Vectors & Zoonotic Disease

Source: Centers for Disease Control and Prevention. Principles of epidemiology, 2nd ed. Atlanta: U.S. Department of Health and Human Services; 1992.

Chain of Infection

How do diseases get passed between species? Explore CDC’s “Chain of Infection” - https://www.cdc.gov/csels/dsepd/ss1978/lesson1/section10.html

Great short description of reservoirs, hosts, and modes of transmission. Make sure you understand the “Chain of Infection” - it is really important.

Or explore it in a different way from this "Chain of Infection" video from a former ESHH 511 student

Snyder, M. (2020, December). Chain of Infection. YouTube (3:13).

Key principles of Chain of Infection

Reservoirs

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.

Mode of Transmission

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

Susceptible Host

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.

Examples of Chain of Infection

Lens: COVID-19 & Vector Biology

From COVID-19 Annotated Bibliography (part of our course materials)

Virus biology section (if you want a refresher) – https://bit.ly/ESHHCOVID19
Also, spoiler alert, it looks like pangolin (left bottom) is not the intermediate host (Li et al., 2020; doi: 10.1002/jmv.25731). Mink is getting more press lately for being asymptomatic carriers with >99.6% viral homology (described in Salajegheh et al., 2020 below).

Read:

  • Salajegheh Tazerji, S., Magalhães Duarte, P., Rahimi, P. et al. Transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) to animals: an updated review. J Transl Med 18, 358 (2020). https://doi.org/10.1186/s12967-020-02534-2 Mink, tigers, cats, dogs – all here

Lens: Monkeypox & Vector Biology

Explore more from WHO on its viral transmission.

Amazing Resource!

Jetelina, K. (2020). Your local epidemiologist. Up to date, translated science for the public. Amazing site that is grounded in science.

Student Recommended Resources

This Podcast Will Kill You

Vector biology podcasts from This Podcast will Kill You

Students love this podcast series for learning vector biology. Engaging and fun. Each around an hour in length. Put it on and go for a walk.

See more about This Podcast Will Kill You at: https://thispodcastwillkillyou.com/episodes/

Click to expand episode list. They have a lot!

Practice

Flashcard sets to test yourself (or practice for the CPH exam)