Attempted transmission of Ehrlichia risticii by field-captured Dermacentor variabilis (Acari: Ixodidae).

This study investigates whether American dog ticks (Dermacentor variabilis), commonly found in horse farms with history of Potomac Horse Fever (PHF), can transmit the disease-causing agent, Ehrlichia risticii, to susceptible horses. The results suggest that this type of tick may not be a vector for this disease.

Research Context and Methodology

• The study takes place in Maryland, USA, where Potomac Horse Fever (PHF) is endemic. The goal is to determine if a common parasite of horses, the adult American dog tick (Dermacentor variabilis), could be responsible for transmitting the disease.
• The researchers collected these ticks from horse farms known to have cases of PHF and allowed them to feed on three test horses that had no prior exposure to the disease.
• Over 500 male and female ticks were attached and fed on these test horses. The researchers then monitored the horses for any signs of PHF.
• To confirm that the horses were indeed susceptible to Ehrlichia risticii, the causal agent of PHF, they were exposed to infective blood at 60 to 65 days after the ticks had fed on them.

Research Findings and Interpretation

• The study found that despite being fed on by over 500 ticks, none of the test horses showed any clinical or serologic evidence of PHF. This suggests that the ticks did not transmit the disease to the horses.
• When the horses were later exposed to Ehrlichia risticii through infected blood, all developed symptoms of PHF and subsequently underwent seroconversion – confirming that they were indeed susceptible to the disease.
• From these results, the researchers concluded that the adult American dog tick may not function as a vector for Ehrlichia risticii. They note, however, that further studies are needed to determine whether other parasites may play a role in transmitting the disease.