The paper reviews some protozoan parasites responsible for infertility in livestock with specific emphasis on neosporosis, sarcocystosis, toxoplasmosis, and trichomoniasis. It highlights the transmission, pathogenesis, clinical signs, diagnosis, prevention, control, and treatment of the individual parasite. It is concluded that these parasites are often overlooked during investigation into causes of abortion in livestock, and they may, however, be responsible for some infertilities in this group of animals which may result in severe economic losses. 1. Introduction Infertility is a major cause of economic losses and a major limitation to achievement of optimum efficiency in the livestock production system largely as a result of loss of milk production in the dairy sector and increase in culling rates. The causes of infertility can be numerous and complex [1]. They may relate to impairment in Graafian follicle development and maturation, onset of estrus, ovulation, successful coitus, fertilisation, implantation, and the development and delivery of the foetus and its membranes. Any temporary state interfering with these routines, such as disease conditions, malnutrition, inadequate herd management, hereditary and congenital factors, hormonal disturbances, or environmental changes, makes the animal infertile [2]. Parasitic infections are typically associated with poor and often marginalized communities in low-income countries [3]. Protozoan parasites are a significant cause of abortion and infertility in domestic ruminants. Toxoplasma gondii and Sarcocystis species are a common cause of extensive abortion in livestock and they both have a two-host life cycle. Carnivorous definitive hosts spread the infection through their feces and domestic ruminants are intermediate hosts. A similar, recently recognized protozoon, Neospora spp., has emerged as an important cause of reproductive disease, especially as an abortifacient in cattle. Neospora is presumed to also have a two-host life cycle, although the definitive host(s) has not been identified. The venereally transmitted Tritrichomonas foetus has also been reported to be an important cause of pregnancy loss in naturally bred cattle throughout the world [4]. 2. Neosporosis Neospora caninum is an obligate protozoan (Coccidian) parasite related to Toxoplasma gondii and was first associated with neuromuscular disease in dogs in the mid-1980s [5]. It is globally distributed and has been identified as a major cause of infertility in cattle resulting in considerable economic losses [6, 7]. The parasite has been
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