Vectors & Zoonotic Disease

Habitat where the infectious agent normally lives, grows, & multiplies

• Human reservoirs - So many diseases associated. May not show any symptoms, yet can pass on to others -- classic example: Typhoid Mary for spreading typhoid fever. Even recovered individuals can sometimes transmit, termed "convalescent carriers."

• Animal reservoirs - Animal to human transmission is known as zoonotic disease. Think brucellosis (cows and pigs), anthrax (sheep), plague (rodents), trichinellosis/trichinosis (swine), tularemia (rabbits), rabies (bats, raccoons, dogs, and other mammals), West Nile encephalitis (birds), and monkeypox (prairie dogs). Bats are a huge animal reservoir for coronaviruses.

• Environmental reservoirs -- plants, water, soil. Think Legionnaire's disease from bacteria found in water in cooling towers & condensers; or botulism spores in soil.

Direct modes:

• Direct contact (skin-skin, kissing, sex); think mono, gonorrhea)

• Droplet spread (short-range aerosols); think pertussis, meningococcal infection

Indirect routes:

• Airborne (think measles)

• Vectors (like mosquitoes, fleas); think plague, malaria

• Vehicles (like food, water, blood, fomites, latter are inanimate objects); think hepatitis A, botulism

Who is a susceptible host? This depends on genetic and immune factors. For example, with SARS-CoV-2, having an ACE2 receptor seems to influence susceptibility because it's a route of entry.

Other influencing factors include mucous membranes, cough reflexes, cilia in respiratory tract, or diseases that influence non-specific immune system (e.g., malnutrition, alcoholism). After all of Oregon's wildfire smoke, I'm particularly interested in how pollution influences susceptibility.

Host stages (for parasites, like tapeworms):

• Intermediate host -- Intermediate hosts are often vectors, since this is a host stage in which a parasite passes one or more of its asexual stages (like larval stage)

• Definitive host (aka final host) a host in which a parasite attains sexual maturity & reproduces.

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