Eperythrozoonosis is caused by Eperythrozoon spp., which are rickettsial organisms that occur epicellularly on erythrocytes and thrombocytes, as well as freely in the plasma of a wide variety of animal species including pigs, sheep, cattle, dogs, cats, mice and rats. Most infections are subclinical, but varying degrees of fever, anaemia and icterus occur (particularly in immunocompromised or splenectomized animals). Natural cases of eperythrozoonosis are generally not of great clinical or economic significance in livestock.

The genus Eperythrozoon was established in 1928 by Schilling75 when he named the parasites he observed in mouse blood Eperythrozoon coccoides.5 In 1934 Neitz et al.63 were the first to describe Eperythrozoon ovis in sheep in South Africa. In this article a note by Neitz and Quinlan was added, which reported that they had also seen an Eperythrozoon sp. in splenectomized calves. In the same year, Adler and Ellenbogen1 described Eperythrozoon wenyoni, which they had observed in blood smears of a splenectomized calf.

Hoyte36 found and named Eperythrozoon teganodes, an organism that occurs free in the blood of cattle. He also suggested that some highly pleomorphic species of Eperythrozoon may eventually prove to be mixed infections of more than one species. Although many scientists have recognized E. teganodes,7, 24, 26, 27, 37, 50, 99 this organism is not listed in Bergey’s Manual of Systematic Bacteriology.48

A third species in cattle, which seems to exclusively infect thrombocytes, was first reported as occurring in Madagascar9 and later in Finland.7, 98 Uilenberg100 named this organism Eperythrozoon tuomii, and it was subsequently concluded that this parasite is a valid species.105 All three species in cattle have been identified in Argentina and in Germany.22, 26 However, E. tuomii, which also occurs in South Africa, does not seem to have earned taxonomic status, even after it was shown to have no antigenic relationship to E. wenyoni.97

Eperythrozoon wenyoni, E. teganodes and E. tuomii are generally regarded as being non-pathogenic for cattle. There is, however, a report of fatal eperythrozoonosis occurring in five- and six-month-old calves.6

Eperythrozoonosis of pigs and sheep are probably the only two diseases of veterinary importance in livestock to be caused by eperythrozoa.24 According to Jansen,43 an eperythrozoon had been identified in blood smears from pigs in Zaire by P.J. du Toit of the Onderstepoort Veterinary Institute in South Africa as early as 1942. In the USA it was found that a condition of pigs associated with icterus and anaemia was caused by an eperythrozoon.86 Splitter 84 named this parasite Eperythrozoon suis and also described another non-pathogenic variety, Eperythrozoon parvum. The latter parasite was also described in pigs in South Africa by Jansen.44 Studies in splenectomized pigs revealed that E. parvum appears to be more pathogenic than had been originally thought.3 Although E. suis is regarded as an economically significant parasite in pigs, it is considered to be unimportant in the relatively small pig industry in southern Africa.

The aetiology of eperythrozoonosis in sheep in South Africa is well documented.59, 63 Extensive reviews have been published on eperythrozoonosis and haemobartonellosis comparing the aetiology and other aspects of these infections.24, 46, 47

Eperythrozoon ovis may play a role in the poorly understood ill-thrift syndrome (Ill thrift) in sheep and goats particularly in the Eastern Cape Province of South Africa.

Eperythrozoon spp. do have a nuisance value in splenectomized donors of live-blood vaccines and in experimental animals. Subclinical Eperythrozoon spp. infections in such animals may result in the occurrence of aberrations in experimental data,21 probably due to the reactivation of latent infections. For example, sheep dosed with Pachystigma spp. plants, the cause of ‘gousiekte’, often develop clinical eperythrozoonosis.21

* For recent changes in the nomenclature of these organisms, refer to the introduction to Section 3, Rickettsial and chlamydial diseases

Aetiology and life cycle

Eperythrozoa are non-motile, prokaryotic, parasitic microorganisms belonging to the order Rickettsiales, family Anaplasmataceae. Morphological differentiation between some Eperythrozoon and Haemobartonella spp. is difficult.24, 47 Depending on the species, these parasites occur epicellularly on erythrocytes and thrombocytes, as well as freely in the blood plasma. Their association with the erythrocyte membrane is used to differentiate between some species.

A summary of the morphological characteristics and location in the blood of the different organisms occurring in domestic stock in South Africa is given in Table 44.1. The characteristic ring forms of Eperythrozoon spp. seen in thin blood smear preparations stained with Giemsa are probably artefacts formed by the spherical organisms during the drying process.24, 45, 46 The proportions of the morphological types of E. ovis apparently change as the parasitaemia increases. 25 Both scanning and transmission electron microscopy have added considerably to our knowledge of their morphology, multiplication and association between them and the cells they parasitize.25, 45, 52, 81, 98, 102, 105

There is little difference between the fine structure of eperythrozoa25, 52, 69...

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