Unit 9 Notes - TindallLand



Unit 25 Notes

Emerging and Re-emerging Diseases

What Are Infectious Diseases?

Infectious diseases are human illnesses caused by viruses, bacteria, parasites, fungi and other microbes. They may be spread by direct contact with an infected person or animal, by ingesting contaminated food or water, by insects like mosquitoes or ticks (disease vectors), or by contact with contaminated surroundings like animal droppings or even contaminated air.

A Problem That Won’t Go Away

With the advent of antibiotics 50 years ago, scientists made sweeping predictions heralding the end of death and suffering from infectious diseases. During the past 25 years, however, microbes have demonstrated their tremendous ability to adapt, survive and challenge us anew. Once thought almost eliminated as a public health problem, infectious diseases remain the leading cause of death worldwide. In 1996, infectious diseases killed about one third of the more than 52 million people who died that year.

In the United States, two of the ten leading causes of death are infectious diseases (HIV and pneumonia/ influenza). The Centers for Disease Control and Prevention (CDC) reports that 160,000 Americans die each year with an infectious disease as the underlying cause of death. Ranging from childhood ear infections to measles to sexually transmitted diseases (STDs), infectious illnesses account for 25% of all physician visits each year, and antimicrobial agents are second only to pain relievers as the most frequently prescribed class of drugs.

Anticipating and coping with these microbial threats requires vigilance. We must maintain global surveillance and a strong public health infrastructure with state-of-the-art laboratories and solid partnerships with colleagues in medical, scientific, and policy arenas. Research and creativity are crucial, as is targeted public education at all levels of society to assure a well- informed public. Knowing that local threats can balloon into national or global problems, partnerships must be formed at all levels to develop both local and global prevention strategies.

Societal costs of infectious diseases are staggering. In the United States, treatment of non-AIDS STDs alone costs $5 billion annually. The yearly price tags of other infectious diseases are $30 billion for intestinal infections, $17 billion for influenza, $1 billion for salmonella, and $720 million for Hepatitis B. Altogether, the cost of treatment and lost productivity associated with illness from infectious agents tops $120 billion each year.

Emerging & Re-emerging Diseases

Although some infectious diseases, such as polio, have been nearly wiped out, the vast majority of these diseases will not be eliminated in our lifetime. Indeed, the World Health Organization reports that at least 30 new diseases have been scientifically recognized around the world in the last 20 years. These emerging diseases include sin nombre hantavirus, first identified in the US in 1993; cryptosporidiosis (a water-borne cause of diarrhea that recently affected more than 400,000 people in a single outbreak in the U.S.); the Ebola virus from Africa; and HIV. Some infectious diseases once thought to be under control are also reemerging. Diseases like tuberculosis, cholera, and even diphtheria are making a comeback.

Why Are Infectious Diseases Emerging and Reemerging?

Many factors are making it easier for infectious diseases to become an even bigger problem in the future.

• Microbe Mutation - The genetic makeup and reproductive ability of many infectious agents allows them to mutate or evolve into more deadly strains against which humans have little resistance. Microbes, because they reproduce so quickly, also mutate quicker than humans. Because our immune system is specific to a specific microbe, every time the microbe mutates, our immune system has to start over producing antibodies.

Also, misuse and overuse of antibiotics have eroded the ability of once-dependable antibiotics to fight common infections. Many microorganisms have become resistant to our most powerful modern drugs. Likewise, disease-carrying insects are becoming resistant to pesticides. Microbes, like all other living things, are constantly evolving. Pathogens can also acquire new antibiotic resistance genes from other, often nonpathogenic, species in the environment.

Many viruses show a high mutation rate and can rapidly evolve to yield new variants. A classic example is influenza. On rare occasions, perhaps more often with nonviral pathogens than with viruses, the evolution of a new microbe may result in a new expression of disease.

• Mass migrations - Refugees bring infectious diseases into new areas. Human population movements or upheavals, caused by migration or war, are often important factors in disease emergence. In many parts of the world, economic conditions are encouraging the mass movement of workers from rural areas to cities. The United Nations has estimated that, largely as a result of continuing migration, by the year 2025, 65% of the world population (also expected to be larger in absolute numbers), including 61% of the population in developing regions, will live in cities. Rural urbanization allows infections arising in isolated rural areas, which may once have remained obscure and localized, to reach larger populations. Once in a city, the newly introduced infection would have the opportunity to spread locally among the population and could also spread further along highways and interurban transport routes and by airplane. Even in industrialized countries, such as the United States, infections such as tuberculosis can spread through high-population density settings (.g., day care centers or prisons)

• Increase in Travel - Global travelers visiting exotic areas bring new diseases home with them. In the past decade, international travel has grown dramatically to the point where more than 500 million travelers cross international borders annually by commercial aircraft alone. Mass migrations of refugees, workers, and displaced persons have led to a steady growth of urban centers at the expense of rural areas. These population movements have been ideal conduits for the global spread of new and reemerging infectious diseases. Virtually any place in the world can be reached within 36 hours, less than the incubation period for most infectious diseases. Infectious agents can spread from person to person directly or to vectors at the traveler's destination. Vehicles of human transport, such as aircraft and ships, also transport the infectious agent and its vector. In addition, mass migrations have facilitated the rapid and immediate spread of communicable disease among refugees and displaced persons.

If travelers are responsible for the spread of infectious agents, they are also ideal sentinels for the arrival of an infectious agent in a new community. In 1969, the first documented outbreak of Lassa fever was noted among American missionaries in Lagos, Nigeria. In 1992, two Peace Corps volunteers contracted neuroschistosomiasis; a subsequent investigation confirmed that Lake Malawi was an important source for the transmission of Schistosoma haematobium. More recently, chloroquine-resistant Plasmodium vivax was documented for the first time in North Africa in U.S. Army troops returning from Somalia.

These and many other examples show that travel is instrumental in the spread of new and reemerging infectious diseases. As international travel grows by more than 10% per year and major population shifts continue because of political, economic, and social instability, public health agencies will have to focus their infectious disease surveillance programs on travelers, migrants, and the vehicles of transport. Recognition that travelers are important sentinels for emerging infectious diseases has stimulated the International Society of Travel Medicine and the Centers for Disease Control and Prevention to carry out a joint surveillance project, Geosentinel, in 22 travel clinics around the world.

• Population Growth and Crowding - Growth of congested urban slums, lacking sanitation and clean water, result in large outbreaks of infections spread by food, water and environmental factors. Institutional settings such as child care centers and hospitals, provide an ideal environment for transmission of infectious diseases because they bring susceptible individuals into close daily contact.

• Ecological and Climate Changes - Population shifts and urbanization disturb natural habitats and increase human contact with remote environments and poorly understood ecosystems that hide many unknown and dangerous microorganisms. Ecological changes, including those due to agricultural or economic development, are among the most frequently identified factors in emergence. They are especially frequent as factors in outbreaks of previously unrecognized diseases with high case-fatality rates, which often turn out to be zoonotic (or animal-related) introductions. Ecological factors usually precipitate emergence by placing people in contact with animals that carry disease. The emergence of Lyme disease in the United States and Europe was probably due largely to reforestation, which increased the population of deer and the deer tick, the vector of Lyme disease. The movement of people into these areas placed a larger population in close proximity to the vector.

Agricultural development, one of the most common ways in which people alter and interpose themselves into the environment, is often a factor. Hantavirus, the cause of Korean hemorrhagic fever, causes over 100,000 cases a year in China and has been known in Asia for centuries. The virus is a natural infection of the field mouse. The rodent flourishes in rice fields; people usually contract the disease during the rice harvest from contact with infected rodents. Junin virus, the cause of Argentine hemorrhagic fever, is an unrelated virus with a history remarkably similar to that of Hanta virus. Conversion of grassland to maize cultivation favored a rodent that was the natural host for this virus, and human cases increased in proportion with expansion of maize agriculture. Other examples, in addition to those already known, are likely to appear as new areas are placed under cultivation.

• Changes in Human Behavior - human sexual behavior and substance abuse expedite the spread of infectious agents. Human behavior can have important effects on disease dissemination. The best known examples are sexually transmitted diseases, and the ways in which such human behavior as sex or intravenous drug use have contributed to the emergence of HIV are now well known. Other factors responsible for disease emergence are influenced by a variety of human actions, so human behavior in the broader sense is also very important. Motivating appropriate individual behavior and constructive action, both locally and in a larger scale, will be essential for controlling emerging infections. Ironically, as AIDS prevention efforts have demonstrated, human behavior remains one of the weakest links in our scientific knowledge.

• Inadequacy of Public Infrastructure - Classical public health and sanitation measures have long served to minimize dissemination and human exposure to many pathogens spread by traditional routes such as water or preventable by immunization or insect control. The pathogens themselves often still remain, but in reduced numbers, in reservoir hosts or in the environment, or in small pockets of infection and, therefore, are often able to take advantage of the opportunity to reemerge if there are breakdowns in preventive measures.

Reemerging diseases are those, like cholera, that were once decreasing but are now rapidly increasing again. These are often conventionally understood and well recognized public health threats for which (in most cases) previously active public health measures had been allowed to lapse, a situation that unfortunately now applies all too often in both developing countries and the inner cities of the industrialized world. The appearance of reemerging diseases may, therefore, often be a sign of the breakdown of public health measures and should be a warning against complacency in the war against infectious diseases.

Cholera, for example, has recently been raging in South America (for the first time in this century) and Africa. The rapid spread of cholera in South America may have been abetted by recent reductions in chlorine levels used to treat water supplies. The success of cholera and other enteric diseases is often due to the lack of a reliable water supply. These problems are more severe in developing countries, but are not confined to these areas. The U.S. outbreak of waterborne Cryptosporidium infection in Milwaukee, Wisconsin, in the spring of 1993, with over 400,000 estimated cases, was in part due to a nonfunctioning water filtration plant; similar deficiencies in water purification have been found in other cities in the United States.

• Technology and Industry - High-volume rapid movement characterizes not only travel, but also other industries in modern society. In operations, including food production, that process or use products of biological origin, modern production methods yield increased efficiency and reduced costs but can increase the chances of accidental contamination and amplify the effects of such contamination. The problem is further compounded by globalization, allowing the opportunity to introduce agents from far away. A pathogen present in some of the raw material may find its way into a large batch of final product, as happened with the contamination of hamburger meat by E. coli strains causing hemolytic uremic syndrome. Bovine spongiform encephalopathy (BSE), or mad cow disease, which emerged in Britain within the last few years, was likely an interspecies transfer of scrapie from sheep to cattle that occurred when changes in rendering processes led to incomplete inactivation of scrapie agent in sheep byproducts fed to cattle.

The concentrating effects that occur with blood and tissue products have inadvertently disseminated infections unrecognized at the time, such as HIV and hepatitis B and C. Medical settings are also at the front line of exposure to new diseases, and a number of infections, including many emerging infections, have spread nosocomially in health care settings. Among the numerous examples, in the outbreaks of Ebola fever in Africa many of the secondary cases were hospital acquired, most transmitted to other patients through contaminated hypodermic apparatus, and some to the health care staff by contact. Transmission of Lassa fever to health care workers has also been documented.

Table 1. Recent examples of emerging infections and probable factors in their emergence

____________________________________________________________________________

Infection or Agent Factor(s) contributing to emergence

____________________________________________________________________________

Viral

Argentine, Bolivian Changes in agriculture favoring rodent host

hemorrhagic fever

Bovine spongiform Changes in rendering processes

encephalopathy (cattle)

Dengue, dengue Transportation, travel, and migration; urbanization

hemorrhagic fever

Ebola, Marburg Unknown (in Europe and the United States, importation of monkeys)

Hantaviruses Ecological or environmental changes increasing contact with rodent hosts

Hepatitis B, C Transfusions, organ transplants, contaminated hypodermic apparatus, sexual transmission, vertical spread from infected mother to child

HIV Migration to cities and travel; after introduction, sexual transmission, vertical spread from infected mother to child, contaminated hypodermic apparatus (including during intravenous drug use), transfusions, organ transplants

HTLV Contaminated hypodermic apparatus, other

Influenza (pandemic) Possibly pig-duck agriculture, facilitating reassortment of avian and mammalian influenza viruses

Lassa fever Urbanization favoring rodent host, increasing exposure (usually in homes)

Rift Valley fever Dam building, agriculture, irrigation; possibly change in virulence or pathogenicity of virus

Yellow fever Conditions favoring mosquito vector

(in new areas)

Bacterial

Brazilian purpuric fever Probably new strain

(Haemophilus influenzae,

biotype aegyptius)

Cholera In recent epidemic in South America, probably introduced from Asia by ship, with spread

facilitated by reduced water chlorination; a new strain (type O139) from Asia recently

disseminated by travel (similarly to past introductions of classic cholera)

Helicobacter pylori Probably long widespread, now recognized (associated with gastric ulcers, possibly other

gastrointestinal disease)

Hemolytic uremic syndrome Mass food processing technology allowing contamination of meat

(Escherichia coli O157:H7)

Legionella (Legionnaires' Cooling and plumbing systems (organism grows in biofilms that form on water

disease) storage tanks and in stagnant plumbing)

Lyme disease Reforestation around homes and other conditions favoring tick vector and deer

(Borrelia burgdorferi) (a secondary reservoir host)

Streptococcus, group A Uncertain

(invasive; necrotizing)

Toxic shock syndrome Ultra-absorbency tampons

(Staphylococcus aureus)

Table 2. Factors In Infectious Disease Emergence

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Factor Examples of specific factors Examples of diseases

__________________________________________________________________________

Ecological changes Agriculture; dams,changes Schistosomiasis (dams);

(including those in water ecosystems; Rift Valley fever (dams,

due to economic deforestation/reforestation; irrigation); Argentine hemorrhagic fever

development and flood/drought; famine;

land use) climate changes (agriculture); Hantaan (Korean hemorrhagic

fever) (agriculture);

hantavirus pulmonary

syndrome, southwestern

US, 1993 (weather anomalies)

Human demographics, Societal events: Population Introduction of HIV;

behavior growth and migration spread of dengue; spread

(movement from rural areas of HIV and other sexually

to cities); war or civil transmitted diseases

conflict; urban decay; sexual

behavior; intravenous drug

use; use of high-density

facilities

__________________________________________________________________________

Factor Examples of specific factors Examples of diseases

__________________________________________________________________________

International travel Worldwide movement of goods "Airport" malaria;

commerce and people; air travel dissemination of mosquito vectors;

ratborne hantaviruses; introduction

of cholera into South

America; dissemination

of O139 V. cholerae

Technology and Globalization of food Hemolytic uremic syndrome

industry supplies; changes in food (E.coli contamination of

processing and packaging; hamburger meat), bovine

organ or tissue spongiform encephalopathy;

transplantation; drugs transfusion-associated

causing immunosuppression; hepatitis (hepatitis B, C),

widespread use of opportunistic infections

antibiotics in immunosuppressed

patients, Creutzfeldt-Jakob disease

from contaminated batches of

human growth hormone

(medical technology)

Microbial adaptation Microbial evolution,response Antibiotic-resistant

and change to selection in environment bacteria, "antigenic

drift" in influenza virus

Breakdown in public Curtailment or reduction Resurgence of tuberculosis

health measures in prevention in the United States;

programs; inadequate cholera in refugee camps

sanitation and vector in Africa; resurgence of

control measures diphtheria in the former

Soviet Union

The Good News About Infectious Disease Control

Many infectious diseases can be prevented through simple and inexpensive methods.

• Wash Your Hands Often

Always wash your hands before, during and after preparing food, before eating, after using the bathroom or changing diapers, and after handling animals or animal waste.

• Routinely Clean and Disinfect Surfaces

Cleaning with soap and water removes dirt and most germs. Using a disinfectant kills additional germs. It is important to thoroughly clean areas where germs are likely to be transmitted, such as the kitchen and bathroom.

• Handle and Prepare Food Safely

Buy and refrigerate perishable foods quickly. Store food properly. Don’t allow juices from meat, seafood, and poultry or eggs to drip on other foods. Wash hands and kitchen surfaces and utensils while preparing food. Wash raw fruits and vegetables. Don’t eat raw eggs. Cook poultry and meat until the juices run clear. Use different dishes for raw foods and cooked foods. Keep cold foods cold and hot foods hot. Don’t leave leftovers out longer than 2 hours.

• Get Immunized

Children, adolescents and adults need immunizations. Make sure the members of your family get the right vaccines at the right time. Keep immunization records for the whole family.

• Use Antibiotics Properly

Unnecessary antibiotics can be harmful and, if misused, can cause bacteria to become resistant to treatment. Antibiotics don’t work against viruses like colds and flu. Use antibiotics exactly as prescribed by your provider.

• Practice Animal Safety

Keep pets healthy by following your veterinarian’s recommendations. Clean litter boxes daily and don’t let children play where animals urinate or defecate. Cover sandboxes. Use insect repellent if engaging in outdoor activities. Avoid contact with wild animals.

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