There are two broadly accepted definitions of vectors. The first considers a vector an arthropod that is responsible for the transmission of a pathogen among vertebrate hosts [33], while the World Organisation for Animal Health (OIE) gives a more stringent definition, that is a living organism capable not only to transmit the pathogen but also to disseminate the related disease [34].
On those bases, the gathered results provide robust evidence of transmission of S. Gallinarum mediated by D. gallinae, thus supporting the vectorial competence of the mite. The investigation also proved that mites were able to transmit not only the pathogen but also the disease, as no differences were found in terms of incidence and severity of FT between infected and infested groups. Since all experimental procedures were carried out in isolators, D. gallinae was the only contact point between infected (A and B) and infested (C and D) groups, the poultry red mite was the only possible route for the introduction of FT. As most of the diseased hens in the infested group developed signs of FT almost simultaneously, 8 and 9 days after the exposure to mites, it is possible to assume that the contribution of the horizontal transmission in spreading FT within the infested groups was minimal. It is not possible to exclude that horizontal transmission occurred, but this could be a secondary event, which took place after the initial transmission from mites.
The capability of D. gallinae to transmit the disease among hens implies that it may inoculate a high enough dose of S. Gallinarum. Several studies were carried out to establish the infectious dose, and they reported that no less than 104 CFU of S. Gallinarum must be administered to trigger FT [35, 36]. Therefore, the number of pathogenic cells that D. gallinae may harbor and inoculate becomes pivotal. In the current trial, as well as in previous studies [20], mites were found to harbor a highly variable amount of S. Gallinarum. This may be explained by considering that mites ingest about 0.3 mg of blood [37] and that the number of bacterial cells in such a small quantity may be far from uniform. However, the estimated pathogen load was quite low after the first infestation, insomuch that, by combining the pathogen load and the EIR, it is reasonable to assume that hens of groups C and D came in contact with a small amount of S. Gallinarum. Nonetheless, such a low quantity was likewise capable to cause disease in most of animals.
Several factors may account for such an apparent inconsistency. First, there is the possibility that the algorithm used to calculate the IR might underrate that parameter when aliquot size is too large, having been devised for mosquitoes, which are usually investigated in smaller pools, or even individually [38]. Then, the amplification factor should also be considered, since the present study, as well as previous investigations [20], revealed that the concurrent presence of D. gallinae, S. Gallinarum, and chickens may increase the circulation of S. Gallinarum among hens and the consequent increment in the pathogen load of mites. Finally, the transmission route from mites to hosts should be analyzed. To our knowledge, the infectious dose of S. Gallinarum has been calculated by orally administering specific amounts of bacterial cells, while no data are available for the introduction of the pathogen into the blood stream. In the latter case, since the bacterial cells do not have to transit through the gastric barrier, it is plausible that a lower dose could be enough for the pathogen to reach the target organs and, thus, to cause disease. The herein presented results do not establish whether D. gallinae transmits S. Gallinarum while biting or by being ingested with hens’ picking. However, the capability to transmit FT despite the low EIR indirectly suggests that S. Gallinarum inoculum may result from the mite’s feeding behavior, but specific investigations should be carried out to confirm this point.
Whatever the route, transmission of FT mediated by D. gallinae may have a deep impact on poultry systems. It is known that, in case of severe infestation, up to 50,000 mites can attack a host each night [39], therefore the EIR can reach very high values. Additionally, weighing further biological features of D. gallinae, the VC reached the remarkable value of 73.79. That is due to the high number of poultry red mites that may attack each host, the high mite survival potential and high feeding rate. To our knowledge, no data are available about the VC of D. gallinae or other mite species, therefore a direct comparison seems not possible. However, the datum becomes remarkable when compared to the VC of mosquitoes in transmitting vector-borne pathogens such as Zika virus, dengue virus and chikungunya virus. In those cases, VC was 0.35 [40], 18.60 and 13.99 [both recalculated from 32], respectively, more than 30 times lower than the value calculated for D. gallinae. However, it should be underlined that the large majority of those models have been developed for insects or ticks, which have population dynamics and biology that can differ substantially from mites. Therefore, the development and implementation of statistical and mathematical tools for the assessment and quantification of vectorial properties of mites are imperative in building understanding of the great volume of data that has been produced about mites.
On the other side, more concrete data are available to classify D. gallinae as a biological or a mechanical vector. The difference resides in the possibility for the pathogen to replicate or not during the vector infection, respectively [41], despite authoritative Authors consider this definition as inadequate [42]. Actually, S. Gallinarum was early found to persist in association with D. gallinae for a long time, even in absence of hosts [19]. The present study, consistently with the recent one [20], confirmed such a subsistence but without detecting an increase in the pathogen load in the absence of a host. Those observations strongly suggest that D. gallinae could influence the life cycle of S. Gallinarum that may at least survive when associated with the poultry red mite, and this would include the arthropod among the biological vectors.
The survival potential of S. Gallinarum might be enhanced by the fact it can survive within D. gallinae, as demonstrated by the positivity of the formalin-washed aliquots. This fact could contribute to protecting the pathogen from adverse environmental conditions such as dehydration, excessive level of ammonium compounds, lack of nutrients, and even the contact with antimicrobial substances.
Those considerations have important outcomes when transposed into the field. It has been already postulated that the infection of D. gallinae could contribute to the reduction in the efficacy of antibiotic therapies during foci of FT [20], since the mites may offer a drug-free environment where S. Gallinarum can survive. The capability of mites to re-transmit the infection to chickens, along with the ubiquitous diffusion of D. gallinae, may provide an additional explanation for the limited efficacy of antibiotic treatments of FT, which has often been described [43].
Altogether, those data reinforce the need for a comprehensive strategy for the control of both infestations and infections. The importance of an integrated pest management approach for controlling D. gallinae infestations has been widely proposed. The approach consists of a coordinated application of good hygiene practices, effective protection against external contaminations, optimization of the chickens’ population density, proper farm design, and the proper administration of the most effective authorized and available acaricide drugs, including a recently introduced drinking water application of fluralaner that offers the potential for improved control of the poultry red mite [44, 45]. A similar approach should be advisable to control and potentially, eradicate FT in affected poultry farms. The presence of D. gallinae may not only provide shelter for S. Gallinarum to avoid the contact with antibiotics, but it also reduces or eliminates the effectiveness of sanitary breaks, considering the high persistence of the pathogen in association with the mite. Therefore, considering current knowledge, the treatment of FT cannot obviate the need for simultaneous actions to remove D. gallinae infestations.