SYMPOSIUM


are found again after elimination.11 Reemergence may relate to change in host range of an infectious agent (e.g. monkey poxvirus and Coronaviruses) or because the organism has undergone alteration, allowing it to be human adaptive as seen in influenza virus. The burden caused by infectious diseases is greatest in low- income countries and regions with reduced socioeconomic level with greater contact with wild animals, making these emerging disease hotspots.12 Many of the infectious


agents


considered emerging produce chronic syndromes13


such as stomach and


cervical cancer, leukemia, Guillain- Barré syndrome, chronic liver diseases, paralytic disease, post-infectious functional bowel disease, and renal disease. Persistent or chronic disease after infections should be sought by careful follow-up and undoubtedly will be seen more commonly than appreciated. Cancers, autoimmune, end organ failure, and gastrointestinal and neurodevelopmental disorders are candidates for explorative studies focusing on basic and epidemiologic research.


In understanding emerging in- fectious disease outbreaks, it is not possible to focus only on the human host. Four integrated factors simulta- neously contribute to infectious dis- ease outbreaks: animals and zoonotic pathogens, microbes in transition, an increasingly susceptible human popu- lation, and climate and environment. (See Figure 1.)


ANIMALS AND ZOONOSES


Zoonoses are diseases and infections transmitted between vertebrate ani- mals and humans. More than 1 billion cases, or two-thirds of all infections of humans, originate in animals14 are referred to as zoonoses.15


and Zoonotic


infectious agents are twice as likely to spread to humans as are non-zoonotic pathogens. Three bacterial organisms of ani- mal origin are currently threatening


32 TEXAS MEDICINE February 2017


our food supply — Shiga toxin-pro- ducing Escherichia coli, Campylo- bacter jejuni, and Salmonella spp.16 The animal reservoirs of human viruses that have the greatest poten- tial for emergence include rodents and ungulates (horses, deer, camels, etc.), primates, carnivores, and bats and birds. Typically, the pathogens do not cause disease in the infected or colonized animals. An important step in emergence for viruses is jump- ing between one host animal species and humans.14,17


More than 60 percent


of the newly identified viruses have been discovered in North America and Europe, not the regions of origin, because of better diagnostic capabil- ity.12


As examples of animal transmis-


sion of viruses, the 1998 Hendra virus outbreak in Australia was traced to infected horses seeking shelter under trees in which naturally infected bats roosted.18


and can lead to later endogenous in- fection. These are especially seen in the hospital setting where the Centers for Disease Control and Prevention (CDC) estimated for 2002 that 1.7 mil- lion health care infections occurred in the United States with a health care cost of $20 billion.8 Microbial mutation allows patho-


gens to better survive but may fa- cilitate disease occurrence. Mutating RNA viruses explain one-third of all emerging and reemerging infections,19 and mutation of M proteins in strains of Streptococcus pyogenes in the 1980s led to outbreaks of invasive Group A Streptococcus infections in Europe and the United States.22


THE HUMAN HOST IN PROMOTING INFECTIOUS DISEASES


The fastest growing U.S. populations are the elderly and infirm23


who are Nipah virus caused viral en-


cephalitis in Bangladesh, and Malay- sia was similarly associated with fruit bats infecting people picking fruit.19 Animal pathogens can be spread


by direct contact via wounds, sexual contact, vertical routes from a moth- er to fetus, or the respiratory tract; indirectly through ingestion of con- taminated food or from environmen- tal surfaces; and by a biting insect.20 Zoonotic infectious organisms show highest risk of transmission when acquired directly and indirectly com- pared with exposure though the vec- tor-borne route.


THE MICROBE IN EVOLUTION


The list of emerging and reemerg- ing infectious diseases is long and includes bacteria, viruses and prions, and parasites. They have been catego- rized into groups A, B, and C depend- ing upon their risk as agents of bioter- rorism.21 Bacterial pathogens are becoming


more resistant in the face of wide- spread use of antibiotics, complicat- ing


therapy. During antimicrobial


therapy of an infection, the gut micro- biota develops resistance to the drug


more susceptible to infectious diseas- es with their reduced efficiency of the immune system. The behavior of peo- ple also contributes to transmission of infectious microorganisms through urbanization and increased interna- tional travel and by purchasing im- ported foods in increasing amounts. In our megacities, health conditions may be reduced due to crowding and more efficient spread of communica- ble infectious agents.24


Urban centers


where 3.9 billion people live are the likely settings for new infectious dis- ease epidemics.25


About 1.1 billion people crossed international boundaries in 2014,26 efficiently carrying


their personal


microbes with them and picking up new ones. International air travel is an important facilitator of pandemic influenza by transporting infected people who will enter susceptible populations.27


The most graphic ex-


ample of the efficiency of global spread of a microbial pathogen was in 2003 when the SARS coronavirus spread from China within weeks to 25 coun- tries across five continents.28


In 2014,


Ebola virus travel from West Africa to the United States by air, during which


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