First Case of Human Rabies in Chile
Caused by an Insectivorous Bat Virus Variant
Myriam Favi,* Carlos A. de Mattos,† Verónica Yung,†
Evelyn Chala,‡ Luis R. López,‡ and Cecilia C. de Mattos†
*Instituto de Salud Pública, Ministerio
de Salud Pública, Santiago, Chile; †Centers for Disease Control and Prevention,
Atlanta, Georgia, USA; and ‡Hospital Clínico Fusat of Rancagua, Rancagua,
Chile
The first human rabies case in
Chile since 1972 occurred in March 1996 in a patient without history of known
exposure. Antigenic and genetic characterization of the rabies isolate indicated
that its reservoir was the insectivorous bat Tadarida brasiliensis. This is the
first human rabies case caused by an insectivorous bat rabies virus variant
reported in Latin America.
In Latin America, rabies in bats was suspected during the 1910s in Brazil and
was definitively diagnosed for the first time in Trinidad in 1931 (1,2). Since
then, rabies has been diagnosed in numerous species of nonhematophagous bats
throughout this region (3). Despite these early discoveries, the important role
of nonhematophagous bats in the epidemiology of the disease remained
overshadowed by the presence of canine and vampire bat rabies in the region (4).
During the past decade, with the control of dog rabies in many urban areas and
the incorporation of antigenic and molecular typing of viral variants into
rabies surveillance programs, an appreciation for the importance of
nonhematophagous bats in rabies epidemiology began to emerge in Latin America
(5-9). Rabies virus has been isolated frequently from insectivorous and
frugivorous bats in cities across Latin America (5,10-12). This situation also
characterizes the current epidemiologic pattern of rabies in Chile, where dog
rabies has been controlled. The last human rabies case in Chile caused by a dog
bite occurred in 1972 (5); since 1985, insectivorous bats have been the main
rabies reservoirs identified. As such, these bats are the most important source
of infection for the sporadic rabies cases diagnosed in domestic animals every
year (5). In 1996, after a period of 24 years with no known human rabies deaths,
the first human rabies case with an insectivorous bat as the source of infection
was reported in Chile (13).
Case Report
 |
Figura 1.
Map of South America showing the
geographic position of Chile and map of Chile presenting the geographic
distribution of the administrative regions of the country. aNumber of the
corresponding administrative region. bMetropolitan region. |
On February 13, 1996, a 7-year-old boy from Doñihue in Administrative Region
VI was admitted to the Hospital Clínico Fusat of Rancagua in the region (Figure
1) with a 2-day history of adynamia and dizziness. On admission, the child was
calm, cooperative, and afebrile. Physical examination revealed anisocoria,
ptosis of the left upper eyelid, and strabismus. There was no sensory loss, but
ambulatory difficulties and abundant sialorrhea were observed. Brain
computerized axial tomography (CAT) scan was normal. Polyradiculoneuritis was
suspected, and gamma globulin was administered intravenously. The presumptive
clinical diagnosis was encephalitis. On February 15, progressive paralysis
developed that evolved to respiratory failure; the boy was connected to a
mechanical ventilator. The patient could still follow simple orders. On February
18, he lapsed into a coma with severe hypotonia and total loss of reflexes. CAT
scan showed diffuse cerebral edema, and the electroencephalogram indicated no
electric activity. Intracranial hypertension developed, and the patient was put
under hyperventilation and treated with intravenous dexamethasone, mannitol, and
acyclovir.
Since a virus was considered
the most probable cause, laboratory tests were conducted to determine the
presence of the following viruses: herpes, measles, Coxsackie, echo, and polio.
All results were negative. Interviews with relatives and the boy's nanny
revealed that bats had been observed in the family's house. The nanny also
reported that she had seen a bat flying away from the child's toy box. Even
though these interviews failed to reveal any direct contact with bats or any
history of an animal bite, this epidemiologic information prompted the
physicians to suspect rabies. On February 26, 1996, a serum sample and corneal
smear were obtained from the patient and sent to the Rabies Laboratory of the
Instituto de Salud Pública de Santiago (ISP). A rabies antibody titer of 1:625
was found in the serum specimen by using the indirect fluorescent-antibody (IFA)
technique (14). The patient had no history of rabies vaccination to account for
the presence of antibodies. The corneal smear was negative for rabies antigen by
the direct fluorescent-antibody (DFA) assay (15). On March 4, a second serum
sample, cerebrospinal fluid, and saliva were obtained. The second serum sample
was tested simultaneously with the first one by IFA assay, and a titer of
1:15,625 was detected. The cerebrospinal fluid showed a titer of 1:125. These
findings confirmed the presumptive clinical diagnosis of rabies. The saliva
sample was negative by DFA assay and suckling mouse inoculation (15,16).
The patient died on March 5,
1996, when artificial respiratory support was disconnected. Postmortem tissue
samples of cerebral cortex, hippocampus, cerebellum, and nuchal skin biopsy were
sent to the ISP Rabies Laboratory for diagnosis. The cerebellum and skin
specimens were positive for rabies virus antigen by DFA assay.
Rabies postexposure prophylaxis with the suckling mouse brain Fuenzalida-Palacios
vaccine was administered to the victim's mother and to 10 health-care providers
who had possible contact with the patient's saliva. The rabies postexposure
prophylaxis schedule used was 2 mL of vaccine, subcutaneously, on each of days
1, 2, 3, 4, 5, 6, 21, and 90. Blood samples were taken from vaccinees on day 14
after the initial dose of vaccine; IFA assay showed that adequate immune
responses had developed.
The virus was isolated from the patient's brain tissue by intracerebral
inoculation of suckling mice (16). To help identify the possible source of
infection, the virus was antigenically and genetically characterized. Antigenic
characterization of the virus was carried out by using a panel of eight
monoclonal antibodies directed against the viral nucleoprotein, provided by the
Centers for Disease Control and Prevention. The MAbs were used in an IFA assay
as described (9,17). These analyses identified a rabies antigenic variant
associated with Tadarida brasiliensis (free-tailed bat) in Chile, which had been
designated as antigenic variant 4 (AgV4) (9,17).
Genetic characterization was done by sequencing a 320-bp portion of the rabies
virus nucleoprotein gene from nucleotide position 1,157 to 1,476, as compared
with the SADB 19 strain (18,19). Briefly, genomic viral RNA was extracted from
infected tissue by using TRIzol (Invitrogen, San Diego, CA, formerly GIBCO-BRL
Inc.) according to the manufacturer's instructions. Complementary DNA was
produced by a reverse transcription polymerase chain reaction with primers 10 g
and 304 (19) and was sequenced by using the Taq Big Dye Termination Cycle
Sequencing Ready Reaction Kit (Applied Biosystems, Foster City, CA), according
to the manufacturer's protocol, on an Applied Biosystems 377 DNA automated
sequencer (Applied Biosystems).
This human rabies virus isolate was compared with viruses obtained from domestic
animals and insectivorous bats in urban centers in Chile from 1977 to 1998 (18).
PileUp and Pretty programs of the Wisconsin Package, Version 10 (Genetic
Computer Group, 2000, Madison, WI), were used to produce sequence alignments and
comparative nucleotide analyses. The programs DNADIST (Kimura-two parameter),
NEIGHBOR (Neighbor-joining method), and DNAPARS (parsimony method) from the
PHYLIP package, Version 3.5 (20), were used in the phylogenetic studies. The
bootstrap method, as implemented by the SEQBOOT program from PHYLIP, was
followed by the use of DNADIST and NEIGHBOR for the distance matrix analyses.
SEQBOOT was also used before employing DNAPARS for the parsimony studies.
Graphic representation of the trees was constructed with the TREEVIEW program
(21).
Although five genetic variants of rabies virus are found in Chile (18) (Figure
2, groups A to E), a reservoir has been identified for only two: T. brasiliensis
(Figure 2, group D) and Lasiurus sp. (Figure 2, group E). Phylogenetic analyses
of the Chilean human isolate demonstrated that it segregated in group D. This
group represents the genetic variant of rabies virus most frequently isolated
throughout the country, formed by viruses from the Metropolitan Region and
Regions IV, V, VI, VII, VIII, IX, and X (Figure 1). The high bootstrap value
that supports the inclusion of this virus in group D and the very close genetic
relationship it has with the other members of this group (average genetic
distance 0.5%) clearly show that T. brasiliensis is the likely reservoir of the
rabies virus isolated in this case.
Conclusions
The absence of a history of an animal bite, the clinical presentation
of the disease without the classic signs of hydrophobia or aerophobia, and the
absence of any human rabies cases for a period of 24 years in Chile were the
primary reasons that rabies was not first suspected and a definitive diagnosis
was delayed in this case. Retrospective studies of human rabies epidemiology
have demonstrated that it is not uncommon to observe rabies cases in which there
is no history of a bite, mainly in situations involving insectivorous bat rabies
variants. For example, of the 17 human rabies cases associated with
insectivorous bats reported in the United States from 1980 to 1996, only one had
clear documentation of a bite (22). Without proper education, patients may not
be aware of the risks from a bat bite. Moreover, the wound may not be
appreciated as a concern because of the limited injury inflicted by the bat's
small teeth (23). Finally, there may not be an opportunity to obtain a history
from a pediatric patient or to discern an exposure that occurs during sleep or
other circumstances (24).
In cases in which a patient shows clinical signs of central nervous system
involvement of unknown or suspected viral origin, health-care providers should
be aware of the importance of conducting a thorough medical history to
appropriately assess the possibility of rabies. With the important changes in
the epidemiologic patterns of rabies in Latin America, this disease should be
included in the differential diagnosis of neurologic diseases characterized by
acute encephalitis and progressive paralysis, even when no previous history of
an animal bite exists and even in regions where canine rabies has been
eradicated.
Acknowledgments
We thank Charles E. Rupprecht and staff of the Rabies Section, Viral
and Rickettsial Zoonoses Branch, Centers for Disease Control and Prevention for
their collaboration, advice, and valuable discussions.
Dr. Favi is the head of the Rabies Laboratory of the Instituto de Salud Pública
de Chile. Her research interests include rabies diagnosis and the study of the
molecular epidemiology of rabies in Chile and other Latin American countries.
Address for correspondence: Carlos A. de Mattos, Rabies Section,
Viral and Rickettsial Zoonosis Branch, Division of Viral and Rickettsial
Diseases, Centers for Disease Control and Prevention, 1600 Clifton Road,
Mailstop G33, Atlanta, GA 30333, USA; fax: 404-639-1058; e-mail:
cdd9@cdc.gov
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