Infectious Diseases and Epidemiology

Chapter 10

An epidemic is a disease that affects many individuals at once, spreading rapidly through an area in which the disease is not permanently found
Throughout man’s history, there have been numerous epidemics
Black plague, Spanish flu (1918), AIDS, small pox, yellow fever, mumps, measles
Epidemics become pandemic when a very large number are affected - over a wide geographic range

Epidemiology

Epidemiology is the application of scientific processes in the study of disease
Make observations about the disease process
Form hypotheses about the origin of the disease and who is most susceptible
Gather information to support or refute the hypothesis - perform experiments*
Report the findings
Goal is to stop the current epidemic or prevent its return
So epidemiology is medical detective work
Besides outbreak investigations, epidemiology relies on case studies, case control studies, and cohort studies
The goal of case studies is to determine the mode of transmission of the disease agent
Extensive interviews of individuals are usually carried out by a trained medical professional
The information obtained from the case studies may lead to a case control study
Here, individuals with similar histories are recruited from the original population
If possible, they should all be at the same stage in the disease progression
Interviews will hopefully shed further light on the mode of transmission
After determining the mode of transmission, a cohort study may follow
Participants are recruited from the affected area
Cohort participants should share common elements important to the study (e.g., smokers, menstruating females, college students, etc.) and be free of the disease and symptoms
Participants are monitored for signs of the disease
As with any study of humans, there are many possible sources of error
Inadequate controls, selection bias, small sample size, inaccurate reporting, etc.

The World Health Organization is the branch of the United Nations tasked with helping all humans attain higher levels of health
Major goals are providing medical care to rural populations and monitoring epidemics
There is constant surveillance for certain diseases deemed capable of causing severe epidemics
Rift Valley fever, monkey pox, Nipah virus, smallpox, AIDS, influenza, polio, measles
Different pathogens enter the host (victim) via different routes
Ebola virus through direct physical contact
Tuberculosis (Mycobacteriumtuberculum) through airborne particles
Malaria (Plasmodiumfalciparum) and HIV through injection into the body

Bacteria

Antibiotics

Most classes of antibiotics are derived from naturally occurring compounds - many are produced by one genus of bacteria (Streptomyces)
Antibiotics work by interfering with
DNA or RNA synthesis
protein synthesis
essential metabolic pathways
cell wall synthesis

Antibiotic resistance

Antibiotic resistance is spreading rapidly among many pathogenic strains of bacteria
Mechanisms by which bacterial populations can evolve resistance to antibiotics
Altered permeability preventing entry of the antibiotic
Altered mechanisms than actively pump the antibiotic back out
Altered target proteins
Chemical modification of the antibiotics
We promote the evolution of resistant bacteria by
overusing antibacterial product (soaps, toothpastes, clothes, etc.)
not taking the full course of prescribed antibiotics
dumping old, unused, or expired antibiotics into water supplies
using antibiotics prophylactically on livestock

Bacterial diseases

Plague (black death or black plague) is caused by Yersiniapestis
Black plague devastated Europe in the middle ages
There was an epidemic in Los Angeles as recently as 1924-1925
The bacteria lives in rodents and is transferred by fleas that bite the rodents and then bite humans
MRSA (methicillin-resistant Staphylococcusaureus)
Unlike most S. aureus strains, MRSA is resistant to most common antibiotics and is capable of causing a serious, life-threatening infection
MRSA was first reported in 1961 but has since become widespread
The only effective antibiotic is vancomycin

Viruses

Viruses do not possess all of the characteristics of life
They can only reproduce within appropriate host cells
They do not carry out metabolism
They are acellular
They are essentially nucleic acid (DNA or RNA) surrounded by a protein coat
Yet they are capable of causing many diseases
Viruses exist in two forms
Extracellular - dormant, essentially a package waiting for proper delivery
Intracellular - capable of completing disrupting and taking over normal cellular processes
Antibiotics are useless against viruses; there are very few effective antivirals
Some viruses are classified as lytic; some lysogenic; and some temperate
Influenza virus is the causative agent of the flu
Responsible for the most devastating epidemic in human history
In 1918, the Spanish flu killed more than 40 million people
We have also seen the Asian flu in 1957 and Hong Kong flu in 1968 which reached pandemic proportions
AIDS (acquired immunodeficiency syndrome) is caused by infection by HIV
HIV transmission occurs through transfer of bodily fluids - blood, semen, vaginal secretions, mother’s blood crossing the placenta, and mother’s milk
HIV targets the helper T cell population, eventually depleting it to the point where the immune system is impaired

Although bacteria and viruses are the main pathogens we deal with, others can also cause disease

Protist infections
Amoeba from contaminated water sources
Amoebic dysentary
Malaria is caused by the protist Plasmodium falciparum
Transmitted through the bite of an infected mosquito (Anopheles)
Major health problem - infecting >500 million worldwide, killing 3-4 million per year
Especially prevalent in parts of Africa

Prions are essentially infectious protein molecules
These misfolded proteins can enter the brain and promote correctly-folded forms of the protein to misfold - leading to more and more of the prion form
Responsible for
Scrapies, kuru, Creutzfeldt-Jacob, and bovine spongiform encephalopathy (BSE, commonly called “mad cow disease”)