Emergent Diseases (Human)

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Emergent diseases (human)

Although many diseases such as measles, pneumonia, and pertussis (whooping cough) have probably inflicted humans for millennia, at least 30 new infectious diseases have appeared in the past two decades, In addition, many well-known diseases recently have reappeared in more virulent or drug-resistant forms. An emergent disease is one never known before or one that has been absent for at least 20 years. Ebola fever is a good example of an emergent disease. A kind of viral hemorrhagic fever, Ebola is extremely contagious and often kills up to 90% of those who are exposed to it. The disease was unknown until about 20 years ago, but is thought to have been present in monkeys or other primates. Killing and eating chimps, gorillas, and other primates is thought to be the route of infection in humans. AIDS is another disease that appears to have suddenly moved from other primates to humans. How pathogens suddenly move across species barriers to become highly contagious and terribly lethal is one of the most important questions in environmental health .

Some of the most devastating epidemics have occurred when travelers bring new germs to a naïve population lacking immunity. An example was the plague , or Black Death, which swept through Europe and Western Asia repeatedly in the fourteenth and fifteenth centuries. During the first- and worst-episode between 1347 and 1355, about half the population of Europe died. In some cities the mortality rate was as high as 80%. It's hard to imagine the panic and fear this disease caused. An even worse disaster may have occurred when Europeans brought smallpox, measles, and other infectious diseases to the Americas. By some calculations, up to 90% of the native people perished as diseases swept through their population. One reason European explorers thought the land was an empty wilderness was that these diseases spread out ahead of them, killing everyone in their path.

Probably the largest loss of life from an individual disease in a single year was the great influenza pandemic of 1918. Somewhere between 30 and 40 million people succumbed to this virus in less than 12 months. This was more than twice the total number killed in all the battles of World War I, which was occurring at the time. Crowded, unsanitary troop ships carrying American soldiers to Europe started the epidemic. War refugees, soldiers from other nations returning home, and a variety of other travelers quickly spread the virus around the globe. Flu is especially contagious, spreading either by direct contact with an infected person or by breathing airborne particles released by coughing or sneezing. Most flu strains are zoonotic (transmitted from an animal host to humans). Pigs, birds, monkeys, and rodents often serve as reservoirs from which viruses can jump to humans. Although new flu strains seem to appear nearly every year, no epidemic has been as deadly as that of 1918.

Malaria , the most deadly of all insect-borne diseases, is an example of the return of a disease that once was thought nearly vanquished. Malaria now claims about 3 million lives every year—90% in Africa and most of them children. With the advent of modern medicines and pesticides, malaria had nearly been wiped out in many places but recently has had a resurgence. The protozoan parasite that causes the disease is now resistant to most antibiotics, while the mosquitoes that transmit it have developed resistance to many insecticides. Spraying of DDT in India and Sri Lanka reduced malaria from millions of infections per year to only a few thousand in the 1950s and 1960s. Now South Asia is back to its pre-DDT level of some 2.5 million new cases of malaria every year. Other places that never had malaria or dengue fever now have them because of climate change and habitat alteration. Gulf-coast states in America, for example, are now home to the Aedes aegypti mosquito that carries these diseases.

Why have vectors such as mosquitoes and pathogens such as the malaria parasite become resistant to pesticides and antibiotics? Part of the answer is natural selection and the ability of many organisms to evolve rapidly. Another factor is the human tendency to use control measures carelessly. When we discovered that DDT and other insecticides could control mosquito populations, we spread them indiscriminately without much thought to ecological considerations. In the same way, antimalarial medicines such as chloroquine were given to millions of people, whether they showed symptoms or not. This was a perfect recipe for natural selection. Many organisms were exposed only minimally to control measures. This allowed those with natural resistance to outcompete others and spread their genes through the population. After repeated cycles of exposure and selection, many microorganisms and their vectors are insensitive to almost all our weapons against them.

There are many examples of drug resistance in pathogens. Tuberculosis (TB), once the foremost cause of death in the world, had nearly been eliminated—at least from the developed world—by the end of the twentieth century. Drug-resistant varieties of TB are now spreading rapidly, however. One of the places these strains arise is in Russia, where crowded prisons with poor sanitation , little medical care, gross overcrowding, and inadequate nutrition serve as a breeding ground for this deadly disease. Inmates who are treated with antibiotics rarely get a complete dose. Those with TB aren't segregated from healthy inmates. Patients with active TB are released from prison and sent home to spread the disease further. And migrants from Russian carry the disease to other countries.

Another development is the appearance of drug-resistant strains of Staphylococcus aureus, the most common form of hospital-acquired infections. Staph A has many forms, some of which are extremely toxic—toxic-shock syndrome is one where staphylococcus toxins spread through the body and sometimes bring death in a matter of just hours. Another strain of staphylococcus, sometimes called flesh-eating bacteria, causes massive necrosis (cell death) that destroys skin, connective tissue, and muscle. For 40 years vancomycin has been the last recourse against staph infections. Strains resistant to everything else could be controlled by this antibiotic. Now vancomycin-resistant staph strains are being reported in many places.

A number of factors contribute currently to the appearance and spread of these highly contagious diseases. With 6 billion people now inhabiting the planet, human densities are much higher, enabling germs to spread further and faster than ever before. Expanding populations push into remote areas encountering new pathogens and parasites . Environmental change on a larger scale, such as cutting forests, creating unhealthy urban surroundings, and causing global-climate change, among other things, eliminates predators and habitat changes favor disease-carrying organisms such as mice, rats, cockroaches, and mosquitoes.

Another important factor in the spread of many diseases is the speed and frequency of modern travel. Millions of people go every day from one place to another by airplane, boat, train, or automobile . Very few places on earth are more than 24 hours by jet plane from any other place. Many highly virulent diseases take several days for symptoms to appear.

Humans aren't the only ones to suffer from new and devastating diseases. Domestic animals and wildlife also experience sudden and widespread epidemics, sometimes called emergent ecological diseases .In 1998, for example, a distemper virus killed half the seals in Western Europe. It's thought that toxic pollutants and hormone-disrupting environmental chemicals might have made seals and other marine mammals susceptible to infections. In 2002, more dead seals were found in Denmark, raising fears that distemper might be reappearing.

Chronic wasting disease (CWD) is spreading through deer and elk populations in the North America. Caused by a strange protein called a prion, CWD is one of a family of irreversible, degenerative neurological diseases known as transmissible spongiform encephalopathies (TSE) that include mad cow disease in cattle, scrapie in sheep, and Creutzfelt-Jacob disease in humans. CWD probably started when elk ranchers fed contaminated animal by-products to their herds. Infected animals were sold to other ranches, and now the disease has spread to wild populations. First recognized in 1967 in Saskatchewan, CWD has been identified in wild deer populations and ranch operations in at least eight American states. No humans are known to have contracted TSE from deer or elk, but there is a concern that we might see something like the mad cow disaster that inflicted Europe in the 1990s. At least 100 people died, and nearly five million European cattle and sheep were slaughtered in an effort to contain that disease.

One of the things all these diseases have in common is that human-caused environmental changes are stressing biological communities and upsetting normal ecological relationships.

[William P. Cunningham Ph.D. ]