Affiliation:
1. Program in Vector-Borne Diseases, Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, Washington 99164-7040
2. Animal Disease Research Unit, U.S. Department of Agriculture Agricultural Research Service, Pullman, Washington 99164-6630
Abstract
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
.
Publisher
American Society for Microbiology
Cited by
65 articles.
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