Emerging Infections of International Public Health Importance

Objectives:

To know the definition of an emerging infection
To be familiar with the paradigm of emergence, and the factors of emergence as described in the 1992 IOM Report
To understand the current state of global disease surveillance and alert systems generally, and the limits of such systems for describing emerging human pathogens.

This lecture is an introduction to the course of Emerging Infections of International Public Health Importance. We will be discussing the paradigm of factors contributing to the emergence or reemergence of infectious diseases world wide.

The Institute of Medicine published its groundbreaking report on the emergence of infections in 1992. This study group, chaired by Dr Joshua Lederberg, consisted of distinguished scientists involved in a broad variety of infectious disease research and care activities.

The Institute of Medicine report defines an emerging infection as a new, reemerging or drug resistant infection whose incidence in humans has increased in the past two decades or whose incidence threatens to increase in the near future.

This definition seems so broad that almost any infection could fit into this framework with the exception perhaps of syphilis. In this context however, are new infections such as HIV/AIDS, variant Creutzfeldt-Jacob disease and human herpes virus 8.

The paradigm of emergence brought forward in 1992 was new. The Institute of Medicine report states that “factors related to human activity are changing the ecology of the microbial world. These changes are affecting the threat to humans of new and emergent infectious diseases.” This report made a link between the macro-changes and ecology that we are seeing in the physical world and the micro world.

We will talk about each of these factors throughout this course and describe more fully how the factors work alone or in combination to foster the emergence of new disease. Infectious disease as a scientific or medical discipline was, in itself, reemerging in 1992. Since the advent of antibiotics, human infection had been perceived to be controlled or controllable. Commitment to infectious disease prevention, investigation and control in public health departments at the state level, at the federal level, and at the international level in developed countries had waned. Infectious diseases were seen as a thing of the past. Complacency had replaced vigilance.

However, this figure looks at "Deaths by Cause World Wide" in 1992, infectious and parasitic diseases remain the major cause of death. This has become even more true with the advent of AIDS deaths.

The emergence of infectious diseases is a global phenomenon. This slide shows the unexpected outbreaks that occurred between 1994 and 1999. This is not a phenomenon that is confined to poor countries. As this slide shows there is broad range of outbreak activity related to the increased incidence of infectious diseases in an unexpected manner. The truly new diseases shown on this out break map include Ebola hemorrhagic fever in Africa, Nipah virus and Hendra virus in Malaysia, Avian influenza in Hong Kong, Variant Creutzfeldt-Jacob disease in the United Kingdom, and West Nile virus in the United States. The remaining outbreaks shown on the map are occurring because infectious disease has changed its ecology and its range of infection within the human community. These agents that are old are being manifested in new populations or new age groups and with an overall increase in their incidence.

Globalization has been one factor in infectious disease emergence and reemergence.

This figure shows the urban population growth. The current population is larger than ever before and is growing more rapidly in developing countries than developed countries. Mega-cities are being created in many of these lesser developed countries which presents many challenges to the existing public health infrastructure. In many places, the infrastructure was inadequate before the population growth. Urban growth has contributed to spread of many diseases such as HIV/AIDS, other sexually transmitted disease, and dengue fever.

There has always been an uneven access to water and sanitation around the world. This is an old slide from 1990 which shows the gap at about 2 billion people without sanitation and a gap of nearly 1.5 billion people without clean water. These gaps remain and in fact, has probably grown larger although no recent studies are available.

This figure shows the international trade and agricultural products over the last thirty years. More than six trillion dollars worth of merchandise cross international borders on an annual basis in the world and much of the increase has been in agricultural products such as fish, live animals, fruits, vegetables and dairy products. The health implications are tremendous. Fresh produce is not easily inspected and assured to be safe when it comes from areas where it is grown with inadequate sanitation and access to clean water. Likewise live animals being shipped across borders can often harbor pathogens and introduce them into new populations. The beef trade, of course, has been implicated in the global transmission of vCJD.

If you look at some of the major global epidemics of new emergent diseases you see how the factors of emergence act together. For example, in HIV the factors of urbanization have changed behavior; transportation has transmitted infection into the interior of many countries; and air transportation has introduced infection from one continent to the other. Technology and trade have been implicated in spreading HIV by the manufacture and export of factor-VIII in the early 1980s. Drug-resistant tuberculosis has been transmitted and introduced into new populations through travel and changes in the microbe itself as it acquires drug resistance. Dengue epitomizes the factor interaction between travel, transportation and technology.

Historically epidemics have often followed routes of transportation and trade. This is the global spread of cholera between 1961 and 1991 in this figure by CDC.

It is believed that epidemic cholera was introduced to the Americas in 1991 through the dumping of contaminated bilge of a tanker from Indonesia into the waters off of Lima Harbor. This slide shows the epidemic as it progressed between 1991, 1992 and 1994 throughout the Americas.

This epidemic curve in 1991 demonstrates the initial epidemic which occurred in Peru. About one hundred thousand cases of cholera were reported.

By 1993 cholera had traveled throughout Latin America as shown here. It was able to do so because it was abetted by transportation and travel of peoples among countries and also by poor public health infrastructure particularly in urban centers.

Dengue also has several factors which have combined and contributed to its current hyperendemicity. The mosquito vectors can travel by barge or in wheel wells of aircraft. The dengue serotypes can travel in human hosts.

This combination of movement of the vector and the serotype of the virus has resulted in this global picture which we call Hyperendemicity where all 4 serotypes of Dengue, Dengue type 1-4, are present in all tropical zones all around the world.

This is the clinical picture of dengue hemorrhagic fever. This is occurring more frequently in persons who are infected with more than one serotype.

For example, in the 1990s Nicaragua called the World Health Organization for help in identifying an agent that was causing a great deal of morbidity and mortality in their population. It turned out that the agent was Leptospirosis. This agent is relatively easy to diagnose and was not new. However, in Nicaragua the capacity to identify it was missing. Syndromic surveillance can certainly be helpful and this is the method that has largely been adopted in outbreak reports to the World Health Organization.

In summary we can say that infectious agents are resilient, highly adaptable to both the micro and macro environment. Humans have facilitated changes in both of these environments and created a threat to ourselves and the global ecology.

This is a final quote from the Institute of Medicine Report and I think as we go through this course you will begin to understand how this increased insight into the paradigm of emergence and the factors of emergence have combined to bring us new microbial threats and you will begin to understand how the world is combating these threats through surveillance and intervention and where the challenges still lie.

Study Questions:

In what ways did the Emerging Infections paradigm represent a new way of thinking about infectious diseases?
What are three key concepts in doing surveillance for the unknown agent?
Give one example of a disease which has changed its epidemiology due to the action of factors of emergence and discuss how the factors affected the disease entity.