Occasional Papers

Museum of Texas Tech University

NUMBER 165 15 MAY 1997

SURVEY OF RABIES AMONG FREE-FLYING BATS
FROM THE BIG BEND REGION OF TEXAS

Franklin D. Yancey, H, Pushker Raj,

Susan U. Neill, and Clyde Jones

In 1951, a 43 year old Texas woman contracted
rabies from a bat bite. This was the first reported hu¬
man case of rabies involving a bat (Sulkin and Greve,
1954; Brass 1994; Clark, 1994). Since then, bats have
been implicated as a major reservoir for the rabies vi-
rus, especially in the Americas (Constantine, 1988).
Understanding the prevalence and distribution of en¬
zootic rabies in bats is fundamental in understanding
the importance of bats in the epidemiology of rabies
(Brass, 1994)

Because of their association with rabies, thou¬
sands of bats arc examined annually by public health
agencies for tlie presence of the virus. Brass (1994)
reported that, on average, about 4% to 10% of bats
submitted to diagnostic laboratories test positive for tlie
rabies virus. Constantine (1988) estimated that in the
United States, about 10 % consistently test positive
annually. In Texas, 11% of 1,758 bats tested by the
department of health between 1984 and 1987 proved to
be infected (Schmidly, 1991), whereas 14 % of 2,931
bats tested betw^een 1990 and 1994 were rabid (Anony¬
mous, 1990; 1991; 1992; 1993; 1994).

Although these figures suggest that a consider¬
able proportion of the bat population is afflicted with

rabies, they arc misleading. Constantine (1988),
Schmidly (1991), and Brass (1994) all recognized a bias
in the sampling techniques of bats submitted to public-
health facilities. These often were sick or dead bats
found near human dwellings (Schmidly, 1991; Brass,
1994). Such bats are considered more likely to be in¬
fected with rabies than those selected from a random
sample of the population (Brass, 1994). Also, if sur¬
veyed bats are collected from topical roost sites, rabid
bats are taken more easily than healthy bats (Constantine,
1988). Therefore, a more reliable and useful survey of
the incidence of rabies among bats requires the sam¬
pling of bats capable of flight (Constantine, 1988),

The Big Bend area is of particular interest because
of its exceptional abundance and diversity of bats
(Easterla, 1973; Schmidly, 1991). Several accounts on
the natiual history of bats in the Big Bend area have
been presented (Baker, 1956; Anderson, 1972; Easterla,
1973; Schmidly, 1977), however, data on the Incidence
of rabies are lacking. Tlie objective of this study was to
examine the incidence of rabies among fi-ee-flying bats
from the Big Bend region of Texas, and to compare the
results with figures reported statewide by the Texas
Department of Health.

2

OCCASIONAL PAPERS, MUSEUM OF TEXAS TECH UNIVERSITY

MATERIALS AND METHODS

All bats assayed in this study were acquired from
within Big Bend Ranch State Park (BBRSP), situated
ill southwestern Texas just north of the Rio Grande in
Brewster and Presidio counties. Tlie park consists of
over 275,000 acres of Chihuahuaii Desert habitat, with
more than 100 springs and several permanent streams.
The numerous w'ater-associated habitats of diis area pro¬
vide an excellent situation for the sampling of bats in
their natural environment.

From May to August, 1994, bats were sampled
using mist nets (Kunz and Kurta, 1988). At dusk, nets
were deployed across springs, streams, or other small
bodies of water. A total of 85 nets was set at 45 locali¬
ties throughout the park. Bats captured were removed
from the nets, identified, and sacrificed. They tlien w'ere
prepared as standard museum skins and skulls (DeBlase
and Martin, 1981). Tliese voucher materials are depos¬
ited ill the collection of Recent manunals in the Natural
Science Research Laboratory’ of the Museum of Texas
Tech University.

During preparation, brain tissue for testing w'as
removed from each cranium using a technique modified
Ifom Greenhall (1965). A 21 gauge hypodermic needle
affixed to a 5 cc syringe with its plunger withdrawn was
inserted into the foramen magnum of each bat skull. The
plunger was then depressed, and the brain, or portion
thereof, subsequently w'as expelled from the cranium
tlirough the foramen magnum via positive pressure. In
Grccnhall’s (1965) technique, tlie brain was aspirated
from tlie cranium. This new procedure pro\ided adequate
quantities of brain tissue for the rabies assay, and yet
avoided damage to the skull, allowing it to be used for
other studies.

Following expulsion, brain tissues temporarily
were stored in liquid nitrogen until they could be depos¬
ited in an ultracold freezer for long-term storage.
Samples of brain from the bats were tested for rabies by
inmiunofluoresccnce as described by Dean and Abclscth
(1973). AJl samples were scored as positive or nega¬
tive for rabies. A 95 percent confidence inter\'al for the
resulting binomial parameter w'as calculated following
Dowdy and Wcarden (1991).

RESULTS AND DISCUSSION

Brain specimens from 171 bats were tested for the
presence of the rabies virus. Species of bats tested (wath
number of each species tested in parentheses), all of
wfrich are known hosts of rabies in tlie United States
(Constantine, 1988; Brass, 1994), were as follows:
Mormoops megalophylla (42), Myotis californicus (8)
M. ciliolabnm {\X M. thysanodes velifer {5),

M. yumanensis (1), Eptesicus fuscus (12), Pipistrellus
hespenis (60), P, subflavus (1), Plecotus townsendii
(4), Antrozouspallidus (27), and Tadarida brasiliensis
(9). All 171 specimens tested negative for rabies.

Sample sizes for each individual species were not
large enough for independent analysis, therefore the gen¬
eral bat population was treated as a whole. The propor¬
tion of positive bats in the entire sample of 171 bats was
zero. Tlius, assuming random sampling, a 95 percent
confidence interv al on the proportion of rabid bats at
BBRSP is [0, 0.0174]. Alternatively stated, assuming
random sampling and given 171 non-rabid bats out of a
sample size of 171, the percentage of rabid bats in tlie
population is < 1.74% (p< 0.05). Therefore, tliere is
only a 5% chance that tlie true prevalancc of rabies in
the population was >1.74% (Fig. 1).

This frequency of rabies in bats at BBRSP (0% to
1.74%) is consistent with that reported in similar stud¬
ies. Constantine (1988) estimated that <0.1% to 0.5%
of bats capable of flight are infected with rabies, and
Baer and Smith (1991), in a siunmary of four similar
studies, reported frequencies of clinically-normal rabid
bats at 0% to <1%.

Undoubtedly, a large amount of the discrepancy
between rabies prevalence in random or free-flying bats
and that obtained from laboratory-submitted bats is due
to the aforementioned bias in samples submitted to health
departments. However, samples obtained from free-fly¬
ing bats conceivably could be biased in the opposite di¬
rection. Bats ill with rabies at an advanced stage may
lose their capacity to fly or behave abnormally, thus
excluding themselves from possible capture
(Constantine, 1988; Baer and Smith, 1991; Brass, 1994).
Presumably, the actual prevalence of rabies in bats lies
somewhere between figures derived from “random” sam-

YANCEY ET AL,-- SURVEY OF RABIES AMONG BATS

3

Figure 1. Probability of finding all bats from a sample size of 171 to be nega¬
tive for rabies, plotted against the true proportion of rabid bats in the population. There
is a <5% chance of observing no positives if the true proportion of rabid bats is >1.74%.

pling of free-flying bats and those based on laboratoiy^-
submitted specimens, and it is essential that data from
both sources be considered in order to understand the
incidence of rabies in bats. Still, according to Brass
(1994:142), “Sampling biases notwithstanding, ‘ran¬
dom’ collection of healthy-appiearing individuals must
serve as our first and best approximation of disease
prevalence in natural populations.”

The incidence of rabies at BBRSP during the sum¬
mer of 1994 was well below the statewide levels re¬
ported by the Texas Department of Health over several
years. However, the results of this study apply only to a
small portion of Texas during a relatively short period
of time. Cycles in sylvatic rabies are known to vary
geographically and temporally, usually predominating
in a single reservoir species (Brass, 1994). Hill and
Smith (1984) suggested that the absence or low level of
rabies in a bat population may indicate tliat bats are at
stable numbers and not overstressed due to overcrowd¬

ing and excessive use of food resources. If this is the
situation at BBRSP, it is conceivable tliat if environ¬
mental conditions change, outbreaks of rabies within bat
colonics could occur, tlius changing the frequency of
rabies in bats du-ougliout the area. Because of the epi¬
zootic nature of rabies, and tlie abimdance of bats and
increase of visitor usage at BBRSP, as well as other
public lands in the area, we suggest continued monitor¬
ing of the bat population in the region.

ACKNOWLEDGMENTS

We tliank Mary Ann Abbey, David Alloway, and
Richard Manning for assistance in the collection of the
bats; Bonny Mayes, Rodney Rohde, and Susan Schmink
for immunofluorescence w'ork on the brain specimens;
and Richard Strauss for statistical assistance. Bats were
collected in Big Bend Ranch State Park in accordance
with scientific collecting permits issued by the Texas
Parks and Wildlife Department (permit numbers SPR-

4

OCCASIONAL PAPERS, MUSEUM OF TEXAS TECH UNIVERSITY

0790-189 and 4-94). Financial assistance was provided
by the Natural Resources Program (Da-^dd H. Riskind,
Director) of the Texas Parks and Wildlife Department.
Logistic support was provided by personnel of Big Bend
Ranch State Park (Luis Armendariz, Super¬
intendent). We are thankful to Michael Bogan for re¬
viewing this manuscript,

LITERATURE CITED

Anderson, S. 1972. Mammals of Chihuahua taxonomy
and distribution. Bulletin of the American
Museum of Natural History, 148:149-410.

Anonymous. 1990. Animal heads diagnosed as posi¬
tive (+) or negative (-) for rabies. Report of
Texas Department of Health Laboratories,
Austin, 18 pp.

_. 1991. Animal heads diagnosed as positive

(+) or negative (-) for rabies. Report of Texas
Department of Health Laboratories, Austin, 19

pp.

_. 1992. Animal heads diagnosed as positive

(+) or negative (-) for rabies. Report of Texas
Department of Health Laboratories, Austin, 17
pp.

_. 1993. Animal heads diagnosed as positive

(+) or negative (-) for rabies. Report of Texas
Department of Health Laboratories, Austin, 17
PP-

_. 1994. Animal heads diagnosed as positive

(+) or negative (-) for rabies. Report of Texas
Department of Health Laboratories, Austin, 16

pp.

Baer, G. M., and J. S. Smith. 1991. Rabies in
nonhematophagous bats. Pp. 341-366 in The
natural history of rabies (G. M. Baer, ed.),
CRC Press, Boca Raton, Florida, 620 pp.

Baker, R. H. 1956. Mammals of Coahuila, Mexico.
University of Kansas Publications, Museum of
Natural Histor}^, 9:125-335.

Brass, D. A. 1994. Rabies in bats. Livia Press,
Ridgefield, Connecticut, xv +335 pp.

Clark, K. A. 1994. Rabies in humans, 1950-present.
Report of Texas Department of Health, Aus¬
tin, 6 pp.

Constantine, D. G. 1988. Health precautions for bat
researchers. Pp. 491-528 in Ecological and
behavioral methods for the study of bats (T. H.
Kunz, ed.). Smithsonian Institution Press,
Washington, D.C., xxii + 533 pp .

Dean, D. J., and M. K. Abelseth. 1973. The fluores¬
cent antibody test. Pp. 73-84 in Laboratoiy
techniques in rabies, 3rd ed. (M. M. Kaplan
and H. Koprowski , cds.). World Health Or¬
ganization, Geneva.

DeBlase, A. F., and R. E. Martin. 1981. A manual of
mammalogy. William C. Brown Company
Publishers, Dubuque, Iowa, xii + 436 pp.

Dowdy, S., and S. Wearden. 1991. Statistics for re¬
search, John Wiley & Sons, New York, xvii +
629 pp.

Easterla, D. A. 1973. Ecolog>' of tlie 18 species of
Chiroptcra at Big Bend National Park, Texas,
The Northwest Missouri State University Stud¬
ies, 34:1-165.

Greenhall, A.M, 1965. Technique for the removal of
the brain in bats. Nature, 208:1014-1015.

Hill, J. E., and J. D. Smith, 1984. Bats: a natural
history. University of Texas Press, Austin, 243

pp.

Kunz, T. H., and A. Kurta. 1988. Capture metliods
and holding devices. Pp. 1 -29 in Ecological and
behavioral Methods for the study of bats (T.
H. Kunz, ed.). Smithsonian Institution Press,
Washington, D.C., xxii + 533 pp.

YANCEY ET AL.- SURVEY OF RABIES AMONG BATS

5

Schmidly, D. J. 1977. The mammals of Trails-Pecos
Texas including Big Bend National Park and
Guadalupe National Park. Texas A&M Uni¬
versity Press, College Station, 225 pp.

. 1991. The bats of Texas. Texas A&M Uni¬
versity Press, College Station, xv + 188 pp.

Sulkin, S. E., and M. J. Grevc. 1954. Human rabies
caused by bat bite. Texas State Journal of
Medicine, 50:620.

Addresses of Authors ;

FRANKLIN D. YANCEY, II,

AND CLYDE JONES

Department of Biological Sciences and the Museum,
Texas Tech University, Lubbock, Texas 79409-3191
e-mail: frank@packrat.musmAtu.edu and
cjones@packrat. musm. ttu. edu

PUSHKER RAJ, AND SUSAN NEILL

Texas Department of Health, 1100 West 49'^ Street,
Austin, Texas 78756

PUBLICATIONS OF THE MUSEUM OF TEXAS TECH UNIVERSITY

ll was tlirough the cITorls of Horn Professor J Knox Jones, as director of Academic Publications, that Texas
Tech University initiated several publications series including the Occasional Papers of the Museum. Tliis and
future editions in the series are a memorial to his dedication to excellence in academic publications. Professor
Jones enjoyed editing scientific publications and served the scientific community as an editor for tlie Journal of
ManiniaIog>; Evolution, Tlic Texas Journal of Science, Occasional Papers of the Museum, and Special Publica¬
tions of the Museum. It is with special fondness that we remember Dr. J Knox Jones.

Institutional subscriptions arc available through the Museum of Texas Tech University, attn: NSRL Publica¬
tions Secretary, Box 43191, Lubbock, TX 79409-319 L Individuals may also purchase separate numbers of the
Occasional Papers directly from the Museum of Texas Tech University.

ISSN0149-175X

Museum of Texas Tech University, Lubbock, TX 79409-3191