rate classication for this parasite falls between
BabesiaandTheileriabased on genomic analysis. 2a
Until additional studies are available that deter-
mine a nal taxonomic classication, for this paper,
we will refer to the parasite asB. (T.) equiorB. equi.
Methods of Transmission
B. caballiandB. equiare transmitted by many of
the same tick vectors, with multiple types of ticks
implicated in the transmission of EP agents on a
global perspective. Ticks serve as a reservoir ofB.
caballi, because the organism persists in the ticks
through several generations, with transtadial and
transovarial transmission occurring in some types of
ticks.
3 In contrast, horses are the primary reser-
voir ofB. equi, with no transovarial transmission
shown to date.
3 Few countries are considered free
from native transmission of EP agents through
ticks, and the prevalence of EP infections is consis-
tent with the distribution of known competent tick
vectors.
3
Dermacentor (Anocentor) nitens, the tropical horse
tick, was reported to be the primary vector ofB.
caballitransmission during the 1960s outbreak of
EP in Florida. A joint United States Department of
Agriculture-Animal and Plant Health Inspection
Services (USDA:APHIS) and State of Florida eradi-
cation program forB. caballibrought the outbreak
under control through a multifaceted approach that
emphasized tick control on horses and equine pre-
mises.
4 Prior to 2009, no tick transmission ofB.
equiwas recognized in the United States.
The EP agents can also be transmitted by iatro-
genic means. Procedures that move blood from an
infected horse to a naïve horse through the reuse of
equipment such as needles and syringes have been
implicated in transmission of EP agents.
5 In addi-
tion, the use of horses that are carriers of the EP
agent as a source of blood for transfusion could re-
sult in transmission.
Clinical Signs
Horses infected with either agent have similar clin-
ical signs. Clinical signs of EP can include fever,
anemia, icterus, and anorexia.
3 Digestive tract
signs can occur, including colic, constipation, or di-
arrhea.
3 It has been reported thatB. equican be
transmitted by intrauterine infection, leading to
abortion or neonatal infection, but how often this
occurs is not well-documented.
6
In countries where EP is endemic, foals born to
infected mares may be protected from clinical dis-
ease through ingestion of protective colostral anti-
bodies; this is called premunition.
3 Thus, in some
regions of the world where infection is common,
little or no clinical disease may be observed in native
horses. However, disease is frequently observed in
adult horses suddenly introduced into areas with
large numbers of infected ticks.
It is important to recognize that unapparent car-
riers represent the majority of infected horses.Because these horses appear clinically normal, diag-
nosis of infection relies on laboratory testing that
will be described in a separate section.
Diagnosis
Clinical signs can alert the veterinarian to the pos-
sibility of EP. However, the clinical signs of EP are
consistent with other diseases including equine in-
fectious anemia (EIA), purpura hemorrhagica, idio-
pathic immune mediated anemia, and intoxications.
In cases having clinical signs consistent with EP,
examination of blood smears can assist in diagnosis.
Giemsa staining of blood smears followed by careful
microscopic examination can reveal the intraeryth-
rocytic parasites in acute cases.B. caballican ap-
pear pyriform-shaped and occurs in pairs (Fig. 1),
whereasB. equiappears as four pyriform parasites
in a Maltese-cross formation (Fig. 2).
3 Because the
parasitemia can be very low in horses that have
recovered from clinical disease, the examination of
direct blood smears may not allow for detection of
the infection in chronic carriers.
To conrm the diagnosis in clinical cases and de-
tect infection in unapparent chronic carriers of EP
agents, serologic tests have been used. There are
three types of serologic tests that are used to detect
antibodies toB. caballiandB. equi. These include
the complement xation test (CFT), the competitive
inhibition enzyme-linked immunoabsorbant assay
(cELISA) test, and the indirect immunouorescent
Fig. 1. Intraeyrthrocytic merozoite form ofB. caballi. (Photo is
courtesy of D. Knowles)
Fig. 2. Intraerythrocytic of trophozoite and merozoite stages of
B. equi.(Photo is courtesy of D. Knowles)
2 2010
Vol. 56
AAEP PROCEEDINGS
IN-DEPTH: EQUINE PIROPLASMOSIS