Location: Arthropod-borne Animal Diseases Research2013 Annual Report
1a. Objectives (from AD-416):
Objective 1: Assess the risk of endemic arthropod vectors to transmit introduced exotic arboviruses in North America. • Sub-objective 1.A. Determine the vector competence of the primary U.S. bluetongue virus (BTV) vector, Culicoides sonorensis, for EU-BTV-8. • Sub-objective 1.B. Create models to assess potential population densities for biting insects that might be involved if Rift Valley fever virus was introduced to North America using ecologic and climatic factors. Objective 2: Identify targets and evaluate tools for vector control and interruption of transmission cycles to protect livestock and humans from vector-borne pathogens. • Sub-objective 2.A. Identify molecular components in insects that can be targeted to interrupt orbivirus transmission cycles. • Sub-objective 2.B. Evaluate insecticide resistance of Culicoides sonorensis to common pesticides used in livestock and agricultural operations. • Sub-objective 2.C. Provide livestock entomologists improved identification tools for North American Culex tarsalis and Aedes vexans.
1b. Approach (from AD-416):
Livestock are often heavily exposed to biting arthropods, causing a number of animal health issues and making them vulnerable to infection with a wide range of insect-borne pathogens. This research program will focus on: 1) improving risk assessments of the potential for introduction of foreign disease agents into the U.S., 2) interrupting transmission cycles at the vector level, 3) identifying viable pesticides for control of vectors, and 4) improving vector identification and understanding of population dynamics to enable more efficient vector control. Determining the vector competence of Culicoides sonorensis for an exotic bluetongue virus (BTV-8) will give an indication of the potential risk for the spread of exotic BTV should it be introduced into North America. Targets for controlling C. sonorensis infection with orbiviruses will come through identification of insect cell receptor(s) for BTV and verification of specific genes associated with orbivirus infection in the insect using RNA interference. Determining the susceptibility of C. sonorensis to common insecticides will identify effective treatments for control of this important livestock pest. Development of predictive models to determine risk of arbovirus transmission, such as Rift Valley fever virus, based on predicted mosquito population densities and distributions will give weeks or months advance notice, allowing preventive measures to reduce or prevent animal and human disease. Understanding the population structures of Aedes vexans and Culex tarsalis through molecular data will provide useful information for field entomologists and agencies developing control strategies and for use in developing models to predict risk of arbovirus transmission.
3. Progress Report:
In FY 2013 the Arthropod-Borne Animal Diseases Research Unit (ABADRU) continued ongoing studies on biting midges and mosquito disease vectors, as well as initiated studies on house flies. Significant findings include: 1) Culicoides sonorensis, one of the major biting midge disease vectors of bluetongue in the US is able to transmit European bluetongue-8, a particularly virulent strain and a major risk to US food security and agricultural interests if introduced. This work was conducted by Pirbright, an International Collaborator in Europe due to limited access to the virus in the US. 2) The first adult female Culicoides sonorensis transcriptome, or expressed gene catalog, was constructed in a collaborative effort with Clemson University Genomics Institute. Using RNAseq, the transcriptome (structure and expression patterns of over 19,000 genes) was explored in order to identify and quantify genes involved in blood and sugar feeding. This approach identified genes involved in key metabolic and physiological processes such as blood feeding, defense, digestion, development and reproduction. These studies will identify key genes or potential “intervention points” involved in the midge biological processes as well as vector competence. 3) To reduce virus transmission by midge populations, combinations of techniques are recommended: a. Larval habitat removal and manure management. b. Residual pesticide barrier treatments. c. Aerial spraying for adults. d. Quarantine of sick animals. Topical pesticide treatments for animals were tested for efficacy of reducing midge bites and killing adult midges. Several over the counter products prevented midge feeding for the 24 hour trial time and/or caused significant midge mortality and will be evaluated further. 4) ABADRU in collaboration with the USDA-ARS-CMAVE, acquired satellite data from NASA then formatted, and analyzed monthly normalized difference vegetation index (NDVI) environmental data for the US, including the calculation of anomalous NDVI values from a 25-year mean. Precipitation and temperature records were correlated to historic mosquito trap captures from Ft. Riley, Kansas to explain the population dynamics of mosquitoes of medical and veterinary importance in the U.S. 5) In collaboration with the Floragenex and the University of Idaho, ABADRU used genetic markers identified last year to generate a map of the historic spread of Culex taraslis, a key disease vector of West Nile virus. The mosquito appears to have originated in California and subsequently radiated up the coast before invading the Rockies. After passing over the Rocky Mountains via Montana, the species then radiated into the Great Plaines. 6) The first immune-stimulated house fly gene library was constructed and analysis revealed several genes associated with the house fly immune defense. These genes are expressed (induced) after bacterial exposure from filthy environments and during grooming. Fly defenses may be involved in not only the house fly’s own ability to fight off bacteria they ingest, but also ultimately in the fate of those bacteria in the house fly gut and therefore the potential for flies to transmit disease.
1. Identification and analysis of immune defense genes in house flies. House flies breed in environments such as garbage and manure and as a result pick up numerous species of bacteria, many of which are pathogenic to humans and livestock. Remarkably, house flies seem to generally be unharmed by these transient microbial residents. The mechanism by which flies protect themselves from bacteria has been understudied, but since fly defenses impact bacterial survival there can be an underlying role of these defenses in vector competence or transmission potential. In collaboration with Clemson University, ARS scientists in Manhattan, KS sequenced genes that were activated in immune-stimulated house flies, and identified several sequences that coded for effector molecules that act in the fly anti-bacterial defense. The role of some of these genes in bacterial fate has been elucidated, and it is clear that house fly defenses such as these have a direct impact on the fate of susceptible species of bacteria and ultimately the ability of flies to disseminate and transmit microbes from filthy environments to humans and livestock. This catalog is now publicly available in GenBank for other researchers to utilize in house fly studies. Understanding how the fly immune system either helps or hinders the fate of bacteria within flies can help us identify new points for interceding in the spread of fly-transmitted pathogens to both humans and livestock.
2. Biting midge gene catalog and expression analysis. Midges transmit deadly diseases impacting our nation’s livestock and wildlife. Unlike many other insect vectors, the genome sequence for this important pest is not yet available. Furthermore, a good understanding about the products of these genes (‘gene expression’) and their role in midge biology is not known. ARS scientists in Manhattan, Kansas, in collaboration with Clemson University Genomics Institute, have generated the first detailed catalog of midge gene products. This catalog has allowed identification of genes involved in key biological processes such as blood feeding, defense, digestion, transport, development and reproduction. In addition, the conditions under which these genes are activated (“gene expression”) were analyzed, and statistical analysis allowed assess the role of diet, for example, in the differential gene expression across conditions such as sugar feeding, blood feeding and fasting. Pinpointing the role certain genes play in the life of the midge may serve as a platform for developing new control approaches and intervention strategies.
Nayduch, D., Cho, H., Joyner, C. 2013. Staphylococcus aureus in the house fly: temporospatial fate of bacteria and expression of the antimicrobial peptide defensin. Journal of Medical Entomology. 50(1): 171-178.