Emerging Infectious Diseases

 

Lecture 2 and Sample Questions

François Elvinger

 

 

Lecture Objectives: Factors that promote the emergence of disease / disease agents populations. Exclusion and / or control of new disease agents from / in populations. Costs of emerging diseases. 

 

 

Operationally, disease emergence has been viewed as a two step procedure (S.S. Morse, EID 1995;1(1)):

 

1. Introduction of an agent into a new host group or population

2. Adoption, i.e. establishment and dissemination of the agent in the new host population.

 

The process by which these agents may be transferred across populations has been called ‘microbial traffic,’ defined as the ‘process by which infectious agents may transfer from animals to humans, or disseminated from isolated groups into new populations.’

 

This definition has been written from the perspective of human health. We need to consider ‘microbial traffic’ as movement of infectious agents among any populations, including for example plasmids (or other entities of genetic information) that are transferred among populations of microorganisms, as well as plant pathogens.

 

A variety of natural conditions and human activities promote microbial traffic. They include

-         changes in human and animal demographics, living / husbandry conditions; behavior. Categorized here would be population growth and migration due to economic or cultural changes, civil unrest and war, disease, poverty; sexual behavior and drug use.

-         ecological changes, including those due to economic development and land use through agricultural and industrial expansion, resulting pollution, global warming?

-         advances in technology and industry, including changes in food production, processing and distribution, advances in medical techniques (transplantation) and products (drugs).

-         increased global travel and commerce

-         microbial adaptation and change.

 

The effects of those changes are enhanced by inadequate public and animal health measures, not only in developing countries, but in developed countries as well (BSE has led to resignation of German health and agriculture ministers on January 9, 2001).

 

Some factors can be included into more than one of these categories. In particular you may want to consider the specifically ‘man-made’ factors, that are either intentional, as in biological warfare and bioterrorism, or unintentional, as would be the case if a highly contagious microorganism escapes from an isolation facility (research, clinical, manufacturing) of Biosafety level (BSL) 3 or 4, or if a new pathogen was introduced through xenotransplantation.

 

The reason for concern about emerging infections obviously is that we need to protect human and animal populations from their impact. Recognition of the problems that emerging diseases may cause is the first step in creating awareness and promoting control and or prevention of such diseases. The US DHHS Centers for Disease Control and Prevention in 1994 published a first bulletin on how to address emerging infectious diseases in the U.S., followed by a second bulletin on prevention strategies for the 21st century. The plan proposed by CDC established four goals:

 

Goal I:             Surveillance and Response

Detect, investigate, and monitor emerging pathogens, the disease they cause and the factors influencing their emergence, and respond to problems as they are identified.

 

Goal II:            Applied Research

Integrate laboratory science and epidemiology to optimize public health practice .

 

Goal III:            Infrastructure and Training

Strengthen public health infrastructures to support surveillance and research and to implement prevention and control programs.

 

Goal IV:            Prevention and Control

Ensure prompt implementation of prevention strategies and enhance communication of public health information about emerging diseases.

 

These goals geared towards human health issues can also be applied to animal and plant health, and activities associated with these goals should not only be directed towards infections that are emerging, but towards contemporary infectious disease problems as well. Prevention of emerging infectious disease in animal and plant populations hinges on the third goal, the presence and availability of an infrastructure that allows recognition and control of emerging disease threats. The 4 objectives listed by the CDC that are associated with the 3rd goal are: the enhancement of epidemiologic and laboratory capacity; the improvement of data and information flow to a central institution that has the capabilities to analyze incoming data and recognize emergence of threats; the enhancement of the nation’s capacity to respond to complex infectious disease threats in the U.S. and abroad, including outbreaks that may result from bioterrorism; and finally, providing training opportunities in infectious disease epidemiology and diagnosis in the U.S, and throughout the world.

 

Regarding animal health, more than 200 private, State, regional, and federal veterinary diagnostic laboratories are spread out across the United States. Diagnostic activities range from offering full services including pathologic, microbiologic, toxicologic, endocrinologic and metabolic investigational capabilities for all animal species to those that specialize in only a few selected tests often restricted to a single species. The creation of a network for sharing information is in its infancy and is hampered by technical and political challenges. Technical problems include varying diagnostic, recording and reporting standards, that do not allow or allow only to a limited extent sharing of information between laboratories. Political challenges include the fact that results from diagnostic investigations often are proprietary and that entities that submit diagnostic material oppose the sharing of information for fear of negative trade repercussions. (Restrictions on trade of animals and animal products have been imposed for reasons other than those that are epidemiologically and scientifically valid.)

 

As in human health, training of veterinary diagnosticians to recognize foreign or exotic diseases is crucial to prevent and reduce losses associated with introduction and spread of such diseases. Important is also that veterinarians in the field and diagnosticians recognize unusual presentation of diseases or clusters of cases that would indicate emergence of a novel disease or disease agent.

 

Losses attributable to emerging diseases may be financial, emotional and political. Financial losses in a year 2000 foot and mouth disease (a foreign animal disease, at this time in the U.S.) outbreak simulation exercise in Texas were estimated at $ 50 million for one county alone. Costs of such diseases are not limited to production losses of the infected cattle, but also include the significant costs that limitation of traffic and trade of animals and animal products from affected regions, States or countries entail. Emergence of a foreign animal disease like foot and mouth may also affect food security of a country. Speed of recognition is crucial for the rapid implementation of control measures such that losses to a country’s economy remain limited.

 

Protection of populations from emerging disease agents has been described in terms of a ‘discovery to control continuum’ (Murphy, Emerg.Infect.Dis., 1998) which proceeds in the following not necessarily consecutive and sometimes simultaneous steps: Discovery of the disease / agent; recognition of the novelty; epidemiologic field investigation to evaluate environment in which agent evolves and risk factors for infection of members of the population, as well as for transmission of the agent; etiologic investigation, once an agent has been detected and the disease may be reproduced; diagnostics development; focused research; technology transfer; training and outreach; control, elimination and / or eradication. Durations for moving along this continuum depend on the agent / disease and the severity of the losses (health, emotional, financial, production), which will determine the funding allocated to reach the last step, i.e. control, elimination and / or eradication.

 

Lecture 2 Sample Questions

 

1) Emergence of infectious diseases is promoted by

 

a)                 medical advances that make possible accidental introduction of infectious agents into naïve populations

 

b)                 microbial adaptation / selection that safeguards populations of infectious disease agents from ‘natural‘ eradication

 

c)            both a and b are correct

 

 

2) Microbial traffic consists of all of the following except

 

a)            introduction of novel infectious agent into a population

 

b)             genetic alteration of infectious agent

 

c)                  transfer of agents across species

 

 

3) The discovery to control continuum includes the following except:

 

a)                 transfer of the disease agent from an ‘old’ to a ‘new’ population

 

b)                 recognition of a novel agent following its discovery

 

c)                  determination of risk factors in order to recognize and design methods / tools for intervention

 

 

4) Financial losses of highly contagious emerging diseases are associated with:

 

a)            costs of quarantine / segregation

 

b)            lost trade

 

c)            both a and b are correct