Abundance, diversity and temporal activity of adult Culicoides spp. associated with cattle in West Bengal, India

Potential roles of Culicoides spp. (Culicoides imicola, Culicoides oxystoma) as biological vectors of bluetongue virus in Yuanyang of Yunnan, P. R. China

Introduction

Culicoides plays a crucial role as an insect vector in the field of veterinary medicine. The transmission of significant viruses such as bluetongue virus (BTV) and African horse sickness virus (AHSV) by this insect poses a substantial threat, leading to the development of severe diseases in domestic animals. This study aimed to explore the Culicoides species, identify their blood-meal sources, and assess the presence of BTV and AHSV carried by Culicoides in Yuanyang County, Yunnan Province. The aim was to gain insights into the potential vectors of these two viruses and elucidate their potential roles in the transmission of pathogens.

Methods

The midges were collected from cattle (Bos indicus), pig (Sus scrofa), and goat (Capra hircus) pens in Yuanyang County, Yunnan Province in June 2020. Initial identification of midges was conducted through morphological characteristics, followed by molecular identification using the cytochrome C oxidase subunit I (COI) gene. The determination of Culicoides blood-meal sources was accomplished using specific primers targeting the cytochrome b (Cyt b) gene from potential hosts. BTV and AHSV RNA were identified in Culicoides pools through the application of reverse transcriptase PCR and quantitative real-time PCR. Nucleotide homology and phylogenetic analysis were performed using MegAlign (DNAStar) and Mega 6.0 software.

Results

A total of 6,300 Culicoides, consisting of C. oxystoma, C. arakawai, C. imicola, and C. innoxius, were collected from cattle, pigs, and goat pens. The engorgement rates for these species were 30.2%, 54.6%, 75%, and 66.7%, respectively. In the cattle pen, the prevailing species is C. oxystoma (100%). In the pig pen, C. arakawai dominates (70%), with C. oxystoma following at 30%. In the goat pen, C. imicola holds the majority (45.45%), trailed by C. oxystoma (25%), C. innoxius (20.45%), and C. arakawai (9.09%). These Culicoides species were identified as feeding on cattle, pigs, goats, chickens (Gallus gallus), and humans (Homo sapiens). The positivity rates for BTV were 20.00% and 11.54% in blood-fed specimens of C. imicola and C. oxystoma, respectively. Conversely, the positivity rates for BTV in non-blood-fed specimens were 0.00% and 6.67% for C. imicola and C. oxystoma, respectively. BTV was not detected in C. arakawai and C. innoxius. The specimens (YY86) from C. imicola that tested positive for BTV had the closest genetic relationship to YTS-4 isolated from Mangshi, Yunnan Province in 1996. All test results for the nucleic acid of AHSV were negative.

Conclusion

The study reveals variations in the species distribution, community composition, blood sucking rate, and blood-feeding sources of Culicoides across different habitats. Notably, C. imicola and C. oxystoma emerge as potential vectors for the transmission of BTV in local animals. Accordingly, this investigation provides crucial insights that can serve as a valuable reference for the prevention and control of BTV in local animals, particularly from the perspective of vector management.

Effect of substrate salinity and pH on life history traits of the bluetongue virus vector Culicoides peregrinus

Habitat selection of Culicoides spp. (Diptera: Ceratopogonidae) is influenced by the physicochemical factors such as temperature, pH, salinity, moisture, conductivity, organic and inorganic compounds of substrates. These factors determine the life history traits of the vectors. We studied the influence of substrate salinity (0-40 parts per thousand, ppt) and pH (pH 1-13) on oviposition, egg hatching, larval survivability, and adult emergence of Culicoides peregrinus Kieffer under laboratory conditions. Most eggs (80.74%) were laid in 0 ppt and 95% in pH 7 but lowered with increased salinity and pH levels. It was observed that the females did not lay eggs in 30 ppt to 40 ppt salinity; pH 1 and pH 13 but interestingly up to 95% of the eggs were retained within the abdomen. Little effect of salinity and pH on egg hatching was observed up to 5 ppt and 10 ppt except at the extreme values of 40 ppt and pH 1, pH 13. Pupation did not occur in rearing plates with high salinities, 30 ppt and 40 ppt, although the few eggs hatched when exposed to such salinity. In low salinity (0 to 2 ppt), occurrence of adult emergence was more and then decreased with increasing salinity. Maximum emergence was seen when the rearing media was alkaline. This study deals with the suitability of breeding substrate of C. peregrinus when exposed to salinity and pH ranges. Our study suggests the ambient salinity and pH ranges to be maintained during laboratory rearing of this vector species.

Molecular identification of Culicoides oxystoma and Culicoides actoni vectors of bluetongue virus

Bluetongue is a non-contagious viral disease causing significant economic losses throughout the world. The bluetongue vectors Culicoides oxystoma and Culicoides actoni, which play a significant role in the transmission of various pathogens, are distributed across different geographical realms. Adults are minute in size with wide phenotypic variation, so morphology-based species identification is severely constrained by preparatory time and shortage of taxonomic expertise. To make the identification process rapid and effective, a specific primer was designed for the identification of C. actoni based on the multiple sequence alignment of ITS1 sequences of 11 Culicoides species. Along with this, a refined version of existing C. oxystoma specific primer was proposed. The primer sets distinguished C. oxystoma and C. actoni from a pooled sample consisting of other Culicoides species as well as closely related genera such as Forcipomyia and Alluaudomyia. Our findings suggest that the primers were species specific, sensitive and have potential to discriminate vector species C. oxystoma and C. actoni from pooled samples. To the best of our knowledge, these are the first ITS1 sequences generated and submitted in GenBank for Culicoides innoxius, Culicoides shortti, Culicoides palpifer and Culicoides anophelis and the first for Culicoides peregrinus, Culicoides fulvus and C. actoni from India.

A Metagenomic Based Approach on Abundance and Diversity of Bacterial Communities Across the Life Stages of Culicoides peregrinus (Diptera: Ceratopogonidae) a Vector of Bluetongue Virus

During larval rearing of Culicoides peregrinus Kieffer (Diptera: Ceratopogonidae) it was obligatory to add a small quantity of mud from larval habitat to nutrient broth in culture plates. This initiated microbial growth in rearing plates which facilitated growth and development of immature. The primary aim was to enumerate gut microbial communities across the different life stages of C. peregrinus. Amplicon sequencing of the V3-V4 hypervariable region (16S rDNA) was done on Illumina Miseq platform to detect gut bacterial communities at different life stages, while ITS regions (18S rRNA) were targeted for fungal communities of the 4th instar larvae. The major findings were: 1) Phylum Proteobacteria and Firmicutes were the most abundant throughout the life stages, along with the highest bacterial alpha diversity in the egg, 2) bacterial compositions were similar to laboratory reared and field collected adults, and 3) abundant fungal phyla associated with the larval gut were Ascomycota and Basidiomycota. Furthermore, analyses of the gut microbiome with METAGENassist might be indicative of their likely function in the natural habitat. Abundant gut-associated bacteria and/or fungal genera detected in the present study could be used as dietary supplements to establish laboratory colonies for further vectorial research. While, individual roles of the bacteria or fungi in paratransgenesis are warned for their possible utilization to frame the management strategy in upcoming works.

Blood meal analysis of Culicoides species associated with livestock in West Bengal, India

Knowledge gaps exist on the feeding pattern and host range of bluetongue virus vectors, Culicoides species, associated with livestock in India. Adult midges were trapped with ultraviolet light traps at 13 household farms adjacent to human biotope. Host DNA was isolated from individual females (n = 101; blood engorged-82, gravid-4 and parous-15) and subjected to PCR amplification targeting CytB and 16S rRNA gene fragments followed by sequencing of amplified DNA samples. However, DNA sequences from only 71 individuals (70.3%) comprising of 10 Culicoides species were obtained. Blood meal analysis revealed at least 10 species that fed on five mammalian hosts including humans, but surprisingly none tested positive for birds. Results revealed that Culicoides innoxius tested positive for four not previously recognized species indicating a potential role as a vector species. Likewise, Culicoides shortti and Culicoides hegneri preferred goat and cattle respectively as hosts, whereas Culicoides palpifer preferred cattle along with buffalo as hosts, which is being reported for the first time. This is the first document on DNA-based blood meal identification and feeding preference of Culicoides midges associated with livestock in India.

Host preference of bluetongue virus vectors, Culicoides species associated with livestock in West Bengal, India: Potential relevance on bluetongue epidemiology

Determination of host choice of Culicoides species (Diptera: Ceratopogonidae), the vectors of bluetongue virus (BTV), is pivotal to ascertain the role of each species in the transmission of pathogens, pest management and enumeration of disease prediction models. Host preference of livestock associated Culicoides midges was investigated in West Bengal, India with four replicates of a 3 × 3 Latin square design during August and September 2021. Adult Culicoides were mouth aspirated from three BTV hosts viz., cattle, sheep and goats. Mouth aspirating was validated by the sweep net collections. The host-baited collections recorded seven Culicoides species; with the highest landing rate on cattle (n = 5,667; 92.9%) followed by sheep (n = 365; 6.0%) and goat (n = 67; 1.1%). Based on the Jacob's selectivity index, all midge species, except for Culicoides fulvus Sen & Das Gupta, encountered, preferred cattle over other mammalian hosts. Culicoides oxystoma Kieffer, the subgenus Trithecoides Wirth & Hubert and Culicoides actoni Smith, predominated on the ventral region (belly/flank) of the cattle. However, Culicoides peregrinus Kieffer and C. actoni were observed to be prevalent in the leg region of sheep. A significantly higher percentage of female (99.9%) with only 0.3% of male were trapped in aspiration based animal baited collections. On the other hand sweep net and light trap catch comprises of 50.7%, 89.7% female and 49.2%, 10.2% male respectively. Surprisingly, DNA based blood meal analysis revealed human blood from the midges trapped in UV-LED light traps. Supplying the first evidence that Culicoides similis Carter, Ingram & Macfie, C. fulvus and Culicoides palpifer Das Gupta & Ghosh, feed on humans.