Article: Jumping species

Sunday May 10, 2009

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By DR Y.L.M.

A look at viruses that can spread from animals to humans.

THERE has been so much talk recently about H1N1, a virus that has mutated from being a swine virus to one that is able to infect humans. Have there been a lot of cases of viruses previously infecting only animals jumping to humans?

Yes. The viruses that are transmissible from animals to humans are called zoonotic viruses. These animals include both arthropods (such as mosquitoes) to vertebrate animals (such as birds and swine). Arthropod borne viruses are called arboviruses. (Ar = arthropod; bo = borne.)

Some of these viruses are transmitted when an infected blood-sucking arthropod vector such as a mosquito or sandfly bites you. Others are transmitted via means of inhalation, skin or conjunctiva contact with an infected animal or through their infected excretions.

The latest theory about HIV is that it originated from a subspecies of chimpanzee. – AP photo

One of the most famous zoonotic viruses is the deadly HIV.

The term “reverse zoonosis” is employed when the virus is transmitted from humans to animals.

Why are these zoonotic viruses becoming more common? Is it because we were not able to detect them previously?

These viruses are not new. It’s just that as the world’s population expands, the frequency of human contact with these viruses also expands. And because humanity has become more populous, these viruses are able to spread easily and rapidly.

Take for example the recent H1N1 contagion. Air travel has resulted in many cases spreading through the four corners of the world.

In the past, when there were smaller human populations and there was no such thing as air travel, the viruses can affect a human being, but because there is very little spread, when the human dies, the virus would die with him, or he may overcome the virus and recover. Therefore, the viral spread is contained. The human here is considered a “dead-end host” and the virus burns out quickly.

But nowadays, when the population is so dense and the world is so connected, viruses can rampage throughout the world unchecked. When man depletes the forests, more and more animals are forced into contact with humans, and so the higher exposure of human to the viruses animals carry.

Call it environmental revenge.

How did the HIV virus jump from animal to man?

HIV infection is now considered a worldwide pandemic. The latest theory about HIV is that it originated from a subspecies of chimpanzee. Scientists believe that HIV might have jumped to humans when the blood of an infected chimpanzee came into contact with a bush meat hunter when he butchered the chimpanzee during the 20th century. This is believed to have happened in southern Cameroon.

The belief is also that HIV has probably infected humans as early as 1900s. People that time didn’t recognise it. And it wasn’t until air travel became common that AIDS became a global epidemic in the 1980s.

From such origins, it is estimated that over 40 million people in the world are now living with HIV.

What is an epidemic? And what is a pandemic?

The word “epidemic” is of Greek origin. (Epi = upon; demos = people.) It is used when new cases of a disease in a certain human population substantially exceeds what is normal for that population. The number of new cases during that specified period of time is called the “incidence rate”.

The word “epidemic” is not confined to infectious diseases. It can also be used to describe diseases such as cancer or heart disease.

The word “pandemic” is also of Greek origin. (pan = all; demos = people.) This means an epidemic of an infectious disease that has spread through populations across a large region. Common example are the SARS virus, HIV, the avian flu. Note than pandemics have to be infectious in origin, unlike epidemics.

What are some of the common zoonotic diseases?

Other than HIV, it is believed that influenza, measles, cholera and diphtheria originated from animals. In the past, the plague has also come down from rats.

Anthrax comes from domesticated herbivorous animals like sheep and cows. Rabies comes from the bite of an infected animal. Psittacosis comes from birds. Toxoplasmosis may come from cats while toxocariasis comes from both dogs and cats.

Recently, we had the Nipah virus, which originated from fruit bats. Common to Malaysia are of course dengue fever (through the Aedes mosquito).

The bird flu comes from wild birds, which then spread to domestic poultry.

There have also been outbreaks of zoonotic infections associated with animal fairs and petting zoos.

Dr YLM graduated as a medical doctor, and has been writing for many years on various subjects such as medicine, health advice, computers and entertainment. The information contained in this column is for general educational purposes only. Neither The Star nor the author gives any warranty on accuracy, completeness, functionality, usefulness or other assurances as to such information. The Star and the author disclaim all responsibility for any losses, damage to property or personal injury suffered directly or indirectly from reliance on such information.

This article was taken from: The Star Online: Home: Health 10 May 2009