Neosporosis

Previous authors: J P DUBEY

Current authors:
J P DUBEY - Senior scientist, M.V.Sc., PhD.,DSc, Animal Parasitic Diseases Laboratory, U.S.Department of Agriculture, Building 1001, Beltsville Agricultural Research Center, 10300 Baltimore Avenue, Beltsville, Maryland, 20705, USA

Introduction

Neospora caninum is a relatively recently recognized protozoan parasite that occurs in a wide range of animals species. Until 1988 it was misdiagnosed as Toxoplasma gondii because of structural and biological similarities.⁵² The disease was first described in dogs by Bjerkås et al.²⁵ in Norway in 1984 but the parasite was not named at that time. Dubey and his co-workers in 1988⁵² described the parasite and proposed a new genus, Neospora, with N. caninum as the type species. In vitro isolation of the parasite in cell culture inoculated with tissues of paralysed dogs contributed to a better understanding of its biology.⁵⁷ In 1988, neosporosis was recognized as a cause of abortion in cattle¹⁶³ and in the subsequent three years infections were induced experimentally in most species of livestock.⁶³ The genome of N. caninum was sequenced in 2012,¹⁴⁶ suggesting that N. caninum and T. gondii diverged from their common ancestor approximately 28 million years ago. A book authored by Dubey⁵⁸ devoted solely to neosporosis listed more than 2000 references: much of the information in this chapter is extracted from that book and only selected references are cited here.⁵⁸

Aetiology and life cycle

Neospora caninum is a coccidian parasite closely related to T. gondii (Figure 1). Dogs (both domestic and feral or wild, Canis familiaris), coyote (Canis latrans), and gray wolf (Canis lupus) are   intermediate and definitive hosts.^{61, 78, 104, 122}  Cattle, sheep, white- tailed deer, European bison and water buffalo are  intermediate hosts, as proven by isolation of  viable parasites.⁵⁸ Clinical neosporosis has been reported in numerous hosts including alpaca (Vicugna pacos), captive antelope (Tragelaphus imberbis), Axis deer (Axis axis), black-tailed deer (Oedocoileus hemionus columbianus), cattle, dog, Eld’s deer (Panolia eldii siamensis), goat, llama (Lama glama), Parma wallaby (Macropus parma), Pine marten (Martes martes), red fox (Vulpes vulpes), white rhinoceros (Ceratotherium simum), and sheep.^{58, 97-101, 171} Antibodies and DNA to N. caninum without evidence of disease have been detected in many mammalian and avian species.⁵⁸

Only an asexual cycle occurs in the intermediate hosts that consists of tachyzoites and tissue cysts. Both tachyzoites and tissue cysts are microscopic in size and intracellular. Tachyzoites are ovoid, lunate or globular and measure 3–7 μm × 1–5 μm, depending on the stage of division (Figures 2 and 3). Tachyzoites are found in the cytoplasm in many cells including neurons, macrophages, fibroblasts, vascular endothelial cells, myocytes, hepatocytes and dermal cells. Tachyzoites are usually located in a parasitophorous vacuole in the host cell. Ultrastructurally,  the tachyzoites of N. caninum resembles the tachyzoites of T. gondii  except that the rhoptries in N. caninum are electron-dense and more numerous, with some extending the entire length of the parasite, whereas in T. gondii, they are electron-lucent, few, and located anterior to the nucleus.⁵⁸

Figure 1  Life cycle of Neospora caninum

Figure 2  Tachyzoites of Neospora caninum

1. Impression smear. Note organisms dividing (arrowheads) into two are bigger than single tachyzoites (arrows). Giemsa stain
2. Tachyzoites in parasitophorous vacuoles in cell culture. Giemsa stain
3. Sections of the skin of a dog. Note suppurative inflammation associated with two groups of tachyzoites (arrows) and individual tachyzoites (arrowheads). Tachyzoites in sections are much smaller than those in smears. Haematoxylin and eosin stain

Figure 3 Transmission electron micrograph of dividing tachyzoites in two parasitophorous vacuoles (Pv). The Pv have many tubular networks. Arrows point to the conoidal end of daughter tachyzoites forming inside the mother tachyzoites. Note electron-dense rhoptries (R), micronemes (Mn), microtubules (Mt) or subpellicullar tubules, conoid (C), host cell nucleus (Hcn). Cardiopulmonary artery endothelial cell culture. (Courtesy of Dr C.A. Speer, Montana State University, Bozeman, Montana, USA)

Tissue cysts are often round to oval in shape (Figure 4). Most tissue cysts studied were from the brain and spinal cord, the majority from naturally infected dogs. Tissue cysts in the brain are round to oval and up to 107 µm in diameter. The cyst wall is 0.5 to 4.0 µm thick, both in live, unstained preparations and in histological sections.⁵¹ The cysts in myocytes are elongated and may be up to 100 µm long, and have a thin cyst wall.^{71, 141} Histologically tissue cysts are rare in other organs or tissues of naturally infected animals. The tissue cyst wall is smooth and up to 4 µm thick, presumably depending on the duration of the infection. Septa are absent in the cysts and there is no secondary cyst wall (Figure 5). The tissue cyst wall is elastic, argyrophilic, has a wavy contour, and stains negative with Periodic-acid Schiff (PAS). Bradyzoites are slender and contain the same organelles as tachyzoites except that there are fewer rhoptries and more PAS-positive (amylopectin) granules in the bradyzoites. Tissue cysts may degenerate and cause an inflammatory reaction by the host (...