In the present study, clinical disease was not observed except for fever. Only a few lambs with high fever (≥41°C) typical for TBF were detected [1, 4]. This could be due to variants of A. phagocytophilum involved [15, 16]. However, the animals were only examined every fourteen days and the fever period in A. phagocytophilum infected sheep normally lasts around one week . In comparison, most primary infections in the field are not observed .
Only a few ticks were found on the animals. The reason for this is unknown, but tick may have attached and detached unnoticeably, since the attachment period on animals for all stages of I. ricinus ticks is normally less than 14 days . In the present study, ticks were not observed on 20% of the lambs that seroconverted to A. phagocytophilum. Attached ticks may also have been overlooked, since a detailed inspection of the whole animal was not performed. However, most I. ricinus ticks on sheep are attached on the head, axillae and inguinal regions [12, 13]. In addition, most ticks were found in May/June and in August/September. Earlier studies indicate that cases of TBF in the same county have a similar distribution with 60.4% occurring in May/June and 27.7% in September/October .
Maternal antibodies were found in 88% of the lambs. In the previous year, maternal antibodies were found in 89% of the lambs (Stuen, unpublished results), indicating that A. phagocytophilum infection is common on the actual pasture. However, the infection seems to occur mainly in the autumn, since the seroprevalence in medio September was only 35%. In the pyrethroid treated group, 30% of lambs were seropositive, which was slightly less than in the control group (40%), although a significantly higher number of ticks were found on the control lambs. The reason for this apparent discrepancy between tick infestation rate and infection prevalence may be due to an unknown number of infested ticks involved (the lambs were only observed every fourteen days) or variation in both A. phagocytophilum variants and infection prevalence in the actual tick population. In an earlier study from the same county in Norway, 24 msp-4 gene variants of A. phagocytophilum were found among 16 lambs during the first grazing season . Serological response may differ significantly between variants of the bacterium , whereas some variants may not even give a detectable serological titre (Stuen, unpublished result). Unfortunately, variants of A. phagocytophilum were not investigated in the present study.
Ticks were found on pyrethroid-treated lambs. This indicates that frequent treatment every second week is not 100% efficient against tick infestation. The present result is in accordance with earlier studies indicating that ticks can be found on animals already 13–14 days after treatment with either cypermethrin or deltamethrin [7, 13]. An earlier field study also supports the present results, since most lambs seroconverted already three weeks after pyrethroid treatment . The main reason for this lack of efficiency is unknown. According to the manufacturer, it may take 1–2 days after tick attachment until therapeutic concentration of pyrethroids in the tick is reached (Enemark, personal information). In addition, an earlier study indicated that sheep infested with ticks at the time of pour-on treatment lost their ticks within two days, which may indicate the kill time and the time for the tick to become detached . Furthermore, treatment of young lambs (≤4 weeks) with flumethrin (Bayticol®, Bayer) may be less effective due to lack of skin fat which in turn impairs the distribution of the compound (Enemark, personal communication). However, the present lambs were more than four weeks old when they were treated.
Acute cases of anaplasmosis were detected in both the control and the treatment group. Transmission of A. phagocytophilum from vector to host generally occurs between 24–48 hours after tick attachment [21–23], but the pathogen have been detected in the salivary glands of questing ticks and may therefore be transmitted immediately when ticks start feeding [24, 25]. The results indicate that Anaplasma can be transmitted by I. ricinus ticks in the time window between attachment and therapeutic concentration of pyrethroids. However, acaricide resistance cannot be ruled out and must be further elucidated. In the present study, resistance seems rather unlikely since pyrethroids have only been used for one grazing season.
No significant weight difference was observed between treated and untreated lambs or between seropositive and seronegative lambs. In contrast, earlier studies indicate a weight reduction of 1.4-3.8 kg in the autumn between seropositive and seronegative lambs [26, 27]. An earlier report also indicates that spring treatment of ewes and lambs with cypermectin pour-on produced 6% more lambs at weaning and the treated lambs were 1.5 kg heavier than untreated lambs . The reason for this discrepancy is unknown, but may be due to small sample size, variants of A. phagocytophilum involved, infection rate in ticks, other tick-borne pathogens, immune status and conditions of the lambs, management and environmental factors such as for instance variation in temperature and rainfall [4, 29].
The present study indicates that the effect of acaricides on the prevalence of A. phagocytophilum is limited. However, acaricide treatment may reduce the losses due to secondary infections. For instance, the incidence of lambs with lameness (tick pyaemia) or sudden death (Bibersteinia/Mannheimia septicaemia) has been reduced due to pour-on treatment . The reason for this effect is unknown. Ticks may carry several infectious agents or several variants of the same agent . Animals treated with acaricides may have reduced infection pressure due to a reduced number of ticks on each animal . For instance, microorganisms not already present in the salivary glands of newly attached ticks may be more vulnerable to hosts treated with acaricides. In order to examine the beneficial effect of acaricides on tick infested pastures, the distribution of A. phagocytophilum variants and other tick-borne pathogens in I. ricinus ticks has to be further elucidated.