Dogs are a common target of tick-borne diseases because of their specific natural behavior of moving through the vegetation during walking, leading to a high incidence of tick infestation. Intense tick infestation in dogs might be assumed to result in a high number of infections, although this could not be confirmed in this study. Only five of the nine most highly infested dogs showed a humoral immune response during the year, which might be explained by a weak individual immune response. Maybe also an above-average alertness and ability of those nine dog owners to detect and remove ticks reduced the infection rate when ticks were removed before transmission occurred. On the other hand, dogs without confirmed tick infestation also seroconverted during the study period. Consequently, the total number of tick per dog and the detection of infections with tick-borne pathogens in the examined dogs did not correlate. Although unexpected, the number of missed ticks and the undetermined number of attached but dead ticks, unable to transmit especially B. canis and B. burgdorferi, for which transmission starts after a feeding time of more than 24 hours, may be the reason for this result.
Irrespective of the total number of ticks per dog, a high risk for contracting a tick-borne infection was demonstrated; according to the presented data, dogs have a high risk to become infected with at least one pathogen during their life. Consequently it is mandatory to apply suitable and effective prophylactic measures against tick infestations and tick-borne infections in dogs, starting early in life.
The risk for the examined dogs to become infected by one of the tested pathogens ranged from 6.9% (B. canis) to 29.1% (A. phagocytophilum), the total risk was calculated to be 54.0%. This high number is in contrast to the low number of clinical symptoms (three cases of canine babesiosis) in these dogs, even in multiply infected dogs (Table 2), leading to the conclusion that symptomatic tick-borne diseases have a low incidence.
The seroprevalences in the dogs in this study are in the range of other studies from Central Europe for TBE virus (25% in Austria;), B. canis (5.7% in Hungary;), B. burgdorferi (28.6% in the Czech Republic;), and A. phagocytophilum (56.5% in Austria;), so that the selected area of investigation can be considered as representative for a rural Central European location.
PCR examinations were all negative for B. canis and for A. phagocytophilum. This is not surprising as the first and the third examination were performed in winter, when tick infestation is rare and therefore also the presence of pathogens in the dog’s blood during acute infection was not expected. PCR of blood samples from 3 dogs with acute babesiosis could not be performed as private veterinarians only sent stained blood smears for verification.
There was no correlation between the number of acaricide applications and infection, although a reduction of transmission events is the major reason to apply such preventive measure[13, 14]. The highest frequency of immune responses in the dogs were found for A. phagocytophilum which is transmitted within the first 24 hours of infestation and B. burgdorferi s. l. which has the highest prevalence in ticks. This outlines the major requirements for spot on preparations: the substances should repel ticks to avoid infestation in large numbers and it should kill the ticks as early as possible, best even before the tick starts feeding.
Spot on application and the type of drug did not influence the total number of infections per dog, as there was no significant difference of this parameter in the three groups. Between permethrin and fipronil + S-methoprene no influence on the total number of infections per dog could be found, although Endris et al. documented a significantly different efficacy in terms of reducing tick infestation, which has also been confirmed in this study.
When testing the efficacy of acaricidal drugs under natural conditions one has to consider several possible influences: the owner’s ability and alertness to apply the spot on to the skin, to search and remove ticks, and different environmental and behavioral conditions for the dogs, such as walking distance and areas with different tick densities, as well as exposure to sunlight or water, including swimming and rain. Both permethrin and fipronil + S-methoprene are described as efficacious in reducing infestation with Rhipicephalus sanguineus, the brown dog tick, under natural conditions, although the effect was less than 100% especially against nymphs. Poor owner compliance in regular application and inappropriate application of the spot on may be reasons for the poor performance of the two drugs in this study, as the application rate in this study was low (on average 3.03 to 3.40 treatments per year), and many owners reported that they applied the spot on when ticks were already observed. Poor owner compliance has also been mentioned and summed up by Stanneck et al.. In the current study 2/3 of dog owners were told to use a spot on acaricide, and they applied it in different frequencies, which is comparable to a study where 74% of owners used a tick and flea control product and only 61% of those used these products year around.
To overcome this irregular application, caused by non-compliance of the owners, leading to this poor protection status, long lasting measures like slow release collars could be a solution. Acaricidal collars provide high efficacy in terms of preventing tick infestation and pathogen transmission and the effect lasts several months compared to several weeks of spot on formulation with similar efficacies. The repellent efficacy of an imidacloprid/flumethrin collar was demonstrated to be faster than the minimal transmission times for Borrelia spp. and A. phagocytophilum, which is comparable to certain spot on formulations.
In 11 dogs (2 untreated, 4 permethrin- and 5 fipronil + S-methoprene-treated) one or more TBD-infections were detected, although owners did not report tick infestation throughout the year. It must be assumed that the owners missed several ticks, which might be explained by the variable ability of owners to search and remove ticks. Identifying ticks visually may provide a better chance of early removal when attached ticks are still small in size, whereas searching ticks by palpating the coat and skin may only result in finding engorged ticks that had already transmitted pathogens. Juvenile tick stages may have been found by some owners but not identified as ticks and therefore have been ignored. Search and removal of ticks are thus additional measures to reduce the events of transmission but cannot prevent infections completely, especially in dog owners with limited ability to find and remove ticks.
The present study provides data on the natural open field efficacy of spot on acaricides when applied by dog owners. The fact that dog owners did not apply the acaricide regularly according to the manufacture’s guidelines is certainly the major reason for the poor performance of prevention measures in this trial. Therefore the repellent and acaricidal effect of both drugs was demonstrated only by the temporary influence on tick infestation but not by any influence on infection incidence during the year. Efficacy studies on acaricidal drugs under laboratory conditions result in a maximum positive measureable effect on tick killing and prevention of pathogen transmission[12, 14, 30], although Estrada-Pena and Venzal Bianchi also described an insufficient prevention of the transmission of Rickettsia conorii after application of fipronil + S-methoprene and permethrin; only amitraz was able to reduce transmission effectively. Especially the permethrin spot on, used in this trial, has been demonstrated to have a proven high efficacy in the protection against TBD transmission when administered correctly either in a regular treatment regime or within the claimed efficacy period. The proven efficacy of such acaricidal substance has a considerably reduced effect when applied incorrectly.