Lecture 4, Zoonoses Flashcards
The word ‘zoonoses’ is derived from the Greek words…?
zōon meaning ‘animal’ and nosos means ‘disease’.
The term ‘zoonoses’ was coined by
Rudolf Virchow during his study on Trichinella in 1855, to indicate the infectious disease link between animal and human health.
As described by the World Health Organization (WHO), ‘A zoonosis is
any disease or infection that is naturally transmissible from vertebrate animals to humans’.
The classic zoonoses which have been well known for centuries and are still causing major socio-economic effects in many parts of the globe. (9)
rabies, plague, leptospirosis, brucellosis, bovine tuberculosis, cysticercosis, echinococcosis, toxoplasmosis and yellow fever
In recent years, new zoonotic entities are posing a serious threat to the globalized world. Name some examples. (min 8)
e.g. Lyme borreliosis, cryptosporidiosis, Ebola, Nipah, severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome (MERS), influenza viruses of animal origin (swine flu – H1N1), hantavirus, etc.
Name 2 zoonotic pathogens that have been used as biologic weapons.
e.g. anthrax and glanders
Zoonoses can be classified in three ways:
Classification of zoonoses based on etiological agents.
Classification of zoonoses based on the reservoir host(s) (and the life cycle of the infecting pathogen).
Classification based on the transmission cycle.
Zoonoses can be caused by a range of pathogens such as
viruses, bacteria, fungi and parasites.
In a study, out of the listed 1415 pathogens known to infect humans, 61% were found to be zoonotic.
Zoonoses can be classified based on the reservoir host(s) and
the life cycle of the infecting pathogen.
The reservoir of an infectious agent is
the habitat in which the agent normally lives, grows and multiplies.
The reservoirs for zoonotic pathogens include
humans, animals and the environment.
Based on reservoir hosts, zoonoses can be classified as follows: (3)
Anthropozoonoses
Zooanthroponoses
Amphixenoses
Anthropozoonoses are
The zoonotic diseases which can be transmitted to humans from lower vertebrates.
Therefore, these infections primarily affect animals but can be naturally transmitted to humans (e.g. rabies, brucellosis, Q fever, leptospirosis, ringworm, etc.).
Zooanthroponoses are
(also known as ‘reverse zoonotic disease transmission’):
Those zoonotic diseases which can be transmitted to lower vertebrate animals from infected humans.
Therefore, these infections are primarily of human origin (e.g. methicillin-resistant Staphylococcus aureus, Cryptosporidium parvum, Ascaris lumbricoides, etc.).
Amphixenoses are
The zoonoses which are maintained in both humans and lower vertebrate animals, which may be transmitted in either direction (e.g. Staphylococcus infection, E. coli infection, salmonellosis, etc.).
Direct zoonoses are
Those zoonotic diseases which are perpetuated in nature by a single vertebrate species (e.g. anthrax, rabies, Q fever, etc.).
Cyclozoonoses are
Zoonotic diseases which require two or more vertebrate hosts to complete the transmission cycle.
Cyclozoonoses can be further classified as follows:
Obligatory cyclozoonoses and
Non-obligatory cyclozoonoses
Obligatory cyclozoonoses are
The zoonotic diseases in which the involvement of humans as a host is compulsory to continue the transmission cycle (e.g. taeniasis).
Non-obligatory cyclozoonoses are
The zoonotic diseases in which humans are accidentally involved in the transmission cycle of the pathogen (e.g. hydatidosis).
Metazoonoses are
The zoonotic diseases which require both vertebrate and invertebrate hosts to continue their transmission cycle.
Saprozoonoses are
The zoonotic diseases which require an inanimate object(s) for the completion of the transmission cycle are known as saprozoonoses.
Saprozoonoses can be classified as:
Saproanthropozoonoses
Saproamphixenoses
Saprometanthropozoonoses
Saproanthropozoonoses are
The zoonoses which can transfer from animals to humans through inanimate substances (e.g. erysipeloid).
Saproamphixenoses are
The zoonoses which can be shared between humans and animals through inanimate objects (e.g. histoplasmosis).
Saprometanthropozoonoses are
These zoonoses require vertebrate hosts and invertebrate hosts as well as inanimate objects for the completion of their life cycle (e.g. fascioliasis).
Classification of Zoonoses According to the Ecosystem in which Pathogens Circulate
Synanthropic zoonoses &
Exoanthropic zoonoses
Synanthropic zoonoses are
The zoonotic diseases which transmit through the urban (domestic) cycle where the sources of infection(s) are domestic and synanthropic animals (e.g. urban rabies, cat-scratch disease and zoonotic ringworm through pets).
Exoanthropic zoonoses are
The zoonotic diseases which transmit through the sylvatic cycle in natural foci through feral or wild animals (e.g. arboviruses, wildlife rabies, Lyme disease and tularemia).
Note: Some zoonoses can circulate in both urban and sylvatic cycles (e.g. yellow fever and Chagas disease).
Direct contact of a susceptible host with infected animals.
Give 3 examples of diseases that transmit in this way.
e.g. scabies, brucellosis, leptospirosis
What is the sylvatic cycle?
also enzootic or sylvatic transmission cycle, is a portion of the natural transmission cycle of a pathogen. Sylvatic refers to the occurrence of a subject in or affecting wild animals.
The sylvatic cycle is the fraction of the pathogen population’s lifespan spent cycling between wild animals and vectors.
Direct transmission through animal bites and scratches.
Give 2 examples of diseases that transmit in this way.
bites (e.g. rabies) and scratches (e.g. cat-scratch fever).
Transmission through contaminated animal food products, mainly due to improper food handling and inadequate cooking practices.
Give 3 examples of pathogens that transmit in this way.
e.g. Salmonella spp., Clostridium perfringens, E. coli, etc.
Feco–oral transmission from animals to humans.
Give 3 examples of pathogens that transmit in this way.
e.g. salmonellosis, E. coli, Toxoplasma gondii, etc.
Vector-borne transmission: Vectors such as mosquitoes, ticks, fleas and lice can transmit zoonotic diseases to humans.
Give 3 examples of diseases that transmit in this way.
e.g. yellow fever, Kyasanur forest disease, plague, etc.
Air-borne transmission: Air-borne transmission results from the inhalation of small particles (droplet nuclei) which are considered to have diameters 5 μm.
Give an example of a diseases that transmits in this way.
e.g. influenza viruses
Indirect transmission through contaminated soil. Allowing the faeces to dry out and disintegrate contaminates the soil which increases the risk of exposure to pathogens.
Give an example of a pathogen that transmits in this way.
e.g. roundworm eggs can survive for years in contaminated soil
Indirect transmission through contaminated water sources.
Give 4 examples of diseases that transmit in this way.
e.g. Cryptosporidium spp., cholera, rotavirus infection, leptospirosis, etc.
In general, viruses account for a significant proportion of
emerging infectious diseases (EIDs).
and the majority have zoonotic origin
what are considered the primary etiological agents of emerging infectious diseases?
and why?
These RNA viruses: ebolaviruses, human immunodeficiency virus (HIV), hantaviruses, Hendra and Nipah viruses, severe acute respiratory syndrome (SARS) coronavirus, influenza A viruses and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
The RNA viruses are characterized by their rapid evolutionary rates due to the frequent error-prone replication cycles. so due to their higher ability to infect new host species with exceptionally short generation times they account for a significant portion of EIDs.
Emerging Infectious Diseases are
the diseases that have not occurred before (e.g. SARS in 2003, COVID-19)
or have occurred previously but affected only small numbers of people in isolated places but now are rapidly increasing in incidence or geographical range (e.g. Zika was discovered in 1947 but the major epidemic was in 2015–2016)
or have occurred throughout the human history but only recently been recognized as a distinct disease due to infectious agent (e.g. the causative agent of Lyme disease was discovered in 1982).
Re-emerging Infectious Diseases are
the diseases that were once major health problems globally or in a particular country, and then declined dramatically, but are again becoming health problems for a significant proportion of the population (e.g. malaria, rabies, cholera, tuberculosis).
Neglected Zoonotic Diseases are
a subset of neglected tropical diseases. The term ‘neglected’ highlights that ‘these diseases affect mainly poor and marginalized populations in low-resource settings’.
Examples include rabies, echinococcosis, taeniasis/cysticercosis, schistosomiasis, etc.
Transboundary Animal Diseases (TADs) are
These may be defined as those epidemic diseases which are highly contagious or transmissible and have the potential for very rapid spread, irrespective of national borders, causing serious socio-economic and possibly public health consequences.
Globalization, land encroachment and climate change contribute to outbreaks of such animal diseases, some of which are transmissible to humans, such as brucellosis, bovine tuberculosis, parasitic illnesses, anthrax, bovine spongiform encephalopathy and influenza viruses.
An endemic is defined as
the habitual presence of a disease within a given geographic area.
It may also refer to the usual occurrence of a given disease within an area (e.g. rabies and brucellosis in India).
An epidemic is defined as
the occurrence of disease above the normal expectancy in a region/country (e.g. Ebola outbreak in West African countries during 2014).
A pandemic refers to
a worldwide epidemic covering larger geographical regions (e.g. H1N1 outbreak in 2009; COVID-19 outbreak of 2019–2021).
define synanthrope
an undomesticated organism and especially an animal (such as a mouse, pigeon, or raccoon) that lives in close association with people and benefits from their surroundings and activities.
Factors Responsible for the Emergence of Infectious Diseases (9)
- Breakdown in public health measures (disasters, war etc.)
- Increasing demand for animal protein
- Unsustainable agricultural intensification
- Increased use and exploitation of wildlife
- Unsustainable utilization of natural resources accelerated by urbanization, land use change and extractive industries
- Travel and transportation
- Changes in food supply chains
- Climate change
- Microbial adaptation and changes
