Equine piroplasmosis

Previous Authors: D T DE WAAL AND J VAN HEERDEN

Current Authors:
R V BHOORA - Senior Researcher, PhD, Agricultural Research Council, Onderstepoort Veterinary Research, 100 Old Soutpan Road, Onderstepoort, Pretoria, Gauteng, 0110, South Africa 
N E COLLINS - Senior Researcher, BSc, MSc, PhD, Faculty of Veterinary Science, Department of Veterinary Tropical Diseases, University of Pretoria, Private Bag X04, Gauteng, 0110, South Africa 

GENERAL INTRODUCTION: BABESIOSES

Introduction

Equine piroplasmosis (EP) is caused by the intra-erythrocytic, tick-transmitted protozoan parasites Theileria equi¹²⁰ and Babesia caballi ,¹³¹ which generally affect horses, mules, donkeys and zebras. These parasites have also been detected in dogs^{14, 31} and, more recently, in camels,^{142, 165} although the epidemiological significance of these detections still needs further investigation. Transmission of equine piroplasmosis can occur either biologically, by ticks of the family Ixodidae, or mechanically, through blood transfusions. This globally significant disease is endemic in most parts of the world and is maintained in equid populations where competent tick vectors are present.¹⁶¹ Both parasites cause disease in equids, which may be either acute or chronic, with mortalities ranging from 5 per cent among horses native to endemic regions, up to and above 50 per cent in naïve horses introduced into endemic regions.¹⁵⁰ Infections with either Theileria equi or Babesia caballi result in the lysis of erythrocytes causing varying degrees of haemolytic anemia. Clinical signs are often non-specific and cannot be used to distinguish between Theileria equi and Babesia caballi infections, thus further complicating the diagnosis. Animals that recover often remain persistently infected carriers that act as reservoirs for the spread of these protozoal pathogens by competent tick vectors.

Piroplasmosis in horses was first reported in South Africa around the turn of the 19th century when it was initially described as ‘anthrax fever’,^{68, 170} ‘biliary fever’,⁸² ‘a bilous form of African horsesickness’¹³⁰ and in West Africa as ‘equine malaria’.⁶⁸ The first detailed study of equine piroplasmosis was the topic of Sir Arnold Theiler’s doctoral dissertation.¹⁷⁰ At the beginning of the 20th century, Laveran (1901)¹⁰³ recognized that the disease was caused by an intraerythrocytic protozoan parasite, which he named Piroplasma equi. A few years later, it was discovered that two morphologically distinct parasites could be identified in horses suffering from what was then known as biliary fever, and the name Piroplasma caballi was given to the larger parasite.⁹⁷ The parasites were later renamed Babesia caballi¹³¹ and Babesia equi.¹⁰³

All cases of equine piroplasmosis are reportable to the World Organisation for Animal Health (OIE: Office International des Épizooties) as it is considered to be a disease of international importance that negatively affects the movement of horses for trade and participation in equestrian sporting events. While certain countries impose stringent import restrictions to prevent the introduction of these parasites into non-endemic areas, endemic countries limit the entrance of infected horses to prevent the introduction of diverse and potentially more virulent strains.¹⁶¹

Aetiology and life cycle

Babesia and Theileria belong to the phylum Apicomplexa and, in reference to intraerythrocytic forms that are pear-shaped in some species, they form a group called the piroplasms.¹⁰⁴ The two piroplasm genera are usually distinguished by the lack of a pre-erythrocytic cycle in Babesia and the lack of transovarial transmission in Theileria. The taxonomy of Theileria equi has remained controversial since its initial discovery in 1901.^{88, 103, 120} The discovery of exoerythrocytic schizogony in Babesia equi;^{125, 156} division into four merozoites (“maltese-cross”) in erythrocytes;⁵¹ the observation of transstadial transmission by ticks;⁵⁸ and resistance to certain babesicidal drugs,,²³ all led to the reclassification of this parasite as belonging to the genus Theileria .¹²⁶ This reclassification may be superseded in future, because phylogenetic analysis of the small-subunit ribosomal RNA gene suggested that Theileria equi belongs to a paraphyletic group distinct from both the Theileira and Babesia genera.⁷ The recent acquisition and analysis of the complete genome sequence of the T. equi USDA Florida strain supports the placement of Theileria equi as a sister lineage to other Theileira spp.⁸⁸ Further data are required, however, before the taxonomic reclassification of this parasite can be finalized, and therefore, for the purposes of this chapter, the parasite will be referred to as Theileria equi.

The piroplasms have complex life cycles that involve morphologically distinct developmental stages that include an asexual haploid stage in the mammalian host and a diploid sexual stage that occurs in the tick vector.¹⁶¹ A typical apicomplexan life cycle is characterized by three stages of reproduction: (a) gamogony, which is the formation and fusion of gametes inside the tick gut, (b) sporogony, asexual reproduction in the salivary glands and (c) merogony, asexual reproduction in the vertebrate host.¹⁰⁴

Theileria equi life cycle

Theileria equi is present in the lymphocytes and erythrocytes of vertebrate hosts and is transmitted exclusively by the bite of vector ticks...