5.1 Zoonoses and Disease-Causing Agents

The word zoonosis can be decomposed into two parts (in Greek):

  1. zoon = animal
  2. noson / nosos = disease / sickness

Hence, a zoonosis is a disease that is transmissible from animals to humans!

“Zoonotic diseases are a group of infectious diseases that are naturally transmissable between vertebrate animals and humans.”

– WHO

“Zoonotic diseases (also known as zoonoses) are caused by infections that spread between animals and people.”

– CDC

One can also consider the two definitions of a zoonosis by the two leading health authorities of the world.

Furthermore, also consider the definitions of the following words:

  1. Zooanthroponosis

    This is a zoonosis that is maintained by humans, but can also be transmitted to other vertebrates (tuberculosis from a human to a primate [e.g., a monkey]).

  2. Anthropozoonosis

    This is a zoonosis that is maintained by animals and is transmissable to humans (e.g., rabies, monkeypox).

5.1.1 Key points of zoonoses

75% of new human diseases discovered in the past decade are zoonotic; this is in contrast to the fact that 61% of all human diseases are zoonotic in origin (over 200 zoonoses)! So far, we know of about 1415 pathogens that are able to infect humans.

On average, a new zoonosis emerges about every four months.

Unsurprisingly, zoonoses have been responsible for some of the human history’s worst disease outbreaks (e.g., HIV, Ebola, SARS, and now, COVID-19). Zoonotic infections have always been a part of the human disease landscape, most of which come from domestic animals (and are viral in nature with bioterrorism capabilities).

Furthermore, humans are either accidental or dead-end hosts; however, illness caused by zoonoses can either be:

  1. Asymptomatic
  2. Mild illness
  3. Chronic conditions, evne death!

5.1.2 Types of zoonoses

There are three main types to be aware of in BS0005:

  1. Emerging zoonosis

    This is a zoonosis that is newly recognized or evolved (e.g., COVID-19, SARS, MERS, Nipah, etc).

  2. Re-emerging zoonosis

    This is a zoonosis that has occurred previously, but shows an expansion in its cases or geographical, host, or vector range (e.g., Nipah virus).

  3. Lingering / neglected zoonosis

    This is a zoonosis that is chronically restricted to one region (e.g., brucellosis and other parasitic diseases).

5.1.3 Classifying zoonoses

There are two main types of zoonoses to be aware of here:

  1. Synanthropic zoonosis

    This applies to animals that live near human civilizations - these animals tend to be pests (e.g., rabies, rat typhus, etc).

  2. Exoanthropic zoonosis

    This applies to animals that live far away from human civilizations - these diseases spread via the sylvatic (i.e., wild) cycle (e.g., arboviruses, rabies, Lyme disease, etc).

5.1.4 Affiliated animals

There are several animals to take note of:

  1. Companion animals

    These include cats, dogs, birds, gerbils, hamsters, rabbits, and so on.

  2. Poultry

    These include chickens, ducks, quails, turkeys, and so on.

  3. Livestock

    These include cattle, goats, pigs, horses, and so on.

  4. Research animals

    Companion animals, poultry, livestock, guinea pigs, ratts, and so on can be considered research animals.

And of course, there is also wildlife and zoological species.

5.1.5 Etiological classifications

Zoonoses can be caused by a variety of agents:

  1. Viruses

    Including rabies, Nipah virus, and Rift-Valley fever

  2. Bacteria

    For instance, E. coli O157:H7, Yersinia pestis (i.e., bubonic plague), Bartonella henselae (i.e., cat-scratch disease)

  3. Fungi

    These include Microspora canis, ringworm, and athelete’s foot

  4. Helminth parasites

    Includes Echinococcus granulosus and Toxocara cati

  5. Protozoan parasites

    Including… Leishmania, Toxoplasma, Cryptosporidia, and Trypanosoma

  6. Unconventional parasites

    For example, prions in CJD or kuru.