Lecture 21 - Portals of Entry and Incubation

Portals of Entry
Depending on the method of entry, pathogens can cause different responses. For example, ingesting tetanus spores will not cause a response but being exposed to it systemically will likely cause death if not vaccinated.

Skin
Skin is a defense mechanism if unbroken. it has several interlocking layers that prevent organisms from getting in. However, certain organisms can penetrate the skin and cause infection. Hair follicles are often infected because of their relatively weakness to other skin.


 * Staphylococcus aureus cause boils


 * Dermatophytes
 * Ringworm
 * Athlete’s foot


 * Neisseria gonorrhea

Streptococcus pyogenes and MRSA can both be part of our normal flora, but can be infectious if skin is broken. Streptococcus pyogenes can cause Necrotizing fasciitis, and MRSA causes lesions that if scratched, causes the MRSA to go deeper in the skin.
 * Chlamydia trachomatis
 * Causes eye infections in infants.
 * Ophthalmia neonatorum
 * STDs that can also be spread through intact skin

Tetanus, malaria, and rabies all need skin to break in order to enter our system.

Respiratory Tract
Another main way of entry is through micro-aerosols, droplets from sneezing, coughing, and talking.

Bacterial infections include Bacillus TB (coughing), Pneumonia (Streptococcus pneumonia), Haemophilus (bacterial meningitis), Pertussis (whooping cough), and Diphtheria.

Viral infections include the common cold, the flu, smallpox, measles (rubeola), and German measles (rubella)

Digestive Tract
Food or water can be contaminated by feces or other pathogens.

Bacteria that enter through the digestive tract are E. coli, salmonella, cholera, shigella, and Staphylococcus.

Viruses that are transmitted through food include polio and hepatitis A.

Genito-Urinary Tract
Bacterial infections include STDs such as gonorrhea, chlamydia, and syphilis, and other infections like Staph (toxic shock syndrome), Strep (childbirth fever), and Pseudomonas aeruginosa UTIs. Vectors include humans, and vehicles include things like thongs, bubblebaths (P. aeruginosa) toilet paper, towels, store underwear, and urinary catheters.

Placenta
Some organism can pass through umbilical cord to baby. These include syphilis, toxoplasmosis, AIDS, Chlamydia trachomatis, hepatitis B, rubella, cytomegalovirus (CMV) and herpes simplex 1 and 2.

Toxoplasmosis is passed through cat feces, and causes encephalitis in AIDS patients and mental disorders or death in babies.

Period of Incubation
A period of incubation is the period during which organisms grow in size or numbers (load). During this stage there are no symptoms, and so the period of incubation is defined as the time between the initial entry and the onset of symptoms. Different things can affect the period of incubation.

Nature of Organism

 * Cold, flu and strep sore throat - 1 to 3 days.


 * Acute childhood infections such as chicken pox or mumps have incubation periods of 10 to 20 days


 * Tuberculosis - 6 months


 * Leprosy - 2-5 years


 * HIV - On average 8 years

Virulence of organism
The more virulent an organism is, the shorter its incubation period. Viruses like chicken pox are known to evolve increased virulence, reducing its incubation period to 7 days.

Resistance of Host
Higher resistance in the host will lengthen the incubation period, often creating a healthy carrier who has a subclinical dose infection. Healthy carriers can transmit disease to others who might not have a strong resistance, causing death. For example, pertussis can be passed on to a child and kill it.

Distance from Entrance to Focus of Action
Rabies can incubate for 1 to 2 months. Its focus of action is the brain, so depending on where the site of infection was. If it is closer to the brain, the incubation period will be shorter than if the site of infection is farther away from the brain,

Amount of Infectious Agent
Generally, the more of an organism one ingests, the shorter the incubation period.

TB has an ID50 of 10 cells, Anthrax' ID50 is 8,000 to 10,000 spores, salmonella's ID50 is 100,000 bacilli to 1 billion bacilli cell