article on disease transmission from animals to humans

[Guest guest]

There's a fascinating article in today's Washington Post " Outlook " section

called " Their Bugs are Worse than their Bite " . It's authored by two

distinguished scientists. For some reason my computer is freezing up when I

try to get it on-line.

It would be worth posting if someone else can access it. Remember that you

can have any article in the WP e-mailed to you and then forward it to the

group.

[Guest guest]

Their Bugs Are Worse Than Their Bite

By E. Fuller Torrey and H. Yolken

Sunday, April 3, 2005; Page B01

Last week, we learned that many children had developed a serious bacterial

infection following visits to Florida petting zoos and that at least seven

of them went into kidney failure. Meanwhile, half a world away in Angola,

more than 100 people have died from the largest outbreak of Marburg

hemorrhagic virus, which is also known as green monkey disease because of

its source. These dramatic stories are playing out against the backdrop of

AIDS (which comes from non-human primates), Lyme disease (from deer), West

Nile virus (spread by birds) and SARS (apparently transmitted by civet

cats). Topping them all are the recent worries about bird flu, which is

currently spreading from chickens to humans and threatens to become the next

pandemic.

Such transmissions aren't new. Experts estimate that at least three-quarters

of all infectious diseases originally came from animals, and last year

L. Gerberding, director of the Centers for Disease Control and Prevention

(CDC), noted that " 11 of the last 12 emerging infectious diseases that we're

aware of in the world, that have had human health consequences, have

probably arisen from animal sources. " What is worrying is both that these

transmissions appear to be on the increase and that many diseases long

believed to be noninfectious (such as multiple sclerosis) may in fact be

attributable to microbes contracted from animals.

In the face of such threats, we should be taking steps to protect ourselves,

ranging from practicing better basic hygiene to realigning our government

agencies. Currently, the CDC does a good job of tracking human-to-human

disease transmission, and the Animal and Plant Health Inspection Service,

under the Department of Agriculture, does a good job of tracking

animal-to-animal disease transmission, but no single agency tracks

animal-to-human transmission. Responsibility for tracking West Nile virus,

for example, is divided among eight federal agencies. Such lack of

coordination is evident also at the international level.

A 1992 report by the Institute of Medicine (IOM) noted that " there is little

coordination among these agencies and organizations regarding infectious

disease surveillance. " In 2002, the IOM again expressed the need for

improved collaboration. What's more, despite the CDC's efforts to educate

the public about these risks, the Bush administration has proposed major

cuts in its budget for the coming year.

What we're seeing is the most recent step in a process that has been going

on for centuries, ever since people began living in close proximity with

animals. Most of the diseases that shaped human history came originally from

animals, including yellow fever, plague, tuberculosis, measles, typhoid

fever, influenza, smallpox and leprosy. Paradoxically, at a time when fewer

and fewer people engage in agricultural work, more and more people are

coming into contact with animal diseases, either through house pets, petting

zoos, takeout food or the congested nature of modern life, which encourages

diseases to spread quickly.

More than 12,000 years ago, though, when humans lived in small groups as

hunters and gatherers, they were afflicted with relatively few infectious

diseases, or heirloom diseases. These infections, which were passed down the

hereditary chain, continue to cause such contemporary illnesses as herpes,

hepatitis A and B, and malaria. Early humans acquired a few other infectious

diseases, such as anthrax from wild sheep and tularemia from rabbits, as

they butchered and ate their kill.

When the relationship between humans and animals changed, so too did human

exposure to microbes carried by animals. Animals that early man had watched

from afar and occasionally hunted now grazed peacefully nearby. Humans began

to share their homes with sheep, goats, chickens and cows -- their most

important possessions -- and still do today in many parts of the world.

The spread of microbes from animals to humans was then inevitable. From

goats, we acquired the bacteria that cause tuberculosis, which has been

identified in human remains more than 7,000 years old and in ancient

Egyptian mummies. In 1680, Bunyan called tuberculosis " the captain of

all these men of death, " and according to one estimate, in the last century

it killed approximately 100 million people.

Measles came to us from cows, a slight modification of the bovine rinderpest

virus. Although now regarded as a relatively benign disease, measles

devastated the Native Americans, who had never been exposed to it, and,

along with smallpox, was a principle reason the invading Spaniards

prevailed. It has been estimated that between 1840 and 1990, measles killed

about 200 million people worldwide.

Both diseases spread rampantly as people moved into villages, then larger

cities. And as humans began traveling between urban areas for trade or

warfare, they carried microbes with them. Rats often came along, too,

carrying fleas infected with the bacteria responsible for plague. The plague

bacteria had existed for thousands of years among marmots and other rodents

in central Asia, causing few problems. But when rats started hitchhiking on

passing caravans, eventually reaching the Mediterranean region in the 6th

century A.D., they boarded ships and initiated the Plague of ian,

which killed an estimated 100 million people. When the plague returned in

the 14th century as the Black Death, it wiped out approximately one-third of

Europe's population.

Other human diseases suspected of being the consequence of the domestication

of animals include whooping cough (pertussis) from pigs, glanders from

horses, typhoid fever from chickens and influenza from ducks. Although the

flu virus has existed in water birds for millions of years, when it infects

pigs or other mammals it is modified. The modified viruses, if spread to

humans, can cause deadly pandemics such as the 1918 influenza outbreak,

which killed more than 20 million people worldwide. The current farming

system in Southeast Asia, in which ducks, pigs, chickens and humans live

beak by jowl, is ideal for fostering such deadly viral strains. This is what

is most worrying about bird flu.

Instead of keeping a safe distance from disease-carrying beasts, Americans

have literally been inviting them to move in. Americans own about 55 million

household dogs, 64 million cats, 31 million caged birds and 7 million

reptiles -- and pet food sales suggest that these numbers are increasing.

The relationship between humans and their pets has become increasingly

intimate, including exchanging kisses, sleeping in the same bed and taking

pets along on family trips. Since 1990, the CDC has reported at least 25

separate outbreaks of E. coli 0157 with associated diarrhea and kidney

failure in children who probably acquired the bacteria learning how to shear

a sheep or milk a cow or simply stroking a goat's back at a petting zoo,

just as seems to have happened in Florida, where one of the affected

children has since died.

Exotic pets have also become increasingly easy to obtain via the Internet.

Anyone with a credit card can order an African pouch rat, bush baby or

kinkajou. A buffalo will set you back $2,000, a reindeer $2,700 and a

kangaroo $6,000. Cases of sometimes-fatal salmonella disease among infants,

transmitted from pet iguanas, are regularly reported. A 2003 outbreak of

monkeypox, carried by pet prairie dogs, is another example.

Technological innovations in food production also contribute to the human

spread of animal microbes. One example has been the transmission of

prions -- the protein-based infectious particles that cause mad cow disease,

or bovine spongiform encephalopathy -- following changes in the commercial

process of slaughtering cattle and preparing bone meal to feed to other

cattle.

Once a microbe spreads from animals to humans, many factors in modern life

contribute to its further dissemination. Air travel, for example, caused the

rapid spread of SARS in 2003 to 30 countries on five continents. The reuse

of disposable plastic syringes without sterilization is common practice in

many developing countries and has contributed to the spread of hepatitis,

AIDS and Ebola and Lassa fevers.

Add to this ecological and population changes. Global warming and

mega-cities of 20 million or more people are both likely to bring about new

patterns of disease transmission. In the United States, a cycle of drought

and heavy rain in southwestern states led to an outbreak of hantavirus,

spread from mice to humans. And the reforestation of northeastern states

combined with suburban living has brought a swelling deer population into

juxtaposition with humans, resulting in an epidemic of Lyme disease.

Finally, there are changes in human behavior that contribute to the spread

of animal microbes. Take our tendency to order in or eat out rather than

cooking for ourselves. Salmonella-infected eggs or chicken products can thus

affect dozens of households simultaneously.

Still, the major source of concern about animal-associated microbes is not

what we know, but what we don't know. There are estimated to be

approximately 5,000 different species of viruses and up to 1 million species

of bacteria, the vast majority of which have not been identified. They

presumably infect all of the known 4,500 species of mammals, of which humans

are only one. Further, we know virtually nothing about the microbes that

infect birds, reptiles, amphibians, fish or simpler forms of life.

Many surprises await us. There has been speculation for many years that

multiple sclerosis may be caused by a virus transmitted from dogs. But what

is the relationship between Chlamydia pneumoniae, a bacterium that may have

been transmitted to humans from horses, and atherosclerotic heart disease?

Our own research is linking schizophrenia to Toxoplasma gondii, a protozoan

carried by cats. And what about some cancers? Liver cancer has been linked

to hepatitis viruses, an heirloom infection. Stomach cancer has been liked

to Helicobacter pylori, which may have been transmitted to humans from

sheep.

The most important thing we can do is to educate ourselves and our families

about animal pathogens. Pets should not sleep in our beds or sit on the

kitchen counter where our food is prepared. Because, yes, Duck really

does carry influenza; Mickey Mouse, hantavirus; Pluto, leishmaniasis;

Garfield, T. gondii; Rocky Raccoon, rabies; Bambi, Lyme disease; and Big

Bird, West Nile virus. Even beloved Barney, like all reptiles for millions

of years, carries salmonella.

The continuing transmission of infectious agents from animals to humans is

inevitable. It is not a question of if there will be additional human

epidemics, merely when. Only by modifying human behavior and using

government resources intelligently can we hope to reduce the incidence and

severity of such epidemics.

Author's e-mail:

torreyf@...

E. Fuller Torrey, an associate director of the Stanley Medical Research

Institute in Bethesda, and H. Yolken, a neurovirologist at s

Hopkins University School of Medicine in Baltimore, are co-authors of

" Beasts of the Earth: Animals, Humans, and Disease " (Rutgers University

Press).