Immediate efficacy and persistent speed of kill of a novel oral formulation of afoxolaner (NexGard^TM) against induced infestations with Ixodes ricinus ticks

Ticks are major ectoparasites of humans and animals. In addition to their direct pathogenic role, they are also known as vectors for a wide range of pathogenic microorganisms. Ixodes ricinus is the most common tick species occurring in Europe. It is the vector of various vector borne pathogens including Borrelia burgdorferi (the agent of Lyme disease), Anaplasma phagocytophilum (the agent of granulocytic ehrlichiosis) or the virus responsible of tick-borne Encephalitis [1, 2]. The geographic distribution of the tick species is expanding in Europe [3] as a result of climate changes or increasing movements of people travelling abroad with their pets [4] and it represents a major threat for human and animal health. As an example, a recent study demonstrated that almost 60% of the ticks collected from humans in Italy were I. ricinus ticks and that 21% of them were carrying at least one pathogenic microorganism [5].

Canine tick-borne diseases are among the most prevalent infectious diseases in the dog population in Europe [6]. The main pathogens transmitted to dogs by I. ricinus ticks are bacteria (B. burgorferi sensu lato, A. phagocytophilum) but a few cases of Ixodes-borne viral diseases (TBE-virus, Louping-ill virus) have also been described in dogs in Europe [1]. Protecting dogs from tick bites remains a primary preventive measure against those diseases. Numerous acaricides have been developed for the protection of dogs against ticks which include topical solutions as spray, collars, or spot-on and, more recently, oral formulations. Due to delayed transmission of many tick-borne pathogens [7], a rapid onset of action of an acaricide is likely to have an impact on pathogen transmission [8] and this feature brings an important added value to any acaricidal solution. According to the standard determined by pharmaceutical regulation worldwide, an anti-tick product for dogs should give a minimal activity of 90% acaricidal efficacy, characterized by a reduction in tick counts 48 h after treatment and the prevention of re-infestation [9]. Faster acting acaricidal activity, i.e. within the first hours post treatment or subsequent infestation, is interesting to evaluate, especially from a perspective of reduction of the risk of diseases transmission.

Orally administrated afoxolaner has demonstrated a persistent efficacy against the main species of ticks infesting dogs with >90% efficacy 48 hours post treatment or infestation for one month after treatment against I. ricinus, I. scapularis, Dermacentor reticulatus, D. variabilis, Rhipicephalus sanguineus, Haemaphysalis longicornis[10–14].

The objective of the present study was to assess immediate efficacy of this novel oral formulation of afoxolaner (NexGard^TM, Merial) against an existing I. ricinus tick infestation and its persistent preventive speed of kill against subsequent infestations with I. ricinus ticks within the first hours post- treatment or subsequent re-infestation.

The study was conducted using a negative controlled and blinded design, with dogs randomly allocated to two groups of 6 dogs (treated and control). Prior to treatment, all dogs underwent a physical examination conducted by a veterinarian and daily health observations by trained personnel to ensure that they were healthy. To detect the presence or absence of any treatment-related or unrelated health abnormality or adverse event, health observations were conducted at hourly intervals for four hours after treatment and daily thereafter throughout all studies.

Treated dogs did not show any adverse effect related to treatment.

The geometric mean number of live female ticks ranged between 14.5 and 24.4 for dogs in the control Group, corresponding to a minimum retention rate of 37%, sufficient to make a valid assessment of tick efficacy [15].

The persistent speed of kill showed a significant reduction in tick counts 12 hours post-infestation when compared to the controls at D7 and D21 (p < 0.05), with efficacy of 76.6%, 41.9%, 36.9% and 38.5% respectively at D7, D14, D21 and D28. The efficacy at 24 hours post-infestation ranged from 91.2 to 100% for each challenge throughout the whole month.

When orally administered, afoxolaner is systematically distributed in the body of the treated dog and plasma concentrations peak quickly within 2 to 6 hours [18]. The immediate efficacy against existing tick infestation is therefore rapid and this is illustrated by the onset of action on experimental infestation with I. ricinus ticks which reached >93.4% efficacy 12 hours post-treatment and 100% 24 hours post-treatment.

Attachment and ingestion of material containing afoxolaner is likely required to affect ticks applied after treatment. Although ticks typically start to effectively take blood only 24 hours post-attachment, with the first hours being dedicated to the attachment process [19], fluid exchanges between the tick and the host still occur during the very first phase of attachment, as ticks will ingest a limited quantity of plasma during this phase [20]. The present study suggests that the quantity of afoxolaner in contact with the tick during this fluid exchange is enough to initiate the killing of the ticks. The onset of action is observed within 12 hours and the efficacy is completed at 24 hours post-infestation for the whole month following NexGard^TM administration.

These results corroborate the hypothesis that afoxolaner rapidly kills ticks as suggested in a recent study that demonstrated the ability of orally administrated afoxolaner to block the transmission of Babesia canis by Dermacentor reticulatus ticks to dogs with 100% of protection of the treated dogs [21]. The level of protection against disease transmission depends on the time required for the transmission of the responsible pathogen: it is probably lower against pathogens in which transmission starts earlier, as soon as salivation begins, within a few minutes or hours of attachment such as it has been demonstrated for viruses [22] or suggested for Rickettsia conorii[23], but is expected to be high against pathogens requiring more than 24 hours of attachment to be transmitted such as Borrelia burgdorferi[24] or Anaplasma phagocytophilum[25].

Dynamics and pattern of seasonal activity of ticks have been shown to be changing dramatically in Europe due to both climate changes and non-climatic factors [3]. The risk of I. ricinus tick infestation and tick-borne disease transmission is not limited to Spring and Autumn [26]. It can even be very high in winter in some areas of Southern Europe [27]. For an optimal protection of the dogs, the period of treatment has to be adapted to each epidemiological situation.

In addition to its demonstrated acaricidal efficacy, afoxolaner, administrated orally in NexGard^TM formulation, has a quick onset of action against I. ricinus, allowing a rapid elimination of the existing infestations and preventing the establishment of further infestations for a month.