Piroplasmosis: Cayenne Tick Ruled Out as T. equi Vector

Approximately 80 tick species live in the United States, and researchers have identified 15 as capable of transmitting the tick-borne disease equine piroplasmosis (EP). A team of researchers from Brazil recently determined that the Cayenne tick (Amblyomma cajennense), which frequents the southern United States and South and Central America, does not transmit one of the disease's primary causative protozoa, Theileria equi (also known as Babesia equi).

Equine piroplasmosis is important to the equine veterinary community because the protozoon attacks and destroys red blood cells in horses, leading to fever, anemia, icterus (jaundice), and anorexia, typically in combination with one another. The mortality rate of EP can be as high as 20% in susceptible horses.

Mucio F.B. Ribeiro, DVM, PhD, of the department of parasitology at the Instituto de Ciencias Biologicas da Universidade Federal de Minas Gerais, in Minas Gerias, Brazil, worked with a team of researchers to test whether A. cajennense could become infected with T. equi after feeding on affected horses and, thus, capable of spreading the disease to subsequent animals it feeds on.

The team used three horses during the study: one adult horse with experimentally induced EP; one foal with naturally occurring EP; and another adult horse chronically infected with EP (i.e., a long-term infection). The team applied a special cotton chamber to each horse's back, which allowed A. cajennense nymphs to feed on the affected horses. After the nymphs detached from the horse and grew into adults, they were examined in one of three different ways for the presence of T. equi:

  • dissected and spread on a microscope slide for visual examination;
  • underwent a DNA examination; or
  • processed for histopathological examination (microscopic examination of tissue).

After evaluating the three tests' results, the team found that none of the ticks in any of the three groups carried T. equi.

"This data suggests that A. cajennense (ticks) cannot become infected with T. equi, and this tick is probably not a biological vector of this parasite," Ribeiro concluded.

Further testing is needed to determine whether the cayenne tick can transmit Babesia caballi, the other causative agent of EP found in the United States.

The study, "Failure of the Amblyomma cajennense nymph to become infected by Theileria equi after feeding on acute of chronically infected horses," was published in Experimental Parasitology. The abstract is available online.

About the Author

Erica Larson, News Editor

Erica Larson, News Editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in three-day eventing with her OTTB, Dorado, and enjoys photography in her spare time.

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