Selection assisted by a BoLA-DR/DQ haplotype against susceptibility to bovine dermatophilosis

Bovine dermatophilosis is a severe skin infection of tropical ruminants inducing a severe loss in productivity and a 15% mortality rate. This disease is caused by the actinomycete bacterium Dermatophilus congolensis associated with the tick Amblyomma variegatum. Currently there are no prospects for a vaccine, and acaricide or antibiotic control is hampered by the development of chemoresistance. Animal breeders have observed that dermatophilosis susceptibility seems to be determined genetically, and we previously identified a BoLA-DRB3-DQB class II haplotype marker for high (R2 = 0.96) susceptibility to the disease. With this marker, we developed a successful eugenic selection procedure for zebu Brahman cattle in Martinique (FWI). Over a period of five years, a marked reduction in disease prevalence, from 0.76 to 0.02 was achieved, and this low level has been maintained over the last two years. The selection procedure, based on a genetic marker system targeting the highly polymorphic BoLA locus, eliminates only those individuals which are at the highest risk of contracting the disease. In the present work, we discuss the properties of this system, including the "heterozygote advantage" and the "frequency dependence" theories, and examine their involvement in the biological mechanisms at the host/pathogen interface. We speculate on the exact role of the MHC molecules in the control of the disease, how the natural selection pressure imposed by the pathogens selectively maintains MHC diversity, and how our results can be practically applied for integrated control of dermatophilosis in developing countries

Colonization of the Mediterranean Basin by the vector biting midge species Culicoides imicola: an old story

Understanding the demographic history and genetic make-up of colonizing species is critical for inferring population sources and colonization routes. This is of main interest for designing accurate control measures in areas newly colonized by vector species of economically important pathogens. The biting midge Culicoides imicola is a major vector of Orbiviruses to livestock. Historically, the distribution of this species was limited to the Afrotropical region. Entomological surveys first revealed the presence of C. imicola in the south of the Mediterranean basin by the 1970's. Following recurrent reports of massive bluetongue outbreaks since the 1990s, the presence of the species was confirmed in northern areas. In this study, we addressed the chronology and processes of C. imicola colonization in the Mediterranean basin. We characterized the genetic structure of its populations across Mediterranean and African regions using both mitochondrial and nuclear markers, and combined phylogeographical analyses with population genetics and approximate Bayesian computation. We found a west/east genetic differentiation between populations, occurring both within Africa and within the Mediterranean basin. We demonstrated that three of these groups had experienced demographic expansions in the Pleistocene, probably because of climate changes during this period. Finally, we showed that C. imicola could have colonized the Mediterranean basin in the late Pleistocene or early Holocene through a single event of introduction; however we cannot exclude the hypothesis involving two routes of colonization. Thus, the recent bluetongue outbreaks are not linked to C. imicola colonization event, but rather to biological changes in the vector or the virus

Exploiting the heterogeneous drop-off rhythm of Amblyomma variegatum nymphs to reduce pasture infestation by adult ticks

International audienceTick distribution depends on the drop-off rhythm of the previous stage and on the suitability for tick survival of the environment where they are disseminated. Studies were implemented in Burkina Faso to assess detachment pattern of engorged Amblyomma variegation nymphs. Experiments were carried out with naturally infested cattle kept in a paddock or monitored when grazing community pasture. In the pasture, 80% of the nymphs detached between 14.00 h and 17.00 h while less than 25% did so in the paddock. Further investigation was implemented to assess whether the density of adult ticks might be modified by herd management. During the early dry season, zebus grazed in 4 plots fenced in natural savannahs and fallows. Two of the plots were used in the morning and the two others in the afternoon. Six months later, zebus were put in these plots, in turn, on 9 occasions. The number of A. variegation adults picked up by the cattle in each plot was highly variable: they captured more ticks in the plots installed on good lush savannah and 3-fold more ticks in those where the herd had grazed in the afternoon during the previous dry season. An integrated tick control strategy taking these results into account is proposed

Survival and moulting of Amblyomma variegatum nymphs under cold conditions of the Malagasy highlands

International audienceAlthough Amblyomma variegatum is now regularly recorded up to 1600 m in altitude in the Malagasy highlands, where it was previously reported not to persist without a constant supply of ticks introduced from lower infested regions, some parts of the highlands remain tick-free. Studies were carried out to verify whether the cold climate prevailing in these areas in June-September could prevent the survival and moulting of nymphs, the tick life stage present in the environment at this period. Cohorts of engorged A. variegatum nymphs were released from June to August in six different sites (three in 2010, altitudes 1200-1415 m; three in 2011, altitudes 1585-1960 m) which were reported to be either tick-infested (two in 2010, one in 2011) or tick-free. The ticks were placed in cages driven into the soil and open at the bottom so that they could hide in the soil or root network. Of the 1975 nymphs released in 2010 and the 1494 released in 2011, 86% and 85% were recovered, respectively. Twenty to 23% of the recovered ticks were dead, and some of them were obviously predated; predation also likely contributed to the disappearance of the non-recovered ticks. When the rainy season started in October, 59% of the newly moulted adults were still alive in the cages. The moulting period lasted up to 20 weeks, depending on the site and release period. As verified in 2011, unfed nymphs could also survive the cold season. Various A. variegatum life stages are thus able to survive the adverse cold and/or dry seasons: unfed nymphs, engorged nymphs in developmental diapause, moulted adults in behavioural diapause as observed previously. Strong variation in mortality and recovery rates was observed between cages, highlighting the importance of the micro-environment and micro-climate for tick survival. The minimum temperature recorded in the field sites varied from 1.1 degrees C to 6.8 degrees C, but the tick-free sites were not the coldest ones; they were, however, those for which the temperature remained below 10 degrees C for the longest time over the study period. Recovery and mortality rates in the tick-free sites were similar to those of the tick-infested sites: the temperatures recorded during the study periods did not prevent ticks from surviving and moulting although it did delay the metamorphosis. Low temperature alone can therefore not explain the persistence of tick-free areas in the highlands. To further monitor survival, cohorts of engorged nymphs were also maintained in an incubator at 3.6 degrees C, 6.2 degrees C or 12.8 degrees C. More than 50% mortality was observed after 6 days at 3.6 degrees C, and after 15 days at 6.2 degrees C, whereas 18 days at 12.8 degrees C only delayed moulting. The collected survival, moulting and climatic data presented in this study should help to develop a predictive model to assess the distribution of A. variegatum according to climate characteristics

Climate change alone cannot explain altered tick distribution across Europe: a spotlight on endemic and invasive tick species.

International audienceThe effect of climate on the evolution of tick populations remains difficult to disentangle from other possible causes and undoubtedly varies depending on the region concerned and local tick species. Large-scale, long-term monitoring is, therefore, necessary to accurately assess climatic impact on tick populations. Climate change can alter tick populations, either indirectly by affecting vertebrate host populations or directly by increasing or decreasing their numbers. These ectoparasites, and in particular hard ticks, spend almost their entire life cycle in the external environment, thus climatic conditions influence their activity, viability and distribution. This expert opinion aims to illustrate the impact of climate change, and its association with other variables, on the distribution and abundance of tick populations in Europe using Ixodes ricinus and Hyalomma marginatum as typical examples of endemic and invasive species, respectively

Climate change alone cannot explain altered tick distribution across Europe: a spotlight on endemic and invasive tick species.

International audienceThe effect of climate on the evolution of tick populations remains difficult to disentangle from other possible causes and undoubtedly varies depending on the region concerned and local tick species. Large-scale, long-term monitoring is, therefore, necessary to accurately assess climatic impact on tick populations. Climate change can alter tick populations, either indirectly by affecting vertebrate host populations or directly by increasing or decreasing their numbers. These ectoparasites, and in particular hard ticks, spend almost their entire life cycle in the external environment, thus climatic conditions influence their activity, viability and distribution. This expert opinion aims to illustrate the impact of climate change, and its association with other variables, on the distribution and abundance of tick populations in Europe using Ixodes ricinus and Hyalomma marginatum as typical examples of endemic and invasive species, respectively

Phylogeography of <em>Amblyomma variegatum</em> (Acari: Ixodidae), the main vector of <em>Ehrlichia ruminantium</em>: preliminary results

International audienc

2. Biologie des tiques

National audienc

Impacts majeurs des tiques en santé animale en Outre-mer

International audienc