laboratories in the PAHO network, using all appropriate techniques. If the specimens were adequate and all were negative, these cases should be considered "not polio" and "discarded." This classification represents a major change from the previous system. Use of the new classification of AFP has been implemented for all patients with dates of paralysis onset since January 1, 1990.
In July 1990, the International Certification Commission of Poliomyelitis Eradication in the Americas*** (5), convened by PAHO, met for the first time to develop the methodology to certify countries that are polio-free. Although the criteria are not finalized, many of the same procedures that PAHO uses to evaluate polio eradication efforts will also be used by the Commission. The burden of diagnosis and, ultimately, the proof that eradication of transmission of wild poliovirus has been achieved rests with the laboratories. Accordingly, countries need to continue to investigate properly all cases of AFP, and stool specimens obtained from persons with suspected polio must be submitted to the laboratory in adequate condition. The current level of effort must be sustained if polio is to be eradicated from the Americas by the end of 1990 and from the world by the year 2000 (6).
there was: 1) laboratory confirmation (wild-type poliovirus isolated from the stool), 2) epidemiologic linkage to another case of AFP or confirmed case, 3) residual paralysis 60 days after onset, 4) death, or 5) lack of follow-up of a case. Cases of AFP were "discarded" if they did not meet these criteria. In July 1989, routine serologic testing was discontinued in favor of efforts to obtain laboratory confirmation by isolating wild poliovirus from stool. ** 525 23rd Street, Washington, DC 20037; telephone (202) 861-3247.
*** The Commission members are: Waldyr Arcoverde, M.D., National Health Foundation, Ministry of Health, Brazil; Isao Arita, M.D., Kumamoto National Hospital, Japan; Rodrigo Guerrero, M.D., Carbajal Foundation, Colombia; Dorothy Horstmann, M.D., Yale University School of Medicine, United States; Jan Kostrzewski, M.D., Polish Academy of Science, Poland; Maureen Law, M.D., International Development Research Center, Canada; Elsa Moreno, M.D., University of Tucuman, Argentina; V. Ramalangaswami, M.D., Nehru University, India; Olikoye Ransome-Kuti, M.D., Ministry of Health, Nigeria; Frederick Robbins, M.D., Case Western Reserve University School of Medicine, United States; Guillermo Soberon, Mexican Foundation for Health, Mexico; and Kenneth Standard, M.D., Caribbean Public Health Association, West Indies.
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