What is zoonosis?

Do you know what's common among SARS-CoV-2, HIV AIDS, Ebola, SARS, MERS, Nipah, H1N1 flu (swine flu) and H5N1 flu (bird flu)? Yes, all of these are zoonoses, that is, they are animal-bome diseases.

A zoonosis is any disease or infection that is naturally transmissible from vertebrate animals to humans. Zoonoses have different modes of transmission. In direct zoonosis, the disease is transmitted from animals to humans through air, bites or saliva. In indirect zoonoses, the transmission occurs via an intermediate species (referred to as a vector), which carries the disease pathogen. These pathogens can be viruses, bacteria, fungi or parasites.

Though the world has seen the emergence of diseases throughout history, in the last 50 years, a host of new infectious diseases has spread rapidly after making the evolutionary jump from animals to humans. According to the World Health Organisation (WHO), 70% of emerging human pathogens come from animals. In the last century, at least 10 infectious diseases jumped from animals to humans, it says.

What has led to this?

Globalisation, deforestation, encroachment of wild environments, human-animal conflicts and wildlife trade have led to the spike in zoonosis outbreak, says a report by the WHO, released in September 2019. It has also warned that the risk of a global pandemic is growing and that zoonotic diseases will continue to emerge and re-emerge.

The deadly ones

As the world grapples with the new coronavirus, which is thought to have spread from bats, let's take a look at some of the major zoonotic diseases and their outbreaks...

Bubonic plague

Bubonic plague was the cause of the Black Death that swept through Asia, Europe and Africa in the 14th Century and killed an estimated 50 million people. It took centuries for some societies to recover. Plague is a bacterial disease caused by Yersinia pestis. It is carried by rodents and cats. The infection in humans is caused by the bite of an infected flea.


HIV, the virus that causes AIDS, originated from chimps and other primates and is thought to have first infected humans at least a century ago. However, it was first recognised only in 1981. By the end of that year, there were 270 reported cases and 121 deaths. HIV destroys the immune system, opening the door to a host of deadly infections. The virus has now mutated to a separate human-only disease. Between 1981 and 2018, the disease caused an estimated 32 million deaths worldwide.

SARS An epidemic of SARS, caused by a strain of coronavirus SARSCOV, affected 26 countries and resulted in more than 8,000 cases in 2003.

SARS-CoV is thought to have spread from bats, which, in tumn, spread to other animals (civet cats) and perhaps first infected humans in southern China in 2002.


The 2014-2016 outbreak in West Africa was the largest Ebola outbreak since the virus was first discovered in 1976. The outbreak started in Guinea and then spread to other parts of the continent. The virus is transmitted to people from wild animals (such as chimpanzees, gorillas, monkeys, antelopes or porcupines). Human-to-human transmission happens through direct or indirect contact with the blood or bodily fluids of infected people. It is thought that fruit bats of the Pteropodidae family are natural Ebola virus hosts.


Nipah virus (Niv) infection severely affects both animals and humans. The natural host of the virus are fruit bats. It can spread from bats to other animals to people or directly from bat to and between people. Even eating a fruit bitten by an infected bat can transfer Nipah to humans. The disease was first identified in 1998 during an outbreak in Malaysia, India has reported three NiV outbreaks in the past. The first two were in West Bengal: Siliguri in 2001 and Nadia in 2007. The third outbreak occurred in Kerala in May 2018.

Picture Credit : Google

What are fleas?

Fleas are tiny bugs. They don’t grow much larger than the tip of a pen, and they range from light brown to almost black in color. They don’t have wings, so they get around by jumping from place to place. Their thin, flat bodies and hard shells mean you often need to squeeze them between fingernails or two hard surfaces to kill them. Even then, where there is one, many often follow. Fleas reproduce quickly, especially if you have pets in the house. But even if you don’t have pets, your yard can potentially play host to fleas, and you may end up with a bunch of mysterious bites. They’re almost impossible to get rid of without a pesticide treatment.

Fleabites are pretty distinctive. They look like small, red bumps in clusters of three or four or a straight line. The bumps remain small, unlike mosquito bites. You might notice a red “halo” around the bite center. The most common places to find these bites are around the legs or ankles. Fleabites are also common around the waist, armpits, breasts, groin, or in the folds of the elbows and knees.

For humans, the risk of contracting another disease from the flea is very small. Yet bacteria can get into your body through the bite and cause an infection, especially if you scratch it. An infected bite will turn red, warm, and it may release pus. Fleabites can also cause an allergic reaction in people who are sensitive to them. Symptoms can range from raised welts on the skin to difficulty breathing. Fleabites can also cause complications in pets, such as allergic reactions and even anemia from blood loss. That’s why it’s important to take animals to a vet if they have fleas.

Picture Credit : Google

How do parasitic worms get into the body of the host?

A person can get it by eating food or drink contaminated with human or animal feces where the parasite is present.

Many parasitic worms enter their hosts by active invasion. Their transmission success is often based on a mass production of invasive stages. However, most stages show a highly specific host-finding behaviour. Information on host-finding mechanisms is available mainly for trematode miracidia and cercariae and for nematode hookworms. The larvae find and recognise their hosts, in some cases even with species specificity, via complex sequences of behavioural patterns with which they successively respond to various environmental and host cues. There is often a surprisingly high diversity of host-recognition strategies. Each parasite species finds and enters its host using a different series of cues. For example, different species of schistosomes enter the human skin using different recognition sequences. The various recognition strategies may reflect adaptations to distinct ecological conditions of transmission. Another question is how, after invasion, parasitic worms find their complex paths through their host's tissues to their often very specific microhabitats. Recent data show that the migrating parasite stages can follow local chemical gradients of skin and blood compounds, but their long-distance navigation within the host body still remains puzzling. The high complexity, specificity and diversity of host-recognition strategies suggest that host finding and host recognition are important determinants in the evolution of parasite life cycles.

Picture Credit : Google

What is amoebiasis?

Amebiasis is a parasitic infection of the intestines caused by the protozoan Entamoeba histolytica, or E. histolytica. The symptoms of amebiasis include loose stool, abdominal cramping, and stomach pain. However, most people with amebiasis won’t experience significant symptoms.

The cause of amebiasis is infection by the protozoan parasite Entamoeba histolytica. It begins when a person drinks contaminated water or eats foods contaminated with the cystic form (infective stage), comes in contact with contaminated colonic irrigation devices or the fecally contaminated hands of food handlers, or by oral-anal sexual practices.

The cystic form changes into trophozoites (invasive form) in the ilium or colon and invade the mucosal barrier, leading to tissue destruction and diarrhea. These trophozoites can reach the portal blood circulation to the liver and eventually go to other organs. It only infects humans, and the CDC does not classify it as a free-living organism.

Picture Credit : Google

What is parasite?

A parasite is an organism that lives in another organism, called the host, and often harms it. It depends on its host for survival.

The parasite uses the host’s resources to fuel its life cycle. It uses the host’s resources to maintain itself.

Parasites vary widely. Around 70 percentTrusted Source are not visible to the human eye, such as the malarial parasite, but some worm parasites can reach over 30 meters in length.

Parasites are not a disease, but they can spread diseases. Different parasites have different effects.

There are many types of parasite, and symptoms can vary widely. Sometimes these may resemble the symptoms of other conditions, such as a hormone deficiency, pneumonia, or food poisoning.

Parasitic infections cause a tremendous burden of disease in both the tropics and subtropics as well as in more temperate climates. Of all parasitic diseases, malaria causes the most deaths globally. Malaria kills more than 400,000 people each year, most of them young children in sub-Saharan Africa.

Picture Credit : Google