Emerging Diseases

Lecture 5: Disease Transmission

5.1 Overview5.2: Routes of Transmission

5.3: Summary

5.1: OverviewThe Germ Theory explains infectious diseases.

They pass from person to person-directly or indirectly through the

environment.

How does this happen?

Overview: Disease Transmission

• The “route of infection” is the way the disease moves from one individual to another

• The disease enters the body through a “portal of entry”.

• Different diseases are characterized by different routes of infection

• Some pathogens (disease-causing agents) utilize more than one route

• To attack your body, the pathogen must attach to a molecule on the surface of your cells called a “disease receptor”

To stop a pathogen you have to block its route of

transmission.This is called “breaking the chain of infection”.

5.2: Routes of Transmission

• Gastrointestinal• Respiratory• Direct Contact• Body fluids• Vector

They are not always clear-cut: sometimes they overlap

Gastrointestinal Route

• Also known as: fecal-oral, food-borne or water-borne, alimentary

• Disease agent enters with contaminated food or water

• Examples: typhoid, polio, Salmonella, E. coli O157:H7, many parasitic diseases

• Raw or undercooked food is usually the problem• Beaver Valley Mall outbreak of hepatitis A is the

classic example in Pennsylvania

Hepatitis A Outbreak Beaver Valley Mall • 660 cases and 4 deaths Oct-Nov. 2003• Largest Hepatitis Outbreak in State History• Health officials issued an alert yesterday, warning the estimated 11,000

customers who ate at the Beaver Valley Mall Chi-Chi's between October 22nd and November 2nd that they may have been exposed to Hepatitis A, and should get immunized as a precaution. January 9, 2004

Bill Vidonic, Times Staff WriterBEAVER - Richard Miller will be on anti-rejection medication for the rest of his life to keep his new liver functioning properly.The 57-year-old said he is struggling to regain the stamina he lost after a liver transplant on Nov. 8, a life-saving procedure made necessary when he contracted hepatitis A after eating at the Chi-Chi's Mexican restaurant in Center Township."I lost my liver for no reason at all," Miller said from his Beaver home Thursday.

What conditions favor the spread of gastrointestinal disease?

• Garbage• Sewage• Bad sanitation• Contaminated water• No refrigeration• Hot weather• Others?

Respiratory Route• Pathogen enters via breathing or via contact

with the respiratory system• Smallpox, influenza, measles, many bacterial

pathogens• Inhalation of dust or aerosols• Irritation of respiratory system leads to

sneezing-droplets carry germs

This is a great way to transmit disease

What conditions favor the spread of respiratory disease?

• Crowded living conditions• Dirty clothes and bedding• Cold weather• Others?

Direct Contact

Pathogens may contaminate inanimate objects from which a healthy person gets the disease: the objects are called fomites.

Fomites

Transmission via Body Fluids(not inhaled)

• “blood-borne”: Hepatitis B, HIV-1 and HIV-2• “sexually-transmitted” or “STDs” or

“urogenital”: Hepatitis B, HIV-1 and HIV-2, syphilis, gonorrhea, chlamydia

• Herpes viruses like Epstein-Barr Virus can be transmitted through saliva (“mono” or “kissing disease”)

What conditions favor the spread of direct contact disease?

• Public facilities not clean• Improper hand-washing • Others?

Vector Transmission

The disease is carried from person to person by an animal:

Usually an insect or close relative such as a tick.

The carrier animal is called a “vector”.

Often, there are host species in addition to humans. These are called “reservoir” species or “reservoir” hosts.

Malaria is the best example of a vector-borne disease. In Pennsylvania, twoImportant vector-borne diseases are West Nile (virus) and Lyme Disease(bacterial-caused by Borrelia burgdorferi).

What conditions favor the spread of vector-borne disease?

• Climate change• Habitat disruption• Others?

More terms• Some pathogens live peacefully on or in your

body until it weakens-then they attack. These are called “opportunistic” pathogens.

• Diseases acquired from animals are called “zoonotic”.

• Diseases acquired as a result of a medical procedure are “iatrogenic”.

• Diseases acquired in a hospital are “nosocomial”.

5.3: Summary-Five Important Routes of Infection

• Gastrointestinal: food-borne or water-borne• Respiratory: air borne, most dangerous• Direct Contact: common• Body fluids: easy to stop• Vector: complex because additional species

involved