A U.S. Isolate of Theileria orientalis Ikeda Is Not Transstadially Transmitted to Cattle by Rhipicephalus microplus

Abstract

Theileria orientalis Ikeda has caused an epidemic of bovine anemia and abortion across several U.S. states. This apicomplexan hemoparasite is transmitted by Haemaphysalis longicornis ticks; however, it is unknown if other North American ticks are competent vectors. Since the disease movement is largely determined by the host tick range(s), the prediction of the T. orientalis spread among U.S. cattle populations requires determination of additional competent tick vectors. Although Rhipicephalus microplus has mostly been eradicated from the U.S., outbreaks in populations occur frequently, and the U.S. remains at risk for reintroduction. Since R. microplus is a vector of Theileria equi and T. orientalis DNA has been detected in R. microplus, the goal of this study was to determine whether R. microplus is a competent vector of T. orientalis. Larval R. microplus were applied to a splenectomized, T. orientalis Ikeda-infected calf for parasite acquisition, removed as molted adults, and applied to two T. orientalis naïve, splenectomized calves for transmission. After 60 days, the naïve calves remained negative for T. orientalis by PCR and cytology. Additionally, T. orientalis was not detected in the salivary glands or larval progeny of acquisition-fed adults. These data suggest that R. microplus is not a competent vector of the U.S. T. orientalis Ikeda isolate.

Dermacentor variabilis Does Not Transstadially Transmit the U.S. Isolate of Theileria orientalis Ikeda: A Controlled Acquisition and Transmission Study

“…Merozoites were first detected within erythrocytes via blood smear cytology on day 38 postinfection. As noted previously [26,35], intraerythrocytic merozoites were pear-shaped and approximately 1-2.5 µm × 0.5 µm. The percent parasitized erythrocytes (PPE) varied from 0.06% to 1.57% for the remainder of the experiment (Figure 1).…”

“…Calf 1 was infected with the U.S. T. orientalis Ikeda isolate as described previously [26,35]. Briefly, 12 mL of cryopreserved T. orientalis Ikeda-infected erythrocyte stabilate was administered intravenously on day 0 (8.4 mL from stabilate batch 1804/3-2-22, parasitemia 2.5%; 2.4 mL from stabilate batch 1697/5-12-20, parasitemia 0.7%; and 1.2 mL from stabilate batch 1726/7-2-20, parasitemia 0.48%).…”

“…Blood was collected via jugular venipuncture. PCR for the T. orientalis major piroplasm surface protein (MPSP) gene was performed as previously described [35]. Packed cell volume (PCV) and percent parasitized erythrocytes (PPE) were assessed via blood smear cytology every 48 h using this equation: ((Total parasites in 5 fields)/(Erythrocyte count in 1 /4 of a field × 20)) × 100, as previously described [35].…”

“…PCR for the T. orientalis major piroplasm surface protein (MPSP) gene was performed as previously described [35]. Packed cell volume (PCV) and percent parasitized erythrocytes (PPE) were assessed via blood smear cytology every 48 h using this equation: ((Total parasites in 5 fields)/(Erythrocyte count in 1 /4 of a field × 20)) × 100, as previously described [35]. To maximize the chance of T. orientalis acquisition by the ticks, D. variabilis nymphs were applied to the calf in two batches: the first batch of 676 nymphs was applied eight days after initial detection of merozoites via blood smear cytology (30 days after initial PCR-positive result), and the second batch of 676 nymphs was applied 21 days after initial detection of merozoites via blood smear cytology (43 days after initial PCR-positive result).…”

“…H. longicornis belongs to the family Ixodidae [32], and evaluation of the competence of other U.S. ixodid ticks for T. orientalis Ikeda is key in the determination of its potential spread. Our group recently showed that the cattle fever tick (also known as the southern cattle tick), Rhipicephalus microplus, is likely not a competent vector of U.S. isolates of T. orientalis Ikeda [35]. In the present study, we assessed the competence of the American dog tick, Dermacentor variabilis, for the U.S. isolate of T. orientalis Ikeda using controlled acquisition and transmission studies in calves.…”