Serological survey for rabies in serum samples from vampire bats (Desmodus rotundus) in Botucatu region, SP, Brazil

The chiropterans constitute 25% of the world's mammal fauna. Due to the destruction of their natural ecosystem, the vampire bats have moved from nature to artificial roosts closer to man and domestic animals. This phenomenon has happened particularly in rural areas. Rabies is a viral anthropozoonosis, 100% lethal, and vampire bats (Desmodus rotundus) represent an important role in its epidemiology. D. rotundus were captured at night with mesh nets in partnership with the Botucatu Defense Office and sent to the Zoonosis Diagnostic Service, at the School of Veterinary Medicine and Animal Husbandry, UNESP. Serum samples from 204 bats were analyzed by enzyme-linked immunosorbent assay (ELISA) and fluorescent antibody viral neutralization test (FAVN) for rabies antibody detection. The results showed 7.4% of sera with titers higher or equal to 0.5 U for rabies antibodies, which demonstrated viral flow circulation among the studied region. Data suggest a need for constant monitoring accomplished by epidemiological and sanitary measures.


INTRODUCTION
The chiropterans are one of the most distinguished mammals as they present special features that make them capable of true flight (19). These animals play an important role in nature, specifically, in several tropical and subtropical areas, since they are responsible for spreading seeds, flower pollination and insect population control.
They constitute somewhat wide range of mammal fauna, in the number of species and population density, representing approximately a quarter of mammal fauna worldwide (23).
The latter is the most severe disease that can be transmitted by bats. D. rotundus, the most common vampire bat of the Americas, can be found in colonies of 20 to 100 animals (16). They live in caves, tree holes, abandoned mines, home basements, drainpipes or roosts (4).
Vampire bats present some adaptations to feed on blood: their saliva enzymes inhibit blood coagulation and two ducts on each side of their tongues enable them to suck blood. Each bat ingests 15 to 25 mL of blood per night and usually feeds on the same prey and the same wound for many nights in a row; additionally, a parasitized animal can be exploited by more than one bat during the same night (18).
As they feed only on blood, they are potential transmitters of rabies, which may cause damage to equine and cattle flocks due to numerous animal deaths (14).
Attacks on humans and rabies transmission by D. rotundus have increased in recent decades, especially in the Amazon area, in Peru and Brazil. Some of these outbreaks have been preceded by local changes, including rapid domestic animal removal, mineral extraction or deforestation, activities that can change the natural environment where the D. rotundus lives (20).  (1). The actions necessary to hinder the disease are well known and there have been many improvements, for example, in feline and canine control in urban areas (13). On the other hand, the chiropteran population has grown in non-rural areas due to lack of metropolitan planning with respect to architectural and landscape projects. Together with the chiropteran population growth, acts of epidemiological vigilance were intensified. As a result, the number of chiropteran rabies cases increased and the virus was found even in areas where the disease had been thought to be under control (12).
Due to the disease expansion, D. rotundus has interfered in the aerial cycle, causing countless rabies cases in non-vampire bats, as well as in the urban cycle, being responsible by eight out of fourteen cases of the disease recorded among dogs and cats (7).  Dis., 2008, 14, 4, p. 655 In order to calculate ELISA titers, the two-graph receiver operating characteristic (TG-ROC) technique was performed. Serum ranging between 0.10 and 0.20 was considered weak positive; 0.20 to 0.30, positive; and greater than 0.30, high positive.
Each high positive serum was tested by RFFIT and the titers were calculated through the analysis of 20 microscopic fields in only one pool, for each serum dilution assessed in the presence of infected cells using a fluorescence microscope (Carl Zeiss Inc., USA) (160x magnification). Titers were calculated using the method of   Souza et al. (22) found, in the same Botucatu region, only 0.1% positivity among 895 studied bats by employing the immunofluorescence technique for brain tissue. On the other hand, Cortês et al. (9), in another study in the same area, found 0.9% positivity.
The results of the present study may indicate subclinical disease, without Negri bodies detected by the immunofluorescence technique. The animals presenting subclinical infections constitute efficient reservoirs because the virus can multiply in their fatty tissues without affecting the central nervous system. In reservoir animals, the rabies virus can endure and is eliminated through both respiratory secretion and saliva, so that they are capable of infecting other bats, humans as well as domestic and wild animals.
The epidemiological characteristics of the disease are still poorly understood.
Consequently, more research and implementation of an epidemiologic vigilance system are necessary (14). D. rotundus plays an important role in rabies circulation in the Botucatu region, being capable of infecting a great variety of animals.

CONCLUSIONS
The studied region presents viral flow circulation through bats, thus a constant monitoring accomplished by epidemiological and sanitary measures is necessary to maintain the control of rabies.