The Re-emergence of Wild Poliovirus in Africa

Abstract & Commentary

Synopsis: Travel Medicine providers must be aware of the need for continued surveillance of their patients for an adequate polio immunization status given the developments in Africa.

Sources: CDC. Wild Poliovirus Importations—West and Central Africa, January 2003-March 2004. MMWR. 2004; 53(20):433-435.

World Health Organization polio experts warn of largest epidemic in recent years, as polio hits Darfur. (WHO Press Release 22 June 2004. Available at http://www.who.int/mediacentre/releases/2004/pr45/en.)

Wild poliovirus (WPV) was imported into 8 countries that were previously felt to be polio-free in west Africa (Benin, Burkina Faso, Cote d’Ivoire, Ghana, Togo) and Central Africa (Cameroon, Central African Republic, Chad) from January 2003 to March 2004. The polio-free period before these recent importations ranged from 28 to 55 months. The MMWR summarized 63 cases, which were all shown to be poliovirus type 1 and linked to strains that circulate in endemic regions of Nigeria and Niger. Nigeria and Niger reported 497 cases of polio type 1 or type 3 during the same period. Two of the 8 index patients in the 8 countries had recent travel to a polio-endemic country, and the other 6 lived near centers where foreign trade with polio-endemic countries occurred. In spite of the supplementary immunization activities (SIAs) organized in all 8 countries, 4 continued to have WPV transmission after at least 2 rounds of immunizations.

In addition, the WHO press release reported re-infection with polio in Sudan, which had been polio-free for 3 years. The report warns of a spreading epidemic of polio through west and central Africa. Apparently, the number of children paralyzed in west and central Africa in 2004 is 5 times those which occurred in the same period in 2003.

Comment by Lin H. Chen, MD

Poliomyelitis is caused by 3 serotypes of poliovirus, which are enteroviruses transmitted via the fecal-oral route. Acute poliovirus infection can be asymptomatic or present as acute poliomyelitis, which is a nonspecific febrile illness followed by aseptic meningitis and/or paralysis.1 Paralysis can be classified as spinal, bulbar, or spino-bulbar disease. Post-polio syndrome, characterized by muscle pain and weakness or paralysis, may develop decades later in up to 40% of persons who had paralytic poliomyelitis in childhood.1 Infection with 1 serotype of poliovirus does not confer immunity to the other serotypes. An inactivated polio vaccine (IPV, Salk vaccine) became available in the United States during 1955, and was widely used until oral polio vaccine (OPV, Sabin vaccine) became available in the 1960s. The introduction of the polio vaccine rapidly reduced the incidence of poliomyelitis.

In 1988, the World Health Assembly established the goal of global polio eradication by 2000. Although the deadline has been postponed until 2005, much progress has been made. Three of the 6 WHO regions (Americas, Europe, and western Pacific) have been certified polio free, and only 6 countries were considered polio endemic in 2003: Afghanistan, Egypt, India, Niger, Nigeria, and Pakistan.2 On the one hand, transmission has become limited within the Eastern Mediterranean Region and South-East Asian Region (see Table).2 On the other hand, the situation has deteriorated in Africa, where an additional 10 countries have reported polio cases in 2003 to 2004, including the 8 countries in west and central Africa noted in the CDC report, as well as Sudan and Botswana.2

Laboratory surveillance has demonstrated importation of viruses from Nigeria/Niger into most of the other African countries. Furthermore, genomic sequencing demonstrated the poliovirus in Lebanon in 2003 originated in India; the poliovirus found in Zambia during 2002 was imported from Angola.3

The current situation reflects a trend associated with routine vaccination coverage in the endemic countries. In Niger and Nigeria, the extent of coverage with 3 doses of oral poliovirus vaccine was estimated to be 25% in 2002, whereas the vaccine coverage was 48% in Afghanistan, 63% in Pakistan, 70% in India, and 97% in Egypt.2 In India, polio vaccine coverage in 2002 was low in the states where polio commonly circulated, Bihar and Uttar Pradesh (21% and 41%, respectively).4

The 144 indigenous cases of polio reported in the United States since 1979 were due to vaccine-strain virus of the live oral poliovirus vaccine (OPV); only 6 additional cases were imported (1979-1993).5 A more immunogenic vaccine, the enhanced-potency IPV was initially licensed in the United States in 1987.6 In 1997, the Advisory Committee on Immunization Practices recommended sequential IPV-OPV for routine childhood immunization.6 In 1999, the recommendation changed to an all-IPV schedule in order to eliminate the potential risks of vaccine-associated paralytic polio (VAPP).1,5

The risk of acquiring polio during travel is low. In the 1990s, polio was estimated to occur at an incidence of 1 symptomatic case and 20-1000 asymptomatic cases per 1,000,000 non-immune travelers visiting developing countries for 1 month.7 By comparison, 3000-6000 per 1,000,000 travelers may contract hepatitis A, 800-2400 travelers may contract hepatitis B, 30 travelers may contract typhoid, and 3 may contract cholera.7

The estimated risk of polio in travelers should be lower at preset, given the global reduction of polio in the past decade. However, travelers visiting any polio-epidemic or polio-endemic areas should be current with their polio immunization: Children should have had their routine polio immunizations (at 2, 4, and 6-18 months, followed by a booster at 4 to 6 years of age), and adults should receive a polio booster (given at age 18 or older) before their trip.1 The progress in polio elimination in the Eastern Mediterranean Region and South-East Asian Region continues to reduce the risk of exposure to polio for travelers visiting these areas. Nonetheless, long-term carriage of polioviruses has been documented in immunodeficient individuals, and questions remain as to whether these rare carriers of poliovirus can reintroduce poliovirus circulation into the population. In summary, given the recent spread of polio in the African region, travelers visiting the area should continue to ascertain that their polio immunizations are up to date, or receive a booster dose of inactivated polio vaccine (IPV).

Dr. Chen, MD, is Assistant Clinical Professor and Harvard Medical School Director.

References

1. CDC. Poliomyelitis Prevention in the United States: Updated Recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR. 2000;49(RR-5):1-22.

2. CDC. Progress Toward Global Eradication of Poliomyelitis.January 2003-April 2004. MMWR. 2004;53(24):532-535.

3. CDC. Laboratory Surveillance for Wild and Vaccine-Derived Polioviruses, January 2002-June 2003. MMWR. 2003;52(38):913-916.

4. CDC. Progress Toward Poliomyelitis Eradication-India. MMWR. 2003-2004;53(11):238-241.

5. CDC. Recommendations of the Advisory Committee on Immunization Practices: Revised Recommendations for Routine Poliomyelitis Vaccination. MMWR. 1999;48:590.

6. CDC. Poliomyelitis Prevention in the United States: Introduction of a Sequential Vaccination Schedule of Inactivated Poliovirus Vaccine Followed By Oral Poliovirus Vaccine. MMWR. 1997;46(RR-3):1-25.

7. Steffen R. Hepatitis A and Hepatitis B: Risks Compared With Other Vaccine Preventable Diseases and Immunization Recommendations. Vaccine. 1993;11(5):518-520.

8. CDC. Progress Toward Poliomyelitis Eradication-Nigeria. January 2003-March 2004. MMWR. 2004;53(16):343-346.