Principles of Disease and Epidemiology

Pathology - study of disease

Etiology - cause of disease

pathogenesis - manner in which disease develops

Infection and disease differ in definition

Infection is the invasion and colonization of the body by a foreign agent

disease occurs when the infections causes a change from a state of health

not all infections lead to a disease state - e.g., HIV virus may be circulating in your blood but not cause a full-blown disease state - AIDS.

In utero we are free of bacteria - we are sterile! During birth and soon after we begin to be colonized. During the birth process, lactobacilli multiply rapidly in the woman’s vagina. These are some of the first to colonize the newborn’s intestine. Other non-pathogenic bacteria are introduced to the new born from various sources. Ultimately some 10e14 bacteria colonize the body, while there are only 10e13 human cells. The permenant bacteria are called normal microflora or microbiota. Others that come and go are called transient microbiota.

The normal microflora protects us from potential infections and disease via microbial antagonisms - ecological considerations. Normal microflora compete for nutrients, produce harmful substances like bacteriocins, affect the pH and oxygen availability. If the balance is upset then infections and disease might set in.

Example Clostridium difficile inhabits the large intestine. Responsible for intestinal infections that follow antibiotic treatment - diarrhea to fatal colitis. The normal microflora of the large intestine inhibit C. difficile possibly by competing for receptors, competing for nutrients or producing bacteriocins.

The relationship between the host and the normal microflora is called a Symbiosis - living together. Many of the organisms are commensals living in a state of commensalism - where one organism is benefitted and the other unaffected. Many of the normal microflora are commensals. Some organisms are mutualistic - living in a state of mutualism. Mutualism is where both partners benefit. E. coli synthesizes vitamin K and some B vitamins that are absorbed into the blood stream for use by the host. The large intestine provides nutrients to the E. coli. Parasitism is where one partner benefits from the host at the expense of the host - many disease causing bacteria are parasites.

Opportunistic microbes are microbes that usually do not cause disease in their normal habitat of a healthy individual and may be mutualistic. For example, E. coli is usually a mutualistic organism, but if it finds its way to the urinary bladder it may cause urinary tract infections.

Etiology of infectious disease

Robert Koch’s postulates are used to determine the etiological agents of infectious diseases.

Review of Koch’s postulates

1. same pathogen must be in every case of disease

2. pathogen must be isolated and cultured in pure culture

3. pathogen must cause disease when inoculated into an otherwise healthy animal.

4. Pathogen must be reisolated from the inoculated animal and be the same as that inoculated into the animal.

There are situations where Koch’s postulates cannot be followed. Some disease agents cannot be cultured at this time. Virulent strains of Treponema pallidum - the cause of syphillis - has not been cultured. So how can we show that these are the cause of syphilis?? Some pathogens have no other known host other than humans - how would you fulfill step 3 of Koch’s postulates??

Classifying infectious diseases

Diseases may be classified as communicable diseases which spread from one host to another either directly or indirectly. Some of these are also contagious diseases that are easily spread between hosts. Examples include chicken pox, common cold, flu, and measles. Noncommunicable diseases do not spread between hosts easily. Example Clostridium tetani produce disease only when introduced into the body via a wound of some sort.

Some diseases are constantly present in a population and they are called endemic diseases - cold virus for example. Others are acquired quickly by lots of people are referred to as epidemic diseases - flu virus for example. Epidemic disease on a world-wide scale are called pandemic disease - flu for example. AIDS is considered pandemic by some authorities.

Emerging diseases

Contributing factors

1. widespread use of antibiotics and pesticides which encourages the growth of more resistant microorganisms and insect vectors that carry them.

2. Global warming may change the distribution and survival of reserviors and vectors of diseases.

3. Modern transportation may help spread new diseases

4. Ecological changes brought about by natural disasters, construction, wars and expanding human settlements.

5. Animal control measures may change the balance of predators of reservoirs and vectors. Lyme disease has increased because of rising deer populations with no predators

6. Failures in public health measures.

Host involvement in disease

local infections are where the infectious agent is restricted to a relatively small area of the body. Example are boils and abscesses. Systemci infections are where microorganisms or their products are spread throughout the body either by blood or lymph. The presence of bacteria in the blood is called bacteremia. If the bacteria actually multiply in the blood the condition is called septicemia. Toxemia refers to the presence of toxins in the blood - e.g., tetanus. And viremia refers to the presence of virus in the blood.

Resevoirs of infection

What are the sources of infectious agents? Resevoirs of infection may be living and may be inanimate. Depends on the organism.

Human resevoir

Many people harbor pathogens and either directly or indirectly spread them to others. Carriers may be infected but show no signs of the disease and spread it unknowingly. Others may be symptom-free - incubation period for example - and be spreading the disease unknowingly.

Animal resevoir

Disease spread from animals to humans are called zoonoses. Some types of influenza (birds), rabies (bats, skunks, dogs and cats) and Lyme disease (deer).

Nonliving resevoirs

soil and water are the two major resevoirs. Soil harbors fungi that cause ringworm and other systemic infectious agents. Clostridium botulinum and tetani are both found in soil. Both are members of the normal microflora of horses and cattle.

Water contaminated with feces of infected individuals is a resevoir of numerous pathogens including Vibrio cholera and Salmonella typhi.

Transmission of disease

Contact transmission

Direct contact transmission like handshaking, kissing or sexual intercourse. Gloves and other protective measures are used to prevent transmission.

Indirect contact transmission occurs when an infected individual contaminates an inanimate object, called a fomite, and a non infected individual touches the contaminated fomite. Fomites include dirty tissues, bedding, diapers, drinking cups, eating utensils, toys, money and thermometers.

Droplet transmission - microbes are spread by coughing, sneezing, laughing or talking. The droplets released during these activities travel up to a meter but usually not much further. They are not considered airborne since the droplets will fall to the ground.

Vehicle transmission - transmission of disease by either water, food or air. Water becomes contaminated with untreated or poorly treated sewage which can carry cholera, shigellosis and leptospirosis. Foods that are inproperly handled, refrigerated, prepared under unsanitary conditions or improperly cooked may lead to food poisoning or tapeworm infections. The air may harbor potential pathogens. The virus causing measles and the bacterium causing tuberculosis may be airborne. Dust may harbor staph and strep pathogens. A number of fungi are transported via spores floating around in the air.

Vectors are disease carrying insects like house flies. Some of the vectors are simply passive vectors called mechanical transmission. The house fly is an example. It moves from contaminated material to uncontaminated materials. Some insects are responsible for the biological transmission of disease. This is a more active process on the part of the vector. Mosquitos are an example.

Nosocomial infections

an infection acquired as a result of a hospital stay. 5-15% of all hospital patients acquire a nosocomial infection and about 20,000 people die yearly from nosocomial infections - NO LAUGHING MATTER!!

Why - nosocomial infections are a result of the variety of pathogenic bacteria and opportunistic bacteria in the hospital full of compromised patients where transmission routes abound. It is just easy to get sick in a hospital.

Most nosocomial infections are due to opportunistic bacteria. In the past they were predominantly gram positive bacteria such as Staphylococcus aureus, today it is E. coli and Pseudomonas aeruginosa. Many of these are resistant to most common antibiotics because of transposons or resistant plasmids that move between bacteria - evolution by natural selection.

Development of disease

incubation period is the time period between infection and the first symptom.

Illness period is when the disease is most acute and the patient shows full blown symptoms.

Decline period is when the symptoms decline. Fever goes down, energy level increases.

Convalescence period is while the body gains its strength back and a full recovery is made.