COVID-19 and Emerging Zoonosis - The Clock is Counting Down: Time for Closer Collaboration between Veterinary and Human Medicine

Factors Associated to the Emergence of Zoonosis

The emergence and maintenance of zoonosis are multifactorial and affected by dynamic interactions among the host, aetiological agent and environment [6, 7, 8]. Regarding human activity and travel, globalization has been considered a factor in increasing the transfer of pathogens to pristine areas, such as tropical forests [9]. Tourism and travel to exotic places have steadily increased over recent years. Although they represent an important economic sector for any country, several health issues have been reported, as the outbreak of Middle East respiratory syndrome coronavirus (MERS-CoV) in South Korea caused by a returnee from the Middle East [10]. Moreover, the transmission of agents of airborne emerging diseases due to commercial airline networks has also been described Mangili A, et al. [11]. One example is the outbreak of SARS in 2003, whose pathogen was disseminated from China to at least 17 countries within a week [7].

History has shown that large-scale epidemics have followed large-scale human migratory movements. Some examples include the Black Death (caused by Yersinia pestis), cholera (Vibrio cholerae), SARS and malaria (Plasmodium falciparum) [12]. Millions of people and animals travel the world annually. The movement of people, animals and food (of animal and vegetable origin) may quickly disseminate pathogenic agents worldwide [13], since they can travel not only inside a sick host, but also by apparently healthy but infected carriers. Before COVID-19 outbreak, it was possible to travel to any part of the world within a few hours and thousands of air journeys were made daily. In addition, trip duration is shorter than the incubation periods of many diseases (especially viral ones) [14], a circumstance which increase the odds of a rapid distribution of infectious agents into new areas. In fact, the last 10 years have shown that zoonotic diseases such as Chikungunya, Dengue, Ebola, MERS, and SARS may have the potential to spread across borders due to rapid human mobility via the global airline network [7]. Since last year, the world assists to the swiftly spread of COVID-19 across the globe, and travel restrictions and border control measures, like never seen before, have been enforced to limit the spread of the outbreak [15].

Pet ownership is another recognized factor. It is known that there are many health benefits, including physical, mental and emotional improvements, linked to pet ownership. New companion animals such as reptiles, birds or insects can also be responsible for zoonotic infections. Additionally, there is an increasing trend (particularly in high-income countries) to breed and maintain wild animals as pets, a situation which can lead to many infectious agents crossing the species barrier into humans [16]. SARS-CoV-2 has been detected in naturally infected dogs and cats in households, most of which lived in close contact with infected humans [17]. Understanding risk factors associated with this scenario and their potential to infect other species requires urgent investigation [18].

The global trade of animals (i.e. pets, livestock and wildlife) and products of animal origin has also been related to the dissemination of non-zoonotic diseases, such as foot and mouth disease, classical and African swine fevers, and zoonosis like brucellosis [19], human monkeypox in North America and H5N1 in the United Arab Emirates [12]. The global trade of illegally imported products of animal origin, and non-compliant or the absence of food and animal control allow vector insects, or other infected animals, to transmit pathogens that can be introduced into areas where there is no host resistance or native predators, and to reintroduce them into previous officially-free regions. Consequently, naïve populations can face large outbreaks and remarkable socioeconomic costs [19]. Several factors influence the ability of a new microorganism to become a public health threat. It is considered that viruses cause about 45% of emerging human diseases. RNA viruses, as SARS-CoV-2, are prone to emerge due to their high rates of replication and mutation, which may produce novel variants responsible for the emergence and re-emergence of infectious diseases [20]. As a consequence, new variants emerge as is the case of the emergent SARS-CoV-2 B.1.1.7 lineage in the United Kingdom defined by a novel set of spike mutations [21, 22].

Anthropogenic practices associated with agricultural production or industrial processes have been identified and are linked to the spread of pathogens, creating new ecological niches and exposing human and animal populations to them [23]. Agricultural expansion contributes to encroachment into wildlife habitats, which causes changes in the ecosystem and implies close proximity of humans and livestock to wildlife and vectors. Such circumstances are responsible for the potential spill over of wildlife pathogens in livestock and humans [24]. Deforestation has accelerated exponentially since the beginning of the 20th century, mainly due to human population growth, agricultural intensification and political and economic interests. This is inextricably linked to habitat destruction and a drastic reduction in biodiversity, since the population of natural predators is no longer sufficient to control rodents, insects and other potential transmitters of infectious agents. This may affect host and pathogen dynamics, forcing reservoirs of zoonotic diseases to become closer to potentially susceptible animal and human populations [25]. One example is the occurrence of the Nipah virus outbreak in Malaysia in the late 1990s, affecting pigs and humans [26].

Increased incidence of zoonosis and/or vector-borne diseases (as well as changes in their transmission pattern) is closely related to drastic changes in the environment and ecosystems caused by climate change associated with global warming [27]. The occurrence of extreme weather events such as heat waves, cold spells, droughts or floods may favour the increase of vectors and their movement from endemic areas, in addition to decreasing their natural predator population, and it have influenced the development, behaviour and survival of vectors as well as the dynamics of pathogen transmission [28].

Building the Bridge between Veterinary and Human Medicine

Global health challenges need an innovative and cooperative approach between human and animal health authorities. In the late 19th century and early 20th century, physicians as Rudolf Virchow and William Osler explained that animal and human health were inextricably linked [29]. Moreover, Virchow indicated the absence of boundaries between human and animal health, and emphasized their crossing lines [30]. Currently, the relationship among humans, animals and environment has gained importance associated with COVID-19 concerns [2]. One of the main problems of emerging and re-emerging diseases is their control, since they are sporadic and unpredictable, often appearing from unexpected reservoirs [26]. Since 60 to 80% of the new human infections likely originated in animals, research and control measures aimed at emerging/ remerging zoonotic diseases should include appropriate veterinary support [31] (Table 1). Their impact is greater in developing countries because of risk factors, such as the lack of medical and human resources to tackle disease outbreaks and the high population growth and abundance of vectors and reservoirs of pathogens. Additionally, a high number of people with a compromised immune system due to the human immunodeficiency virus (HIV)/acquired immune deficiency syndrome (AIDS) or parasitic diseases, a dependence on animals as the main source of livelihood, low socio-economic status and hygienic deficits are considered important risk factors [7].

A One Health integrative approach for COVID-19 and new zoonosis must be implemented in order to develop epidemiological surveillance and to establish disease control mechanisms to limit zoonotic transmission [2]. Long-term veterinary experience with infections caused by CoVs, such as infectious bronchitis virus (IBV) in poultry, feline infectious peritonitis virus (FIPV) or swine CoVs, needs to be included for a better understanding of the origin and spread of SARS-CoV-2. The same is true regarding the use of animal models to study vaccines for humans [2, 32]. Among the distinctive areas of action to anticipate ultimate threats caused by biological agents, four of them stand out and can be applied to SARS-CoV-2. The first one is related to the discovery of pathogenic agents through continuous longitudinal surveillance of wildlife and domestic animals, considering the animal origin of SARS-CoV-2 and its spill over into humans [2]. The second one refers to the quantifiable identification of the risk that a microorganism as SARS- CoV-2 may represent to humans as well as their transmission pathway to humans and other animals [33]. The third one is the ability to respond adequately to COVID-19 outbreaks by strengthening countries skills to detect them with effective rapid diagnostic tests [34] and vaccines [35]. Currently, many efforts have been directed towards the development of effective vaccines and therapies to prevent transmission person-to-person, between health professionals or the elderly [36]. The fourth is based on the reduction of human risk by minimizing human behaviors and practices that increase the probability of agent’s transfer and amplification. In relation to COVID-19, the main measures are wearing a mask, frequent hand washing, promoting social distance and compliance with lockdowns [37].

Due to COVID-19, public awareness has reached an unprecedented scale. In the face of the current pandemic, the public is required to understand and trust the entities that are making recommendations to limit the exposure and spread of the disease. General public must trust on science and collaborate with public health services through practicing all measures and policies implemented to solve the pandemic problem. The battle against COVID-19 has not yet been won, and people’s adherence to prevention and control measures are essential to ensure ultimate success, including collaboration between veterinary and human medicine through the One Health strategy for prevention and control of COVID-19, as well as other emerging zoonosis.