Endemicity

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Endemicity

Introduction

An endemic disease is one that occurs naturally in a community. This is opposed to an epidemic disease, in which the rate of infection suddenly increases in a community. Endemicity can be measured by determining how common an infection is, or by determining the change in rates of infection over time.

The endemicity of a disease may be altered by a number of factors. Human intervention has led to many previously endemic diseases being eradicated from specific regions. This has been achieved by vaccination, as well as by the elimination of the cause of the disease, such as a vector (the organism that aids in the transmission of the disease).

However, endemic diseases can also develop in previously non-endemic regions, or can develop into epidemics. This often occurs when infections are introduced, undergo mutation, or when conditions within a community change due to events such as wars or natural disasters. The extent to which the change persists influences whether the change in endemicity will be long term.

History and Scientific Foundations

An endemic disease is one with a constant rate of infection in a community. When new individuals are born into that community, they become infected, cured, and eventually recover and retain the infection for life, or obtain immunity. Conversely, an epidemic occurs when a disease is introduced to a community and it multiplies, or when the rate of infection of an existing disease increases and causes an excess of cases in a community.

Endemicity can be measured by examining the prevalence rate or incident rate of a disease. (Prevalence refers to how common the disease is within a given community.) Some communities may have higher levels of endemicity, which indicates a higher prevalence of infection. The incident rate refers to the change in the level of infection over time. Many infections tend to show seasonal incident rates, in which the level of infection increases during certain periods. When the incident rate increases above a certain threshold, the disease becomes an epidemic, that is, the rate of infection causes an excess of cases.

Within a community, there can be “foci,” or areas of increased prevalence. Host focality refers to areas in which hosts have more severe infections than other hosts. For example, the infection schistosomiasis is characterized by infection with parasite eggs. Some hosts suffer heavier parasite loads due to more severe infections. When these heavier infections occur in specific areas, they form host foci. Geographic focality refers to a higher prevalence rate of the disease in certain regions. For example, malaria tends to show varying prevalence rates in urban versus rural regions.

The foci of a disease affect the treatment and eventual containment of the disease. If treatment methods aim to treat the entire community to the same extent, foci will maintain the infection. On the other hand, targeting foci will ensure that the infection is contained.

Applications and Research

Endemic diseases may not always remain endemic. In some cases, transmission of the disease may increase, causing the disease to become an epidemic. On the other hand, transmission of the disease may decrease, causing the number of cases in the community to go below an endemicity threshold. Many factors can influence the endemicity of a disease. Eradication techniques have played a major role in decreasing the endemicity of certain diseases in the world. Within the United States, measles, which was endemic prior to 1997, is no longer considered an endemic disease due to vaccination efforts. Similarly, malaria, which is still endemic in some regions of the world, has been eradicated from the United States and some western European countries following large scale eradication efforts.

WORDS TO KNOW

EPIDEMIC: From the Greek epidemic, meaning “prevalent among the people,” is most commonly used to describe an outbreak of an illness or disease in which the number of individual cases significantly exceeds the usual or expected number of cases in any given population.

FOCI: In medicine, a focus is a primary center of some disease process (for example, a cluster of abnormal cells). Foci is pleural for focus (more than one focus).

GEOGRAPHIC FOCALITY: The physical location of a disease pattern, epidemic, or outbreak; the characteristics of a location created by interconnections with other places.

HOST FOCALITY: Host focality refers to the tendency of some animal hosts, such as rodents carrying hantavirus and other viruses, to exist in groups in specific geographical locations, acting as a local reservoir of infection.

INCIDENCE: The number of new cases of a disease or injury that occur in a population during a specified period of time.

PREVALENCE: The actual number of cases of disease (or injury) that exist in a population.

The viral disease measles is highly communicable among humans. As a result, it is endemic in many regions of the world. In the United States, measles was once a common childhood disease with over 90% of children under the age of 12 infected. However, following the introduction of a measles vaccine in 1963, measles outbreaks have decreased. Aside from outbreaks occurring following introduction of the disease from other countries, measles no longer circulates in the United States. Vaccination is an effective way of increasing the immunity of a population and causing a decrease in the transmissibility of an infection. As a result, when most or all of a population is vaccinated against a certain disease, that disease does not retain its endemic state.

Vaccination is one eradication technique employed against infectious diseases. However, vaccinations have not been developed for all infectious diseases. As a result, other methods must be used to control some endemic diseases. Malaria is an example of an endemic infectious disease that cannot be controlled by vaccination. This disease is transmitted via mosquitoes, which infect new hosts when they feed on them. Eradication efforts involved spraying human living spaces with dichloro-diphenyl-tri-chloroethane (DDT), a toxic insecticide that kills mosquitoes (DDT was banned from use in most developing countries in the 1980s). This technique was designed to remove the mode of transmission for the disease (in this case, the mosquito vector), with the expectation that this would prevent the spread of the disease.

Malaria was once a major endemic infectious disease worldwide. However, since the late 1940s, malaria is no longer endemic in the United States nor in many countries of Western Europe. However, worldwide efforts to completely eradicate malaria have not been as successful. A variety of problems, such as mosquito tolerance to DDT, banning of DDT use, outbreaks of war, lack of funding, and population movements, have hindered efforts to eradicate malaria worldwide. Health authorities now attempt to control outbreaks of malaria, rather than to eradicate it completely.

Impacts and Issues

Endemicity can develop in countries in which the disease did not previously exist or only existed in low numbers. A disease may be introduced to countries with no history of the disease, and thus no immunity against it. Endemicity may develop as the disease spreads unchecked throughout the community. If transmission continues to infect an increasing number of people, the endemic disease may develop into an epidemic.

Endemicity may also develop when a disease that is usually only transmitted from animal to human, begins to be transmitted between humans, causing an increased rate of human infection. For example, avian influenza, or bird flu, tends to be predominantly spread between birds, and occasionally from bird to human. However, the virus that causes this disease may mutate, allowing it to be transmitted more easily between birds and humans, and, perhaps, between human hosts. Therefore, avian influenza is being closely monitored in order to keep it from becoming an endemic—and possibly—epidemic.

Not only can certain conditions cause a disease to become endemic, but some conditions may prompt an endemic disease to develop into an epidemic. Climate change and disasters, such as floods or wars, may cause changes that favor disease transmission. Some diseases, such as malaria, are dependent upon an arthropod vector to spread among hosts. A change in the climate, such as increased temperature or moisture, may be favorable to the vector, causing an increased number of vectors in a region. This increases the chance that a human will become infected. If a disease is already endemic in the region, an increase in the number of cases may result in an epidemic.

Disasters, such as war or floods, may also cause other more favorable conditions for a disease. For example, during war, or following a flood or earthquake, a large number of people are often required to live together in close quarters, often with only very basic sanitation. As a result, diseases are more easily transmitted. Airborne diseases benefit from the close proximity of people, orally transmitted diseases benefit from the poor sanitation conditions, and vector-borne diseases benefit from conditions that promote vector breeding. Therefore, diseases may erupt during these times. However, when people are allowed to return to their homes, conditions change again and may no longer favor the transmission of infectious diseases. This can cause the transmission rate to decrease and thus a disease may no longer be endemic or epidemic.

IN CONTEXT: ENDEMIC DISEASE AND THE PANAMA CANAL

Endemic diseases such as yellow fever, plague, and malaria had frustrated earlier French attempts to build a canal through the Isthmus of Panama by disabling and killing thousands of project workers and managers. Dr. William Crawford Gorgas (1854–1920), chief of sanitary affairs for the American project, made the canal possible by organizing public health and sanitation efforts. It was not ignorance of public health principles that had doomed earlier efforts to build the canal, but a lack of effective public health organization and the thorough implementation of disease control measures.

BIBLIOGRAPHY

Books

Arguin, P.M., P.E. Kozarsky, and A.W. Navin. Health Information for International Travel 2005–2006. Washington, DC: U.S. Department of Health and Human Services, 2005.

Nelson, Kenrad E., and Carolyn F. Masters Williams. Infectious Disease Epidemiology: Theory and Practice. 2nd ed. Sudbury, MA: Jones & Bartlett, 2007.

Webber, R. Communicable Disease Epidemiology and Control. New York: CABI Publishing, 2005.

Web Sites

Centers for Disease Control and Prevention. “Avian Influenza (Bird Flu).” June 30, 2006. <http://www.cdc.gov/flu/avian/gen-info/pdf/avian_facts.pdf> (accessed April 10, 2007).

Centers for Disease Control and Prevention. “Malaria.” April 23, 2004. <http://www.cdc.gov/malaria/index.htm> (accessed April 10, 2007).

Tony Hawas

Infectious Diseases: In Context