Chapter 19: Epidemiology

Outbreak

A cluster of cases occurring during a brief time interval and affecting a specific population; an outbreak may herald the onset of an epidemic

Placebo

A mock drug. Effectiveness of treatment in experimental studies are compared to placebos.

Factors that contribute to emergence and reemergence of disease: Microbial evolution

A new serotype of Vibrio cholerae, O139, has gained the ability to produce a protective capsule. Immunity against earlier strain doesn't guarantee immunity against the new one. Scientists are concerned that avian influenza virus will evolve to spread from person to person. Resistance to antimicrobial drugs is contributing to the reemergence of many diseases, including malaria.

Reservoir of infection

A pathogen must have a suitable environment in which to live. The natural habitat, "reservoir of infection" may be on or in an animal, including humans, or in an environment such as soil or water. The reservoir affects the extent and distribution of a disease. Once the reservoir is identified, people can be prevented from coming in contact with the disease source. The US does not have epidemics of plague, in part because populations of wild rats, mice, and prairie dogs (natural reservoirs of Yersinia pestis, the bacterium that causes plague) are controlled.

Portal of exit

A pathogen must leave its reservoir to be transmitted to a susceptible host. If the reservoir is an animal, the body orifice or surface form which a microbe is shed is called the portal of exit. Microorganisms that inhabit intestinal tract are shed in the feces. Pathogens such as vibrio cholerae, which cause massive volumes of watery diarrhea, can contaminate drinking water and food. Pathogens such as mycobacterium tuberculuosis and various respiratory viruses exit the body in droplets of saliva and mucus when people talk, laugh, sing, sneeze or cough. Organisms that inhabit the skin are shed on skin cells. Genital pathogens such as Neiserria gonorrhoeae can be carried in semen and vaginal secretions. Hantavirus is found in the saliva, urine and droppings of deer mice

Common-source epidemic

A rapid rise in numbers of people who become ill suggesting they were all exposed to a single, common source of infectious agent such as contaminated chicken at a picnic.

Descriptive study

A study done by collecting data that characterizes the occurrence, from the time and place of the outbreak to the individuals affected. The information is used to compile a list of possible risk factors involved in the spread of the disease

Healthcare Airborne transmission

Airflow to operating rooms is usually regulated so that it is supplied under slight pressure, preventing contaminated air in the corridors from flowing into the room.Floors are washed with a damp mop or floor washer rather than swept in order to avoid resuspending microbes into the air. To exclude airborne microorganisms and viruses from rooms in which extremely susceptible patients reside, i.e. those who have recently undergone a bone marrow transplant, HEPA filters are used that filter out most airborne particles including microorganisms.

World Health Organization (WHO)

An agency of the united nations with 193 member states. Devoted to achieving the highest possible level of health around the globe. Has four main functions 1) provide world-wide guidance in the field of health; 2) set global standards for health; 3) cooperatively strengthen national health programs; and 4) develop and transfer appropriate health technology. Disseminates information through periodicals and books. I.e.the weekly epidemiological record. WHO hopes to eradicate polio, measles, and dracunculiasis soon. Political and social upheaval, complacency, and lack of financial support can result in a resurgence of diseases unless the pathogens are completely eliminated.

Host factors that influence the epidemiology of disease: Religious and cultural practices

An infant who is brest fed is less likely to have infectious diarrhea because of the protective antibodies in the mother's milk. Groups who eat traditional dishes made from raw freshwater fish are more likely to acquire the tapeworm Diphyllobothrium latum that is normally killed by cooking.

Epidemic

An unusually large number of cases in a population. May be caused by diseases not normally present in a population, such as cholera being reintroduced to the Western Hemisphere, or by fluctuations in the incidence of endemic diseases such as influenza and pneumonia

Vectors

Any living organism that can carry a disease causing microbe, most commonly they are arthropods such as mosquitoes, flies, fleas, lice and ticks. A vector can carry a pathogen externally or internally. Diseases such as malaria, plague, and lime disease are transmitted via arthropods that harbor pathogen internally. The vector can inject infectious agent while taking a blood meal or deposit the pathogen when it defecates on the skin where it can be inoculated when the individual scratches the bite. Fleas inject Yersinia pestis while attempting to take a blood meal. Prevention of vector-borne diseases relies on arthropod control. Malaria was eliminated from the nation through a combination of mosquito elimination and prompt treatment of infected patients, worldwide eradication efforts failed.

Direct contatcct

Can be simple as a handshake or as intimate as sexual intercourse. Sometimes the transfer of few microbes can initiate an infection. Shigella species, intestinal pathogens have an infectious dose of 10 to 100 cells, this can easily be transferred by shaking hands.

Factors that contribute to emergence and reemergence of disease: War and civil unrest

Can disrupt the infrastructure on which disease prevention relies. Refugee camps that crowd people into substandard living quarters lacking toilet facilities and safe drinking water are hot-beds of infectious diseases such as cholera and dysentery. War also disrupts disease eradication efforts

Factors that contribute to emergence and reemergence of disease: Complacency and Public health efforts

Complacency can develop as infectious diseases become less of a concern. The plan to eliminate tuberculosis brought early success, which diverted attention and money to other more common diseases. At the same time social welfare programs were curtailed. As a result, poor health and living conditions put more people at risk of developing active tuberculosis and the disease reemerged as an increasing threat. New public health measures have brought the disease back under control in the US

Factors that contribute to emergence and reemergence of disease: Development

Dams have inadvertently extended the range of certain diseases. Transmission of the disease schistosomiasis relies on the presence of an aquatic snail that serves as a host for the Schistosoma parasite. Construction of dams such as the Aswan Dam on the Nile River has increased the habitat for the snail, extending the distribution of the disease

Factors that contribute to emergence and reemergence of disease: Changes in human society

Daycare centers, where diapered infants mingle, oblivious to sanitation and hygiene are relatively new. These centers can be hotbeds of contagious diseases . Giardia and Shigella, which have low infectious doses can easily spread because infants often explore through taste and touch and are likely to ingest fecal organisms. Many young children have not yet acquired immunity to communicable diseases such as colds and diarrhea that re readily transmitted among this susceptible population

Analytical studies

Designed to determine which of the potential risk factors in a descriptive study are actually relevant to the spread of the disease

Contagious or communicable diseases

Diseases that can be transmitted form one host to another, such as measles, colds, and influenza. Transmission is determined by interactions etween the environment, the pathogen and the host. Control of any of those factors may break the infection cycle, hindering or preventing the disease.

Non-Communicable diseases

Diseases that do not spread from one host to another. Microorganisms that cause these diseases most often arise from an individual's normal microbiota or from the environment. Clostridium tetani, can enter a host from the environment but is not then transmitted to another host

John Snow

Documented that the choler epidemics plaguing England from 1849 to 1854 were due to contaminated water supplies by carefully comparing the conditions of households that were affected by cholera to those that were not, eventually determining that the primary difference was their water supply. His analysis saved countless lives

Public health departments

Each state has a public health lab responsible for infection surveillance and control as well as other health-related activities. Individual states have authority to mandate which diseases must be reported by physicians to the lab. The prompt response of health authorities in Washington that helped stop an E. coli O157:H7 outbreak in 1993 by contaminated burgers was partly because they were one of the few states with reporting measures for the organism. The epidemic had started in other states but gone unrecognized. The fungus Cryptococcus gattiii appeared in Washington in 2007, causing lung infections and meningitis in healthy persons, and is now closely watched by health authorities in oregon and california

Category C agents

Emerging pathogens that could be enginered for easy dissemination. Include Nipah virus, first recognized in 1999 and hantavirus, first recognized in 1993

Endemic

Endemic diseases are constantly present in a give population. The common cold is endemic in the united states.

Preventing healthcare-associated infections

First detect their occurrence and then establish policies to prevent their development. Nearly every hospital has an infection control committee, composed of representative of various professionals in the hospital, including nurses, physicians, dietitians, housekeeping staff and microbiology lab personnel and a hospital epidemiologist, specially trained in hospital infection control. Hospitals may employ an infection control practitioner (ICP) whose role is to perform active surveillance of the types and numbers of infections that arise in the hospital. The infection control committee and the ICP, drafts and implements preventive policies following the guidelines suggested by the standard precautions ad the transmission-based precautions. The CDC has established the Healthcare Infection Control Practices Advisory Committe (HICPAC) to provide advice to hospitals and recommend guidelines for surveillance, prevention, and control of nosocomial infections.

Mechanical vector

Flies that land on feces can pick up intestinal pathogens such as E. coli and Shigella species on their legs. If the fly then lands on food and transfers the microorganisms, it serves as a mechanical vector, carrying the microbe on its body from one place to another.

Reservoirs of infectious agents in healthcare settings: Healthcare environment

Gram negative rods, particularly Pseudomonas aeruginosa, can thrive in healthcare environments such as sinks, respirators, and toilets. It is resistant to many disinfectants and antimicrobial drugs, and requires few nutrients enabling it to multiply in environments containing little other than water. Many nosocomial infections have been traced to soaps, disinfectants, and other aqueous solutions contaminated with the organism.

Direct Transmission--Medical Personnel

Handwashing between contact with individual patients helps prevent the spread of disease, but this is often overlooked. Healthcare personnel should routinely wash or disinfect their hands after touching one patient before going to the next. Should perform a more thorough hand scrubbing lasting 10 minutes with a strong disinfectant before participating in a surgical operation, or when working in a nursery or an intensive care or isolation unit. Healthcare workers are instructed to wear gloves when they have contact with blood, mucous membranes, broken skin, or body fluids.

The standard precautions of the Hospital Infection Control Practices Advisory Committee (HICPAC)

Hang hygiene, Personal protective equipment, Respiratory hygiene/cough etiquette, Patient placement, Patient-care equipment and instruments/devices, Care of the environment, Textiles and laundry, Safe injection practices, Infection control practices for special lumbar puncture procedures, Work safety.

Factors that contribute to emergence and reemergence of disease: Population expansion

Humans can expand into areas where they come in contact with new reservoirs of disease (like with the Ebola virus) or have increased contact with reservoirs. Building homes along forest borders exposes humans to more deer that carry ticks, the host for Borrelia burgdorferi, the cause of lyme disease.

Propagated epidemic

If the numbers of ill people rise gradually, the disease is likely contagious, with one person transmitting it to several others and so on.

Biological vector

In Malaria caused by Plasmodium, the mosquito transmits the parasite and plays an essential role in its reproducive life cycle. The pathogen multiplies to high numbers within this vector.

Other components of the Public Health Network

Includes public schools, which report absentee rates, and hospital labs, which report on the isolation of pathogens pathogens with epidemiological significance. The news alert the public of the presence of infectious diseases

Human Reservoirs

Infected humans are a significant reservoir of most communicable diseases. Humans can be the only reservoir but in some cases pathogens can exist in humans, other animals and in the environment. Diseases can be easier to control when humans are the only reservoir because it is more feasible to set up programs to prevent and control the diseases in humans than in wild animals. The eradication of smallpox was due to the combination of widespread vaccination programs resulting in fewer susceptible people, and isolation of those who became infected. The virus no longer had a reservoir.

Reservoirs of infectious agents in healthcare settings: Healthcare Workers

Infected personnel who report to work with even a mild case of influenza can expose patients to an infectious agent that has serious or fatal consequences to those in poor health. A healthcare worker who is a carrier of a pathogen such as staphylococcus aureus or Streptococcus pyogenes are more troublesome, they may not realize they pose a risk to patients until they are implicated in an outbreak. Carriers who are surgical team members pose a threat because inoculation of pathogen directly into a surgical site can result in a systemic infection.

Healthcare-associated infections (HAIs) or Nosocomial infections

Infections individuals acquire while receiving treatment in a healthcare setting. One of the top 10 causes of death in the United States. Modern medical practices have increased the incidence of these infections. 5 to 10% of patients admitted to a hospital in the United States develop a nosocomial infection, adding billions to the price of healthcare. Many infections originate from the patient's own normal microbiota, but approximately one-third are potentially preventable. These infections can range from very mild to fatal. Some arise during the stay, but some aren't discovered until after discharge. UTIs are the most frequent type.

Host factors that influence the epidemiology of disease: genetic background

It is difficult to determine the relative importance of genetic, cultural and environmental factors. However, the genetic basis for resistance to a few infectious diseases is known. Many people of black African ancestry are not susceptible to malaria caused by Plasmodium vivax because they lack a specific red blood cell receptor used by the infecting organism. Some populations of North European ancestry are less susceptible to HIV infection because they lack a specific receptor on their white blood cells.

Descriptive studies: The person

It is important to determine the profile of those who become ill. Age, gender, ethnicity, occupation, personal habits, previous illnesses, socioeconomic class and marital status may all give clues about risk factors for developing the disease. I.e. the swiss ski resort epidemic, the cases only occurred in tourists because the natives mostly drank wine instead of water.

Droplet transmission

Large microbe-laden respiratory droplets fall to the ground no farther than a meter from release. People in close proximity can inhale those droplets. Physical contact is not necessary, droplet transmission is considered contact transmission because of the close range. This form of transmission is important as a source of contamination in densely populated buildings such as schools and military barracks. Desks should be more than 1.5 meters apart to minimize the transfer of infectious agents. Spread of respiratory diseases is minimized if people cover their mouths when they cough or sneeze.

Center for Disease Control and Prevention (CDC)

Located in Atlanta, Georgia. Part of the U.S. Department of health and human services. Provides support for infectious disease laboratories in the United States and abroad and collects data on diseases that impact public health. The CDC publishes the Morbidity and Mortality Weekly Report (MMWR) weekly that summarizes the status of a number of diseases. The number of new cases of over 50 notifiable diseases is reported to the CDC by individual states. The list of diseases is determined through collaboration of the CDC and state health departments. The diseases are usually of high incidence or pose danger to public health. The MMWR reports this data and historical numbers to reflect any trends. The CDC also conducts research relating to infectious diseases and can dispatch teams worldwide to assist with identifying and controlling epidemics.

Host factors that influence the epidemiology of disease: General Health

Malnutrition, overcrowding, and fatigue increase people's susceptibility to infectious disease, enhancing its spread. Infectious diseases are a bigger problem in developing areas where individuals crowd together without proper food or sanitation. Good general health results in increased resistance to diseases such as tuberculosis. When infection does occur in a healthy individual, it is more likely to be asymptomatic or result in mild disease.

Reservoirs of infectious agents in healthcare settings: Patient Microbiota

Many nosocomial infections originate from the patient's own microbiota. Nearly any invasive procedure can transmit these organisms to otherwise sterile body sites. When intravenous fluids are administered, Staphylococcus epidermidis, a common member of the skin microbiota, can potentially access the bloodstream causing bacteremia. The immune system can usually eliminate these organisms, but the underlying illness of many patients compromises their immunity and they can develop bloodstream infection. Patients who undergo intestinal surgery are prone to surgical site infection by their normal bowel microbiota. Patients who are on certain medications or have impaired cough reflexes can inadvertently inhale their normal oral miicrobiota resulting in health-care associated pneumonia. Immunocompromised patients, i.e. people who have undergone chemotherapy or are on immunosuppressive drugs, are prone to activation of latent infections their immune system could previously control. Latent infections of Toxoplasma gondii, a protozoan parasite commonly acquired during childhood, can become activated and cause a lifethreatening disease.

Double-blind

Neither the researcher nor the patients know who is receiving the experimental treatment. Done to prevent bias

Fomite Transmission--Medical Devices

Nosocomial infections most often result from medical devices that breach first-line barriers of normal host defense. Urinary tract catheterization can introduce microorganisms into the normally sterile bladder, because urine is a great growth medium, the tract often becomes infected. IV catheters can introduce microorganisms into the bloodstream. When normal skin microbiota colonize the tip of an indwelling catheter or when environmental organisms contaminate IV fluids or the lines that deliver them. Normally benign skin microbiota can cause life-threatening bacteremia upon gaining access to the bloodstream. Respirators that assist breathing by pumping air into the trachea can deliver microorganisms. If a nutritionally versatile organism like Pseudomonas species gains access to water droplets in the machine it can multiply and be pumped into the lungs. Inadequately sterilized instruments can also transmit infectious agents. Endoscopes and other heat-sensitive instruments are often treated with chemical sterilants to render them microbe-free. Improper use of these chemicals can result in the survival of some organisms.

Fecal-oral transmission

Once on the hands an organism can inadvertently be ingested. Handwashing is important in preventing this type of spread of disease. Even washing in just water reduces the numbers of pathogens on the hands, which decreases the possibility of transferring or ingesting enough cells to establish an infection. Routine handwashing is considered to be the most important measure for preventing the spread of infectious disease. Alcohol gels may provide some protection but are not effective against all pathogens. Pathogens that cannot survive for extended periods in the environment must generally be transmitted through direct contact. Treponema pallidum (syphilis) and Neiserria gonorrhoeae both die quickly when exposed to a cold, dry environment and require sex for their transmission

Waterborne Disease

Outbreaks can involve large numbers of people because municipal water systems distribute water to widespread areas. he 1993 waterborne outbreak of cryptosporidium parvum in milwaukee wisconsin contaminated over 400,000 people. Prevention of waterborne diseases required water filtration and disinfection and proper disposal and treatment of sewage.

Food and water

Pathogens that infect the digestive tract can be transmitted through contaminated food or water. Foods can become contaminated in many ways. Animal products can have pathogens that originated from the animal's intestinal tract. I.e. poultry contaminated with salmonella or campylobacter and hamburger contaminated with E. coli. Pathogens can be inadvertently added during food preparation. S. aureus carriers who don't wash their hands can contaminate the food

Reservoirs of infectious agents in healthcare settings: Other patients

Patients are often hospitalized because they have severe infectious diseases. These pathogens can be discharged into the environment via skin cells, respiratory droplets, and other body secretions and excretions. Scrupulous cleaning and the use of disinfectants minimize the spread of these pathogens

Symptomatic infections

People with symptomatic illnesses are a source of infectious agents. Although people generally are cautious and stay home and rest to avoid transmitting their illness to others disease can still be spread. I.e. people who are in the incubation period of mumps shed virus before symptoms appear.

Host factors that influence the epidemiology of disease: Herd immunity

Previous exposure or immunization of a population to a disease agent or an antigenically related agent decreases incidence of the disease.A disease is unlikely to spread in a population in which 90% of the individuals are immune to the disease agent. Herd immunity protects non-immune individuals when infectious agent can't spread in a population because most potential hosts are immune.Some infectious agents can undergo antigenic variation and overcome herdimmunity. Humans have no immunity to the H5N1 (avian) influenza strain

Air

Respiratory diseases are often transmitted through the air. When particles larger than 10 um are inhaled, they are trapped in the mucus lining of the nose and throat and eventually swallowed. Smaller particles can enter the lungs, where any pathogens they carry can cause disease. Droplet nuclei and other airborne particles, including dead skin cells, household dust and soil disturbed by the wind can also carry respiratory pathogens. An air conditioning system can distribute contaminated air. The number of viable organisms in the air can e estimated using a machine that pumps a measured volume of air, including any suspended dust and particles, against the surface of a nutrient rich medium in a petri dish. The number of bacteria in sampled air rises in proportion to the number of people in a room. Airborne transmission is very difficult to control.Ventilation systems can help. High efficiency particulate air (HEPA) filters remove airborne organisms.

Environmental Reservoirs

Some pathogens have environmental reservoirs. Clostridium botulinum (botulism) and clostridium tetani (tetanus), are widespread in soils. Pathogens that have environmental reservoirs are difficult or impossible to eliminate

Asymptomatic carriers

Someone can carry a pathogen without any ill effects. They can shed the organism intermittently or constantly for months, years or a lifetime. Some carriers have an asymptomatic infection; their immune system is actively responding to the invading microorganism but they have no obvious clinical symptoms. They don't consider themselves a reservoir so they don't worry about spreading the pathogen. People with asymptomatic infections are a complicating factor in the control of sexually transmitted infections (STIs) such as gonorrhea. Up to 50% of women infected have no symptoms and unknowingly transmit the disease. Infected men are more often symptomatic and seek treatment. Gonorrhea infections are treated with antibacterial drugs. Some potentially pathogenic microbes can colonize the skin or mucosal surfaces becoming part of the microbiota. Many people carry staph aureus as part of their nasal or skin microbiota, these carriers may never have illness or disease because of this, but they remain a potential source of infection to themselves and others. Ridding a colonized carrier is often difficult even with antimicrobial drugs.

Case-control study

Starts with the disease and attempts to identify the causative chain of events leading to it. If an activity or event is common among the cases, but not among the controls it may have been a factor in the development of the disease. It is important to select controls that match the cases with respect to variables not thought to be associated with disease such as age, gender, and socioeconomic status to ensure accuracy.

Cohort groups

Study groups that have a known exposure to the risk factor

Pathogen factors that influence the epidemiology of disease: Virulence

Successful pathogens have virulence factors that increase their ability to cause disease. These may allow pathogens to adhere to a host or penetrate host cells. Neiserria gonorrhoeae bind to mucosal epithelial cells; Shigella species adhere to intestinal epithelial cells and promote their own uptake. Other factors allow pathogens to thwart immune defenses of the host. Capsules such as those of Streptococcus pneumoniae protect microbial cells from phagocytosis. Mycobacterium tuberculosis can survive within activated macrophages. Pathogens damage the host, perhaps by production of toxins or destructive enzymes, or perhaps by stimulating inflammatory response that damages host tissue. E. coli produces a cytotoxin that destroys cells that line blood vessels. Clostridium perfringens, (gas gangrene) destroys host cell membranes with a phospholipase. Organisms that possess a variety of virulence factors are more likely to cause severe disease

Cross-sectional study

Surveys a range of people to determine the prevalence of such characteristics as infection, presence of risk factors associated with disease, or previous exposure to a disease causing agent. This assessment may suggest associations between risk factors and disease. Does not attempt to establish causes of disease

Factors that contribute to emergence and reemergence of disease: Advances in technology

Technology can inadvertently create new habitats for microorganisms. I.e. contact lenses. Contact lenses of people who don't employ proper sanitation techniques resulted in new types of eye infections

Category A agents

The CDC separates bioterrorism agents into three categories based on the ease of spread and severity of disease. Category A agents pose the highest risk because they are easily spread or transmitted from person to person and result in high mortality. Include: Bacillus anthracis, Botulism, Yersinia pestis, Smallpoxx, Francisella tularensis, Viruses that cause hemmorhagic fevers

Bioterrorism

The deliberate release of infectious agents or their toxins as a means to cause harm

Pathogen factors that influence the epidemiology of disease: Incubation period

The extend of the spread of an infectious agent is influenced by its incubation period. Diseases with long incubation periods can spread extensively before the first symptomatic cases appear. The spread of typhoid fever from a ski resort in switzerland in 1963 as many as 10,000 people were exposed to drinking water containing small numbers of salmonella enterica serotype typhi. The long incubation period of the disease, 10 to 14 days allowed the organism to be spread by the skiers, who flew home to various parts of the world before they became ill. More than 430 cases of typhoid fever appeared in at least 6 countries.

Index case

The first case in a propagated epidemic

Incidence

The incidence of a disease is the number of new cases in a specific time period in a given population. Provides a useful measure of the risk of an individual contracting the disease

Morbidity

The incidence of disease in a population at risk

Descriptive studies: The Place

The location of disease acquisition identifies the general site of contact between the person and the infectious agent, helping to pinpoint the exact source. This may also give clues about potential reservoirs, vectors or geographical boundaries that might affect disease transmission. I.e. malaria can only be transmitted in regions with the mosquito vector

Mortality

The overall death rate in a population

Case-fatality rate

The percentage of a population that dies from a specific disease, diseases such as plague and ebola hemorrhagic fever are feared because of their high case-fatality rate. As the case-fatality rate for AIDS has decreased the prevalence rate of AIDS has gone up.

Attack rate

The percentage of people who become ill in a population after exposure to an infectious agent. The attack rate reflects many factors, including the infectious dose of the organism and the immune status of the population

Prevalence

The prevalence of a disease is the total number of cases at any time or for a specific period in given population. Reflects the overall impact of a disease on society because it includes old and new cases, taking the duration of the disease into account.

Pathogen factors that influence the epidemiology of disease: Dose

The probability of infection and disease is lower when someone is exposed to a small number of a pathogen. A certain minimum number of cells of the pathogen must enter the body and produce enough damage to cause disease. Because host defenses are mobilized at the same time and are racing to eliminate the invader, small doses often result in asymptomatic infections--the immune system eliminates the organism before symptoms appear. There are few if any infections for which immunity is absolute. An unusually large exposure to a pathogen such as can occur in a lab accident may produce serious disease in a person who is ordinarily immune to that microbe. Even immunized persons should take precautions to minimize exposure to infectious agents.

Non-Human Animal Reservoirs

The source of many pathogens. Poultry are a reservoir of gastrointestinal pathogens such as campylobacter and salmonella. Raccoons, skunks and bats are reservoirs of the rabies virus. Plague is still occasionally transmitted in southwestern states where Y. pestis is endemic in prairie dogs and other rodents. Rodents, particularly the deer mouse are the reservoir for hantavirus.

Epidemiology

The study of disease patterns in populations

Descriptive studies: The Time

The timing of an outbreak may yield clues. The time between onset of symptoms reflects the incubation period of the disease. The season in which the epidemic occurs may be important. Respiratory diseases including influenza, respiratory syncytial virus infections, and the common cold are more easily transmitted in crowded indoor conditions during the winter. Vector and foodborne diseases are more often transmitted in warm weather when people are more likely to be exposed to mosquitoes and ticks, or eat picnic food that has not been properly stored

Host factors that influence the epidemiology of disease: Age

The very young and the elderly are more susceptible to infectious agents. The young child immune system is not fully developed. They are more vulnerable to diseases such as meningitis caused by Haemophilus influenzae. Hib vaccine introduced in the late 80s dramattically decreased incidence of meningitis in this age group. Elderly are more prone to disease because immunity wanes over time. Influenza outbreaks in nursing homes often have high case fatality rates. Older adults are less likely to update their immunizations. To prevent tetanus, a booster vaccine is needed once every 10 years.

Retrospective Study

This type of study is done following a disease outbreak. The actions and events surrounding clinical cases (individuals who developed the disease) are compared to those surrounding appropriate controls (those who remained healthy)

Prospective study

This type of study looks ahead to see if the risk factors identified by the retrospective study predict a tendency to develop the disease. Cohort groups are selected and followed over time. The incidence of disease in those who were exposed to the risk factor and those who were not is then compared. By following cohort groups, the study attempts to determine if the suspected cause does correlate with the expected effect. Less prone to bias of inaccurate recall than retrospective study but more expensive and time-consuming, especially when a disease has a long incubation period

Emerging diseases

Those that are novel or have recently increased in incidence. Include new or newly recognized diseases such as severe acute respiratory syndrome (SARS), mad cow disease, and avian flu, as well as familiar ones such as malaria and tuberculosis that are reemerging after years of decline

Portal of entry

To complete the cycle of infection, the pathogen is transmitted to the next host and enters through a body surface or orifice called the portal of entry. To cause disease a pathogen must be transmitted to a new host and colonize a surface of or enter that new host. Cells of a Shigella species can be transferred by a handshake but they will only cause disease if they are transferred to the mouth or to food that is then eaten. The mouth is then the portal of entry. Respiratory pathogens released in the air during a cough generally cause disease only when they are inhaled. The nose is then the portal of entry. Many organisms can cause disease if they enter one body site, but are harmless if they enter another. Enterococcus faecalis can cause bladder infection if it enters the urinary tract but is harmless when ingested and colonizes in the large intestine becoming part of the normal microbiota.

Vertical transmission

Transfer of a pathogen from a pregnant woman to her fetus or from a mother to her infant during childbirth or breast feeding

Indirect contact

Transfer of pathogens via fomites. Carriers of S. aureus may inoculate their fingers with the organism when touching a skin lesion or nostril, organisms on the finger can be transferred to a fomite. When someone comes into contact with said fomite they can acquire the pathogen. Handwashing prevents spread by these means

Horizontal transmission

Transmission of a pathogen from one person to another through the air, by physical contact, by ingestion of food or water, or via a living agent such as an insect

Experimental study

Used to judge the cause-and-effect relationship of risk factors or preventive factors, and the development of disease. Done most frequently to assess the value of an intervention or treatment.

Factors that contribute to emergence and reemergence of disease: Climate changes

Warm temperature favor reproduction and survival of some arthropods, which can serve as vectors for diseases such as malaria and West Nile encephalitis. Heavy rainfall and flooding caused by the effects of El Nino may be related to surges of cholera in Africa

Pandemic

When an epidemic spreads worldwide, as AIDS hs

Sporadic

When cases occur only from time to time

Cross contamination

When pathogens from one food are transferred to another. A cutting board used to cut raw chicken then cooked potatoes can transfer salmonella cells from the chicken onto the potatoes. Microorganisms can multiply to high numbers if contaminated food is not refrigerated because foods are a rich nutrient source. Sanitary preparation as well as thorough cooking and proper storage can prevent foodborne diseases.

Droplet nuclei

When people talk, laugh, sing, sneeze or cough they continually discharge microorganisms in liquid droplets. Large droplets fall to the ground, but small droplets dry, creating droplet nuclei composed of microbes attached to a thin coat of the dried material. These can remain suspended indefinitely in the presence of even slight air currents.

Factors that contribute to emergence and reemergence of disease: Mass production, widespread distribution, and importation of food

Widespread distribution of food contaminated with pathogens can result in a broad outbreak of disease. Contaminated hotdogs shipped from a single food manufacturing plant in Canada in 2008 leading to a country-wide epidemic of listeriosis that killed 21 people

Host factors that influence the epidemiology of disease: Gender

Women are more likely to develop urinary traact infections because their urethra (tube that connects the urinary bladder to the external environment) is relatively short. Microbes can ascend the urethra into the bladder. Pregnant women are more susceptible to listeriosis caused by Listeria monocytogenes.

Ignaz Semmelweis

Working for professor Johann Klein he noticed incidence of puerperal fever wars four times higher in Dr. Klein's section of the hospital than the midwives, suggested that doctors wash their hands with strong disinfectants before attending patients. puerperal fever rate dropped to one-third its previous level but people got mad at him instead of admitting responsibility.

Zoonoses

Zoonotic diseases. Diseases such as plague and rabies that can be transmitted to humans but primarily exist in other animals. Are often more severe in humans than in their animal host because infection in humans is accidental, there has been no evolution toward balanced pathogenicity that exists between host and parasite.

Fomites

inanimate objects such as clothing, table tops, doorknobs and drinking glasses that are involved in the transfer of pathogens

Category B agents

pose moderate risk because they are relatively easy to spread and cause moderate morbidity. Include organisms that cause food and waterborne illness, Brucella species, Burkholderia mallei and pseudomallei, Coxiella burnetii and chlamydophila (chlamydia) psittaci.