Bovine anaplasmosis

Previous authors:F T POTGIETER AND W H STOLTSZ

Current authors:
SM Noh - Research Veterinary Medical Officer, DVM, PhD, DACVP, USDA-ARS Animal Disease Research Unit, Washington State University, Pullman, Washington, USA
KA Brayton - USDA-ARS Animal Disease Research Unit, Washington State University, Pullman, Washington, USA

GENERAL INTRODUCTION: ANAPLASMOSES

Introduction

Bovine anaplasmosis is a tick-borne disease caused predominantly by Anaplasma marginale, though a few reports indicate that Anaplasma centrale can also cause clinical disease. This pathogen has a worldwide distribution and is endemic in temperate, subtropical and tropical regions throughout the world. Bovine anaplasmosis is generally characterized by fever and progressive anaemia. In some instances, rapid destruction of erythrocytes by the reticuloendothelial system results in icterus, though this is not consistent. Infections can vary from clinically inapparent to severe, with clinical disease ranging from mild, self-limiting disease in small numbers of animals to larger herd outbreaks characterized by severe anaemia and unexpected death. Disease severity tends to increase with age and is typically inapparent in cattle less than nine months of age, although other, unknown factors, likely play a role in determining disease severity. Once infected, if animals survive initial infection, they become lifelong inapparent carriers and thus serve as a pathogen reservoir for ongoing transmission.

Figure 1 Anaplasma marginale : blood smear

Historical perspective

‘Peripheral coccus-like bodies’ and ‘marginal points’ were observed in erythrocytes of cattle by Sir Arnold Theiler in 1910, which was long before he identified these inclusion bodies as being the causative organism of the specific disease known by local farmers as gallsickness^{168, 169, 170}.  Mixed infections of Babesia bigemina and A. marginale, which frequently occur, led earlier workers to believe that these ‘coccus-like bodies’ represented a stage in the life cycle of B. bigemina¹⁵⁸. In 1911, Theiler¹⁷¹  referred to the following serendipitous discovery as a ‘lucky coincidence’. In 1908, cattle due to be imported from England to South Africa were vaccinated with a South African isolate of B. bigemina. On their arrival in the province then known as Transvaal, Theiler challenged them with blood from immune local cattle. Fifty per cent of these animals died, having shown fever, icterus, severe anaemia, ‘marginal bodies or points’ in the erythrocytes but no haemoglobinuria, and this led him to believe that these deaths were due to an agent other than B. bigemina. He concluded that the ‘marginal points’ were independent organisms responsible for the acute signs observed and called them anaplasmata because of their lack of cytoplasm, and the disease caused by them anaplasmosis. The specific organisms were called A. marginale because most resided near the margin (Figure 1) of the erythrocyte⁹.

Figure 2 Anaplasma centrale : blood smear

In 1911 Arnold Theiler¹⁷¹ discovered a second, similar bacterium, which proved to be invaluable in the control of anaplasmosis. He named this related organism A. marginale variety centrale (Figure 2). By virtue of an erroneous report by Ristic who stated that Theiler had named this organism as a separate species, the name A. centrale was taxonomically validated, but it was only in 2018 that evidence was published establishing A. centrale was indeed a separate species from A. marginale^{67, 74, 143}.  Theiler based the distinction between A. marginale and A. centrale on: the different position taken up by the two organisms within the erythrocyte, the lower virulence of A. centrale, and the incomplete cross-immunity to A. marginale demonstrated in animals that recovered from A. centrale infections.

Theiler immediately realized the tremendous potential of A. centrale for use in a vaccine, and the difference in the lengths of the incubation periods of B. bigemina and A. centrale made it possible and practical to vaccinate imported cattle with a combined B. bigemina (redwater) and anaplasmosis vaccine, which was sold for a shilling¹⁷².

The A. centrale isolate that Theiler made has been used extensively in a live-blood based vaccine in southern Africa, Australia, Israel and South America. Therefore, the distinct possibility exists that any subsequent isolations of A. centrale in these countries may represent reisolation of the original vaccine strain. A more detailed account of the history of anaplasmosis in the southern African region and elsewhere is given by Henning⁶¹.  

Aetiology and life cycle

Anaplasma marginale is a Gram negative, obligate intracellular bacteria in the family Anaplasmataceae, which includes the genera Anaplasma, Ehrlichia, Neorickettsia and Wolbachia⁴⁴.  Anaplasma spp. and Ehrlichia spp. are tick transmitted pathogens that cause disease in a variety of animals and humans. Less is known about Neorickettsia spp, which have a complex life cycle that involves multiple invertebrates. Disease in domestic animals or humans, when it occurs, tends to be due to inadvertent infection, rather than a natural component of the pathogen life cycle. Wolbachia are found in many arthropods, and have a complex and variable relationship with the host, often having an endosymbiotic relationship. Interestingly, organisms in this family are often...