Researchers at the Basque Institute of Agricultural Research and Development, Neiker-Tecnalia, have undertaken a study of ticks, tick-borne diseases and the reservoirs of tick-borne pathogens, which has confirmed the importance of continous surveillance programs to monitor these arthropods. T
he research was undertaken in the Basque Country and has revealed an increase in the abundance of ticks due to the rise in the average winter temperatures. It has also pointed out that the most abundant species in the area is Ixodes ricinus, considered to be the vector of various pathogenic agents. The study is part of the PhD thesis by veterinarian Jesϊs Fιlix Barandika Iza, who has confirmed the need to undertake epidemiological monitoring programmes for ticks in order to help the implementation of efficacious measures for reducing the risk of infections transmitted by ticks to people and animals.
The Neiker-Tecnalia research focused on aspects of seasonality of the tick populations and their relation to bioclimatic characteristics, and the prevalence of various zoonotic agents, as well as the role of micromammals as reservoirs for some of these pathogens.
No encephalitis virus
The most widespread species in the Basque Country is Ixodes ricinus. Unlike what happens in regions of Central Europe, the study revealed that this tick is not a vector of tick-borne encephalitis virus (TBEV) in the Basque Country or in La Rioja, possibly due to the fact that the climatic conditions of both zones do not favour TBEV maintenance in this environment. To arrive at this conclusion, the activity of ticks of the I. ricinus species was studied over three years in the two regions mentioned above, with the aim of determining if the necessary conditions exist for the presence of the virus, as well as assessing -- by molecular methods -- the presence of the virus in the collected ticks.
Besides TBEV, ticks are able to transmit numerous zoonotic bacteria, such as Borrelia burgdorferi (causative agent of Lyme's disease), Anaplasma phagocytophilum (human granulocytic anaplasmosis), Coxiella burnetii (Q Fever) or several Spotted Fever Group rickettsias (microorganisms that have features in common with viruses and bacteria). Hence, with the purpose of estimating the prevalence of these agents in ticks collected in the Basque Country, 691 adult ticks belonging to six different species were analysed using molecular methods.
With the exception of the Dermacentor reticulatus species, in which the presence of Rickettsia raoultii was detected in 50% of the samples, infection prevalence for the remaining pathogens was generally lower than in other European countries. Despite its low pathogenicity, R. raoultii has been occasionally associated with disease, and this is the first time it has been detected amongst ticks from the Basque Country.
As regards seasonality, winter was the season when the number of ticks infected with R. raoultii was higher, while summer represented the highest risk for A. phagocytophilum and B. burgdorferi infection. The transmission of C. burnetii in the Basque Country does not seem to be produced by bites from ixodid ticks, since C. burnetii DNA was only detected in 0.1 % of the ticks.
Different species depending on bioclimatic conditions
Tick species distribution, activity and abundance depend largely on bioclimatic conditions. Knowledge of all these aspects is essential from the sanitary point of view since different diseases are specifically transmitted by certain tick species, and each tick species has a particular seasonality.
In order to study the effect of the climate and vegetation on the distribution of ticks in the Basque Country, samples of these arthropods were monthly sampled for several years. The results were compared with those obtained by researchers at the Veterinary Faculty in Madrid from samplings performed in Central Spain (Madrid-Toledo) at the same time and using the same methodology. In total 12 different species of ixodid ticks were identified, with a predominance in the North of those species adapted to live in wet environments (I. ricinus and Haemaphysalis punctata); while in the Centre species that usually occupy drier habitats predominated (Hyalomma lusitanicum and Dermacentor marginatus).
The data from the Basque Country was compared to that obtained a decade previously in the same zone, with an observed increase in tick abundance over time -- a fact associated with higher winter temperatures registered during this latter period. An established population of Haemaphysalis concinna was detected for the first time, a species which did not appear to be present in the Basque Country previously.
Micromammals as reservoirs of bacteria
Finally, to assess the possible role of micromammals as reservoirs for these tick-borne bacteria, 253 domestic and wild micromammals were investigated by Neiker-Tecnalia researchers. The results showed for the first time that the shrew species Crocidura russula and Sorex coronatus can act as possible reservoirs of A. phagocytophilum and B. burgdorferi, respectively. The presence of C. burnetii DNA was also detected in domestic and wild mice. These results point out the important role of micromammals in our region as reservoirs of tick-borne bacterial pathogens.
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