AN OVERVIEW OF OUTBREAKS OF ANIMAL ORIGIN (ZOONOTICS)

Introduction

By now billions of people around the world are following advice to practice social distancing and "shelter at home" to prevent the alarming spread of a new coronavirus disease, COVID-19. The virus joins a growing list of emerging and reemerging zoonotic diseases worldwide.  Now, what are the zoonotic diseases? Let’s look a bit of history!

Rudolf Virchow (1821–1902), one of the foremost 19th century German leaders in medicine and pathology, noted a relationship between human diseases and animals and then introduced the term “zoonosis” (plural: zoonoses) in 1880. Later, the World Health Organization (WHO) in 1959 specified that “zoonoses are those diseases and infections which are naturally transmitted between vertebrate animals and man”. Venkatesan and co-authors reported that the term zoonosis is derived from the Greek word “zoon” = animal and “noso” = disease. Zoonotic pathogens causing different kinds of diseases are of major public health issues worldwide.

Zoonotic infections are not new. In fact, they existed even before the introduction of the word “zoonosis”. They have always featured among the wide range of human diseases and most, e.g. anthrax, tuberculosis, plague, yellow fever and influenza, have come from domestic animals, poultry and livestock. However, with changes in the environment, human behavior and habitat, increasingly these infections are emerging from wildlife species.

An epidemiological study confirmed that about 61% of the total number of microbial diseases affecting man is zoonotic.  Moreover, another study suggested that animals are the major sources of human zoonotic infections, globally and that among all the emerging infectious diseases, almost 75% are considered to be caused by animals. During the last three decades, rats have been increasingly implicated in several emerging and reemerging human outbreaks of zoonotic diseases and have accounted for ~75% of the new zoonotic diseases in nature. Other animals include bats, pigs and birds.

Viral zoonotic diseases.

Usually viruses are the host specific, means that they can only live in their specific living organisms. But, when another animal specific virus gets the capacity to infect human (cross-species transmission), it is the most significant cause of novel(new) diseases emergencies.

Even before COVID-19 outbreak, The WHO and most infectious disease experts agreed that the source of the next human pandemic is likely to be zoonotic, and wildlife is emerging as the primary source.

While others have only recently emerged or have only recently been linked to wildlife reservoir species, many viral zoonoses from wildlife, including avian influenza and rabies, are well established. In many respects, bats represent an ideal reservoir for most zoonotic viral pathogens. One example is the Ebola virus, which, after decades of research, was recently linked to cave-dwelling bats in Africa. Similarly, the severe acute respiratory syndrome (SARS) coronavirus in 2003 which claimed over 800 lives and cost over 60 billion dollars globally, emerged from bats to civets before ultimately affecting humans in the wet markets and restaurants of southern china. The novel coronavirus(COVID-19) also appears likely that bats are the primary source although whether COVID-19 virus is transmitted directly from bats or through some other mechanism is unknown.

Frequent mixing of different animal species in the markets in densely populated areas, and the human intrusions into the natural habitats of animals, have facilitated the emergence of novel viruses.

Some of zoonotic viral diseases include; Rabies, Middle East Respiratory Syndrome (MERS-CoV), influenza virus (IFV), Alkhurma hemorrhagic fever, Crimean-Congo hemorrhagic fever (CCHF), Rift Valley fever (RVF), West Nile fever (WNV) and dengue fever virus and recently, COVID-19.

Other zoonotic infections/diseases.

1.    Bacterial diseases like anthrax (from cattle), brucellosis (from cattle) and bovine tuberculosis (from cattle).
2.   Fungal diseases like dermatophytosis (from dogs, cats), sporotrichosis (from cats), and histoplasmosis (from birds, bats).
3.     Protozoan diseases like Toxoplasmosis (from cats, sheep) and trypanosomiasis (From cattle)
4.     Parasitic diseases like cysticercosis (from pigs) and toxocariasis (from dogs, cats)

Drivers for the emergence of zoonotic diseases.

Emergence of new zoonotic diseases is primarily due to environmental changes on the interactions between pathogens and their hosts and between the host and other species, including other wildlife, livestock and humans. These changes are driven by driven by;

1. ·         Modernization of farming practices, particularly in the developing world.
2. ·         habitat destruction
3. ·         human encroachment and
4. ·         climate change

A point to note is that the emergence of a zoonotic disease is a multifactorial event; it may involve changes in, among others, human behavior, farming and trading practices, vector distribution and the genetics of microbes. It is equally important to recognize that different drivers play distinct roles in the emergence of different viruses, which can be the case even for viruses from the same family.

Modes of animal to human transmissions

There are five broad but distinct transmission pathway categories:

 (1) Direct contact (skin-to-skin contact; scratches; animal bites; contact with body fluids, organs, and tissues; direct large droplet exposure);

 (2) airborne transmission (via dust particles and airborne small droplets);

  (3) vector-borne (by biting or mechanical transfer by insects);

 (4) oral transmission (consumption of contaminated food or water); and

(5) contaminated environment or fomite (indirect contact with soil or vegetation, contact with water, indirect transmission by contaminated inanimate objects).

People are at risk of getting zoonotic diseases are those who work closely with animals. These include veterinarians, slaughterers/butchers, farmers, researchers, pet owners (e.g., through bites and/or scratches of owners of indoor pet-animals), and animal feeders in animal companies using animal products, via animals used for food (e.g., meat, dairy, eggs, birds, infected domestic poultry, and other birds).

Furthermore, transmission can also occur through animal vectors (e.g., tick bite, and insects like mosquitoes or flea).

What to be done.

(i) Environmental preservation and prevention of climate change. This will preserve animal habitats and reduce interspecies interactions which is the core for interspecies transmissions.

(ii)     Building effective collaboration between animal and human health sectors. As it is difficult to predict when or where the next zoonotic disease will emerge, close collaboration between veterinary and public health specialists is important. The goal of this inter-sectoral collaboration would be to enhance inter-personal and inter-organizational communication.

(iii)   Improving surveillance for early detection of disease threats in humans. As most of the emerging zoonotic infections have reservoirs in animals or/and in arthropods, and the occurrence of such diseases in humans often cannot be precisely predicted, investigation at the first sign of emergence of a new disease in animals that has the potential to jump species barrier is particularly important to early detect any disease threats from zoonoses.

(iv) Strengthening laboratory diagnostic capacities for novel pathogens.

(v)      Improving case management and infection control. It is an essential component. Standard precautions should be used in the care and treatment of all infected patients. If consistently applied, the standard precautions would help prevent most transmission through exposure to blood and body fluids before any zoonotic disease with unknown origin are recognized. As the clinical manifestations of many of the emerging zoonoses are often indistinguishable, leading to confusion and misdiagnosis by health-care workers, the use of a clinical decision algorithm for acute febrile illnesses with a more sensitive case definition may be useful for early detection of any suspected cases. Use of such a decision tree will help in guiding initial therapeutic decisions and trigger the protocol steps for further laboratory diagnosis and follow-up. Implementation of this clinical algorithm needs to be backed up with training of health-care workers on case management and infection control measures as well pre-positioning of strategic supplies.

(vi)        Vector control including rodents and bats.

(vii)Reducing transmission through sociocultural and behavioral interventions. This includes proper using and handling of livestocks and their products.

Discussions

An important challenge to developing effective prevention and control strategies for zoonotic emerging infectious diseases is identifying the relevant transmission routes between reservoir hosts and humans. In many cases, primary pathways for transmission are identified during outbreak investigations after a disease has already emerged. However, it is likely that many more novel transmission pathways remain undescribed for pathogens that pose a potential human health risk.

The emergence of diseases like severe acute respiratory syndrome [SARS], pandemic influenza H1N1), zoonotic viruses of pandemic potential Middle East respiratory syndrome [MERS] coronavirus) and recently emergence of COVID-19, make the targeting of surveillance programs to early stages of emergence a crucial tool for combating pandemics.

Elinda Kuhoga