Diseases caught from animals on the increase around the world

Daily Yomiuri Online/Science, May 2, 2000

Saori Kan Yomiuri Shimbun Staff Writer

One after another, new varieties of infectious diseases have been emerging throughout the world. Some bring on severe symptoms and result in sudden death, threatening to wreck havoc in countries in which they become widespread.

Most of the diseases are zoonoses, which means they naturally spread from animals to humans. One major factor behind the emergence of these diseases is the development of remote regions, which has brought humans into contact with previously unencountered animals. What is more, illegal trade in wildlife has brought some infected animals to urban areas. In Japan, fears are mounting as lethal viruses are increasingly detected in imported species.

By Saori Kan Yomiuri Shimbun Staff Writer It started with a fever. One after another, residents in the rural community of Sungai Nipah outside Kuala Lumpur fell ill last March. Out of about 250 stricken, more than 100 died.

The diagnosis was encephalitis. At first, locals suspected Japanese encephalitis because the disease has cropped up in the area several times before. But as research by the government and independent scientists continued, the "culprit" was identified as an unknown virus.

Most of those who fell ill were hog raisers, and prior to the outbreak there had been reports of pigs dying mysteriously. Suspecting zoonoses, scientists began analyzing the regurgitated contents of pigs' stomachs. In the end, they discovered a new and ferocious virus, naming it Nipah virus which is capable of infecting the human brain and destroying nerves.

After the Malaysian military slaughtered about 1 million pigs, there were no new reported cases. Indeed, by April last year, it was thought that the virus had been effectively wiped out. But much damage was already done. Aside from the human toll, the region's all-important hog exporting industry was dealt a crushing blow.

The World Health Organization defines zoonoses as infectious diseases communicable under natural conditions between humans and vertebrate animals. There are more than 200 identified to date. In fact, most newly emerging infectious diseases discovered over the past 20 years are zoonoses.

The pathogenic organisms constituting zoonoses vary in size. They range from bacteria and viruses visible only through an electron microscope to parasites that grow to several meters.

Recently, previously undetected zoonoses, such as a new strain of Creutzfeldt-Jakob Disease, have been attracting media attention. Creutzfeldt-Jakob causes atrophy of the brain through infectious protein particles called prions, which are transmitted through beef contaminated by so-called mad cow disease.

However, most of the newly emerging zoonoses that result in high death rates--including Nipah disease, Ebola hemorrhagic fever and Lassa fever--are caused by viruses. Plenty of questions remain concerning channels of infection for a number of zoonoses, including Marburg disease, which is communicable from apes, and hantavirus syndrome, picked up from mice.

A virus is a simple structure, comprising a gene and protein shell enveloping the gene. Genetic information is contained in the form of DNA or RNA, but it lacks the ribosomes necessary to synthesize proteins. It also lacks mitochondria, or the cellular organelles found outside the nucleus that produce energy for the cell through cellular respiration. As a result, the virus cannot multiply and has no choice but to parasitize other creatures' cells.

Most viruses live innocuously in the bodies of their so-called natural hosts--in a sense realizing that to cause damage to the host is a sure ticket to destruction. However, when a creature` comes into contact with a previously unencountered animal, the new animal effectively becomes an unnatural host. Viruses transmitted in such cases can be far from benign.

The natural host of the HIV virus that causes AIDS is a type of chimpanzee living in a specific part of Africa. It is commonly accepted that the virus spread from the chimpanzees to humans, becoming the deadly disease we know today.

In the case of influenza, natural hosts are domestic animals and birds. Experts in relevant countries keep a watchful eye on domestic animals and exchange information that is useful for predicting the prevalence of influenza every year.

At the same time, there are exceptions to the rule that all "unnatural" infections are best avoided. In 1980, the WHO was able to declare the eradication of the smallpox virus, thanks to the development of a vaccine that made use of certain characteristics of the cowpox virus, which resembles the smallpox virus. Living naturally in cows, the cowpox virus does not spread to humans.

Understanding the risks The route of transmission of Nipah virus is hard to trace. Researchers discovered that dogs and goats living near pig pens were also found to have contracted the virus. A hog enclosure near where the first outbreaks were reported had been built close to a newly infringed-upon natural forest. As the virus was subsequently detected in small bats living in caves near the  pig pen, experts think the bats are the likely sources of the virus. They probably passed it on to the pigs.  Tsuneo Kamiyama, chief of the zoonoses research room at the Health and Welfare Ministry's National Institute of Infectious Diseases in Shinjuku Ward, Tokyo, said, "Viruses do not suddenly appear. Infectious diseases emerge as a result of the fact that human and domestic animals enter places where pathogenic organisms exist and encounter them (for the first time)."

According to Kamiyama, most of the areas where newly emerging infectious diseases are prevalent center on farms built after natural forests are felled.

But newly emerging infectious diseases are also cropping up in urban areas. Last year, 13 people died in New York City after contracting the West Nile virus, which uses birds as its natural hosts. It has been said that birds illegally imported from Africa for pets are the most likely culprits for the mass infections that shocked the city.

In 1997, a researcher died in Florida after contracting B virus, one of the herpetoviruses communicable from apes, from a crab-eating macaque raised for experimental purposes.

There are only 40 known examples of humans being infected with B virus--but 70 percent of those cases have resulted in death. In Japan, about 40 percent of monkeys raised at national universities have tested positive for the B virus antibody.

"All the ape research facilities are aware of the dangers. More worrying is the fact that ordinary people are raising apes as pets without knowing the risks," said Akio Yamada, a member of a Health and Welfare Ministry research group studying the B virus.

In fact, the new Infectious Disease Law, which took effect in April last year, made it mandatory for the first time to inspect imported apes for symptoms of zoonoses such as Ebola hemorrhagic fever and Marburg fever. Currently, 53 crab-eating macaques are housed at an inspection facility designated by the Ibaraki prefectural government. However, the B virus is not covered by the new law.

Other potential sources of infectious diseases are raccoons and prairie dogs, which can spread rabies and other diseases and are routinely sold at pet stores. A number of pet magazines even publish photographs of readers kissing their pets. Experts are increasing warning of the dangers of contracting infections.

"Now and in the future, humans will encounter unknown pathogenic organisms, including various forms of viruses. Zoonoses are certain to increase," Kamiyama warned.

Copyright 2000 The Yomiuri Shimbun