Epidemiology (derived from the Greek epi or upon, demos meaning people, and logos as a field of study) is the study of how diseases are distributed and caused in populations. Epizoology is the study of disease patterns in animals.
An epidemic is a disease that appears as a new case in a population, in a time period, and at a rate that exceeds the expected rate of infection, based on what is known of the disease. An epizootic would be the equivalent in animal populations.
An epidemic, by definition, is a subjective call, as it is based on how a disease acts in relation to what is known of the disease. Epidemics can occur in a small area, a general area, or may be continental or global in nature. In these cases, they would be referred to as an outbreak, epidemic, or pandemic, respectively. With this in mind, a rabies epidemic may be experienced with only a few cases, while multiple cases of a cold may not be, as it is a common dilemma.
A disease that is found within a population is classified as "endemic" or "enzootic". Chronic Wasting Disease is said to be endemic to some areas, as it is common to find a certain portion of the wildlife population with the disease.
Where people are concerned, the World Health Organization, states that a pandemic can start when these three conditions are met:
- a disease is introduced that is new to a population.
- people are infected and become seriously ill.
- the disease spreads easily and is sustainable in people.
There have been a number of significant pandemics in history, including:
- Peloponnesian War, 430 BC. Typhoid fever killed a quarter of the Athenian troops and a quarter of the population over four years.
- Antonine Plague, 165–180. Possibly smallpox brought back from the Near East; killed up to five million in all.
- Plague of Justinian, started 541. The first recorded outbreak of the bubonic plague. It started in Egypt and reached Constantinople the following spring, killing a quarter of the human population of the eastern Mediterranean.
- The Black Death, started 1300s. Starting in Asia, the disease reached Mediterranean and western Europe and killed twenty million Europeans in six years.
- Cholera with eight pandemic episodes in the 1800s and 1900s, in which millions have died.
- Influenza, including "Asiatic Flu", "Spanish Flu", "Asian Flu", and "Hong Kong Flu" beginning in the 1500s and continuing through today have killed millions of people.
- Typhus, a disease that spread rapidly during wars and in military prisons.
- Colonization diseases such as smallpox have killed millions of people who had no resistance.
Potential epi- or pandemics
Ebola virus or other highly contagious and deadly diseases have a theoretical potential to become pandemics. These diseases spread quickly, but require close contact with an infected vector for transmission.
Antibiotic-resistant diseases such as tuberculosis have arisen from what was thought a conquered problem. Hospital infections are an increasing area of concern. Many enter the hospital for one treatment and catch unrelated infections, equating to more than 90,000 deaths in the US each year.
HIV is the virus that causes AIDS and is considered a worldwide pandemic. HIV infection rates in some areas are as high as 25% and are rising in Asia and the Americas.
Quick work by the World Health Organization checked the spread of SARS prior to becoming a pandemic. The disease is still present and continual monitoring is prudent to detect spread.
Since February 2004, avian influenza virus has been a constant source of concern for health and wildlife officials worldwide. It was first detected in birds in Vietnam and has subsequently been detected in domestic and wild birds in most of the world outside of the Americas. The fear is that the influenza strain infecting birds will combine with a strain that infects people. A new strain could cause a pandemic similar to "Asiatic Flu", potentially killing millions of people. There have been cases of people dying of H5N1, but the virus has not been able to sustain itself for easy spread from person to person. To date, this virus requires close, prolonged contact with infected patients.
Potential epi- or panzootics
Again, an epizootic is the wildlife equivalent of an epidemic. West Nile Virus is an epizootic that infects primarily mosquitoes and birds, but occasionally other hosts such as horses and people.
Yersinia pestis causes the plague enzootic which is a vectorborne disease infecting fleas, rodents (i.e., prairie dogs, chipmunks) and predators (i.e., black-footed ferrets, coyotes), but can also be transmitted to humans. In wildlife the plague is referred to as sylvatic plague. Infection in people usually occurs in the form of bubonic plague (lymph node infection), but can be septicemic (infection of the blood and organs) or pneumonic (infection of the lungs).
Hantavirus pulmonary syndrome (HPS) is carried by New World rats and mice of the family Muridae, subfamily Sigmodontinae. While there are numerous hantavirus, some are pathogenic for humans.
As with epidemics, a major contributing factor in epizootics is high population density. As population increases, so does the potential for animal-to-animal contact, which favors disease spread. As can also be seen, the more contact there is between wildlife and people, the potential for disease spread to the human population increases. (Sources the Centers for Disease Control and Wikipedia)
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