Neosporosis

Neosporosis

J P DUBEY

Introduction

Neospora caninum is a recently recognized protozoan parasite of livestock and companion animals. It is not a new parasite42 but until 1988 was misdiagnosed as Toxoplasma gondii because of structural and biological similarities.34 The disease was first recognized in dogs by Bjerkås, Mohn and Predsthus15 in Norway in 1984 but the parasite was not named. Dubey and his co-workers34 subsequently 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 led to newer knowledge of its biology.38

Aetiology and life cycle

Neospora caninum is a coccidian parasite, closely related to Toxoplasma gondii (Figure 23.1). Dogs are both an intermediate host and a definitive host.34, 76 In addition to dogs, cattle,31 sheep,37 goats,10, 33 horses,48 deer49, 102 and water buffaloes (Bubalus bubalis),31, 54, 59 are also intermediate hosts. Antibodies to N. caninum have been found in foxes (Vulpes vulpes),23 coyotes (Canis latrans),69 camels (Camelus dromedarius)55 and white-tailed deer (Odocoileus virginianus), >39 suggesting that these hosts may be natural intermediate hosts for N. caninum. Cats, mice, pigs, rats, foxes, gerbils, and monkeys may be induced to be experimental intermediate hosts.31, 66 Only an asexual cycle occurs in the intermediate hosts which consists of tachyzoites and tissue cysts. Both tachyzoites and tissue cysts are microscopic and intracellular. Tachyzoites are ovoid, lunate or globular and measure 3–7 μm × 1–5 μm, depending on the stage of division (Figure 23.2). In infected animals, tachyzoites are found in many cells including neurons, macrophages, fibroblasts, vascular endothelial cells, myocytes, hepatocytes and dermal cells. Tachyzoites are usually located within the host cell cytoplasm within a parasitophorous vacuole. Ultrastructurally, N. caninum tachyzoites are similar to T. gondii tachyzoites except that the rhoptries in N. caninum are electron-dense whereas in T. gondii they are electron-lucent.42, 71

Tissue cysts are often round to oval in shape (Figure 23.4), up to 107 ?m long, and have been observed only in neural tissues (brain, spinal cord, nerves and retina)34 with the single exception of a solitary tissue cyst in the ocular muscles of a foal.72 The tissue cyst wall is smooth and up to 4 ?m thick, presumably depending upon how long the infection has existed. In most tissue cysts, the cyst wall is 1 to 2 ?m thick. Septa are absent and there is no secondary cyst wall (Figure 23.5). Bradyzoites are slender (6–8 ?m × 1–1,8 ?m) and contain the same organelles as are found in tachyzoites except that there are fewer rhoptries and more periodic-acid Schiff (PAS)-positive (amylopectin) granules in the bradyzoites. Tissue cysts may degenerate and cause a host reaction (Figure 23.6).

Dogs fed tissue cysts may shed unsporulated oocysts.76 Oocysts can sporulate outside the host within 24 hours. Sporulated oocysts contain two sporocysts each with four sporozoites. Neospora caninum oocysts are 10 to 11 ?m in diameter (Figure 23.7) and are morphologically indistinguishable from Hammondia heydorni found in canine faeces, and Toxoplasma gondii and Hammondia hammondi in cat faeces. At present, nothing is known regarding the frequency of shedding of oocysts, the survival of the oocysts in the environment, and whether canids other than domestic dogs are also definitive hosts for N. caninum.

Susceptible hosts can become infected by ingesting food and water contaminated with N. caninum oocysts from dog faeces. Experimentally, animals may become infected lactogenically.26, 98

Neosporosis in cattle

Neosporosis affects both dairy and beef cattle.47, 58, 96 Bovine N. caninum infection has been reported from Europe, Australia, Asia, Africa, New Zealand, and the Americas.22, 31, 79, 81, 82, 96 It is a major cause of abortion in dairy cattle in the USA,1, 2, 8, 96 New Zealand,97 and the Netherlands. 103

Figure 23.1  Life cycle of Neospora caninum

Figure 23.2  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

Based on serological surveys, up to 100 per cent of cattle in some herds have been exposed to N. caninum. 40, 85–88, 101 Neoporosis has been reported in cattle from South Africa,62, 63 and Zimbabwe.64

Neospora caninum is efficiently transmitted vertically in cattle, even for several generations, but horizontal transmission is necessary to introduce new infections in the herd.3, 17

Neospora caninum can be transmitted transplacentally in naturally infected cattle, sheep, goats, deer and horses, and experimentally in several other species of animals.31 Transplacental infection can occur repeatedly in the same animal and through its progeny for several generations. The mechanisms of primary and repeat congenital transmission of infection are unknown.

Whether the repeat congenital infections which occur in dogs and cattle are due to relapse of the primary...

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