At first sight, the relatively low (<0.5 IU/ml) maAb titers observed during the first three weeks after birth were surprising. The great variance in individual maAb-titres during the first three weeks of cubs (Fig.
1) was similar to that observed in older fox cubs (Fig.
2). Large differences in maAb-titres were observed even among littermates; these could be a result of difference in suckling behaviour among the cubs [
16]. The initial level of maAb is influenced by many factors; e.g. quality and quantity of colostrum and milk-intake as well as body constitution (condition, birth-weight) [
16‐
19]. In another canids species, the domestic dog (
Canis familiaris), major transfer of maAb takes place during ingestion of colostrum and milk by the new-born [
16,
17,
20]. In these animals, a limited transfer of maAb also occurs in utero [
13,
16]. Additional studies have been initiated to clarify whether or not this also takes place in foxes. Unfortunately, these and other possible factors are, most of the time, extremely difficult to assess, mainly due to the previously-mentioned extreme susceptibility of the vixen to disturbance immediately prior and after parturition. Pollock & Carmichael [
21] mentioned that increasing dog litter-size negatively influenced maAb-levels in puppies. This effect, however, could not be observed in foxes [
9]. The model presented here clearly identified another important parameter determining maAb-levels: the nAb-titre of the mother animal. A direct proportional relationship between the serum titre of the mother and her offspring has been identified in many studies on maternally derived immunity [
18,
20‐
22]. Our results indicate that the subsequent disappearance rate of maAb in fox cubs was independent of the nAb-titre of the vixen. The exponential decline of maAb against rabies in foxes corresponded with the maAb-decline observed for other viruses in canine animal species, whereby maAb persist for up to 8–10 weeks on average [
12,
13,
16]. The half-life of maAb against rabies in foxes was estimated to be 9.34 days, and is similar to that observed for maAb against canine distemper virus (8.4 days) and canine parvovirus (9.7 days) [
13,
21]. However, the disappearance of maAb is also linked with the sensitivity of the serological techniques and the threshold to distinguish between positive and negative used (Fig.
4). At an international level nAb at concentrations < 0.5 U/ml representing an arbitrarily defined threshold are considered positive whilst such nAb below this threshold cannot be distinguished from unspecific reactions [
10]. Following rabies vaccination of female dogs, maAb in puppies could be detected up to 6–7 weeks post partum, on average [
16]. Taking an estimated mean time period of 23 days into account during which maAb can be distinguished from unspecific reactions in RFFIT (nAb-titre ≥ 0.5 IU/ml) after birth, there is evidence that maAb-titres in fox cubs are not as high as in puppies and therefore appear to decrease more quickly than in dogs. Further considering spring whelping activity, maternal immunity against rabies in young foxes is very difficult to detect under field conditions. Thus, the percentage of 9–20% of young foxes having nAb following spring vaccination campaigns [
23‐
25], may result exclusively from active immunization by bait-uptake. It has been shown, however, that the detection of rabies maAb by immunoblotting is much more sensitive than the RFFIT, and consequently, by using the former method, maAb could be detected for a longer period of time (Müller, unpublished results). The relatively longevity of maAb at a low level results in an interference between passively and actively acquired immunity up to 8 weeks post partum which affected more severely the ability of fox cubs to resist a virus challenge [
9]. Taking this into account, concerning spring vaccination campaigns baits should not be distributed in previously baited areas before most cubs are more than 8 weeks of age. Therefore, to reach optimal immune response in young foxes, depending on the geographical region vaccination campaigns should be adjusted accordingly. In areas vaccinated for the first time, however, baits can be distributed earlier, while 5 weeks old cubs are already immunocompetent [
14].