Zoonotic Infectious Diseases: Local Origins, Global consequences

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On December 31, 2019, twenty-seven cases of pneumonia of unknown origin were reported in Wuhan, China. By the second week of January 2020, the first case outside China was reported in Thailand. On January 30, 2020, the WHO declared an international public health emergency. Since those events transpired none of us have escaped the effects of the waxing and waning of SARS-CoV-2 as it has raged around the world over the past 18 months. 

If there is a positive fallout from this event, it is the explosion of international scientific efforts to find a way to control this deadly virus. The first sequence of this coronavirus was publicly available in January 2020, and vaccines were created within the next six months, both achievements as epic as the urgency created by this unprecedented (at least in our lifetime) international crisis. Simultaneously, the origin of this virus is being investigated, and expanding upon the knowledge that bats are the natural host of previous coronaviruses that caused human epidemics, namely SARS-CoV and MERS-CoV, it appears that SARS-CoV-2 managed to jump to humans from bats. In order for the jump to human infectability to occur, mutations occurring in the virus genome create a viral surface protein that can bind specifically to a human cell surface protein; in the case of SARS-CoV-2, this could be a mutation that allows the viral Spike protein to bind the human cell surface ACE2 protein and cause infection. 

COVID-19 structure
COVID-19 structure

This process of zoonosis, involving the adaptation and transmission of infectious agents from a primary host that is either a mammal or a bird to humans, is an evident aspect of over a hundred infectious diseases known to afflict humans. The infectious agent involved could be bacteria, fungi, parasites, or viruses and in addition to known diseases, there is a continuous roster of emerging zoonotic diseases as these opportunistic microorganisms try to find new hosts to live and breed in. Transmission from animal to human may be through direct contact through potential scratches or bites, airborne through droplets for instance, through vectors such as mosquitoes, ticks, and lice, ingestion of contaminated food or water, and by contact with infected vegetation, soil, water, wild animals, etc. Transmission of pathogens across oceans and borders after they have adapted to humans can, unfortunately, become a reality with the ease of international travel, especially if they can achieve efficient human-to-human transmission and become highly infectious, as in the case with SARS-CoV-2. 

Commonly known extant zoonotic diseases include rabies, plague, chagas disease, brucellosis, anthrax, bovine tuberculosis, Japanese encephalitis, zika virus, ebola, and AIDS. All these, and many more, are of direct relevance to India and other tropical and sub-tropical countries including south-east Asia, Africa, South America, western Pacific islands, and parts of Australia where they can be a burden on the public health system and economy. In India, 13 zoonoses are associated with 2.4 billion cases of human disease, and 2.2 million deaths per year. The National Center for Disease Control in India coordinates efforts at early diagnosis and effective containment, and a specific focus is in the handling of animals and regulation of human-animal contact. Peri-urban areas have grown rapidly in India, and are a link between agricultural areas and densely populated sites. They present a risk as there is unregulated livestock-based food production in these areas to meet the increased demand for food products. 

In addition to these existing illnesses, it is estimated that 60-80% of emerging infectious diseases are zoonotic diseases. Change in land use is thought to be a major underlying cause of this especially in Southeast Asia and in tropical and developing countries, coupled with wildlife diversity. Depending on the use that the deforested land is co-opted for, be it monoculture forests, crops. poultry, livestock. housing, etc., different groups of zoonotic species came to the fore. For instance, strong associations of vector-borne diseases were found with monoculture plantations (for instance, rubber), and bacterial and viral diseases are among others associated with livestock farming. In India, which is one of the hot spots for emerging zoonotic diseases, potential reasons for the emerging disease include changing land use, dairy farming, rodent infestations, wild-animal trading, climate change, and improper farming practices. Coupled with these conditions there is a lack of awareness, poverty, and poor access to medical and diagnostics services. Endemics, epidemics, and emerging zoonotic diseases in Australia have been a constant presence between livestock, horses, and humans. These are mostly viral and vector-borne diseases, and a few examples are Nipah virus, Menengle virus, and JE virus

Examples of zoonotic diseases and their affected populations.
Examples of zoonotic diseases and their affected populations.

Triggered by the ongoing SARS-CoV-2 pandemic, the World Wildlife Fund has published a report on the zoonotic disease risk posed by wildlife markets in southeast Asia that are involved in wildlife trade and consumption. They urge governments to impose regulations on these activities and reduce demand for high-risk wildlife products. Other comprehensive and phased efforts to prevent and control known and new pathogens have been reported from Congo (monkeypox virus), Ethiopia (rabies), and Georgia (a new zoonotic virus). Of particular concern to India is the potential for it to become a hotspot for future variants of SARS-CoV-2, with global consequences. With its density of population, a priority is to exercise COVID19-related behaviors of masking, social distancing, and vaccination. A second priority is sequencing variants as they arise and following them epidemiologically with outbreaks of COVID19. 

In urban settings, most contact with animals is relegated to pets, household pests, and the consumption of meat and dairy products. Obviously, food needs to be handled with care and cooked well, and pests ranging from rats to mosquitoes and flies need to be eradicated. Although specific viruses can infect dogs and cats, there is currently no evidence that these transmit to humans and cause disease. However, there is some evidence that pets can test positive for SARS-CoV-2, with infection transferring from infected humans. Import and close continuous contact with exotic and wild animals as pets is not recommended. 

L. Iyengar has lived and worked in India and the USA. A scientist by training, she enjoys experiencing diverse cultures and ideas, and writing. Her short story will be included in an anthology showcasing a group of international women writers, to be published in 2021 by The Nasiona. She can be found on Twitter at @l_iyengar and www.liyengar.com.


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