Karine Huber

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 1970s. 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.

MLST scheme of Ehrlichia ruminantium: genomic stasis and recombination in strains from Burkina-Faso.

Heartwater, caused by the intracellular bacterium Ehrlichia ruminantium, is a major tick-borne disease of livestock in Africa also introduced in the Caribbean. The main problem encountered with the control of this disease is the lack of efficient vaccine in the field. This is thought to be related to the high genetic diversity of strains circulating in a same area. A set of eight circulating strains was isolated from a herd of cows in a small locality in Burkina-Faso and analyzed along with two reference strains, i.e. ERGA and ERWO, for which full-length genome was available. A MLST analysis was developed based on the genes gltA, groEL, lepA, lipA, lipB, secY, sodB and sucA. Phylogeny analysis was conducted both on concatenated MLST loci and on each individual locus. This showed differing phylogenies for each individual target gene. Most of the recorded polymorphism was borne by three strains: 331, 469 and 623. The neutrality hypothesis could not be rejected. Recombination and linkage disequilibrium were shown to have occurred. A core of seven strains displayed little polymorphism and signs of most likely ancient recombination events. The two reference strains, one from the Caribbean separated from west African strains three centuries ago and another one isolated in South Africa, were very closely related to the core strains whereas the three differing strains displayed recombination and most of the parcimony informative sites. These data suggest that some strains are in genomic stasis, as expected for intracellular parasites, while others emerge in the same area with DNA polymorphism. This work also shows that the MLST scheme developed can discriminate between these two kinds of strains.

Clonal origin of emerging populations of Ehrlichia ruminantium in Burkina Faso

Cowdriosis or heartwater is a major tick-borne disease on ruminants in Africa and the Caribbean. The causative agent is Ehrlichia ruminantium, an intracellular bacterium. Development of vaccines against heartwater has been hampered the limited efficiency of vaccine in the field, thought to be a consequence of the high genetic diversity of strains circulating in a same area. A sampling scheme was set to collect ticks over 2 years in a delimited area and well identified flock. Prevalence was low at about 3%. A set of 37 strains was considered for MLST analysis along with two reference strains, i.e. ERGA and ERWO, for which full-length genome was available, using a previously described scheme based on the genes gltA, groEL, lepA, lipA, lipB, secY, sodB and sucA. Two populations were identified both with limited genetic variability but with differing evolutionary patterns. Population 1 is in genomic stasis, in agreement with the paradigm for intracellular bacteria. The two reference strains, one from the Caribbean separated from West African strains three centuries ago and another one isolated in South Africa, belong to Population 1. Population 2 is on expansion following a recent clonal emergence from Population 1. The founder strain was identified as strain 395. Strain 623 displays a particularly high rate of mutations in groEL. Owing to the chaperone function of GroEL, this might indicate another clonal emergence under way. This work brings further insight in the genomic plasticity of E. ruminantium and its impact on vaccine strategy.

Range expansion of the Bluetongue vector, Culicoides imicola , in continental France likely due to rare wind-transport events

The role of the northward expansion of Culicoides imicola Kieffer in recent and unprecedented outbreaks of Culicoides-borne arboviruses in southern Europe has been a significant point of contention. We combined entomological surveys, movement simulations of air-borne particles, and population genetics to reconstruct the chain of events that led to a newly colonized French area nestled at the northern foot of the Pyrenees. Simulating the movement of air-borne particles evidenced frequent wind-transport events allowing, within at most 36 hours, the immigration of midges from north-eastern Spain and Balearic Islands, and, as rare events, their immigration from Corsica. Completing the puzzle, population genetic analyses discriminated Corsica as the origin of the new population and identified two successive colonization events within west-Mediterranean basin. Our findings are of considerable importance when trying to understand the invasion of new territories by expanding species.

New data regarding distribution of cattle ticks in the south-western Indian Ocean islands

Recent studies have produced new insight into the origin and distribution of some cattle ticks in the south-western Indian Ocean islands. Rhipicephalus appendiculatus, introduced from Tanzania in 2002, is now well established on Grande Comore but has not yet reached the other islands of the archipelago (Mohéli, Anjouan and Mayotte). Only one of the two clades identified in Africa has settled so far. Amblyomma variegatum, which was not supposed to be able to persist in the Antananarivo region (1300 m) nor in other Malagasy regions of high altitude without regular introductions of ticks by infested cattle, is now endemic as a general rule up to 1600 m although other regions of lower altitude (1400 m) are still free of the tick. This species remains confined in a small area of the west coast on La Reunion Island. On the contrary, Hyalomma dromedarii could not settle on Madagascar where it was introduced in 2008 and Rhipicephalus evertsi evertsi is not yet present in Grande Comore despite regular introductions by infested cattle from Tanzania. A phylogeographic approach has been carried out at an intra-specific level for A. variegatum. This study has led to the identification of two main lineages, one covering all species distribution and one restricted to East Africa and the Indian Ocean area. These two lineages are in sympatry in Madagascar where a high genetic diversity has been described, whereas a lower genetic diversity is observed on other islands. These results seem to agree with the historical data concerning the introduction of the tick in the Indian Ocean area.

A preliminary study of natural and experimental infection of the lesser mealworm Alphitobius diaperinus (Coleoptera: Tenebrionidae) with Histomonas meleagridis (Protozoa: Sarcomastigophora)

Histomonas meleagridis is a protozoan parasite causing histomoniasis (histomonosis), a disease of gallinaceous fowl. In order to determine whether the lesser mealworm, Alphitobius diaperinus, is capable of harbouring H. meleagridis, the presence of the parasite was tested by polymerase chain reaction in lesser mealworms collected in the field. Parasite DNA was detected in two larvae from two farms undergoing an outbreak of histomoniasis. Insects were also artificially infected, killed after incubation and analysed by polymerase chain reaction for the presence of parasite DNA. After 4 days, two larvae (out of 20) remained positive. In another experimental infection to investigate the viability of histomonads in the larvae, living parasites were detected in five of 20 larvae 4 days after infection. These results indicate that although A. diaperinus can become infected with H. meleagridis it appears to have a low susceptibility to infection and would probably not be a major route of contamination between flocks.

Modelling population dynamics and response to management options in the poultry red mite Dermanyssus gallinae (Acari: Dermanyssidae).

The poultry red mite Dermanyssus gallinae is a major pest and widespread ectoparasite of laying hens and other domestic and wild birds. Under optimal conditions, D. gallinae can complete its lifecycle in less than 10 days, leading to rapid proliferation of populations in poultry systems. This paper focuses on developing a theoretical model framework to describe the population dynamics of D. gallinae. This model is then used to test the efficacy and residual effect of different control options for managing D. gallinae. As well as allowing comparison between treatment options, the model also allows comparison of treatment efficacies to different D. gallinae life stages. Three different means for controlling D. gallinae populations were subjected to the model using computer simulations: mechanical cleaning (killing once at a given time all accessible population stages), sanitary clearance (starving the mite population for a given duration, e.g. between flocks) and acaricide treatment (killing a proportion of nymphs and adults during the persistence of the treatment). Simulations showed that mechanical cleaning and sanitary clearance alone could not eradicate the model D. gallinae population, although these methods did delay population establishment. In contrast, the complete eradication of the model D. gallinae population was achieved by several successive acaricide treatments in close succession, even when a relatively low treatment level was used.

Strong evidence for the presence of the tick Hyalomma marginatum Koch, 1844 in southern continental France.

Hyalomma ticks can transmit several human and animal pathogens in Eurasia and Africa. Interest in Hyalomma marginatum has increased since the recent (re)emergence of Crimean-Congo Hemorrhagic fever in the Palearctic region. Until now, continental France has been considered free of this tick species. Nevertheless, the existence of incomplete and occasionally incorrect records has maintained confusion about its status. Based on several tick sampling campaigns conducted on horses and birds from 2007 to 2016, we provided very strong evidence for the presence of reproducing populations of H. marginatum in parts of southern continental France. We also confirmed the introduction of immature developmental stages of H. marginatum, as well as H. rufipes, into France probably through trans-Mediterranean bird migrations.

Recombination Is a Major Driving Force of Genetic Diversity in the Anaplasmataceae Ehrlichia ruminantium

The disease, Heartwater, caused by the Anaplasmataceae E. ruminantium, represents a major problem for tropical livestock and wild ruminants. Up to now, no effective vaccine has been available due to a limited cross protection of vaccinal strains on field strains and a high genetic diversity of Ehrlichia ruminantium within geographical locations. To address this issue, we inferred the genetic diversity and population structure of 194 E. ruminantium isolates circulating worldwide using Multilocus Sequence Typing based on lipA, lipB, secY, sodB, and sucA genes. Phylogenetic trees and networks were generated using BEAST and SplitsTree, respectively, and recombination between the different genetic groups was tested using the PHI test for recombination. Our study reveals the repeated occurrence of recombination between E. ruminantium strains, suggesting that it may occur frequently in the genome and has likely played an important role in the maintenance of genetic diversity and the evolution of E. ruminantium. Despite the unclear phylogeny and phylogeography, E. ruminantium isolates are clustered into two main groups: Group 1 (West Africa) and a Group 2 (worldwide) which is represented by West, East, and Southern Africa, Indian Ocean, and Caribbean strains. Some sequence types are common between West Africa and Caribbean and between Southern Africa and Indian Ocean strains. These common sequence types highlight two main introduction events due to the movement of cattle: from West Africa to Caribbean and from Southern Africa to the Indian Ocean islands. Due to the long branch lengths between Group 1 and Group 2, and the propensity for recombination between these groups, it seems that the West African clusters of Subgroup 2 arrived there more recently than the original divergence of the two groups, possibly with the original waves of domesticated ruminants that spread across the African continent several thousand years ago.

Spatio-temporal genetic variation of the biting midge vector species Culicoides imicola (Ceratopogonidae) Kieffer in France.

BackgroundIntroduction of vector species into new areas represents a main driver for the emergence and worldwide spread of vector-borne diseases. This poses a substantial threat to livestock economies and public health. Culicoides imicola Kieffer, a major vector species of economically important animal viruses, is described with an apparent range expansion in Europe where it has been recorded in south-eastern continental France, its known northern distribution edge. This questioned on further C. imicola population extension and establishment into new territories. Studying the spatio-temporal genetic variation of expanding populations can provide valuable information for the design of reliable models of future spread.MethodsEntomological surveys and population genetic approaches were used to assess the spatio-temporal population dynamics of C. imicola in France. Entomological surveys (2–3 consecutive years) were used to evaluate population abundances and local spread in continental France (28 sites in the Var department) and in Corsica (4 sites). We also genotyped at nine microsatellite loci insects from 3 locations in the Var department over 3 years (2008, 2010 and 2012) and from 6 locations in Corsica over 4 years (2002, 2008, 2010 and 2012).ResultsEntomological surveys confirmed the establishment of C. imicola populations in Var department, but indicated low abundances and no apparent expansion there within the studied period. Higher population abundances were recorded in Corsica. Our genetic data suggested the absence of spatio-temporal genetic changes within each region but a significant increase of the genetic differentiation between Corsican and Var populations through time. The lack of intra-region population structure may result from strong gene flow among populations. We discussed the observed temporal variation between Corsica and Var as being the result of genetic drift following introduction, and/or the genetic characteristics of populations at their range edge.ConclusionsOur results suggest that local range expansion of C. imicola in continental France may be slowed by the low population abundances and unsuitable climatic and environmental conditions.