Ever since the emergence of life on this planet, human beings have co-existed with numerous other animals. From sharing habitats with them to creating our civilizations and safeguarding ourselves from them, we have come a long way. A significant part of this transition was the domestication of animals beneficial to our survival. Though this transition has proven helpful, domesticating cows, dogs, and some species of birds has given rise to several zoonotic diseases spreading to human civilizations.
Zoonoses, or zoonotic diseases, in simple terms, are infections that are spread to humans by animals through the germs that they carry, and vice versa. These diseases can be caused by bacteria, viruses, fungi, and parasites; depending on the pathogens that infect people, the disease could range from mild to life-threatening. A recent literature survey aiming to quantify the relevance of these infections identified 1,407 species as pathogens, 58% of which are zoonotic [1]. One of the most common and well-known zoonotic diseases is rabies, caused by a virus transferred through the bite of infected dogs, cats, monkeys, and bats, among other mammals. According to a 2018 study, 95% of the deaths due to rabies occur in the Asian and African continents. Furthermore, according to WHO surveys, India contributes to one-third (approximately 20,000) of the world's deaths due to the life-threatening disease each year. The universal way of treating an infected rabies bite is by administering rabies immunoglobulin once at the start of the treatment, along with anti-rabies vaccines for 14 days starting from the day of the bite.
The five major zoonotic diseases around the world. (Credits: https://www.healthforanimals.org/global-challenges/zoonoses/)
Human Immunodeficiency Virus (HIV) is also a virus that is being extensively studied. Most strains of HIV affecting human beings can be traced back to chimpanzees The African population who originally contracted this disease seem to be highly susceptible to it. In the sub-Saharan region alone, around 70% of the population is accounted to have contracted HIV infection/AIDS. Host genetic polymorphisms play a significant role in the HIV infection/AIDS spread among the African population [3].
The plasma levels of HIV-1 suppressive and proinflammatory chemokines, such as RANTES (Regulated upon Activation, Normal T cell Expressed and Secreted) and CCL5 (a chemotactic cytokine protein), are influenced by the Duffy antigen receptor for chemokines (DARC) on red blood cells (RBCs). The causative agent of malaria, Plasmodium vivax, uses this DARC as its receptor. Africans with the DARC 46C/C genotype (conferring a negative phenotype) are resistant to malaria. DARC-46C/C has also been linked to a 40% increase in the risk of HIV-1 in African-Americans. This can further be extrapolated to African populations and may be responsible for 11% of their HIV-1 load. DARC-negative RBC status, on the other hand, is linked to a slower course of the illness after infection. [3]
Though anyone can get sick from a zoonotic disease, the most susceptible groups are children, senior citizens, and people with chronic health conditions. The probability that a zoonotic infection in humans will lead to an epidemic depends on the human-to-human transmissibility of the pathogen. These transmissions can occur through oral and fecal-oral routes, as well as pathogen vectors in the environment. The most common diseases transmitted through these paths are influenza, CoVID-19, salmonellosis, mosquito-borne diseases, and E. coli infections.
Emerging cases of zoonotic infections from animals have been a global cause of alarm. The spread of the bubonic plague in the early 1300s and its re-emergence in July 2020 shows that though zoonotic infections cannot be eradicated, they can be contained for long periods. More recent outbreaks include Rift Valley fever (in 2018 in Japan and 2021 in Kenya), SARS from 2002 to 2004, influenza H1N1 in 2009, West Nile virus ( in 2011 and 2012), and Middle Eastern respiratory syndrome coronavirus (MERS-CoV) in 2012. COVID-19, first detected in a 55-year-old Chinese national, is the most recent zoonotic infection.
In the past few years, many air-borne infections have been traced back to pathogen-carrying bats. Chiropterans (an order of mammals capable of true flight, e.g. bats) are now regarded as the largest reservoir of infectious, transmissible diseases; they are known to carry over 60 viral diseases, most of which have RNA as their genetic material. Bats act as a favourable long-term host for these viruses, mostly credited to a mutation in a highly conserved residue of serine (S358) in STING (Stimulator of Interferon Genes), an adaptor protein. A study by Jiazheng Xie et al. aimed to reverse this mutation by introducing S358 restored STING. They succeeded, resulting in interferon activation and virus inhibition [2]. It is no surprise that with over 290 species of bats, most of the rabies infections in Latin America are caused due to contact with infected bats rather than dog bites.
Zoonotic diseases, when escalated to the community level, can cause grave impacts on the economy and on the general well-being of people; CoVID-19 has made us all the more familiar with this. The loss of workforce and earnings, the clinical complications and long-term effects of the infection, and disruption of daily-life patterns altogether can take serious tolls on one’s mental and physical wellness. Furthermore, healthcare systems are burdened with patients and are often forced to treat people despite the inadequate facilities. For example, the 1994 bubonic and pneumonic plague outbreak in Surat, Gujarat, resulted in an air travel ban, leading to a loss of INR 816 crores. A similar incident in 2021 in Southern Australia, the mice plague, led to several reports of rat-bite fever which, when left untreated, can lead to further infections or even sepsis.
The treatment for zoonotic infections (Credits: https://www.uspharmacist.com/article/zoonotic-infections-your-pets-can-make-you-sick)
The best and most effective precautions we can take to prevent getting infected by these pathogens are washing our hands, preventing bites from animals, and ensuring that pets are regularly vaccinated for the diseases most easily spread by them. Most zoonotic infections can today be treated with antibiotics and vaccines developed for them. More precautions are necessary to counteract new emerging infections since there is a high chance that the causative agents of these zoonotic infections might mutate into newer and more resistant forms. Personal judgment plays an essential role in preventing these infections, allowing us to lead a healthier life with fewer complications.
References
[1] Woolhouse, M.E. and Gowtage-Sequeria, S., 2005. Host range and emerging and reemerging pathogens. Emerging infectious diseases, 11(12), p.1842.
[2] Mboowa, G., 2014. Genetics of sub-Saharan African human population: implications for HIV/AIDS, tuberculosis, and malaria. International journal of evolutionary biology, 2014.
[3] Xie, J., Li, Y., Shen, X., Goh, G., Zhu, Y., Cui, J., Wang, L.F., Shi, Z.L. and Zhou, P., 2018. Dampened STING-dependent interferon activation in bats. Cell host & microbe, 23(3), pp.297-301.
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