We share a common environment with all other plants and animals. This is not merely a co-existence, but at times a competition for resources such as food and habitat. Our growing numbers, and expansion in available land for settlements, cultivation, and often adventure brings us in a close to the animal kingdom.
Micro-organisms live within us, and so do they live inside many animals. These tiny viruses, bacteria, or protozoa are ancient creatures who continue to evolve and adapt for their survival. While many of them have a dependent (or parasitic) existence, some others live freely in air, water or soil. They are quite numerous, 40 trillion of them are inside each human body, and a similar or even a larger number inside every member of the animal species. In fact we all live inside a microbial cosmos.
Zoonoses: An Outcome of animal and human interaction
Most microbes are beneficial, both for human and animal life. A small minority is harmful. These disease causing or pathogenic microbes are in a constant conflict with our immune system, which usually takes care. Some of these harmful tiny beings, were circulating in specific animal species for years. Animal immune systems had found a way to adapt and tolerate their presence. At a particular time or a place of its choosing, a microbe managed to shift its base to humans. Enabled by human-animal interaction, this became a Zoonosis.
Zoonoses are diseases that either circulate across species between humans and animals, or has jumped from an animal species for a long term existence in the human world. As we understand our biological world, this interplay has become more and more apparent.
Microbes have an intricate life cycle
Microbes are invisible to the naked eye, and so are the instances when they move from one organism to another. Most disease causing microbes and their life cycles were discovered in the last century. Many such microbes or parasites that cause disease amongst animals, have a transmission cycle. Such a cycle often involve insects (flies, fleas, mosquitoes, or ticks) and environment (water, or soil) as vehicles (or vectors).
Some animal parasitic diseases have a counterpart amongst humans. For instance, Ascaris (roundworm) is a parasite, that affects animals as well as human beings. The organism is released from body through excreta, and gets back into the body, through contaminated soil. Excreta in this case is mostly human, but could also be of a pig. On the other hand Ancylostoma species (hookworm) that infect canines (eg dogs) and those that infect humans are distinct. However, humans may also get cross-infected by animal species. Both of these are ancient human or animal parasites.
A complex interaction parasite, reservoir and human being
Various parasitic diseases that affect human beings not only cross-infect animals, but also have animal reservoirs. This means that these organisms live and multiply in animal species, mostly as a peaceful co-existence. However, reservoirs become a source of the organism for human disease. Insects such as mosquitoes, flies or ticks can transmit disease from an animal reservoirs to a humans.
Leishmania infects human beings, and the disease is transmitted from on person to another through sand-flies. These sand-flies also bite dogs, who also act as a reservoir of infection
Plague: From Rodents to humans
Plague is an ancient disease known in history of mankind for its global pandemics. The disease was also known as “black death” as mortality was high, and many civilizations perished in these pandemics. The organism causing plague was discovered in a 1884 epidemic in Hong-Kong by a French scientist Alexendere Yersin. This plague causing bacteria was hence named after its discoverer as Yersinia pestis.
Yersinia sp have a rodent life-cycle. About 6000 years ago, mutation in the organism, caused human disease. Subsequent mutations have led to human pandemics. The organism is still maintained in rats, and transmitted from one rat to another through fleas. However, a mutation can make the organism more furious, and rats begin to die. When rat population begins to get scarce, fleas infected with Yersinia, turn to an alternate host – human beings. Thus, excess rodent deaths are a precursor to transmission of disease in humans.
Rabies: First Zoonotic disease with a vaccine
Many mammalian wild animals harbor Rabies virus. It infects their brains after a variable incubation period. Infected animals develop brain inflammation, and have a pro-drome, excitatory and paralytic stages, and the condition is fatal. The excitatory form may be intense and furious, and such infected animals bite others to spread the disease. The virus can also spread to domesticated or stray canines, who have the same fate. Human beings can acquire infection, once they get bitten by a wild animal or even a domesticated canine.
For Rabies virus, human beings are a dead-end host. Which means that the virus cannot transmit further (unless the furious infected person, end up biting another human). Rabies infection has no cure, and the infected human is destined to die. Thus, vaccination is the only means of survival. Every bite is potentially infectious, and we must protect ourselves against this invariably fatal disease.
See this previous blog for history behind first Rabies Vaccine
Flu Pandemics: From Birds to us
Influenza viruses cause flu, and since 1918, we have seen and recorded major global pandemics. Various bird species are the natural reservoirs of influenza A viruses. These viruses undergo a mutation quite frequently. These mutations enable the virus to infect other animals such as pigs, hens, dogs, or horses. Such animal influenza strains often assort themselves to cause human disease.
World has seen four major Influenza epidemics, due to H1N1 strain in 1918, H2N2 strain in 1957, H3N2 in 1968, and again H1N1 in 2009.
Human influenza pandemic of 2009, was preceded by a bird-flu outbreak. This outbreak started in South East Asia in the year 2004 and spread to other countries in Asia and the world over next two years. This bird-flu was due to H5N1 strain.
We were worried about the threat of bird-flu, as such strains can pass on to humans. Fortunately human disease in bird-flu pandemic was rare, and the epidemic passed, without much human impact.
Coronaviruses also have their origin in animals
In 2002, first coronavirus infection affected humans. This was a virus that had originated from bats. First cases occurred in China, and over next year SARS affected more than 8000 people in 29 countries. About 700 died, and the pandemic mysteriously disappeared. This was first Severe Acute Respiratory Syndrome (or SARS) pandemic due to a coronavirus.
In 2012, another coronavirus, named Middle East respiratory syndrome (MERS)-CoV, emerged in Saudi Arabia. While exact source of this coronavirus is not known, it is believed to have its origin in domesticated camels. This zoonotic transmission was limited to the middle east Asia, due to reasons that are unexplained.
COVID-19: Coronavirus jumped from the bats
The 2019 coronavirus pandemic (COVID-19) was most sinister of them all. Known as SARS-Cov2 this virus originated from bats, and probably spread to some exotic animal species in the Wuhan wet market. However, this virus had a rapid spread, and caused a more severe disease.
SARS-CoV2, unlike its ancestor SARS-CoV from 2003 had a much wider spread. It reached all continents and countries within a few months, and affected most population. About 7 million people died due to this virus, across the world.
COVID-19 zoonosis was disruptive. In addition to a huge mortality, this pandemic was also marked by lockdown, disruption in travel, trade and global economic slowdown. Towards the end of the year 2020 we started a vaccination campaign. The pandemic was contained only by the end of the year 2023.
See previous blogs on Global COVID Philately
Gallery of COVID special covers from India
COVID postcards from across the world
Dengue virus jumped from Monkeys to humans
Dengue viruses originated in monkeys in Africa and Southeast Asia. They jumped to humans about 500 to 1,000 years ago, but mostly circulated between monkeys and mosquitoes in forests. However, human expansion and increased travel, changed the virus to a predominantly human-mosquito lifecycle.
In 1943, two scientists Ren Kimura and Susumu Hotta were studying blood samples of patients during an epidemic in Nagasaki, Japan. They first identified dengue virus, which was DEN-1 serotype. In the aftermath of World War II, rapid urbanization in Southeast Asia led to increased transmission and hyper-endemicity.
From nine countries that had experienced severe dengue epidemics in 1970s, the virus now circulated in more than 100 countries. Factors such as Globalization, trade, urbanization, travel, inadequate domestic water supplies and warming temperatures all help the virus and its vector Ades mosquito to spread. In 2019 World Health Organization estimated more than 52 million Dengue cases worldwide.
More viruses Ebola and Zika
Ebola virus originated in fruit-bats. The virus can get into the human population when people have close contact with the blood, secretions, organs or other bodily fluids of infected animals such as fruit bats, chimpanzees, gorillas, monkeys, forest antelope or porcupines.
The rain forest habitat of Ebola virus, let to its spread in Western Africa in the year 2013-16. Guinea, Liberia, and Sierra-Leone were the most affected countries. Half of about 30,000 infected individuals died.
Similar to Dengue, Zika virus also originated in monkeys in Africa. After a few sporadic cases in Africa, the virus spread to South America in 2007. In 2015-16 a large epidemic occurred in Brazil. While the virus initially causes a self-limiting fever – It leads to neuropathies and myelitis. It also affects unborn fetus, and causes malformations in the unborn. A typical presentation is a small sized head or microcephaly.
Why a focus on Zoonoses now
While Zoonotic diseases were always there, the risk of zoonoses became more visible after the Covid-19 pandemic. Factors beyond health, such as increased mobility, intercontinental movement of humans, greater contact with rural environments, wildlife, climate change, and natural disasters have all played their part. We now recognize one world one health concept, implying that we share our environment, and so do we share our diseases with a variety of animals. The infected animals act as a reservoir, and help spread the disease.
The list of Zoonoses is long, leaving aside only a few infections that do not involve animals in any way. Some infections that have origins in humans, infect animals. This reverse zoonosis is also on the rise. We are still on a learning curve, especially when it comes to our understanding of zoonoses. Our understanding also brings in humility, that we as a species are just one of the many in the animal world.