EQUINE | Disease
EQUINE | Disease
Equine Piroplasmosis in South Africa
Dr Rick Last – BVSc , M . Med . Vet ( Pathology ) Specialist Veterinary Pathologist
Introduction
Equine piroplasmosis is a tick transmitted disease that results from infection with either Theileria equi or Babesia caballi . There have been very rare instances of Babesia bovis infection documented in horses . Infection has been reported in horses , mules , donkeys and zebra with zebras acting as an important reservoir of infection in Africa . Equine piroplasmosis is reported to be widespread in South Africa , with the highest prevalence of disease in Africa reported in South Africa . Based on the findings of extra erythrocytic stages within equine peripheral blood mononuclear cells , morphological , biochemical and genetic evidence , Babesia equi was reclassified as Theileria equi in 1998 . Babesia caballi currently remains classified as a classical Babesia .
Epidemiology
Equine piroplasmosis is a tick transmitted disease of equids which requires the presence of a competent arthropod vector to persist within equine populations . Infected animals may remain carriers for long periods and act as sources of infection to other ticks . The introduction of carrier animals into areas where competent tick vectors are prevalent can result in epizootic spread of the disease . Over 14 species of ticks in the genera Boophilus , Rhipicephalus , Hyalomma and Dermacentor act as vectors of equine piroplasmosis . In South Africa Rhipicephalus evertsi evertsi is the main vector of both Theileria equi and Babesia cballi .
Life cycle and transmission .
Tick transmission of disease can occur in 3 forms namely intrastadial , transtadial or transovarial . Theileria equi zygotes do not multiply in ticks and thus transovarial transmission is not thought to occur . Ticks transmitting these protozoa become infected as larvae and transmit the infection as nymphs or become infected as nymphs and transmit the infection as adults ( transtadial transmission ). In certain species of ticks such as Rhipicephalus microplus intrastadial transmission of T . equi occurs wherby the same stage of tick that acquires the infection transmits it . Babesia caballi can multiply within ticks and invade many of the ticks organs including ovaries with B . caballi being readily passed to the next generation of ticks in the egg ( transovarial transmission ). In addition Babesia caballi is also effectively transmitted transtadially by its vectors .
Theileria equi sporozoites inoculated into the equine host via a tick bite , invade lymphocytes and undergo development to form intra-lymphocytic schizonts . Merozoites released from these schizonts invade red blood cells and transform into trophozoites divide and grow into the classic pear-shaped tetrads “ Maltese crosses ” ( figure 1 ). Babesia caballi sporozoites transmitted to equines via tick bites on the other hand , invade erythrocytes directly with no involvement of the lymphocytes , and here they transform into trophozoites which grow and divide form round , oval or pear shaped merozoites which are capable of infecting new red blood cells
8 • Equine Health Update •