Know Your Enemy
Strait, G. Carroll, The World and I
The world of infectious diseases and the agents that cause them is not so foreign if we take a little time to explore it.
Most of us will probably catch a cold this year. Someone in our family may come down with the flu as well. Those of us over 35 probably remember fevers, headaches, stomachaches, itching skin, and swollen glands as we suffered through the inevitable childhood diseases: mumps, measles, chicken pox, and perhaps others. We learned that by having the disease as children, we were building up immunity so we wouldn't have to suffer later.
Today, most American children are immune to those diseases. Thanks to childhood vaccinations, they have built up immunity without becoming ill. On the other hand, many common illnesses such as coughs and earaches afflict these children and cause sleepless nights for their parents. As treatment, children receive a course of antibiotic, prescribed for a period of days. However, when the symptoms fade in a few days, parental anxiety is relieved and the natural tendency is to stop giving the antibiotic. Thus parents and children treating a cough or ear infection, along with countless others who are prescribed an antibiotic course, unwittingly assist the microbial world to overcome our defensive weapons.
By discontinuing the treatment prematurely, we fail to kill all the microbes, leaving behind those survivors that are best able to withstand the antibiotic. Our priorities do not include fighting invisible and apparently harmless enemies. But the microbes are relentless. Their constant priority is to seek out advantageous living conditions and exploit available resources to achieve maximum growth and multiplication. Thus they turn every human lapse in treatments to a microbial advantage, adapting now just as they have been doing for hundreds of millions of years.
Deseases and disease agents
The roster of infectious diseases is imposing: measles, malaria, hepatitis (A, B, C, E, or G), sleeping sickness, cholera, leishmaniasis, AIDS, tuberculosis, dengue fever, and river blindness. The list could continue for many more lines. Some of the diseases, like AIDS, are new, while most are old. In the twentieth century, significant progress has been made in overcoming such diseases as measles, malaria, and tuberculosis. But in the last 20 years these three scourges have been occurring in increasing numbers.
Each infectious disease is caused by a particular microorganism, and their diversity is perhaps even more impressive than the variety of diseases themselves. In simplified terms, we may categorize disease agents according to their level of development of a cell-like structure. Thus, they can be acellular, unicellular, or multicellular, with sizes increasing correspondingly. Our survey of microbial disease agents will progress roughly from the smallest to the largest, covering viruses, bacteria, fungi, protozoa, and helminths (worms).
Viruses are acellular agents ranging in size from 20 to 200 millionths of a millimeter (nanometers). They consist essentially of a core of genetic material (RNA or DNA) surrounded by one or two layers of protein. Despite their small size, viruses exhibit a surprising variety of shapes and survival strategies, and they cause a comparable diversity of disease effects. Most viruses are so small that they could not be viewed until the electron microscope became available after World War II. Viruses are true parasites, requiring a host for replication. They cause diseases ranging from AIDS to smallpox, rabies, polio, measles, and the common cold. Measles is the second-largest cause of death by an infectious disease, even though an effective vaccine exists. Most of these deaths occur among children in India and Africa.
Single cells without a nucleus
The unicellular agents are divided into those that have no nucleus (the prokaryotes) and those that do (the eukaryotes). The multitudinous types of prokaryotes that cause human disease are encompassed in the large category of bacteria. Within the bacteria, a large subset called the "true bacteria" includes most of the medically significant species. These occur in such diverse shapes as rods, spheres, and spirals, with individual cells varying from 1 to 20 thousandths of a millimeter (micrometers) in length and from 0.4 to 1.5 micrometers in width.
True bacteria can exist as single cells, as clusters of cells, or as chains that are either branched or straight, a fact used by scientists in classifying the bacterial world. In addition to shape, scientists also classify bacteria according to the thickness of their cell wall, as indicated by the cell's response to a cell-marking procedure called the Gram stain. (Thus we may hear of Gram-positive and Gram-negative bacteria.) Diseases caused by true bacteria include tuberculosis, pneumonia, toxic shock, and cholera.
Tuberculosis, caused by Mycobacterium tuberculosis, can lie dormant for years before bursting into activity that threatens the life of its host. Tuberculosis symptoms are coughs, weight loss, night sweats, low-grade fever, and chest pains. Tuberculosis spreads through bacteriacontaining, airborne droplets that can remain aloft for two hours. Taking more than 2 million lives each year, the tuberculosis bacterium is the champion killer among infectious microbes today and one of the reemerging diseases in the United States and around the world.
Other bacteria of medical interest but not belonging to the true bacteria include the spirochetes plus the closely related pair of the rickettsiae and the chlamydiae. They tend to be somewhat smaller than the true bacteria, and all three groups stain Gram negative.
Spirochetes are slender, flexible spiral filaments that can move but lack a "tail," or flagellum. A spirochete, Borrelia burgdorferi, is the cause of Lyme disease, which is transmitted by tick bites.
The rickettsiae and chlamydiae may be rod-shaped or spherical, or may even have two or more shapes. Rickettsia prowazekii, acquired by contact with the feces of infected body lice, has caused epidemic typhus during warfare in Europe for centuries. Chlamydia trachomatis can cause urinary tract infections. Chronic chlamydia infections in women can lead indirectly to infertility or ectopic pregnancy through the scarring of reproductive organs.
Cells with a nucleus
More biologically advanced are the fungi, which have not only a well-defined nucleus but also other internal, membranebounded structures. Fungi exist both as single cells, the yeasts, and in filamentous forms, the molds. Yeasts replicate asexually, while molds can replicate either sexually or asexually. Perhaps the best-known infectious fungi are the candida yeasts, which can grow to excess in the gastrointestinal or genitourinary tracts if the microbial balance is disrupted. Fungi of the genus Cryptococcus cause a form of meningitis that can encompass internal organs and lead to death if untreated.
Protozoa are all unicellular, but they occur in a great variety of shapes and in widths ranging from 2 to 100 micrometers. Protozoa and the multicellular helminths (worms) are often classified together as parasites. The single-celled protozoa are complete units capable of performing functions that in multicelled organisms require specialized cells. A group of four species of protozoa, in the genus Plasmodium, causes malaria, which kills roughly 1 million people each year, most of them children and pregnant women in the tropics. Its complex life cycle involves asexual development in human red blood cells coupled with sexual development in the gut of a female mosquito.
Also widely distributed in the tropics are about 20 species of protozoa of the genus Leishmania, which are transmitted by the bite of infected female sand flies and maintained in a reservoir of dogs and rodents. The disease takes many forms, from self-healing ulcers to hideous destruction of facial features and fatal illness. Many U.S. soldiers contracted the disease during the Desert Storm operations in the Middle East, triggering the introduction of new measures to protect the blood supply from contamination by Leishmania parasites.
The largest and most complex of the infectious disease agents are the multicellular metazoa, or helminths which pass through complex life cycles involving often-microscopic eggs and larvae and much larger or very much larger adult stages. Intestinal tapeworms, the largest of the metazoan disease agents, grow in human intestines to lengths of 3-10 meters (10-34 feet). Animal hosts for tapeworm larvae can include fish, cattle, or pigs.
Blood flukes, another type of metazoan, cause schistosomiasis, a major public-health problem affecting more than 200 million people in Africa, Asia, and South America. Death or chronic disability can result as adult flukes (6-20 millimeters in length) resident in such sites as the intestines, liver, and urinary bladder release eggs, some of which are excreted while others become lodged in the organs. Scar tissue that forms around the lodged eggs can block blood vessels, causing them to rupture, leading to death or urinary obstruction. Eggs that are shed in the feces or urine enter the water and develop into larvae in freshwater snails. They then exit into water, where they swim until they contact a human, penetrate the skin, and migrate through blood vessels to sites where they will grow to be adults.
New and reemerging diseases
The new infectious diseases, those that are reappearing after a decline in incidence, and those whose causal agents are developing resistance to drugs are all defined as emerging. These emerging infectious diseases are a diverse lot, with all major types of disease agents represented. In Emerging Infections: Microbial Threats to Health in the United States, published in 1992 by the Institute of Medicine, there is a list of 54 emerging diseases, including many that have been mentioned above and many more that have not. These include 17 emergent bacteria, 26 emergent viruses, and 11 emergent protozoa, helminths, and fungi.
Studying the list offers an insight into the field of infectious diseases. We may tend to think that each disease has a name and that a disease agent must be found to correspond with it. But new diseases don't come with a name. People have symptoms, then doctors need to determine if they correspond to some existing disease state, or if it may be a new complex of symptoms suggesting a new disease agent and hence a new disease.
Thus, in the virus group, the name of a virus is usually associated with a cluster of symptoms but not a named disease. For example, beside a named virus, hantavirus, we see "abdominal pain, vomiting, hemorrhagic fever." Associated with human papillomavirus we see "skin and mucous membrane lesions (often, warts); strongly linked to cancer of the cervix and penis." For some viruses, the associated disease has the same name as the virus. Thus, for example, measles virus causes measles, and the yellow fever virus causes yellow fever. AIDS is distinctive as a named disease caused by a virus,--or in this case, two viruses, HIV-1 and HIV-2--with a different name from the disease.
Among the bacterial group are more named diseases. These include: Legionnaires' disease (caused by a bacterium distributed through air-cooling systems and water supplies), Lyme disease (caused by a spirochete conveyed by the bite of an infected tick), and hemorrhagic colitis (caused by the renegade bacterium E. coli 0157:H7 and acquired through ingestion of contaminated food or water). Peptic ulcer, scarlet fever, cholera, and cellulitis are all included among the bacterially caused emergent diseases.
The protozoa, helminths, and fungi, though represented by fewer diseases on the Institute of Medicine's list, are far from being minor factors in infectious disease. With protozoa causing malaria and acute pneumonia and fungi causing meningitis and candidiasis, these disease agents remain potent threats to health in the United States. Even the helminths cannot be discounted. The rising popularity of eating raw fish, for example, increases the possibility of ingesting larvae of nematodes of the genus Anisakis, which can cause severe abdominal pain, vomiting, nausea, and fever.
With such a rich abundance of disease-causing agents arrayed against us, it may seem remarkable that humanity has survived and even multiplied rapidly in this last century. Indeed, it is remarkable. Medical science and modern hygiene have transformed the struggle between microbial disease agents and humanity in the twentieth century, rendering immeasurable support to an already remarkable human immune system.
The challenge of the next century is to spread this support to all people, pushing the disease-causing viruses, bacteria, fungi, protozoa, and helminths out of the human arena.
For a comprehensive treatment of the diverse types of disease agents and the diseases they cause, look for a text on medical microbiology. One of several good sources:
David Greenwood, Richard Slack, and John Peutherer, eds., Medical Microbiology: A Guide to Microbial Infections: Pathogenesis, Immunity, Laboratory Diagnosis, and Control, 15th ed., Churchill Livingston, Edinburgh and New York, 1997.
On the Internet:
http://vm.cfsan, fda.gov/~mow/ intro.html
G. Carroll Strait is an editor in the Natural Science section of The World & I.…
Questia, a part of Gale, Cengage Learning. www.questia.com
Publication information: Article title: Know Your Enemy. Contributors: Strait, G. Carroll - Author. Magazine title: The World and I. Volume: 13. Issue: 10 Publication date: October 1998. Page number: 160+. © 1999 News World Communications, Inc. COPYRIGHT 1998 Gale Group.
This material is protected by copyright and, with the exception of fair use, may not be further copied, distributed or transmitted in any form or by any means.