CHUCK OCHLECH鈥檚 annual fishing tournament was unique. People flocked from
around the globe to The Yellow Rose, his bar in East Baltimore, to see locals
take their tackle and bait into the surrounding alleys to fish for rats. The
winner might expect to net an animal over half-a-metre long with nothing more
than a conventional fishing rod and a piece of rancid bacon. But this year there
will be no tournament. Animal rights campaigners have put an end to the event,
claiming the sport is cruel.
If rats make your flesh crawl, you might like to see rat fishing take off.
But at least you draw comfort from the thought that the rodents are someone
else鈥檚 problem. They belong to another place, another time. Rats are at home on
farms, in shanty towns, in the squalor of modern housing projects and
plague-ridden mediaeval cities. They don鈥檛 inhabit your nice, sanitised
neighbourhood. Or do they?
Rats and humans have always lived shoulder to shoulder, but now, it seems,
the rodents are really muscling in on us. As well as the back alleys of
Baltimore, they are staking a claim in the parks of Paris and London, and the
commercial heartland of the world鈥檚 major cities. Nowhere is sacred. This summer
they came out in their hundreds to dine on discarded pizza in Washington鈥檚
Lafayette Square, across from the White House. The authorities claimed it was an
isolated incident, blaming the mild, wet weather for the surge in rodent
numbers. But, though long-term records are in short supply, a suspicion is
growing that rodent populations are on the increase in many parts of the world.
Even if this is not the case, rodents are becoming more visible and new medical
evidence suggests our relationship with them and the diseases they carry is much
more intimate than anyone had suspected.
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According to a recent report from the WHO, plague is on the increase. The WHO
recorded 2935 cases in 1994, from countries throughout Africa, the Americas and
Southeast Asia. In the US, 13 states reported plague between 1984 and 1994,
compared with just three states 40 years earlier. Evgueni Tikhomirov, a plague
specialist at the WHO, believes that the real numbers worldwide are at least
four times the official ones. Many cases go unreported, he says. And plague is
difficult to diagnose once a patient has begun a course of antibiotics.
Many of the diseases that humans contract from rodents are even more elusive,
although advances in biomedical techniques are starting to show how
underreported some of them are. Often diseases we catch from rodents begin with
flu-like symptoms. People frequently recover without ever realising that they
have had a close encounter with rats or mice. But if symptoms don鈥檛 clear up the
resulting illnesses are often severe or even fatal.
鈥淟eptospirosis is probably the most common unrecognised rat-borne disease in
the US,鈥� says Joe Vinetz, a doctor from Johns Hopkins School of Medicine in
Baltimore. He has just started using a new test in routine screening for the
disease. The most recent figures suggest that around 50 people contract
leptospirosis each year in the US. Vinetz found four cases in a blanket
screening of patients at a single Baltimore hospital. Most cases are missed
because the symptoms of leptospirosis are often mild, but they can develop,
resulting in kidney and liver failure. The illness, then known as Weil鈥檚
disease, is fatal in 10 per cent of cases.
Infected rats carry leptospira in their kidneys and excrete them in urine.
The bacteria reach humans via moist soils and water. Vinetz says that in
inner-city Baltimore people chiefly catch leptospirosis because of poor and
unsanitary housing. 鈥淵ou can see trash heaped up and kids running up and down
alleys,鈥� he says. Children splashing through puddles can quite easily become
infected if they get the bacterium in their eyes. One study carried out in the
early 1980s in Detroit, found that almost a third of children under the age of
six had high levels of antibodies to leptospira, from exposure to the
bacterium.
More recent research shows antibodies in 16 per cent of adults. In 1992 Jim
LeDuc, an expert in haemorrhagic diseases at the Centers for Disease Control
(CDC) in Atlanta, and Greg Gurri Glass, associate professor of molecular
microbiology and immunology at Johns Hopkins, published results from tests on
blood samples taken from 1150 people visiting a clinic for sexually transmitted
diseases in Baltimore. They found 185 people with antibodies to leptospira, yet
in the five years prior to the study, only three cases of leptospirosis had been
registered for the entire city.
Hidden threat
Gurri Glass believes that the underreporting of leptospirosis is just the tip
of the iceberg. He cautions that in the US fears about diseases waiting to
鈥渟neak across the border鈥� have blinded people to the threat on their doorstep.
Rodent-borne infections are here and killing people, he says. 鈥淔airly minor
changes in how humans and rodents interact may lead to an explosion in
诲颈蝉别补蝉别.鈥�
Many rodent-watchers fear that such changes are already under way.
Homelessness, urban squalor, overcrowded cities and civil unrest all draw
rodents into human settlements and sustain them there. But rodents are superbly
adaptable and thrive equally well on the discarded remains of affluent consumer
societies鈥攔ats, for example, eat anything from fast food to faeces. 鈥淩ats
have piggy-backed on our success and followed us around the world,鈥� says James
Childs, head of epidemiology in the branch of viral and rickettsial zoonosis at
the CDC. In addition, pressure of numbers is forcing people to to move into new
territories that may already be occupied by rodents. Changing circumstances seem
to be tipping the balance in favour of rodents. Plague will kill rats, but most
of the parasites, viruses and bacteria they carry make humans sick but leave
rodents healthy. And some are only now starting to emerge.
One of these diseases, Lymphocytic Choriomeningitis Virus (LCMV), which
infects house mice, was until recently thought only to be a risk for
immunologists who use it as a model for study. Last year, researchers at the
Pasteur Institute in Paris reported that pet hamsters are also carriers. And in
the past few years three separate teams of researchers in the US have discovered
that five per cent of the general population are infected with LCMV. In many
cases symptoms are mild, but some researchers believe that infected pregnant
women could have babies with serious malformations including hydrocephalus, or
water on the brain. Pierre Rollin, from the special pathogens group at the CDC,
says that LCMV may be responsible for some of America鈥檚 TORCH kids, so-called
because their birth defects are thought to be the result of infection in the
womb with toxoplasmosis, rubella, cytomegalovirus or herpes. 鈥淲e should pay more
attention to this and try to get some data,鈥� says Rollin.
Rodents carrying LCMV excrete it in their saliva and urine, but people can
become infected by inhaling airborne particles containing the virus. And it is
just one of a whole range of diseases passed from rodents to people via
aerosols. Research published this year suggests that if rodent populations are
increasing, so will the number of diseases humans are exposed to. Joanne Webster
and David Macdonald, zoologists from the University of Oxford, showed that the
more heavily infested an area, the more parasites an individual rat is likely to
carry. They found that rats trapped on farms in Oxfordshire, Hampshire and Wales
carried 23 different parasites, 13 of which cause disease in humans. Individuals
from the most overcrowded populations harboured almost all these disease-causing
agents. In smaller populations, the rats carried fewer.
But are rodent numbers really on the increase worldwide? Good data are hard
to come by. The National Rodent Survey, carried out by the Central Science
Laboratory and the Chartered Institute of Environmental Health in 1993, is the
only nationwide study ever carried out in England and Wales. Researchers
inspected a random sample of 10 929 properties from the community charge
register. The survey suggested that rat infestation in large towns was up 43 per
cent since the 1970s, with a 48 per cent rise in rural areas.
Controlling interest
In the US data on rodent numbers are even more sparse. Anecdotal evidence,
however, suggest that numbers are on the increase. 鈥淣obody has systematically
followed rodent populations over the long term,鈥� admits Childs. He points out
that federal control programmes are being scaled down in favour of local
initiatives, many of which are inadequately funded. 鈥淚t may be contributing to
the overall problem,鈥� says Childs.
Rodent control is fraught with difficulty at the best of times. It relies
heavily on food baited with chemicals that prevent blood clotting, and rats, in
particular, are quick to develop resistance to these anticoagulants. Rats can
also make a link between illness and any new food they have tried recently and
will avoid it in future. 鈥淩ats are amazing and seem to get round whatever you
do,鈥� says Webster. 鈥淎nd they reproduce so quickly, there are always some left.鈥�
Female brown rats (Rattus norvegicus) first mate at two or three
months. When conditions are good, they give birth to 10 to 12 pups at a time and
they might have six to eight litters in a year. That鈥檚 between 70 and 100 pups,
though half may die in the first few months, according to Childs. He has tagged
animals and followed them for up to 18 months, showing that they can survive
much longer than the 10-month average. If numbers are increasing then so is the
threat to human health from rodent-borne diseases. The signs are already
there.
In Europe and Asia there is a long history of human disease caused by
hantaviruses carried by mice. This family of viruses tend to cause kidney
failure and sometimes haemorrhagic fever, killing around 10 per cent of the
people it infects. But hantavirus was not known in the US until 1993. Then
researchers identified a new strain, sin nombre virus, which is fatal in more
than half of cases. Since then, 144 cases of hantavirus have been diagnosed.
That first recognised outbreak has been linked with increased rodent
populations. During the spring and summer of 1993, southwestern states were
deluged with unusually heavy rains that produced lush vegetation, increasing
rodent food reserves. Data from the Sevilleta field station in Socorro County,
New Mexico, showed that in some places deer mouse populations were 10 times
higher in May 1993 than they had been in the previous year. This coincided with
the peak of the hantavirus outbreak and the disease waned over the summer as
rodent numbers declined.
Hantavirus in the US has left a legacy of fear out of all proportion to its
effects. The idea that a virulent new disease might threaten the entire human
population is given voice in movies such as Outbreak, starring Dustin
Hoffman and an ebola-type supervirus. In reality, the threat from diseases
carried by rodents is much more insidious. It is more a case of death by a
thousand cuts. In the past decade, for example, teams of researchers in
Argentina, Bolivia and Venezuela looking for the cause of haemorrhagic fevers
have tracked down three different arenaviruses鈥攖he same family as
LCMV鈥攅ach with a specific rodent host and all transmitted to humans via
aerosols. Corn mice carry Junin virus, causing Argentinian haemorrhagic fever.
In Venezuela cotton rats carry Guanarito virus and in Bolivia vesper mice carry
the Machupo virus.
Back in Hollywood, Hoffman eventually tracks down the virus鈥檚 monkey-host,
cultures the antibody and saves the world. In real life, the vast range of hosts
and pathogens associated with rodent-borne diseases make prevention a real
headache. In Argentina, trials with a new vaccine over the past decade have
reduced the cases of haemorrhagic fever by 95 per cent. But no pharmaceuticals
company is prepared to manufacture it because only a million or so doses are
required, making it a commercial nonstarter. Even where vaccines are a viable
tool against rodent-borne infections, the sheer number of different variants of
some diseases limits their success. China, for example, has a massive programme
to immunise its rural population against leptospirosis. But the vaccine works
against only the most prevalent and severe of the 20 or so strains of the
bacterium found in China.
The alternative to vaccines, making houses rodent-proof, is extremely
difficult. Mice can squeeze in through the smallest cranny and like to set up
home in a pantry or food store. Black rats (Rattus rattus)鈥攆ound
mostly in the tropics鈥攁re expert climbers and nest in the walls of
buildings. They are a pest in cities like Los Angeles, particularly when they
gnaw through electrical insulation, fusing whole tower blocks. Old sewers make
ideal homes for brown rats. They are outdoor creatures and tend to infest the
area around a building rather than the building itself. But they often make
their presence felt by urinating on food supplies. Brown rats have amazing teeth
that allow them to burrow through soil and even concrete. They can turn up
almost anywhere.
Traditionally, humans have picked up diseases by living close to rodents, and
poor people were most at risk. Now, however, our affluent lifestyles are opening
up new routes to exposure. Leisure activities are increasing our proximity to
rodents. People with the time and money to spend on water sports, for example,
are more likely to contract Weil鈥檚 disease. Incidents of Lyme
disease鈥攃arried by ticks that live on rodent hosts among others鈥攎ay
also be rising as new woodlands spring up, either planted specifically for
recreational use or left to regenerate where farmers set aside land. And many
researchers believe that global warming鈥攖hat quintessential product of
affluent, industrialised society鈥攚ill fuel the spread of rodent-associated
disease.
Lyme disease was first described in the late 1970s in Lyme, Connecticut. Last
year there were 11 700 cases in the US alone. Researchers have identified eight
strains of Borrelia burgdorferi, the bacterium that causes it. B
burgdorferi lives in ticks. In the US their most important host is the
white-footed mouse and in Britain grey squirrels and mice carry the ticks.
Childs says that changes in land use in the US are already contributing to the
large numbers of people getting Lyme disease. Land right down to the
Mississippi, for example, was once used exclusively for farming. Now it is a
patchy mix of fields and woodland鈥� perfect tick country.
Sarah Randolph, a researcher in the department of zoology at the University
of Oxford, is also concerned that global warming may influence ticks. 鈥淭heir
survival depends on the climate being clement,鈥� she says. 鈥淚t鈥檚 a tremendously
important effect.鈥� Randolph and David Rogers, a fellow zoologist at Oxford, have
combined satellite data from Africa with biological data on tick survival to
track how climate affects their distribution. The models produced so far suggest
that in temperate areas tick survival is related to temperature and relative
humidity, whereas at the equator, rainfall is the most important factor. In
short, ticks thrive in warm moist conditions and, says Randolph, climate change
is very likely to alter their distribution. Whether that means more or less Lyme
disease is not clear, however.
Climate change may also affect diseases transmitted directly from rodents to
people. Webster says that rising temperatures are likely to increase rat
numbers. 鈥淥ne of the few things that gets the numbers down is a cold winter.
They breed less then,鈥� she says. Other researchers believe that variants of
diseases now limited to tropical areas could survive in areas that are now
temperate. Vinetz adds that if climate change means warmer temperatures and more
rain, bacteria excreted in rat urine are more likely to survive in puddles, and
be passed on.
To Gurri Glass, all this sounds like procrastination. Rodents are already
here, and in large numbers. Rat numbers probably equal human numbers worldwide,
he says. 鈥淧eople confuse absence of evidence with evidence of absence.鈥� Your
neighbourhood may seem rodent-free, but the chances are that you have recently
had a brush with the uninvited guest at the table of human civilisation.
* * *
Natural born killers
According to the classic text, Rats, Lice and History, written by
Hans Zinsser in 1934, black rats (Rattus rattus) arrived in Europe from
the East some time between AD 400 and 1100. They spread across the continent
鈥渨ith a speed superior even to that of the white man in the Americas鈥�, he wrote.
In the early 18th century, however, Europe鈥檚 black rats were all but wiped out
by a second influx of brown rats (Rattus norvegicus) from Asia. The
same pattern of annihilation seems now to be happening in the US.
Zinsser draws some unflattering comparisons between humans and rats. Both are
ferocious, omnivorous and adaptable to all climates. Both breed year-round.
Unlike ants, bees, some birds and fish, neither humans nor rats have achieved
social, commercial or economic stability. They are, however, the most successful
animals of prey. 鈥淭hey are utterly destructive of other forms of life,鈥� writes
Zinsser.
鈥淕radually these two have spread across the Earth, keeping pace with each
other and unable to destroy each other though continually hostile. They have
wandered from East to West, driven by their physical needs, and鈥攗nlike any
other species of living things鈥攈ave made war upon their own kind. The
gradual relentless, progressive extermination of the black rat by the brown has
no parallel in nature so close as that of the similar extermination of one race
of man by another.鈥�