2004—American Society of Tropical Medicine and Hygiene Meeting Highlights

Special Feature By Lin Chen, MD

David Heyman, executive director of Communicable Diseases for the World Health Organization and Representative of the Director-General for Polio Eradication, recently spoke on Infectious Disease Challenges: From Polio to SARS to Avian Influenza in a Plenary Session. Since WHO established a goal in 1988 to eradicate polio, the incidence of polio has decreased dramatically. There were 350,000 cases in 125 endemic countries in 1988, compared to 784 cases officially reported from 6 endemic countries in 2003. Eradication strategies included: 1) routine childhood immunizations; 2) national immunization campaigns; and 3) surveillance of acute flaccid paralysis. Despite these efforts, vaccine coverage globally is about 73%, and coverage is low in Africa, The Eastern Mediterranean Region, and Southeast Asia Region.5

As of November 2004, 10 countries are endemic for polio. Persistent transmission in Nigeria has led to the re-establishment of polio in 4 additional countries: Sudan, Chad, Burkina Faso, and Côtes d’Ivoire. In Asia, the number of cases has decreased, and may reach zero transmission by early 2005. Some regions that were free of wild poliovirus have identified cases of circulating vaccine-derived poliovirus infection: China (2 cases in 2004), Hispaniola (22 cases in 2000), Madagascar (4 cases in 2002), and Philippines (3 cases in 2001). Additionally, there are 24 cases of individuals with primary immunodeficiency syndromes who are long-term excretors (> 12 months) of vaccine-derived poliovirus (VDPV).

WHO has formulated an Emergency Response Plan. Risks for polio, after eradication, include that from vaccine-associated paralytic polio (VAPP), long-term excretors of vaccine-derived poliovirus in immunodeficient persons (iVDPV), circulating vaccine-derived poliovirus (cVDPV), lab accidents, and deliberate release of poliovirus. Vaccine-associated paralytic polio (VAPP) poses the greatest risk with the first dose, ie 2-4 per million birth cohorts, or 250-500 cases/year. Potential oral polio vaccination (OPV) cessation may occur about 3 years after disappearance of wild polioviruses. Some conditions are required prior to OPV cessation: confirmation of interruption, containment of all polioviruses, global surveillance, stockpile of monovalent OPV, and readiness of post-OPV plans.

Dr. Heyman reviewed the SARS outbreak, which was initially a respiratory illness noted concurrently with influenza activity in Guangdong Province, China, in November 2002. An outbreak of atypical pneumonia was reported in Guangdong in February 2003, followed shortly by the report of a 33-year-old male from Hong Kong and his 9-year-old son diagnosed with H5N1 linked to Fujian Province. When a physician became infected in Vietnam and died, the disease was recognized as a new entity, and the first global alert was issued concerning Vietnam and Hong Kong on March 12, 2003. The disease was soon diagnosed in Ontario and Singapore. On March 15, 2003, an ill physician from Singapore flew to New York and then Europe, which led to another global alert and containment strategies. Additional airline passengers who were subsequently symptomatic for SARS, led to recommendations for travelers on April 2, 2003.

SARS was an example of international amplification; an outbreak driven by health care workers and spread to the community. Airport screening in Hong Kong identified 2 confirmed cases, prevented their travel, and restored some level of confidence. Strategies that contained SARS are case identification, isolation, contact tracing, surveillance, quarantine, travel recommendation, and restriction. Some good luck was apparently around at the time, such that SARS did not spread to weak economies lacking public health infrastructures.

Dr. Heyman also discussed concerns regarding Avian Influenza. Influenza A, H1N1 (Spanish Flu), that caused a pandemic in 1918-1919, infected more than half of the world’s population. The pandemics of 1918, 1957 (Asian flu), and 1968 illustrate the concern for reassortment of human influenza genomes with those of avian influenza strains. One example of the risk in developing countries is Madagascar, which experienced an outbreak in 2002 involving 13 of 111 districts. There were 27,519 cases from July to September, and case fatality was < 1%.

In 2003, the circulating avian influenza strains included H5N1, H7N7, H9N2, H5N1, and H7N2. Now, a pandemic of avian influenza is occurring in Asia. Ducks are asymptomatic shedders, and are vectors in the transmission of avian influenza. Non-immunized humans may serve as intermediate hosts in a similar fashion. Strategies to intervene are:

1. Vaccination of chickens with H5N1, although they can still secrete virus.
2. Culling, although migratory birds and animals (ducks, pigs, cats, tigers) can still shed the virus.
3. Vaccination of cullers with human influenza vaccine.

Prevention and control will face many challenges. Vaccine production takes 6-8 months. Vaccine efficacy changes depending on the strains involved. Antivirals are limited in supply and high in cost.