Diseases emerge, re-emerge in a cycle driven by worldwide environment

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Scott Norton, MD, MPH, MSc: “Every year, every month, every week new diseases … are emerging.”

Lost amid news of new biologic therapies, the migration of the nine-banded armadillo from Mexico across Texas and to the southeastern U.S. is not something the average dermatologist is tracking on Twitter. Yet, bacteria on those armadillos have been linked to nine new cases of leprosy in Florida — and is an example of the new paradigm of emerging and re-emerging diseases.

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“Leprosy seems to be a footnote in many medical textbooks but it may soon again be a well-recognized disease,” said Scott Norton, MD, MPH, MSc. “The disease isn’t changing. What is changing is the complex inter-relationship among bacterial pathogens, the host animals, the climate, and human influence on the environment. When we look at conditions that are considered emerging or re-emerging diseases, there are many reasons they may show up on our radar.”

Leprosy is just the latest example of an almost-forgotten disease re-emerging, just as other “new” diseases emerge to baffle public health officials. Dr. Norton discussed the cyclical nature of disease Friday in his Plenary presentation, “Emerging and Infectious Disease,” and in a separate interview.

“We may spend time talking about new lasers, or treating congenital lesions, or new treatments for psoriasis, but underlying skin disorders have been with us since time immemorial. But we are also seeing — every year, every month, every week — new diseases that are emerging,” said Dr. Norton, chief of dermatology at Children’s National Medical Center, Washington, DC, who also has worked with the Centers for Disease Control and Prevention on research into infectious diseases for 15 years.

One example of a new disease is AIDS, which gained notice in the early 1980s as it spread from the HIV infection, but is now controlled successfully with combinations of drugs. A second example is eosinophilia-myalgia syndrome, which raged for several years around 1990, but then disappeared almost as quickly as it started. A third recent disease is nephrogenic systemic fibrosis, which in the mid- to late-1990s destroyed skin, muscle, and internal organs, and in some cases was linked to deaths.

“People got very ill with a crippling muscle disorder, and some people even died of it,” Dr. Norton said of eosinophilia-myalgia syndrome. “It turned out to be caused by an obscure contaminant in a vitamin supplement. Nephrogenic systemic fibrosis, we now know, was caused by a gadolinium compound that was added to IV contrast used during MRI examinations.

“Some newly emerged diseases turn out to be among the most significant events in human culture, like the rise of HIV/AIDS. Other newly emerging diseases appear and then vanish within a few years. Some diseases, like chickenpox, mumps and measles, were once ubiquitous but may also vanish because of vaccines.”

Another class of disease are those “discovered,” even though they have been around for centuries.

“Many emerging diseases have been around forever but are just newly being recognized, identified, and described. Others have been in nonhuman animals for time immemorial — but are new arrivals in human populations,” Dr. Norton said. “Sometimes new diagnostic tests help us realize that what we once thought was a single disease may actually have several different causes.

“There are a lot of genetic conditions that we once thought were one condition, but now we know there are several genes that cause nearly identical phenotypes, or there are several different species of bacteria that can cause nearly identical diseases. On the flip side, one bacterium can cause several disease manifestations. Think about the protean expressions of Lyme disease and syphilis.”

The scientific and public health communities view the emergence or re-emergence of a variety of diseases not as random acts, but as being studied in the concept of One Health, which recognizes that the health of humans is inextricably connected to the health of animals and the environment.

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Dr. Norton: “One Health is a public health concept that is becoming more and more important.”

“One Health is a public health concept that is becoming more and more important,” Dr. Norton said. “To understand how a disease emerges or re-emerges, we can’t just look at a numerical scorecard of the number of humans who have a disease. We must look at where the pathogen is found in the environment, its animal reservoirs and insect vectors, and how it is transmitted. What is different in animal populations or their behaviors that allows disease to enter the human population?”

Although dermatologists may not treat patients with these diseases, they still have an impact on the specialty, he said. HIV was first recognized by dermatologists, and leprosy has for centuries been managed by dermatologists.

“It is false … it is a parochial notion to think of stovepipe, non-overlapping categories of disease. It is a false construct to think of pure skin diseases, or exclusively human diseases, or just tick-borne diseases. We live in a world that is a fabric where every strand has importance,” Dr. Norton said. “Diseases will rise and fall. A century ago, half of every dermatology practice was dedicated to syphilis and tuberculosis, but we don’t see a lot of those diseases now. They exist, but they have been controlled. The history of humankind shows that we strive to control diseases, but then, they can re-emerge.”

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