Bat
Rabies in the United States
Sources
of U.S. Human Rabies Infection
James
A. Wilkerson III, MD
In the
late 1940s and through the 1950s, most human rabies in the
United States resulted from bites by dogs or cats. Of 146
infections in the years 1946 to 1961 for which a source
of exposure could be identified, dogs were responsible for
120 and cats for nine. Other animals involved included foxes
(seven), skunks (five), and bats (five).
When
rabies in domestic animals was controlled, human rabies
resulting from bites by pets disappeared. Since 1966, all
but two of the human rabies infections resulting from documented
exposures to rabid dogs have been incurred outside of the
continental United States. Prior to 1965, the Centers for
Disease Control and Prevention (CDC) had recorded no human
rabies occurring within the United States that had been
acquired outside the country.
Until
recently, the number of infections acquired outside the
United States has been steadily increasing: three of fifteen
(20%) between 1965 and 1970, six of twenty-three (26%) in
the 1970s, seven of ten (70%) in the 1980s, but only five
of twenty-eight infections (21%) from 1990 through 2000.
Although lack of knowledge about the risk of rabies in developing
countries has led some travelers to disregard animal encounters
and not obtain rabies immunoprophylaxis, some of these infections
have been in children who did not inform their parents of
the animal contact.
Identifying
the source of human rabies infections in the United States
is a long-standing problem because no animal exposure incident
ó or even an opportunity for animal exposure ó can be identified
for many infected persons. An animal exposure could not
be identified for 35% (eighty-four of 230) of the human
rabies infections occurring in the United States between
1946 and 1961, for 16% (six of thirty-eight) of the human
infections between 1960 and 1970, or for 60% of the infections
(six of ten) between 1980 and 1989. However, in recent years,
including the 1980 , 1989 decade, monoclonal antibody typing
or nucleotide analysis of the infecting organisms has allowed
the source of human rabies infections to be determined when
no animal exposure incident can be identified. Such studies
have clearly demonstrated that bats are now the major source
of human rabies in the United States.
Rabies
in Bats
The
incident that drew widespread attention to bat rabies in
the United States occurred in Tampa, Florida, on June 23,
1953. The seven-year-old son of a ranch hand was looking
for a baseball near some shrubbery when a lactating female
yellow bat suddenly flew out of the bushes and bit the boy
on the chest, remaining firmly attached until knocked off
by the boy's mother. The ranch owner had heard of rabies
in vampire bats in Mexico and insisted the bat be examined
for infection. Negri bodies were found in smears of the
brain, and the diagnosis was confirmed by mouse inoculation
of brain tissue. The boy was treated and survived, and the
publicity given this event led to many more bats being submitted
for rabies examination.
Subsequently,
rabies has been found in bats in every state, except Hawaii,
and in eight Canadian provinces. Large areas of the United
States, particularly the Pacific Northwest report rabies
in bats but in no other species. The estimated incidence
of rabies in all bats is 0.5 to 1.0 %; the incidence in
bats that appear ill or injured is much higher, 7 to 50%.
Transmission
of Bat Rabies to Humans
Twenty-six
of the twenty-nine human rabies infections acquired within
the United States since 1980 have been produced by virus
strains associated with bats, eighteen by the strain associated
with silver-haired and eastern pipistrelle bats. Five were
produce by Brazilian (Mexican) free-tail bat strains, one
by a big brown bat strain, and the associated species for
two could not be determined.
The
information available about these incidents is conflicting,
but two of the individuals had a definite history of being
bitten by a bat. A second may have been bitten. Ten to twelve
of the victims had a history of apparent contact with a
bat, but no bite is known to have been inflicted. In seven
of ten individuals no exposure to bats could be identified.
No instance of more than one bat-associated rabies infection
in a single household has been reported.
The
etiology of all of the infections was established with monoclonal
antibodies or by nucleotide analysis of viral isolates.
The route of transmission of the infection remains an enigma.
Vampire bats have long been known to bite sleeping persons
without waking them. The small, very sharp teeth of other
bats may be capable of inflicting similar bites.
Preventing
Rabies Transmission by Bats
The
problem of rabies transmission by bats is best addressed
through education. Reducing the bat population is not an
acceptable approach, is probably impossible, and would almost
certainly be an ecological disaster because bats play such
a major role in insect control. The following measures have
been recommended:
- Dwellings
should be "bat-proofed"by
carefully covering all possible entrances, particularly
roof ventilation openings, with wire screens. Protection
from bats in unscreened dwellings or when sleeping outdoors
can be achieved with mosquito netting.
- Contact
with bats should be assiduously avoided, particularly
bats that are behaving unusually. Bats are nocturnal and
any activity during daylight hours should be considered
abnormal. Diseased bats often are unable to fly. Bats
should be caught with nets; disabled bats should be scooped
into a container. These animals must never be picked up
or handled with unprotected hands.
- Any
person who has contact with a bat,regardless of
whether a bite is thought to have been inflicted,
should receive postexposure prophylactic therapy
unless the bat can be caught and tested for rabies.
- Any
person who awakens from sleep and finds a bat in their
room should receive postexposure prophylaxis.
To
learn more about bats, contact your local wildlife conservation
agency or the Bat Conservation International at
www.batcon.org, or visit the Center for Disease
Control at www.cdc.gov.
Jim
Wilkerson is a long time member of WMS. He has just finished
chairing the Annual Meeting in Park City and is the past
chair of the Continuing Medical Education Committee. Jim
has a pathology practice in Merced, California.
Wilderness
Medicine Letter, Volume 18, Number 2, Spring 2001
