Ability of the vector tick Boophilus microplus to acquire and transmit Babesia equi following feeding on chronically infected horses with low-level parasitemia

Abstract

The protozoan parasite Babesia equi replicates within erythrocytes. During the acute phase of infection, B. equi can reach high levels of parasitemia, resulting in a hemolytic crisis. Horses that recover from the acute phase of the disease remain chronically infected. Subsequent transmission is dependent upon the ability of vector ticks to acquire B. equi and, following development and replication, establishment of B. equi in the salivary glands. Although restriction of the movement of chronically infected horses with B. equi is based on the presumption that ticks can acquire and transmit the parasite at low levels of long-term infection, parasitemia levels during the chronic phase of infection have never been quantified, nor has transmission been demonstrated. To address these epidemiologically significant questions, we established long-term B. equi infections (>1 year), measured parasitemia levels over time, and tested whether nymphal Boophilus microplus ticks could acquire and, after molting to the adult stage, transmit B. equi to naive horses. B. equi levels during the chronic phase of infection ranged from 10(3.3) to 10(6.0)/ml of blood, with fluctuation over time within individual horses. B. microplus ticks fed on chronically infected horses with mean parasite levels of 10(5.5) +/- 10(0.48)/ml of blood acquired B. equi, with detection of B. equi in the salivary glands of 7 to 50% of fed ticks, a range encompassing the percentage of positive ticks that had been identically fed on a horse in the acute phase of infection with high parasitemia levels. Ticks that acquired B. equi from chronically infected horses, as well as those fed during the acute phase of infection, successfully transmitted the parasite to naive horses. The results unequivocally demonstrated that chronically infected horses with low-level parasitemia are competent mammalian reservoirs for tick transmission of B. equi.

FIG. 1.

Comparison of B. equi levels in the peripheral blood during the ascending phase of parasitemia detected by real-time PCR and by microscopic examination.

FIG. 2.

Determination of B. equi levels over time in four chronically infected horses.

FIG. 3.

(a) Detection of B. equi-positive salivary glands from individual B. microplus adults fed as nymphs on a chronically infected horse. (b) Confirmation of amplicon identity of positive B. equi salivary glands from a duplex nested PCR with a digoxigenin-labeled ema-1 probe. SG1 to SG11, DNA amplified from salivary glands of individual ticks; H072, DNA amplified from B. equi Florida isolate-infected erythrocytes as a positive control. The predicted amplicon sizes for B. equi ema-1 and B. microplus α-tubulin are indicated in the right margin. Molecular size markers are in the left margin.