Assessing anti-rabies baiting--what happens on the ground?

Abstract

Background: Rabies is one of the most hazardous zoonoses in the world. Oral mass vaccination has developed into the most effective management method to control fox rabies. The future need to control the disease in large countries (i.e. Eastern Europe and the Americas) forces cost-benefit discussions. The 'Increase bait density' option refers to the usual management assumption that more baits per km2 could compensate for high fox abundance and override the imperfect supply of bait pieces to the individual fox.

Methods: We use a spatial simulation, which combines explicitly fox space use (tessellation polygons) and aeroplane flight lines (straight lines). The number of baits actually falling into each polygon is measured. The manager's strategic options are converted into changes of the resulting bait distribution on the ground. The comparison enables the rating of the options with respect to the management aim (i.e. accessibility of baits).

Results: Above 5% (approx. 10%) of all fox groups without any bait (at most 5 baits) relate to the baiting strategy applied in the field (1 km spaced parallel flight lines, 20 baits per km2 distributed) under habitat conditions comparable to middle and western Europe (fox group home-range 1 km2, 2.5 adults; reference strategy). Increasing the bait density on the same flight-line pattern neither reduces the number of under-baited fox group home-ranges, nor improves the management outcome and hence wastes resources. However, reducing the flight line distance provides a more even bait distribution and thus compensates for missed fox groups or extra high fox density.The reference strategy's bait density can be reduced when accounting for the missed fox groups. The management result with the proper strategy is likely the same but with reduced costs.

Conclusion: There is no overall optimal strategy for the bait distribution in large areas. For major parts of the landscape, the reference strategy will be more competitive. In situations where set backs are attributed to non-homogeneous bait accessibility the distribution scheme has to be refined zone-based (i.e. increase of the flight line length per unit area). However, increase in bait density above the reference strategy appears inappropriate at least for non-urban abundance conditions of the red fox.

Figure 1

The 'Non-homogeneous bait coverage' hypothesis Schematic representation of the hypothesis of violated 'Homogeneous bait coverage' due to randomly missed fox group home-ranges. After Breitenmoser & Müller (1997, [12]).

Figure 2

Schematic representation of the model Schematic representation of the components of the model: Randomly generated landscape of fox group home-ranges (including high and low density areas), the parallel flight lines randomly intersected with the 'fox landscape', and the measurements in the model (i) length of flight line segment in each polygon, (ii) number of empty polygons.

Figure 3

Frequency distribution of local bait densities Full frequency distribution of local bait densities found under the reference or field scenario (pBaitDens = 20; pLineDist = 1 km; pN = 2500; MaxX*MaxY = 2500 km²), whiskers represent MIN and MAX of 100 repetitions. The segments of the inserted bar diagram illustrate the percentage of spatial units with a particular number of baits (red 0; orange 1–5; light orange 6–10; yellow 11–15; green 16–20; dark green > 20 baits). Management aim of 20 b.p.km² is exceeded on the group home-range level where mean bait density is measured as 21.1 b.p.km².

Figure 4

The effect of group home-range size The effect of mean area of family group home-range (i.e. fox density) on the distribution of absolute bait numbers in individual fox groups under the field strategy (pBaitDens = 20; pLineDist = 1 km). Bar segments depict the percentage of spatial units with a particular number of baits (red 0; orange 1–5; light orange 6–10; yellow 11–15; green 16–20; dark green > 20 baits).

Figure 5

The effect of varying bait density The effect of varying bait density (i.e. management aim) on the distribution of absolute bait numbers in the individual fox group under the field strategy (pLineDist = 1 km; mean fox group area ~1 km²). (a) Bar segments depict the percentage of spatial units with a particular number of baits (red 0; orange 1–5; light orange 6–10; yellow 11–15; green 16–20; dark green > 20 baits). (b) Bar segments depict frequencies of the relative deviation of the resulting local bait density from the aimed density of the campaign (pink -100% to -75%; lilac -74% to -50%; aubergine -49% to -25%; blue -25% to +50%; dark blue above +50% deviation of management aim of 20 b.p.km²).

Figure 6

The effect of varying flight line distance The effect of varying flight line distance (i.e. management strategy) on the distribution of absolute bait numbers in the individual fox groups under the field strategy (pBaitDens = 20; mean fox group area ~1 km²). Bar segments depict the percentage of spatial units with a particular number of baits (red 0; orange 1–5; light orange 6–10; yellow 11–15; green 16–20; dark green > 20 baits). Very small flight line distances cannot improve the ground pattern further, as small sized home-ranges can only contain a maximum number of baits even with perfectly uniform distribution.