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A Primer on Zoonotic Diseases

Zoonotic Diseases


Zoonotic diseases are infections that can be transmitted from animals (such as birds and mammals) to people. These diseases can come as bacteria, fungi, viruses or single-celled organisms.

There are more than 150 known zoonotic diseases. Some of the newer zoonotic diseases including bird flu and swine flu, SARS, HIV, Ebola and Nipah infections have received considerable media coverage.

Many older diseases that are causing a lingering problem in developing countries are less well known, despite affecting many people each year. The World Health Organization calls zoonotic diseases that public-health systems do not prioritize “neglected zoonotic diseases”.

One example is rabies, which is usually spread to people who are bitten by an infected dog. The virus has been eliminated in some European countries but still kills tens of thousands of people each year in places such as India.

In most zoonotic diseases, people are infected directly – such as by skin contact with an infected animal or its parts, by eating infected substances, or by being bitten by an infected animal or an insect that has recently fed on one.

People can also be infected indirectly by coming into contact with contaminated water or soil,

Food is a major source of exposure to zoonotic diseases. Each year millions of people suffer from infections of parasitic worms such as tapeworms or bacteria such as Salmonella through eating infected animal products.

A zoonotic infection can become serious if it also spreads easily between people, sparking an epidemic or even a global pandemic.

This happened recently when the H1N1 swine flu virus began to infect people in Mexico and then rapidly spread to infect people around the world.

Scientists have discovered a number of new zoonotic diseases in recent years and have warned that such diseases are a growing world problem — about 75 percent of all new diseases are zoonotic.

Various factors contribute to this trend, such as changes to farming practices, eating habits and changes in disease-causing organisms themselves.

One theory is that as human populations grow and place greater pressure on the natural world, people are increasingly coming into contact with wild species.

This can happen as human activities move into wild areas or when wild animals move towards people following disturbance of their natural habitat.

In 1999 intensive pig farms in Malaysia spread into the natural habitat of fruit bats carrying the Nipah virus. The virus spread to the pigs and then to people, and this led to 105 human deaths.

Some zoonotic diseases – such as Lyme disease and West Nile Fever – can spread between people and animals with the help of intermediary animal – such as a tick or a mosquito.

These examples are distinct from vector-borne diseases such as malaria, which spread from person to person but only when carried by an animal vector (in the case of malaria, it is a mosquito).

Many zoonotic diseases are easy to treat or prevent but in developing nations especially public health systems are not well prepared for monitoring, diagnosing and treating these diseases.

Basic hygiene is an important way to reduce risks, especially in situations where people and animals come into close contact. This includes market places, farms, zoos and – in the case of risks from pets – in the home. Hunters and consumers of bush meat may also be at higher risks (see case study).

Some research suggests that a high diversity of wild animals in an area might reduce the likelihood that some diseases will transfer to humans.

Journalists who cover zoonotic diseases should take care to be accurately when reporting on risk (see Communicating Risk). Stories relating to disease are of great interest to the public therefore journalists must responsibly convey the information.

Many zoonotic diseases are very common in nature, but extremely rare in people or produce such mild symptoms that the overall health risk to people is very low.

SciDev.Net has produced a practical guide for journalists reporting on disease outbreaks and pandemics.

Ebola is an emergent viral disease that has periodic outbreaks in parts of Africa. It was first recorded in 1976.

The virus causes fever, vomiting, diarrhea and massive internal bleeding. There is no treatment or vaccine and up to 90 per cent of infected people die.

In 2001 researchers in Gabon led by Eric Leroy showed that the virus could spread from infected wildlife, such as gorillas, chimpanzees, gorillas and forest antelopes, which can also succumb to the disease.

Four years later, Leroy’s team found three species of bats in the wild that were carrying the virus without showing any symptoms of illness. Leroy warned that people should avoid capturing and eating bats to reduce their risk of infection.

However, for rural communities bats can be an important source of protein. They are often shot and then sold covered in blood. According to research Leroy published in March 2009 in the journal Vector-borne and Zoonotic Diseases, in 2007 a man in Gabon bought bats from a local market and soon fell ill with Ebola. He survived but his daughter and 11 other family members died.


CDC on Zoonotic Diseases
World Health Organization on Zoonotic Diseases