"Leptospirosis may be the most common disease that rats
carry and transmit to humans in the United States," comments Dr.
Vinetz, "but it's simply underrecognized."
Although leptospirosis is not new to the United States, it is
difficult to diagnose, hence its prevalence is unknown. The current
study is based on findings obtained from using a sensitive
experimental diagnostic test.
Dr. Vinetz urges clinicians to consider the diagnosis of
leptospirosis in urban patients who complain of flu-like symptoms
(especially during the warm months outside the flu season) and could
have been exposed to the urine of rats or to pools of infected water,
which can be present in alleys and parks in the inner city and possibly
in the suburbs.
Human leptospirosis infection usually causes a mild syndrome
of aches, pains and fever that resolves spontaneously or after
treatment with antibiotics. But up to 10 percent of infected persons
develop a serious systemic form of the illness, Weil's disease, which
can result in high fever, jaundice, aseptic meningitis, acute kidney
failure, internal bleeding and, occasionally, death.
In their report, Dr. Vinetz and his co-authors from NIAID's
Laboratory of Parasitic Diseases and Johns Hopkins Schools of
Medicine and Hygiene and Public Health describe three sporadic
cases of severe leptospirosis infection diagnosed within a year in an
urban hospital in Baltimore. The authors could remember no cases
diagnosed there in the previous decade, and no cases had been
diagnosed by the Maryland State Health Department for at least five
The investigators confirmed the link to infected rodents when
they identified Leptospira interrogans in 19 of 21 rats trapped in the
alleys where the patients had sustained cuts before their illnesses
developed. The disease agent--a thin, spiral-shaped bacterium--
enters the body through cuts in the skin and through the linings of the
eyes, nose and throat.
Although a variety of mammals worldwide harbor the infection,
in the United States the most common carriers are rats, dogs and
livestock. Humans do not transmit the infection to others. It is
common practice for livestock and dogs to be immunized against the
disease, but even vaccinated animals can shed the bacteria in their
urine for long periods of time and be an occasional source of human
Surveys of rat populations have not been conducted since the
1940s and 50s. City officials today often estimate two rats for every
resident, but this is not known to be based on any hard data. An
indirect gauge of the problem is the tally of complaints about rats
received from residents. Most recent annual figures show New York
City with about 40,000 complaints, Philadelphia with 10,000,
Baltimore with 5,000 and Washington, D.C., with 4,000.
People traditionally recognized at highest risk for leptospirosis
have been those engaged in certain occupations (farmers,
veterinarians, sanitation and sewer workers, and meat processors) or
recreational activities (campers, freshwater swimmers) as well as
individuals exposed to flooding as a result of hurricanes or other
natural disasters. Travel to endemic areas (most often in tropical
climates) also is a risk factor. The Annals report is the first study
to establish the risk to residents of U.S. inner cities.
However, at least two previous surveys provided indirect
evidence that unrecognized leptospirosis might be common in city
dwellers. A 1992 survey from a Baltimore sexually transmitted
diseases clinic found that 16 percent of blood samples taken from
1,150 patients for reasons unrelated to leptospirosis were positive for
exposure to leptospiral antigen. An earlier study found that about
one-third of children tested in Detroit had antibodies to Leptospira.
More inner-city children had antibodies than did children from the
suburbs, yet none of the inner-city children had been diagnosed with
"These serosurveys indicated that the risk to city dwellers was
there," says Dr. Vinetz, "but nobody could make use of this data until
someone could identify patients with acute disease."
Dr. Vinetz and his colleagues accomplished this by using an
experimental polymerase chain reaction (PCR) test, which can detect
within 24 hours tiny amounts of the bacterium's genetic material.
Currently available diagnostic tests for leptospirosis detect
antibodies to the bacterium. Local laboratories, however, ordinarily
do not perform these tests. Blood samples must be sent to the state
health department first, and if positive, to the Centers for Disease
Control and Prevention laboratory for confirmation. The results
typically come back weeks to months later, according to Dr. Vinetz,
much too late to impact on clinical care.
"What the PCR test allowed us to do," comments Dr. Vinetz,
"is connect the seroepidemiology data to the clinical data and make it
All three patients in their study lived in inner-city Baltimore.
None had recently traveled outside the city, had pets or had been
exposed to farm animals. Although Dr. Vinetz recognized that the
first patient had the classic symptoms of leptospirosis, to confirm the
diagnosis, he had to establish that she had been exposed to the
After a week in the intensive care unit, the patient finally
recalled that 10 days before the onset of her illness she had been
walking barefoot in an alley behind her house and had stepped on
some broken glass. "That's the perfect incubation period," comments
Dr. Vinetz. "I checked, and she had a healing gash on the bottom of
her right foot. As soon as I got that information, for all intents and
purposes, the diagnosis was established. Nobody has ever shown
that you can get leptospirosis just by walking around in a city," he
notes. "That was novel." Likewise, the second patient told the
investigators he cut his hand on glass in a rat-infested alley about 10
days before he developed his illness. The third patient acknowledged
walking barefoot in rat-infested alleys near the Baltimore harbor.
More research is needed to validate the PCR assay as the
best way to diagnose leptospirosis. If the assay is adopted, the
authors say, its widespread use would allow physicians to initiate
specific therapy earlier and to avoid unnecessary invasive and
expensive tests to rule out other diseases.
To accelerate the validation of the PCR test for leptospirosis,
Dr. Vinetz encourages physicians to send sera, urine and
cerebrospinal fluid from patients with suspected leptospirosis, along
with clinical and epidemiologic information, to the Clinical
Microbiology Laboratory at Johns Hopkins. For more information, call
Dr. Vinetz at 301-496-3655.
The co-authors on the report were Gregory E. Glass, Ph.D.,
Charles E. Flexner, M.D., and Paul Mueller, M.D., of Johns Hopkins;
and David C. Kaslow, M.D., of NIAID.
NIAID, a component of the National Institutes of Health,
conducts and supports research to prevent, diagnose and treat
illnesses such as HIV disease and other sexually transmitted
diseases, tuberculosis, asthma and allergies. NIH is an agency of the
Public Health Service, U.S. Department of Health and Human
Farr RW. Leptospirosis. Clin Infec Dis 1995;21:1-8.
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