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SHORT COMMUNICATION |
-Maganja1
e Grom1
University of Ljubljana, Veterinary Faculty, Gerbieva 60, 1115 Ljubljana, Slovenia
2 Corresponding author (email: peter.hostnik{at}vf.uni-lj.si)
ABSTRACT:
Red foxes (Vulpes vulpes) are the main reservoir of rabies in Slovenia, whereas cases of rabies in other wildlife species occur sporadically. In 1995, a program of oral vaccination of wildlife in Slovenia was initiated; baits with oral vaccine were distributed by air at a density of 20 baits/km2. During 1995, when the oral vaccination program was started, 1,089 cases of rabies (including both wild and domestic animals) were reported. Five years later (1999), only six positive animals were detected among 1,195 tested (0.5%). Despite an increase in bait density (25 baits/km2) during the years 2000 and 2001, reported rabies cases increased to 115 and 135, respectively. In 2003, following initiation of a new bait-dropping strategy, which incorporated perpendicular rather than parallel flight lines, the number of rabies cases decreased to eight.
Key words: Rabies, Slovenia, vaccination, wildlife.
Rabies is a zoonotic disease of mammals caused by a negative-stranded RNA virus belonging to the Lyssavirus genus of the family Rhabdoviridae; it is transmitted most frequently via saliva following a bite from an infected animal (Bourhy et al., 1999). The genus Lyssavirus is divided into seven genotypes. Genotype 1 viruses are distributed worldwide and generally are found in terrestrial mammals (Bourhy et al., 1999; Finnegan et al., 2002). In central and southeastern Europe, red foxes (Vulpes vulpes) are the primary wildlife reservoir (Lontai, 1997; Office International des Epizooties [OIE], 2000a), and in red fox–mediated rabies epizootics, the numbers of cases vary over season and year (Müller, 2000). Oral vaccination of red fox as a method of rabies eradication started in Switzerland, and by 1984, this method was adopted by several other European countries. In central and eastern European countries, including the Czech Republic, Slovak Republic, Poland, Slovenia, and Hungary, cases of rabies were vastly reduced after 9 yr of oral vaccination (Matouch and Vitasek, 2002). Rabies cases peaked in Europe in 1989, when 24,377 cases were reported (Potzsch et al., 2002). As a result of oral vaccination programs since 1989, several European countries, based on World Health Organization principles, are rabies-free in terms of terrestrial animals (Müller, 2000); these countries include Finland (1991), The Netherlands (1991), Italy (1997), Switzerland (1998), France (2000), Belgium (2001), and Luxemburg (2001) (Potzsch et al., 2002). In other eastern and southern European counties where oral vaccination programs were not implemented, the number of reporting cases has increased.
In 1995, after preliminary experimental vaccination trials (Curk and Carpenter, 1994), a national eradication program to eliminate rabies throughout Slovenia was initiated. The objectives of this report are to describe the implementation of this rabies eradication program, to analyze the rabies situation during the vaccination period from 1995 to 2003, and to describe the use of two different airborne bait-delivery models (a parallel-line model and raster model) in these eradication efforts.
According to Slovenian legislation, all veterinary organizations are required to submit samples from animals with neurologic signs of rabies for laboratory testing. Hunter organizations also have collected samples from red fox and other wildlife species and submitted them to the local veterinary service for rabies testing. All samples were tested with a fluorescent antibody test (FAT) using impression smears of Ammon’s horn and cerebellum as described in the Manual of Standards for Diagnostic Tests and Vaccines (OIE, 2000b). A commercial immunofluorescent conjugate (Bio-Rad, Marnes la conquette, France) was used in this test. Samples with questionable FAT results were retested using virus isolation on murine neuroblastoma cells (Webster and Casey, 1996). Only laboratory-confirmed cases were included in this analysis.
A commercial vaccine containing attenuated rabies strain SAD (Steck et al., 1982; Schneider and Cox, 1983) was used. Vaccine delivery protocols were based on preliminary studies conducted from 1988 to 1992. During the first 2 yr of these studies, baits were distributed manually; during the last 2 yr, they were distributed by helicopter and plane. Based on the results from these trials, a required bait density of between 16 and 20 baits/km2 was estimated for the vaccination area. A countrywide vaccination campaign within Slovenia started in 1995, and baits were distributed by plane at a height of between 300 and 500 m. Pilots used a global positioning system for navigation and orientation, and flight paths were 1,000 m apart. Bait discharge was monitored using the computer program FI-CO3J® (constructed by computer engineer Aleksander Modic S. P., Ajdovscina, Slovenia), which calculated the density of distributed baits based on route and flight time.
Vaccination efficiency was monitored by detection of the biomarker in the bones (lower jaw) of the captured red foxes. A mineral saw (Buehler Isomet, Wusterhausen, Germany) was used to prepare bone sections (thickness, 1.0 mm), and a fluorescent microscope was used to determine the presence of biomarker. Samples for biomarker detection were collected each year in winter (from November to March). Results are presented in Table 1
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From 1992 to 2003, a total of 22,082 samples from different animal species were tested, and 3,269 of them were positive by FAT for rabies (Table 2). Most of the positive cases (95.2%) were wildlife species, whereas domestic animals accounted for only 4.8% of positive cases. The incidence of rabies in wildlife populations (3,112 cases) was 19.7-fold higher than that in domestic animals (158 cases). Among domestic animals, cats (51.3% of rabies cases in domestic animals) were positive more often than dogs (37.3% of rabies cases in domestic animals).
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). The distribution of cases in 1995 is shown in Figure 2, and a rapid decline of positive cases was evident after 1995, when the nationwide oral vaccination program began (Fig. 1). In 1995, a total of 1,089 cases of rabies were registered, predominantly in red foxes (Table 2), and from 1995 to 1998, the highest prevalence of rabies was detected in the central part of Slovenia. In 1998, only 14 cases were detected, and 12 of these occurred in the center of the vaccinated region, within an area with a radius of 30 km around aerodrome Brnik (a restricted flight area, 30 x 15 km). The remaining two occurred near the Croatian border. In 1999, only six cases were detected (Table 2). In 2000 and 2001, rabies cases were restricted to the southern part of Slovenia, along the border with Croatia, and during these years, the number of positive cases increased (115 in 2000 and 135 in 2001). The number of cases declined to 15 in 2002 and then to eight in 2003. All of the 2000 and 2003 cases were detected near the Croatian border (Fig. 2).
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eleznik and Janc, 1977), and as a preventive measure, intensive hunting was performed in this region. Vaccination of dogs, pasture cattle, sheep, and horses at the expense of the state became compulsory, and voluntary vaccination of cats was recommended. Nevertheless, rabies was detected throughout Slovenia, and case data indicated that red foxes were most affected. During this study (1995–2003), wildlife rabies accounted for 95.2% of all cases, and most of these involved red fox. Among domestic animals, cats and dogs were most affected, but overall, cases in domestic animals represented a small proportion (4.8%) of total rabies cases in Slovenia. This low percentage demonstrates the effectiveness of the domestic animal vaccination programs.
During the past 25 years, the success of oral mass vaccination in European wildlife has been demonstrated repeatedly (Brochier et al., 1988; Masson et al., 1996; Vos et al., 2000). The management aim of an oral vaccination program is to provide accessible vaccine baits to every fox within the treatment area. Consequently, bait density has been designed empirically, according to population estimates and minimum bait number per capita (Breitenmoser and Müller, 1997; Selhorst et al., 2001). The recommended strategy of applying approximately 20 baits/km2 (Brochier et al., 1988; Müller and Schlüter, 1998; Thulke et al., 2004) was adopted in Slovenia when the wildlife vaccination program started in 1995, and in that year, rabies cases were distributed throughout Slovenia (Fig. 2). After 3 yr of vaccination, a marked decrease in rabies cases was observed; only 14 rabies cases were recorded in 1998. The persistence of rabies within a 30-km radius area in the center of the vaccinated area (12 cases) is believed to be related to the failure to place baits in this area as a result of restricted airspace. The two additional cases were registered near the border of Croatia, which was not vaccinating wildlife at that time. Rabies persisted in this region for three additional years. In 1999, all six positive cases were found in this same area, and despite an increase in bait density (from 20 to 25 baits/km2), the number of positive cases dramatically increased in the years 2000 and 2001. With single flight lines separated by 1,000 m, it was suspected that the area had not been covered with vaccine baits effectively, so the parallel-line model was replaced with the raster model (double-baiting system using perpendicular flight lines). During the next 2 yr, the number of rabies cases decreased; only 15 and eight cases were registered in 2002 and 2003, respectively. Historically, rabies cases in Slovenia (Fig. 1) have peaked in approximately 7-yr intervals, but after oral vaccination started in 1995, the disease appears close to being eradicated. These activities have greatly aided in preventing the spread of rabies to rabies-free parts of central Europe.
Red foxes are solitary, and the home range of adults can vary in size, depending on the quality of the habitat. Home ranges may cover between 5 km2 and 12 km2 in quality habitats to between 20 km2 and 50 km2 in poorer habitats. An individual red fox can move up to 8 km per night (Corbet and Harris, 1991). It is possible that in some areas, perpendicular flight paths would not transect an individual’s home range, and we believe that this is the basis for the improved results when the raster model was used (Fig. 3). This strategy seemed to be especially effective when the landscape was configured with high hills and deep valleys, which may have influenced the shape and dimensions of home ranges. The configuration of dispensed baits also may have affected the results, because the baits have a very intense smell that is easily detected by red foxes from a distance of 100 m (Corbet and Harris, 1991). The single-flight strategy with the density of 25 baits/km2 (Fig. 3) resulted in baits being dispensed 40 m apart, which may have increased the chance of multiple baits being eaten by the same fox following the transect line. When baits were distributed by double-baiting using perpendicular flight lines, a more uniform delivery was achieved, which may have resulted in more baits being accessible to more animals.
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Slovenia is densely covered with forests and has rich biotypes favorable to potential wildlife vectors. Although the incidence of rabies has been greatly reduced because of the success of the vaccination program, a continuation of surveillance and oral vaccination of wildlife each year as well as the coordination of vaccination campaigns between countries are highly important.
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Received for publication 9 August 2004.
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