Factors Affecting the Risk of Ticks and Biting Flies

Background

Bluetongue is a viral disease of ruminants. Cattle do not usually become sick but the disease can be fatal to sheep. Bluetongue cannot spread directly from one animal to another. It can only move from one animal to another animal if it is carried by a biting fly (Culicoides sonorensis). Bluetongue is found in the U.S. It does not have a foothold in Canada, although it’s biting fly vectors is found in parts of southern BC, AB, and likely SK.

Until recently, concerns about the health of Canada’s sheep herd resulted in import restrictions on US feeder cattle. Pre-import bluetongue testing is no longer required. Laboratories are required to notify the CFIA when they confirm a bluetongue diagnosis to monitor the presence or spread of the disease.

Climate change can also make conditions favourable for disease outbreaks in new areas. To prepare for these possibilities, it is important to identify potential vector species and develop information on their seasonal abundance, biology, and population dynamics. This information can be used to develop sound strategies to control vectors minimize or prevent disease spread. But the first step is to identify the best way to study the flies.

Objectives

• Compare standard trapping methods for sampling abundance of biting flies; and
• Determine the relationship between trap captures and biting rates on livestock.              

What They Did

Three types of black-light (found in bug zappers) traps were compared at two sites in Southern Alberta. One trap used only black light to attract flies. The other two traps also had carbon dioxide (CO₂) to attract insects. Like humans, cattle inhale cool air and oxygen and exhale warm air and CO₂.  Biting flies and mosquitos have CO₂ sensors that help them locate cattle to feed on. The CO₂ traps took advantage of this behavior. One trap simulated the CO₂ output of a calf, and the other trap simulated the CO₂ output of an adult.

The traps were operated for 27 nights during the summer, and the numbers of Culicoides and mosquitoes captured were compared each day.  One trap of each type was able to segregate the catch into 3 hour intervals to determine the optimal time of night to trap in the next study.

In the second year, they compared captures using black light and CO₂ traps to the numbers of insects attracted to sheep.   

What They Learned

Adding CO₂ to black light traps increased capture of most biting flies. C. sonorensis and C. gigas and C. palmerae accounted for 78 – 89% of the Culicoides captured regardless of trapping method.  Adding CO₂ increased the percentage of C. sonorensis and C. palmerae captured while C. gigas declined.  These three species were active at different times of the night. C. sonorensis appeared to be most active between 6PM and midnight.

Even though black light traps captured fewer C. sonorensis than either of the CO₂ traps, a detailed analysis indicated that they are as sensitive as 300 ml/min CO₂ traps at detecting the presence of C. sonorensis if operated for 7 nights.  These traps are much easier to use, and would provide reliable information on the presence of C. sonorensis in an area. 

C. sonorensis and C. gigas accounted for 92% of the Culicoides captured on sheep. The black light and CO₂ traps will trap other Culicoides species that are not attracted to sheep. But only C. sonorensis is not known to be a vector of any pathogen, these other species of Culicoides are likely not important and do not need to be counted.

What it means

Unsophisticated black light traps can be used to monitor whether C. sonorensis are present in an area, how common they are, and how active they are in efforts to monitor the risk of bluetongue transmission.