Epidemiology and Public Health Microbiology

Epidemiology is the study of the occurrence, distribution, and control of disease in populations.
Epidemiologists are interested in the life history of pathogens. What it does to the host, the spread of diseases, where did it start and what is its mode of transmission.

Terms used by epidemiologists
Prevalence is the percentage of individuals infected with the disease. The incidence of a disease is the number of diseased individuals in a population.

Epidemic occurs when a disease occurs in high numbers in a community at the same time. Pandemic is a widely distributed epidemic. Endemic is a disease constantly present in a population usually in low incidence.

Mortality expresses the incidence of death in a population. Morbidity refers to the incidence of disease in a population and includes both fatal and nonfatal.

Progression of disease
A. Infection
B. Incubation period - time between infection and symptoms. Can be short or a long period of time. Depends on inoculum size, virulence of pathogen, and resistance of the host.
C. Acute period - disease at its height.
D. Decline period - disease symptom are subsiding.
E. convalescent period - regain strength and health

Disease reservoirs (see table 22.2)
origins of the infectious agent. Can be inanimate or animate.

Zoonosis - a disease that is primarily in animals but may go to humans. Maintenance of the pathogen in nature requires animal to animal transfer.
Two examples include bovine tuberculosis and brucella. Eliminating infected animals was central in their control. Also pasteurization of the milk controlled tuberculosis since this was a primary mode of transmission.

Carriers - a carrier is someone without clinical signs but carriers of the disease agent. May be individuals in the incubation period - referred to as acute carriers. Respiratory infections are common among these since we are non clinical for a while before coming down the symptoms. Chronic carriers may be recovered patients or subclinical infections. These individuals may appear to be healthy but they harbor and transmit the agent - typhoid mary for example.

Transmission of pathogens
Host to host - respiratory route (for example the common cold or flu) or direct contact are common. Pathogens that are transferred by direct contact frequently cannot survive outside the host for significant periods of time. Examples include sexually transmitted diseases such as gonorrhea or syphilis. They are both extremely sensitive to drying effects and do not last on exposed surfaces for even the briefest period of time. Direct contact required for many skin pathogens such as Staphylococci or fungi.

Indirect host to host transmission - vectors such as arthropods (such as fleas, ticks, or mites) or vertebrates (such as dogs or rodents). Many of the arthropods require a blood meal where they acquire the pathogen from an infected individual. In the arthropod, the pathogen population builds up and now becomes sizable.

Inanimate objects such as toys, books, bedding are transmit disease. These objects are called fomites.

Epidemics (See figure 22.5) can be divided into common-source epidemics and host-host epidemics. In common source epidemics a large number of individuals become infected through a common source such as food or water. Pathogens are usually intestinal and are due to fecal contamination of food or water sources. Characterized by a rapid rise to a peak number of individuals infected and a slower still rapid decline.

Host-host epidemics shows a relatively slow, progressive rise in infected individuals.

Host community - Excellent example of coevolution of the host and pathogen. Originally the host population is sensitive to the pathogen and epidemics are common. But with each epidemic, the number of resistant hosts increases and finally there is a balance between the host and the pathogen. The host usually develops resistance to the pathogen and the pathogen is trying to overcome the resistance. Example of this is a virus and rabbits in Australia.
Rabbits were rampant during the 1950s after their introduction in the 1800s. Myxoma virus, which is pathogenic to the European rabbit at the time but non pathogenic to its host the south American rabbit, was introduced into Australia. Transmitted by mosquitoes. In a brief time (6 years) both the virus and host rabbits had changed. During the first year about 95% of the infected rabbits died. in 6 years the mortality rate dropped to 84% and the virus was less virulent and changes in the host resistance were noted. There were genetic changes in both the host and virus.

Hospital infections - Nosocomial infections occur in about 5% of all patients up to 10% in intensive care units. In all there are about 2 million nosocomial infections each year leading directly or indirectly to about 80,ooo deaths!!! Due to disease sources and selection for certain agents by the hospital environment. Most nosocomial agents are endemic not epidemic. E. coli, P. aeruginosa and Enterococcus are ommon urinary tract pathogens.

Staphylococcus aureus is common in septicemia, surgical wounds, and lower respiratory infections. Other species of Staphylococcus are also found in septic patients and wound infections. The source is generally a healthy carrier such as hospital personel. Staphylococci are resistant to drying and can survive long periods of time on dust and other fomites..

Pseudomonas aeruginosa is important in lower respiratory and urinary tract infections and burn victims. These organisms can show high resistance to antibiotics due to plasmids they carry often multiple resistance. Staphylococcus is generally less resistant and Escherichia are most susceptible.

How do we control epidemics? A number of factors have reduced the incidence of certain diseases. Generally there has been an increase in the general well-being of the population. Better nutrition, less crowding, and lighter work loads have contributed to this increase in well being. There have been successful public health measures that have reduced the incidence of specific diseases such as typhoid fever, diphtheria, polio and burcellosis.
Control diseased domesticated animals by eradicating diseased animals and immunization procedures. It is difficult to take the same measures against wild animals that are reservoirs for things like rabies.

Insect reservoirs have been controlled by pesticide sprays. The malaria spreading mosquito has been controlled in North america by originally spraying DDT which is now banned from use. Sometimes the measures we use to control vectors/reservoirs have been unhealthy themselves.
Control the transmission of pathogens. In japan, many people wear face masks to prevent the transmission of respiratory infections. The spread of water-borne pathogens is controlled by water purification procedures implemented.
Immunization schedules have gone a long way in controlling smallpox, diphtheria, tetanus, polio, and pertussis (whooping cough). We shouldn't become laxed about these either, we have not erradicated the viruses or bacteria responsible for these diseases!

Quarantine is another measure to prevent the spread of disease. Simply restrict the movement of an infected individual about the population. There are six diseases that are quarantinable by international law - smallpox, cholera, plaque, yellow fever, typhoid fever, and relapsing fever.

Emerging and resurgent infectious diseases
Emerging diseases are those that suddenly become prevalent and include not only "new" diseases but old ones that have become less well controlled. Syphilis, the plague, influenza, legionellosis, AIDS and lyme disease are examples. Why have these emerged? Outline some factors:

Human demographics
Populations are more dense than 1800's therefore disease spreads more easily.

Human behavior
Sexual promiscuity, intravenous drugs use

Technology and industry advances
Includes advances in medicine and hospitals where we have explosive increase in nosocomial infections. Increase in antibiotic resistance and multiple resistances. Transportation, bulk processing and central distributions have led to increases in some diseases such as E. coli strain O157:H7 in meat.

Economic development
Awan High Dam in Egypt has increased Rift Valley fever incidence since there is an increase in mosquito breeding grounds.

Land use
Lyme disease has become more common. Reforestation practices have helped increase the deer population and the reservoir of lyme disease. More ticks are carrying the disease. Also more people like to live and recreate in forests so there is more contact with these ticks.

International travel and commerce
Travel to and from exotic places could lead to the movement of endemic disease agents.

Microbial adaptation
Evolution of new strains contributes to their emergence. RNA viruses constantly mutate since they use reverse transcriptase to make the replicative intermediate. Reverse transcriptase has no editing function so when errors are introduced they become incorporated into the new virus. Examples include the HIV virus and influenza virus.
Bacteria also evolve using mechanisms such as transposons, bacteriophages and plasmids to facilitate their evolution.

Breakdown of public health measures
This happens occassionally. The cholerae epidemic of Peru was due to a breakdown in water sanitation. Their municpal water supply was contaminated. Similarly in Milwaukee, WI a chlorine resistant protozoa, Chryptosporidium, caused 370,000 cases of intestinal disease and 4000 required hospitalization.
Vaccination programs are neglected. Measles and whooping cough have made a resurgence due to lack of proper immunization programs.

Abnormal natural occurrence
Changes in the weather patterns for example. Hantavirus infects rodents and is a human pathogen. In SW US recently there were some lethal cases reported. Due to abundant mice since there was adequate rains and a long growing season coupled with a mild winter. More mice were around, leaving their droppings that people came in contact with which resulted in more people coming down with the sickness.