Emergence and Reemergence of Zoonotic Diseases

Emerging diseases are commonly defined as illnesses that have increased in incidence during the past two decades or are likely to increase in the near future. Many of these diseases are zoonotic.

A zoonotic disease can emerge as the result of increased human contact with the animal hosts, animal tissues, vectors, or environmental sources of the pathogens. It may also result from an increased prevalence of the agent in domesticated or wild animals or in vectors. Many currently emerging and reemerging diseases have reservoirs in wildlife or are foodborne.

Emerging Coronaviruses

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Factors that can cause emergence include altered human demographics or behavior, ranging from societal upheaval (eg, war) that cause humans to leave urban areas to simple changes in food preferences. For example, outbreaks due to enterohemorrhagic Escherichia coli (E coli O157:H7) have been linked to prewashed greens.

Breakdowns in public health measures such as sanitation and vaccination also increase the transmission of disease.

Changing land use patterns may alter the number of reservoir hosts, increase incidence of infection in these animals, encourage genetic changes in the pathogen (eg, recombination with other strains), or bring animal hosts or disease vectors into closer contact with humans. Because many mosquitoes preferentially breed along the edges of forests rather than deep among the trees, deforestation can increase their numbers, potentially resulting in increased exposure to some mosquito-transmitted diseases. Degradation of their natural habitats, as well as the ready availability of food near human dwellings, can encourage wildlife to move into suburban areas.

The growth of the human population also exerts pressures that ultimately result in increased contact with wildlife.

Changing climates can sometimes be a factor in disease emergence, particularly for arthropod-borne pathogens such as Rickettsia spp. A warmer climate not only allows vectors to survive the winter but also permits a longer transmission season.

Technological and industrial changes in food production can contribute to disease emergence by increasing the concentration, movement, and mixing of animals. Long-distance transport of animals has been associated with increased shedding of enteric pathogens, including Salmonella. Decreased genetic diversity may eliminate species, breeds, or individuals with innate resistance to a disease. The development of large-scale farms and food-processing facilities has led to the exposure of greater numbers of humans to a contaminated food source. Increased mobility of humans, animals, and goods allows diseases to spread quickly. Viruses that formerly died out after affecting small numbers of animals or humans can now find many susceptible hosts within a short period. Severe acute respiratory syndrome (SARS), for instance, spread to nearly 30 countries on 6 continents within months of the initial outbreak.

Occasionally, the pathogen itself may become more virulent or better adapted to humans, or it may undergo changes that affect transmission patterns.

Increased human susceptibility has also contributed to the emergence or recognition of some opportunistic pathogens. The number of immunocompromised humans has been increased by factors such as the AIDS epidemic, the success of organ transplantation programs, and advancements allowing those with primary and secondary immunodeficiencies to live longer. Modern medicine also allows more humans, many of whom develop chronic conditions, to survive to an advanced age.

Finally, some diseases are emerging not because they are more common but because they are better recognized. Increased recognition can result from improved diagnostic techniques, increased use of laboratories for identification of specific pathogens, and better awareness among physicians. Marburg virus, for example, was once thought to be a very rare and less virulent relative of ebolaviruses. However, it was later found to have caused hemorrhagic disease in workers at one African mine since the 1980s or earlier. This focus on bat-transmitted infections was only recognized when highly fatal outbreaks affected hundreds of humans in the Democratic Republic of the Congo between 1998 and 2000. Likewise, some species of Rickettsia are emerging, in part, because the increased use of molecular techniques facilitates their identification and allows them to be distinguished from similar organisms.