Chapter 19 Natural History of Infectious Disease

What are the components of the chain of infection?

• Microorganisms

• Reservoir/source

• Port of exit

• Modes of transport

• Port of entry

• Susceptible host

Microorganisms

Pathogens - bacteria, virus, fungus, parasite

Reservoir/source

Place where pathogens survive and multiplies (ex. food, water, body fluids, environmental surfaces, equipment, etc)

Port of exit

How pathogens leaves reservoir/source

Secretions - blood, saliva

Excretions - urine, feces 

Modes of transport

How pathogens move from reservoir/source to susceptible host 

Direct vs. indirect transmission

Direct - airborne, bite, skin contact, needlestick

Indirect - contaminated equipment or medication, vectors, food, water, fomites 

Port of entry

Body opening where pathogen can enter (ex. mouth, respiratory tract, eyes)

Susceptible host

Person who is at risk (ex. unvaccinated person)

Factors affecting susceptibility

Age, health, co-morbidities, immune system function, etc.

Infection

Person with disease organism in their body

Endemic

Constant/usual presence of a particular disease/infectious agent in a specific population and geographic area

Sporadic

Situation where a specific disease occurs irregularly and infrequently in a population and geographic area 

Cluster

Situation where the number of cases grouped in place and time is suspected to be greater than the expected number 

Epidemic

When there is an increase (often sudden) in number of cases of a specific disease, above the expected number of cases in that area

Pandemic

Epidemic that has spread across multiple countries or continents, usually affecting a large number of people 

Viable balance in host-pathogen relationship

• Natural selection applies to pathogens too - survival and spread based of reproductive success

• Best not to kill a high % of hosts or kill them too quickly - improved reproductive success if pathogens are less virulent/takes longer to kill host

• Natural selection favor pathogens that allow both pathogens and hosts to survive and reproduce 

Common cold viruses vs. Ebola

Common cold viruses - relatively easy to transmit, infect millions, kill few hosts 

Ebola - transmits slowly, kills more hosts quickly → unlikely to spread worldwide 

“Smart” microbes definition

• Adapt to host

• Cause little or no disease

• May even help host (ex. gut microbiome)

CFR (Case Fatality Rate)

Percent of diagnosed cases that are fatal

Smart or dumb: Bubonic plague

Dumb

CFR = around 60-100%, burns out in human population (today treatable with antibiotics) but relatively smart in rats and fleas

Smart or dumb: Ebola

Dumb 

CFR = 41-85%, kills quickly, burns out in human population, retreats to sylvan host and survives 

Smart or dumb: Polio

Smart

CFR = 2-5%, coexistence in human population for millennia, natural lifelong immunity conveyed to those exposed 

Smart or dumb: Malaria

Smart

CFR = very low (~0.47%), 4 major species and 3 less virulent, ~30 million years of evolution 

FDA Bad Bug Book

• Washing your hands before and after handling food, and in between handling different foods, is one of the most important steps you can take. Do the same with equipment, utensils, and countertops

• Wash raw fruits and vegetables under running water. These nutritious foods usually are safe, as you probably know from the many times you’ve eaten them, but wash them just in case they’ve somehow become contaminated. For the most part, the less of a pathogen on a food – if any – the less chance that it can make you sick

• Cooking food to proper temperatures kills most bacteria, including Salmonella, Listeria, and the kinds of E. coli that cause illness, and parasites

• Keep any pathogens that could be on raw, unwashed foods from spreading by keeping raw and cooked foods separate. Keep them in different containers, and don’t use the same equipment on them, unless the equipment is washed properly in between. Treat countertops the same way

• Refrigerate food at 40°F as soon as possible after it’s cooked. Remember, the less of a pathogen there is in a food, the less chance that it can make you sick. Proper refrigeration keeps most types of bacteria from growing to numbers that can cause illness (although if a food already has high numbers of bacteria when it’s put in the refrigerator, it could still cause illness)

Which food pathogen sickens most people

Norovirus (61%) followed by salmonella (12%)

Which food pathogen cause the greatest economic burden

Salmonella 

What is included when calculating economic burden

Cost of medical treatment and value of lost wages

Other food safety concerns

• Allergens not listed on labels

• Extraneous materials (plastic bits, metal)

• Food fraud (ex. fish improperly labeled as a different species)

• Chemical contaminants (ex. lead in applesauce)