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HLTH 2000: Chapter 9 Notes
Terms in this set (36)
How Were Infectious Diseases "Conquered"?
These infectious diseases were largely conquered through public health measures, including purification of water, proper disposal of sewage, pasteurization of milk, and immunization, as well as improved nutrition and personal hygiene. The discovery and introduction of antibiotics in the 1940s also played a role.
What Causes Epidemic Diseases?
The major epidemic diseases are caused by bacteria, viruses, or parasites.
Robert Koch, a German physician, developed techniques to classify bacteria by their shape and their propensity to be stained by various dyes.
It was necessary to develop a set of rules that could be used to prove that a specific organism caused a specific disease. These rules, called "Koch's postulates," are
(1) The organism must be present in every case of the disease;
(2) The organism must be isolated and grown in the laboratory;
(3) When injected with the laboratory-grown culture, susceptible test animals must develop the disease;
(4) The organism must be isolated from the newly infected animals and the process repeated.
Koch applied these rules in his proof that tubercle bacilli were the cause of tuberculosis. Koch identified another bacillus, Vibrio cholera, as the cause of cholera. Other disease-causing bacilli identified during that period were those that cause plague, typhoid, tetanus, diphtheria, and dysentery.
Round-shaped bacteria, called cocci, include streptococci, which cause strep throat and scarlet fever; staphylococci, which cause wound infections; and pneumococci, which cause pneumonia.
Syphilis is caused by a cork-screw shaped bacterium called a spirochete. All these bacteria were identified by the beginning of the 20th century.
Smallpox, for example, was known to be transmitted from a sick person to a healthy person by something in the pus of the patient's lesions. The agent that caused the disease could pass though the finest available filters and could not be observed in any existing microscope. Smallpox was recognized to be one of a number of disease caused by such "filterable agents" or viruses. It was not until 1935, when the American scientist WM Stanley crystallized tobacco mosaic virus, that the nature of viruses was demonstrated.
It was not until 1935, when the American scientist WM Stanley crystallized tobacco mosaic virus, that the nature of viruses was demonstrated.
While bacteria are living, single-celled organisms can grow and reproduce outside the body if given the appropriate nutrients. Viruses are not complete cells, they are simply complexes of nucleic acid and protein that lack the machinery to reproduce themselves. Various kinds of viruses infect not only animal cells but also plant cells and even bacteria. They can survive extreme conditions. They reproduce themselves by taking control of the cell's machinery, often killing the cell in the process. The human diseases caused by viruses include smallpox, yellow fever, polio, hepatitis, influenza, measles, rabies, and AIDS, as well as the common cold.
Protozoa and other Agents
Human diseases can also be caused by protozoa, or single-celled animals that can live as parasites in the human body. Malaria, cryptosporidiosis, and giardiasis, also known as "beaver fever" are examples of protozoal diseases. Other parasites, such as roundworms, tapeworms, hookworms, and pinworms, are the most common source of human infection in the world.
Means of Transmission: Aerosols
Bacteria and viruses that cause respiratory infections are transmitted through the air on aerosols, water droplets produced when an infected person coughs or sneezes.
Means of Transmission: Touching Surfaces
They can also be transmitted from an infected person to objects he or she touches, to be picked up by the next person to touch the contaminated object and transferred by hand to the nose.
Means of Transmission: Fecal-Oral Route
Gastrointestinal infections are generally spread by the fecal-oral route, by which fecal matter from an infected person reaches the mouth of an uninfected person. This may occur as a result of poor personal hygiene or by contamination of drinking water.
Means of Transmission: Vectors
Vector-born diseases generally use a more complex route from one person to another, most often through an insect.
Patterns of Development
Each disease has its own pattern of development after a person is infected, and the time during which the patient is capable of transmitting the infection to others varies from one disease to another. Some diseases are most likely to be transmitted during the most symptomatic phase. Others are most communicable during the day or two before noticeable symptoms develop. A few diseases can exist in a carrier state, in which the infected person can transmit the disease without having symptoms.
Mary Mallon worked as a cook in a series of wealthy New York homes at the beginning of the 20th century. After an increasing number of family members in these homes became sick with typhoid fever, suspicion fell on the cook. Mary resisted medical tests and, when finally proven to be a carrier of the bacteria, refused to accept the results. She remained in the custody of the New York City Health Department for the rest of her life.
Chain of Infection
3) Method of Transmission
4) Susceptible Host
Chain of Infection: Pathogen
The Pathogen is a virus, bacterium, or parasite that causes the disease in humans.
Chain of Infection: Reservoir
The reservoir is a place where the pathogen lives and multiplies. Some pathogens spread directly from one human to another and have no other reservoir. Others, however, may infect nonhuman species, spreading from them to humans only occasionally. Contaminated water or food may also serve as reservoirs for some human diseases.
Chain of Infection: Method of Transmission
The pathogen must have a way to travel from one host to another, or from a reservoir to a new host. Food-born diseases are transmitted when a person eats contaminated food; water-born diseases are transmitted when someone drinks contaminated water. Many respiratory diseases are transmitted by aerosol. Other diseases are transmitted by sexual contact.
Chain of Infection: Susceptible Host
Even if the pathogen gains entry, a new potential host may not be susceptible because the host has immunity to the pathogen. Most microorganisms are specifically adapted to infect certain species.
Breaking the Chain of Infection
Public health measures to control the spread of disease are aimed at interrupting the chain of infection at whichever links are most vulnerable. At link 1, the pathogen could be killed by using an antibiotic. At link 2, one could eliminate a reservoir that harbors the pathogen. At link 3, transmission from one host to another could be prevented by quarantining infected individuals. At link 4, the resistance of hosts can be increased by immunization. Other links are often included separately as part of the chain of infection when it is useful to consider them as sites for public health intervention. Public health measures to control the spread of infectious disease include both routine prevention measures and emergency measures to control an outbreak once it has begun. Many of the measures referred to above—especially those concerning links 2 and 3—come under the category of environmental health. Immunization—link 4—is a major weapon that has had great success against the dread diseases that created the epidemics of the past. However, vaccines do not exist for all diseases.
Chain of Infection and Epidemiologic Surveillance
Epidemiologic surveillance is the system by which public health practitioners watch for disease threats so that they may step in and break the chain of infection, halting the spread of disease.
In the early history of public health, the solution was often quarantine. Quarantine is still used occasionally, when the disease is serious and there is no effective vaccine. More often, the pubic health response when an outbreak is detected by surveillance is to locate people who have had contact with the infected individual and to immunize them or give them medical treatment, as appropriate.
More often, the pubic health response when an outbreak is detected by surveillance is to locate people who have had contact with the infected individual and to immunize them or give them medical treatment, as appropriate. Contact tracing is also routinely used for controlling sexually transmitted diseases. The challenge for public health is to identify those with the disease through screening programs. Once a case is identified, public health workers try to discreetly alert those who have been exposed. The public health worker asks the person who has been diagnosed to identify sexual contacts.
The classic public health measures of surveillance and quarantine were key components in combating severe acute respiratory syndrome (SARS), a highly infectious new disease that first broke out in southern China in November 2002. Because China did not at first report the disease, it was not recognized as a major threat until March 2003, when the World Health Organization (WHO) issued a global alert and a travel advisory. WHO had been alerted by Dr. Carlo Urbani. Dr. Urbani himself soon contracted the disease and died. Epidemiologic detective work found that patients had all stayed in the same hotel in Hong Kong where a traveler from southern China had spent 1 night before falling ill with the syndrome. More than a dozen guests at the hotel had been infected by that one traveler, and they carried the disease to several other countries. By July 5, 2003, when WHO declared that SARS had been contained, the disease had infected people in 30 countries and had killed 812 people. Although a virus was identified, lab tests could not diagnose the disease until weeks after a patient had developed symptoms. No drug has been found effective against the virus, and treatment requires intensive respiratory therapy. SARS was contained by old-fashioned measures. The epidemic had severe economic impact wherever it broke out. There was concern that the disease would be seasonal and would break out again in 2004, but this did not occur. Since 2004 there have not been any known cases of SARS anywhere in the world.
Rabies, a fatal disease of the nervous system caused by a virus, kills an estimated 55000 people around the world each year, usually contracted through a dog bite. Although there is an effective vaccine against rabies, routine immunization of everyone is not recommended. Human exposure to the rabies virus in this country is relatively rare, and the vaccine is expensive and inconvenient to deliver. The rabies virus infects only mammals, and it is almost always transmitted when a rabid animal bites another animal or a human. Since the animal most likely to bite a human is the dog, mandatory immunizations of dogs against rabies is the first line of defense in the protection of people. The public health system has defined clear guidelines for responding to a report of a person's being bitten by a domestic or wild animal, depending on the likelihood that the animal is rabid. If the biting animal is wild, or if there is other reason to suspect that it is rabid, it must be killed and its brain tested for signs of rabies virus infection. Rabies virus affects the brain and from there travels to the salivary glands and is secreted in saliva. An animal capable of transmitting the virus in its saliva will already have brain involvement, exhibit symptoms, and be dead within a few days.To control rabies, public health practitioners conduct surveillance for rabies in wildlife. Bats are the most dangerous rabies threats to humans. Even in parts of the country where the disease is not endemic among most wildlife, rabid bats are likely to be found. Because the animals are nocturnal and elusive, contact with bats may go unnoticed. The rabies surveillance system has been remarkably successful. The cost of rabies control is significant, however.
Smallpox was a particularly feared disease that is believed to have first emerged in Asia about the time of Christ and tended to spread in major epidemics that claimed millions of lives in China, Japan, the Roman Empire, Europe, and the Americas. It was highly contagious, spread by aerosol or by touch. The concept of vaccination originated with smallpox: the observation that survivors of the disease were immune to future infection inspired the idea that people could be protected against serious illness by inoculating them with small amounts of infected matter from a person suffering a mild case. In 1976, the practice of immunization became less risky when the British physician Edward Jenner proved the inoculation with cowpox matter, which was harmless to humans, provided immunity against smallpox. By 1958, routine immunization had eliminated smallpox in the US and other industrialized countries. However, it was still widespread in 33 underdeveloped countries. With support from both the US and the Soviet Union, WHO developed plans for a program to eliminate smallpox. The last case was found in Somalia in October 1977. Now the smallpox virus officially remains in only two places, stores in laboratories at the CDC and in a Russian laboratory in Siberia. Plans for destruction of the smallpox virus have been put on hold, and research priorities have focused on developing an improved vaccine and finding drugs that would be effective against the virus.
A British physician who observed that milkmaids were immune to smallpox, because they had contracted cowpox. He proposed that people could be immunized against smallpox with a cowpox vaccine.
Poliovirus, like smallpox virus, infects human beings only, and polio similarly has the potential to be eradicated. In 1988, WHO set a goal of eradicating polio by the year 2000. This goal was not met, but substantial progress has been made against this crippling disease: polio has been essentially eliminated from the Western Hemisphere, Europe, and the Western Pacific. Only Southern Asia and Sub-Saharan Africa still have a significant incidence of polio. An important reason that eradication did not succeed in these countries is that rumors spread in 2003 among Muslims, especially in Nigeria, that the polio vaccine had been deliberately contaminated to cause AIDS or infertility. Now, only three countries continue to have endemic polio—Nigeria, Pakistan, and Afghanistan but eradication from these countries has proven extremely difficult. As long as the disease exists in these countries, it tends t spread to neighboring countries.
Why is Polio Harder to Eradicate than Smallpox?
There are several reasons why polio is proving more difficult to eradicate than smallpox. Unlike smallpox, there are many invisible cases of polio, in which children may be infected, able to spread the virus by the fecal-oral route but not show symptoms. The vaccine is imperfect and must be administered several times to become effective. Political upheaval in parts of Pakistan and Afghanistan has interfered with immunization campaigns in those countries.
Measles, another viral disease that could in theory be eradicated, offers an example of what happens when public health relaxes its vigilance. Before a vaccine was available, almost all children contracted measles. A vaccine became available in 1963, and the number of cases in the US dropped precipitously. In 1978, the US Department of Health and Human Services set a goal to eradicate measles from this country by 1982. That ambition proved overly optimistic. Two problems interfered. First, outbreaks of measles began to occur among high school and college students who had been vaccinated as babies. It became clear that the immunity conferred by vaccination in infancy wears off and that a booster vaccination is necessary in older children. The greater problem was that too many children were not being immunized until it was required for entry into school. This was particularly true in large cities among poor African American and Hispanic children. More than 27,000 American children contracted measles in 1990, and 89 died. Even the surveillance system was doing poorly. The public health system was shaken by this evidence of failure, and a better job is now being done on measles immunization.
An attempt to eradicate an eradicable disease can backfire if it is not conducted with sufficient political will, knowledge, and resources. This was the case with malaria, which was the target of an international eradication campaign in the 1950s and 1960s. There is no nonhuman reservoir for the malaria-causing parasites, but the route of transmission is a vector. The primary weapon in the eradication effort was the pesticide DDT. While the campaign produced dramatic results, funding ran out before the objective was achieved, and there was a resurgence of the disease with greater impact than ever. A combination of factors contributed to the calamity: DDT- resistant mosquitoes emerged; the pathogen developed resistance to the main antimalarial drug, chloroquine; and populations in former malarial areas lost their immunity to the disease because of lack of exposure. The disease occurs mainly in tropical and subtropical areas and has been largely eliminated in the US, but global climate change and international travel could contribute to the re-emergence of malaria as a public health problem in the South.
Fear of Vaccines
Autism often becomes apparent at about the age when the vaccine is given. Similarly, unfounded stories about side effects of the pertussis (whooping cough) vaccine—that it might cause sudden infant death syndrome (SIDS)—led many parents to resist the vaccine. Some vaccines do have real risks. These risks are much smaller than the risks of the diseases in an unvaccinated population. Because of the success of vaccinations, people have never seen these diseases and thus no longer fear them. Another drawback of people's fear of vaccines is that pharmaceutical companies have become reluctant to invest in developing them. Parents' tendency to blame a recent immunization for any serious health problem suffered by their children leads them to sue the company that made the vaccine. This experience, together with the fact that prices that can be charged for vaccines tend to be low, has caused many companies to drop vaccine production altogether.
It is often in wealthy communities that parents refuse to subject their children to the small risk of immunization. They count on the fact that most other children are vaccinated to protect their own children from being exposed. However, much of the protection afforded by a high rate of immunization in a population comes from "herd immunity," the phenomenon by which those who refuse to be immunized are unlikely to be exposed to a disease because the majority of the population is immune.
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