Infection, Infectious Disease, and Epidemiology: Reservoirs and Transmission

Reservoirs of Infectious Disease

Introduction to Reservoirs

Reservoirs are sites where pathogens are maintained as a source of infection. Most pathogens cannot survive for long outside their host, so reservoirs are critical for the persistence and transmission of infectious diseases. There are three main types of reservoirs: animal, human, and nonliving.

• Animal reservoirs: Animals harbor pathogens that can be transmitted to humans (zoonoses).

• Human reservoirs: Infected individuals, including asymptomatic carriers, can transmit pathogens to others.

• Nonliving reservoirs: Soil, water, and food can serve as sources of infection, often due to contamination.

Animal Reservoirs and Zoonoses

Zoonoses are diseases that are transmissible from vertebrate animals to humans. Transmission can occur through direct contact with animals or their waste, ingestion of contaminated animal products, or via bloodsucking arthropods.

• Direct contact: Handling animals or their waste.

• Ingestion: Eating infected animal products.

• Vector-borne: Transmission by arthropods such as mosquitoes or ticks.

Examples of Zoonotic Diseases

Disease	Causative Agent	Animal Reservoir	Mode of Transmission
Anthrax	Bacillus anthracis	Domestic livestock	Direct contact, inhalation
Bubonic plague	Yersinia pestis	Rodents	Flea bites
Lyme disease	Borrelia burgdorferi	Deer	Tick bites
Rabies	Lyssavirus sp.	Bats, skunks, foxes, dogs	Bite of infected animal
Malaria	Plasmodium spp.	Monkeys	Bite of Anopheles mosquito
Ringworm	Trichophyton spp., Microsporum spp., Epidermophyton floccosum	Domestic animals	Direct contact

Human Reservoirs

Humans can act as reservoirs for infectious diseases. Carriers are individuals who harbor pathogens without showing symptoms but can still transmit the disease to others. Some carriers eventually develop illness, while others remain asymptomatic due to effective immune responses.

Nonliving Reservoirs

Nonliving reservoirs include soil, water, and food. These environments can become contaminated with pathogens, often through feces or urine, and serve as sources of infection for humans and animals.

• Soil: Can harbor pathogens such as Clostridium species.

• Water: May contain pathogens like Vibrio cholerae or Giardia.

• Food: Improperly handled or cooked food can transmit various pathogens.

The Nature of Infectious Disease: Transmission

Modes of Disease Transmission

Pathogens can be transmitted from reservoirs to susceptible hosts through several modes:

• Contact transmission: Direct, indirect, or droplet contact.

• Vehicle transmission: Airborne, waterborne, or foodborne spread.

• Vector transmission: Mechanical or biological vectors (e.g., insects).

Contact Transmission

Mode	Examples of Diseases Spread
Direct contact (e.g., handshaking, kissing, bites)	Anthrax, genital warts, gonorrhea, herpes, rabies, syphilis
Indirect contact (e.g., fomites like drinking glasses, toys)	Common cold, influenza, measles, pneumonia, tetanus
Droplet transmission (e.g., sneezing within 1 meter)	Whooping cough, strep throat

Vehicle Transmission

Mode	Examples of Diseases Spread
Airborne (dust or droplets >1 meter)	Chickenpox, influenza, tuberculosis
Waterborne (streams, pools)	Cholera, Giardia diarrhea
Foodborne (poultry, meat, seafood)	Botulism, hepatitis A, tapeworms

Vector Transmission

Mode	Examples of Diseases Spread
Mechanical (on bodies of flies, roaches)	E. coli diarrhea, salmonellosis
Biological (lice, mosquitoes, ticks)	Chagas’ disease, malaria, plague, yellow fever

Invasion and Establishment of Disease

Events in Establishing an Infectious Disease

The process of infection involves several key events:

• Encounter: Host comes into contact with the pathogen.

• Entry (and multiplication): Pathogen enters the host and begins to multiply.

• Disease process: Pathogen causes damage, leading to symptoms.

• Spread: Pathogen may disseminate within the host or to new hosts.

• Damage: Host tissues are harmed by the pathogen or immune response.

• Outcome: Recovery, chronic infection, or death.

Portals of Entry

Pathogens enter the body through specific portals of entry. The three major pathways are:

• Skin: Through cuts, abrasions, or direct penetration.

• Mucous membranes: Lining the respiratory, gastrointestinal, and urogenital tracts.

• Placenta: Some pathogens can cross the placenta to infect the fetus.

Parenteral Route

The parenteral route is not a true portal of entry but refers to the direct deposition of pathogens into tissues beneath the skin or mucous membranes, often through punctures, bites, or injections. This route circumvents the body's usual barriers to infection.

Key Terms and Concepts

• Fomite: An inanimate object involved in the indirect transmission of pathogens (e.g., doorknobs, utensils).

• Airborne transmission: Spread of pathogens via droplets or dust particles that travel more than 1 meter from the source.

• Vector: An organism, typically an arthropod, that transmits a pathogen from one host to another.

Example Questions

• Chagas disease is transmitted by a bug with mouthparts that penetrate blood vessels. Which type of exposure does this represent? Answer: Parenteral route

• Fomites are: Inanimate objects involved in the indirect transmission of pathogens

• If one person sneezes, transmitting a virus to another person more than 1 meter away, that is an example of: Airborne transmission

Additional info: This summary integrates and expands upon the provided lecture slides, adding definitions, examples, and context for microbiology students studying infectious disease epidemiology.