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The U.S. Surgeon General said in 1965, “It is time to close the book on infectious diseases.” However “old” infectious diseases are re-emerging and new ones are emerging along with increased fears of worldwide epidemics. Many infectious diseases that we’ve forgotten about in this country remain a major problem worldwide. Diseases such as malaria and tuberculosis could very well, given the right conditions, reappear as serious problems. In addition, there are rich new opportunities for microorganisms to jump into the human population and cause new variant diseases. We’ve seen that with HIV and SARS. Now, new strains of influenza loom on the horizon.
Concern over emerging infectious diseases are being expressed as they pose a biological threat to all persons regardless of borders or socioeconomic status, and can cause staggering economic disruption to societies. Biological agents such as viruses, bacterium, parasite, fungus, spores, prions, or bioengineered agents cause infectious disease. These diseases are easily spread through direct or indirect contact. Although some diseases have been “conquered” through the use of antibiotics, vaccines, and other advances, new infectious diseases are constantly emerging and shifting.
It appears that the balance between humans and infectious microorganisms is changing. Microorganisms are among the greatest threat to our survival. Let’s face it: bacteria have been around at least three billion years. Just because we have six billion people in the world doesn’t mean we humans will win. Stunning breakthroughs in public health have enabled people to live longer. Clean water, sanitation, and healthy nutrition along with the development of highly effective vaccines/antibiotics/pharmaceuticals, have increased average western life expectancy by an amazing 35 years.
However, in the past two decades, infectious diseases that had nearly been conquered are returning with a deadly twist, cleverly evading the once toughest antibiotic weapons. Microbial resistance to many pharmaceutical agents is rising, threatening people everywhere with the re-emergence of “drug resistant” diseases, or, catching us with no prior immunity.
Whether naturally occurring, or induced by calculated bio-weaponry, infections more often than not cause illness, disability, and death in people. Wider infections disrupt populations, economies, and governments. National borders offer trivial impediment to such threats, especially in our extremely mobile world; microbes move faster than ever in our instant jet travel world. Visitors and immigrants arrive from other nations daily and may be carriers of disease. Insects from thousands of miles stow away on passenger and storage compartments of cars, boats, and planes. Plants and animals contaminate cargo. Restaurants boasting of their imported specialty foods introduce microbes to new places.
Compounding the danger posed by infectious diseases are other important trends. Examples are: Immunity to antimicrobial resistance (prevalent in hospitals and pervasive worldwide), genetic engineering, and sadly, the intentional use of biological agents to do harm. Also, there appears to be a diminished capacity to recognize and respond to emerging diseases, particularly those originating elsewhere. There are also potential problems because of political cover-ups, and poor communication.
Although the United States has shown leadership in the past by strengthening its own and others’ capacities to deal with infectious diseases, the present reality is that public health and medical communities are inadequately prepared. More needs to be done now to improve our ability to prevent, detect, communicate, isolate, and control emerging, as well as resurging, microbial threats to the health of individuals and communities. Even though there was a so-called long-term warning system in place, bureaucratic ineptitude as seen with “Katrina,” (the 2005 hurricane and levee disaster in New Orleans), was a prime example of poor communication and insufficient preparation
Beyond the general ecology of many developing countries, a number of social and economic factors contribute to the high rates of infectious disease. Poverty, lack of access to health care, antibiotic resistance, evolving human migration patterns, new infectious agents, changing environmental and development activities all contribute to the expanding impact of infectious diseases.
Every year, 700,000 people die from food or water borne diseases in the Asia-Pacific region alone, including the Philippines. Medical Microbiologist, Dr. Alice-Alma C. Bungay said that “one factor that triggers the emergence of emerging new micro-organisms is the unprecedented change in microbial population, which leads to their evolution making them more virulent, stronger, and dangerous: for these newly evolved micro-organisms have already acquired a very strong resistance against commercially available antibiotic drugs.”
There is a growing danger of food-related outbreaks particularly in Asia-Pacific where most of the people are engaged in backyard swine and poultry enterprises. In addition to the victims’ proximity to the animal dwellings, unclean households and surroundings, and the improper handling and cooking of foods trigger enteric disease.
A research study entitled, The Emerging Foodborne Bacterial Zoonoses, conducted by three Filipino medical researchers (Drs. Methusyla J. Estacio and Calvin S. Delos Reyes, Bungay) shows that the rise of virulent microorganisms poses a serious health problem with high mortality rates via food and water contamination.
Emerging infectious diseases are those diseases where the incidence in humans has increased in the past 2 decades or threatens to increase in the near future. Common bacterial pathogens (malaria, enterococcus, pneumococcus, tuberculosis, staphylococcus, and streptococcus) have become resistant to many of the antibiotics used over the past 20 years and leave the medical community with no effective treatment for a growing list of infectious diseases. Re-emerging infectious diseases are communicable diseases that were previously known and believed to be eradicated or controlled and are now again causing illness and death.
Examples of new emerging diseases include AIDS, lyme disease, Escherichia coli O157:H7 (E. coli), hantavirus, SARS, West Nile, Ebola, and variant influenzas. Examples of re-emerging diseases include malaria, measles, polio, tuberculosis, cholera, and pertussis,
Emergence may be due to many causes. These include the intentional or accidental release of a new agent; a previously undetected infection that has been present in the population (e.g., hepatitis C or hepatitis G); or the realization that an established disease has an infectious origin. Emergence may also be used to describe the reappearance of a known infection after a decline in incidence. Imagine a country filled with a majority of sick people. Ghana is a place like that, harboring many of the worst infectious diseases. The main diseases there are cholera, typhoid, tuberculosis, anthrax, pertussis, yellow fever, hepatitis, trachoma, and malaria. Other diseases include schistosomiasis, guinea worm, dysentery, onchocerciasis, venereal diseases, and poliomyelitis. Malnutrition has also increased. The incidence of acquired immune deficiency syndrome (AIDS) is the second highest in West Africa and is rising.
Heart disease has been the number one killer in the United States for several decades (28.5% deaths per annum). The United States also has the highest incidence of cancer. Cancer is the second leading cause of death (22.8%) in the world.
In China, the number one killer has been strokes, (18.4%), with pulmonary disease being the second killer at 11.5%. It has been generally accepted that heart disease is caused by a combination of genetics and environmental factors, such as poor diet and lack of exercise. But increasingly, researchers are associating viral and bacterial infections with heart disease and heart failure. Several pathogens have been tied to heart disease for example:
In some other parts of the world, infectious diseases are the number one killer. Are “run away” infections such as those found in Africa a glimpse into our future? Look at Africa, where 10% of the world’s population exists, infectious diseases are the main killers of the people, and as of this writing, avian flu has been seen in the poultry in some African countries.
The latest figures show that Africa has 64% of the people living with HIV, or about 25 million. HIV/AIDS is responsible for 20.4% of deaths there, and 2.3 million deaths per year (74%). Researchers believe that the next generation will have an even higher rate of infection, with approximately 80% of the people HIV positive.
More than 90% of the deaths from infectious diseases worldwide are caused by only a handful of diseases. These diseases create lower respiratory infections, HIV/AIDS, diarrheal diseases, tuberculosis, malaria and measles. While not major killers, a number of the world’s “neglected” infectious diseases such as lymphatic filariasis, trachoma, and intestinal parasites create chronic disability for millions of people.
Person-to person spread is the simplest method of transmission. The major routes of entry for the natural spread of disease are inhalation (results in the deposition of infectious or toxic particles within alveoli and provides a direct pathway to the systemic circulation) and ingestion of contaminated food or water. Other methods of acquiring disease include percutaneous inoculation via breaks in the skin (including insect bites), mucous membrane penetration, contact with infected blood and body fluids.
There is great apprehension that common bacterial pathogens (i.e., TB, malaria, shigella, salmonella, streptococcus, pneumococcus) have become resistant to many of the antibiotics commonly used in the past 20 years. The increase in drug resistance has resulted from several practices, including inappropriate use (prescribing an antibiotic for a viral episode, i.e., colds), over-prescribing, poor patient compliance in completing entire treatment series, and the widespread use of antiseptics. (USDHHS (1998). Preventing Emerging Infectious Diseases: A Strategy for the 21st Century, Atlanta: Centers for Disease Control and Prevention).
Today we are seeing malaria reappear in the United States. For example, The Palm Beach County Health Department in Florida confirmed the seventh case of malaria in August of 2003.
Humans co-evolved with microbes. Thus the body is like an ecosystem, or a biosphere. From food digestion to fighting pathogenic wars, symbiotic microbial alliances exist within each of us. On just one intestine about a hundred trillion microbes can be found. A single gram of typical human feces contains 1 billion viruses, not affected by chlorine. (J. L. Melnick and T. G. Metcalf, “Distribution of Viruses in the Water Environment” NEW YORK: Cold Spring Harbor Laboratory: 1985, 95-102).
Scientists have tried to “grow” snippets of DNA and have concluded that each individual has somewhere between 500 to 1,000 species of microbes, representing about 8,000 sub-species. This adds up to a huge amount of microbials. We have a very long road to classify even a small percentage of them. According to David Relman, microbiologist from Stanford University, the internal microbial world varies so distinctly from person to person that we are genetic fingerprints, so to speak, each one unique.
Disease results when the balance of health is in imbalance (pathogenesis). Respiratory infections and gastrointestinal infections cause more deaths worldwide than all other diseases added together.
Microbes (minute life forms) can mutate every 20 minutes. A pathogen is a disease-producing organism. A parasite is an organism that grows, feeds, and lives on or in another organism without contribution and can cause disease. Some parasites are actually commensals, in that they neither benefit nor harm their host (for example, Entamoeba coli). Medical parasitology traditionally has included the study of three major groups of animals: parasitic protozoa, parasitic helminths (worms), and those arthropods that directly cause disease or act as vectors of various pathogens.
The unicellular parasites (protozoa) and multicellular parasites, (helminths, and arthropods) are antigenically and biochemically complex, as are their life histories and the pathogenesis of the diseases they cause. Infectious diseases as we know them are caused by pathogenic microbes. These microbes fall into five main groups:
Zoonoses are animal diseases that can be transmitted to humans.
A partial list of animals that can transmit disease includes:
A vector is defined as “a living organism that carries disease-causing organisms to new hosts. Vectors can be either biological vectors or mechanical vectors. A biological vector is persistently infected and replicates. Viruses are biological vectors. A mechanical vector is a vector in which, or upon which a parasite is transported, but the parasite does not replicate. A housefly, for example, can pick up Salmonella on its feet from feces and then deposit it on human food. Flies carry pathogens from faeces to food and are therefore, mechanical vectors.
Infections of humans caused by parasites number in the billions and range from relatively innocuous to fatal. Biological vectors are more likely to introduce exotic disease agents of animals to new areas than are mechanical vectors. An example of a disease agent suspected to be introduced to a new area by a biological vector is West Nile virus’ introduction into the northeastern U.S. by a mosquito vector.
Arthropods are insects and insect-like animals. Insects often cause great losses in agriculture, attack stored products, parasitize humans and domesticated animals and plants, and serve as important carriers of disease organisms. Arthropod vectors are responsible for the transmission of a variety of diseases, though often each vector (i.e., arthropod species) can transmit only a limited number of pathogens, and each pathogen may be transmitted by only a limited number of arthropod vectors. Important disease-transmitting arthropods include mosquitoes, ticks, lice, fleas, and flies. They can also be beneficial, producing honey and silk and pollinating the flowers of the majority of flowering plants.
Vectors more or less permanently associated with the human host:
Those vectors only transiently associated with the human host (usually to feed):
Examples of important mosquito-borne diseases include:
Examples of important tick-borne diseases include
Example of a louse-borne disease is:
Epidemic typhus (bacteria) Rickettsia prowazekii)
Epidemics of louse borne diseases usually occur under crowded, unsanitary conditions. All louse-borne disease agents enter the body when louse feces are scratched into bite wounds.
“Parasites have killed more humans than all the wars in history,” reported The National Geographic in its award-winning documentary, The Body Snatchers. “Every living thing has at least one parasite that lives inside or on it, and many, including humans, have far more. The notion that tiny creatures we’ve largely taken for granted are such a dominant force is immensely disturbing. We are a collection of cells that work together, kept harmonized by chemical signals. If an organism can control those signals — an organism like a parasite — then it can control us. And therein lies the peculiar and precise horror of parasites.”
Parasites are used to describe parasitic protozoa, helminths (worms) and arthropods. Humans fall victim to many different internal animal parasites. A virus is a parasite, which requires a living host in order to survive. In the words of the National Geographic, “a sinister world of monstrous creatures that feed on living flesh: parasites.”
An ectoparasite is a parasite that lives on the surface of a host. Arthropod vectors are responsible for the transmission of diseases. An endoparasite is a parasite that lives within a host (this includes most protozoas and helminthes). In developing nations and throughout pre-industrial history, animal endoparasites were by far the most common and troublesome of human pathogens, causing immense suffering and death. Behaviors that help prevent infection by parasites have become ritualized in some cultures such as the widespread avoidance of pork.
It is the purpose of this section to provide a basic understanding of fungi and the diseases they cause. Of the approximately 70,000 recognized species of fungi, about 300 are known to cause human infections. In addition, some fungi have economic importance as plant and animal pathogens. Fungal diseases of healthy humans tend to be relatively benign, but the few life-threatening fungal diseases are causing increasing concern.
Fungal diseases are an increasing problem because of the use of antibacterial and immunosuppressive agents. Individuals with an altered bacterial flora or compromised defense mechanisms (e.g., AIDS patients) are more likely than healthy people to develop opportunistic fungal infections such as candidiasis. Consequently, opportunistic fungal pathogens are increasingly important in medical microbiology.
The fungi are a group of eukaryotic microorganisms, some of which are capable of causing superficial, cutaneous, subcutaneous, or systemic disease. Fungi have cell walls that are composed of chitin. Like animals, fungi are extracellular digesters of their food and are also nutrient absorbers. Typically fungi reside within the food they are consuming. Often that food is plant material, but their food can be animal in origin as well. Most fungi are saprophytes (organisms that consume and digest dead organic matter and organic waste).
Fungi are heterotrophic and essentially aerobic, with limited anaerobic capabilities. They possess a nucleus enclosed by a nuclear membrane, a rigid cell wall, endoplasmic reticulum, and mitochondria like those of plant and animal cells. These structures differ substantially from those of bacteria. Host defenses against fungi are similar to those utilized against bacterial diseases, except that the cell-mediated response is extremely important. Nonspecific immunity and cell-mediated immunity seem to be the most important means by which humans resist or eliminate fungal pathogens.
Fungi tend to be able to do the same kinds of things bacteria do, especially gram-positive bacteria, but do so in dryer, more acidic, more highly osmotic environments than most bacteria prefer. Fungi can grow as yeasts and/or as molds. Yeasts are single-celled forms that reproduce by budding, whereas molds form multicellular hyphae. Fungi have developed many mechanisms to colonize human hosts. The ability to grow at 37°C is one of the most important. Production of keratinase allows dermatophytes to digest keratin in skin, hair and nails. Dimorphism allows many fungi that exist in nature as molds to change to a yeast form in the host and thus become pathogenic.
Of all the fungi that have been implicated in human disease, only the six agents that cause the systemic mycoses have the innate ability to cause infection and disease in humans and other animals. The primary site of infection is the respiratory tract. Conidia and other infectious particles are inhaled and lodge on the mucous membrane of the alveoli, where they encounter macrophages and are phagocytosed.
Yeasts. Yeasts are single-celled fungi (i.e., yeasts tend to be more closely related to molds than to other yeasts).
Molds. Molds are multi-celled micro-fungi.
Spores. Many fungi produce spores. These spores typically represent the dispersal stage. They are relatively environmentally resistant cells that can create new hyphae, mycelia, or cultures. Depending on the fungi as well as the stage of the life cycle, spores may be produced sexually (meiosis).
Diagnosis of Fungal Infection. The first suspicion of a fungal infection is often the result of the clinical presentation. Microscopic examination of skin scrapings, a vaginal discharge or bronchoscopic washings might reveal dermatophytes, candidiasis or histoplasmosis, respectively.
Protozoa are a group of single-celled organisms, such as amoebas, ciliates, flagellates, and sporozoans. The nutrition of all protozoa is holozoic; that is, they require organic materials, which may be particulate or in solution. Amebas engulf particulate food or droplets through a type of temporary mouth, perform digestion and absorption in a food vacuole, and eject the waste substances.
Many protozoa have a permanent mouth, the cytosome or micropore, through which ingested food passes to become enclosed in food vacuoles. Pinocytosis is a method of ingesting nutrient materials whereby fluid is drawn through small, temporary openings in the body wall. The ingested material becomes enclosed within a membrane to form a food vacuole.
Several protozoan infections are not apparent, or mild in normal individuals but can be life-threatening in the immune suppressed individual. For example, evidence suggests that many healthy persons harbor low numbers of Pneumocystis carinii in their lungs. However, this parasite produces a frequently fatal pneumonia in immune suppressed patients such as those with AIDS. Toxoplasma gondii, a very common protozoan parasite, usually causes a rather mild initial illness followed by a long-lasting latent infection. AIDS patients, however, can develop fatal toxoplasmic encephalitis.
Cryptosporidium was described in the 19th century, but widespread human infection has only recently been recognized. Cryptosporidium is another protozoan that can produce serious complications in patients with AIDS. Microsporidiosis in humans was reported in only a few instances prior to the appearance of AIDS. It has now become a more common infection in AIDS patients. As more thorough studies of patients with AIDS are made, it is likely that other rare or unusual protozoan infections will be diagnosed.
Unlike most viral and bacterial infections, protozoan diseases are often chronic, lasting months or years. When associated with a strong host immune response, this type of chronic infection is apt to result in a high incidence of immune pathology. Protozoa have metabolic pathways similar to those of higher animals and require the same types of organic and inorganic compounds. In recent years, significant advances have been made in devising chemically defined media for the in vitro cultivation of parasitic protozoa.
Worms are relevant to microbiology because they cause disease. The two types of worms that parasitize humans are flatworms and roundworms (segmented worms, like the earthworm, are not parasitic). Helminths can have complex life cycles involving more than one host or host species.
Flatworms, Flukes, Hookworms, and Tapeworms. Tapeworms are flatworms and lack a digestive tract and acquire nutrients from their host’s digestate. Flukes are Flatworms that tend to live in tissues other than the lumen of the digestive tract.
Roundworms (Hookworms, Pinworms). Roundworms are more sophisticated than flatworms, possessing not just a separate mouth and anus, but a circulatory system as well. Round worms are small but important (and extremely numerous) components of most ecosystems. Most, but not all parasitic round worms live in the intestinal tract. Parasitic round worms include the hookworms and the pinworms. Trichinosis is an example of a disease caused by a parasitic roundworm. Sushi (raw fish) can harbor parasitic roundworms.
Hookworm (Ancylostomiasis). One billion people get infected per year. Hookworm is an intestinal parasite of humans that usually causes mild diarrhea or cramps. Heavy infection with hookworm can create serious health problems for newborns, children, pregnant women, and persons who are malnourished. Found in the larvae in soil, hookworm penetrates the skin (often through bare feet) or can be accidentally ingested through contaminated food.
The main forms of human schistosomiasis are caused by five species of the flatworm, or blood flukes, known as schistosomes: Schistosoma mansoni causes intestinal schistosomiasis and is prevalent in 52 countries and territories of Africa, the Caribbean, the Eastern Mediterranean, and South America
Nine out of ten people living in the depressed areas of Africa south of the Sahara still have nothing to drink but meager quantities of impure water, thus exposing themselves to dracunculiasis. This parasitic disease causes dreadful suffering and disability among the world’s most deprived people. Schistosomiasis is endemic in 74 tropical developing countries. Some 600 million people are at risk of becoming infected. It is estimated that 200 million people are already infected.
Extreme poverty, the unawareness of the risks, the inadequacy or total lack of public health facilities plus the unsanitary conditions in which millions of people lead their daily lives are all factors contributing to the risk of infection.
The female lays from 200 to 2000 eggs per day over an average of 5 years, depending on the species. In the case of intestinal schistosomiasis, the worms reside in the blood vessels lining the intestine. In urinary schistosomiasis, they live in the blood vessels of the bladder. Safe and effective oral medications are now available for the treatment of schistosomiasis.
The Guinea worm disease is a parasitic infection caused by Dracunculus medinensis (a long, thin worm). This worm is the largest of the tissue parasites affecting humans. The adult female, which carries about 3 million embryos, can measure 600 to 800 mm in length, lives in subcutaneous tissues, and causes severe pain, especially when it occurs in the joints. The major source is infected water. Infection is seen when the adult worm emerges through the skin of an infected person.
The worm can grow to be 3 feet long. Sometimes the worm can be pulled out completely within a few days, but this process usually takes weeks or months, the average being 3 months. No medication is available to end or prevent infection.
Onchocerciasis is an infection caused by the parasite Onchocerca volvulus (worm), spread by the bite of an infected black fly. The World Health Organization’s (WHO) expert committee on onchocerciasis estimates it affects approximately 17.7 million people, of whom about 270,000 are blind and another 500,000 have visual impairment. About 99% of infected persons are in Africa; the remainder are in Yemen, Mexico, Guatemala, Ecuador, Colombia, Venezuela, and Brazil. Onchocerciasis is commonly treated with oral medicine.
The ecological balance is shifting. New disease is emerging - or re-emerging. Despite medical advances, new trends such as massive frequent jet travel, spread microbes. Vectors normally found in one part of the world are being imported to others where little immunity exists. Immunity to exotic microbes does not work predictably as in the past, while genetic engineering creates the potential of unknowns.
Our health care system needs to become increasingly visionary to impede cascading problems. Communication systems need to be established without creating fear. It’s as if a time bomb is ticking as the microbial world grows beyond human ingenuity with a seemingly archaic medical system to fight it.