Attempted transmission of Ehrlichia risticii (Rickettsiaceae) with Stomoxys calcitrans (Diptera: Muscidae).

This research explores the potential for Ehrlichia risticii, the bacteria behind Potomac horse fever, to be transmitted via stable flies (Stomoxys calcitrans) under experimental conditions. Their results determined that these flies, given different feeding schedules, were unsuccessful in transmitting E. risticii.

Experimental Setup

• The research was conducted through experimentation — looking at the transmission of Ehrlichia risticii, a bacterium responsible for causing Potomac horse fever, by stable flies (Stomoxys calcitrans).
• Two feeding schedules were used in these experiments. Schedule A saw 140 flies feed once on an experimentally infected pony and once more 24 hours later on a recipient pony. This was done daily with a new set of flies for a period of 12 days.
• Schedule B allowed 240 flies to feed daily on the donor pony for 12 consecutive days, before feeding on the recipient pony for five consecutive days.
• Ehrlichia risticii was found to be present in the blood of the infected donor pony throughout these feeding schedules.

Results and Analysis

• The recipient pony, which was the subject of a potential passive transmission of the bacteria via the stable flies, did not show any symptoms of Potomac horse fever, and remained seronegative to E. risticii up to 60 days after the last instance of fly feeding. This indicated that the disease was not transmitted to the recipient pony by the stable flies.
• Mice were also a part of this study, being injected with emulsions of flies from both feeding schedules (A and B). There was no indication of Ehrlichia risticii in the mice 30 days after they were inoculated, supporting the result that the disease was not transmitted by the stable flies.
• Furthermore, there was no presence of ehrlichial organisms found in the digestive tracts of the stable flies— showing that stable flies could not carry these bacteria in their systems, at least under the experimented conditions.

Conclusion

• This research ultimately concluded that stable flies (Stomoxys calcitrans) did not transmit Ehrlichia risticii under the controlled conditions of their experiments.
• While this bacterium is known to cause Potomac horse fever, this disease was not transmitted to the recipient pony or mice, indicating that these flies may not be a vector for this bacterium and its subsequent disease.