NeosporosisÐthe ®rst decade of research

This paper reviews the history of Neospora caninum for the past decade, and provides a prospective for research for the next decade. # 1999 Australian Society for Parasitology Inc. Published by Elsevier Science Ltd. All rights reserved.

The year 1988 was eventful for me because of the discovery, naming and in vitro cultivation of the protozoan parasite Neospora caninum, and the development of a serological test to distinguish N. caninum from the related parasite Toxoplasma gondii [1,2]. Until 1988, N. caninum was misdiagnosed as T. gondii because of structural similarities. The early history of Neospora research and the progress made since 1988 are summarised in Table 1.
Neosporosis is not a new disease; retrospective studies have shown that severe disease due to N. caninum was found in a group of dogs in the USA as early as 1957 [3]. Historically, BjerkaÊ s et al. [4] ®rst recognised a neurological disease in dogs in Norway associated with a T. gondii-like parasite. Notably, antibodies to T. gondii were not found in aected dogs, nor was the parasite from dogs infective to mice. A similar parasite was found in dogs in the USA and a new genus Neospora, and Neospora caninum were proposed for the canine parasite [1]. It is noteworthy that the parasite was named based on formalin-®xed canine tissues. Therefore, in vitro isolation of the parasite, development of diagnostic tests, ful®lment of Koch's postulates, recognition of its role in abortion, and ®nally the elucidation of the life-cycle ( Fig. 1)Ðall in the past 10 yearsÐare remarkable (Table 1). Two recent reviews have summarized structure, life-cycle, biology, host range, clinical signs, diagnosis, and control of Neospora and neosporosis in animals [5,6].
The publication of this special issue of the International Journal for Parasitology devoted entirely to the biology of neosporosis is a tribute to the progress made by many researchers worldwide. Although N. caninum and T. gondii are structurally and antigenically related, they are biologically distinct. For example, N. caninum is a major cause of abortion in cattle [7±10], whereas T. gondii is not known to cause abortion in cattle. Neospora is not considered a human International Journal for Parasitology 29 (1999) 1485±1488 0020-7519/99/$20.00 # 1999 Australian Society for Parasitology Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 0 -7 5 1 9 ( 9 9 ) 0 0 1 3 4 -4 pathogen, whereas T. gondii can causes loss of sight or even death in humans.
Neospora caninum can cause severe clinical signs in dogs, and should be considered in dierential diagnosis of any neurological disorder in dogs, especially pups. The most serious clinical signs of neosporosis are observed in congenitally infected dogs. Although N. caninum tachyzoites may be found in many organs, it encysts only in the CNS. As yet, there are no drugs to kill tissue cysts of N. caninum.
Neospora caninum-induced abortion and stillbirth occur in both dairy and beef cattle. Approximately 25% of abortions in dairy cattle in California, USA, and the Netherlands are attributed to N. caninum infection [7,8]. Abortions can occur thoughout the year and at the same rate in housed cattle in California vs pastured cattle in the Netherlands. The median gestation age at the time of abortion is 5±6 -months. At present, there is no proven vaccine or drugs to kill N. caninum tissue cysts.
The epidemiology and control of bovine neosporosis will be exciting areas of research for the next millennium. How cattle initially become infected in nature is still an enigma. The recent discovery of the N. caninum oocyst in dog faeces [11] can explain postnatal infection in cattle. However, in limited experiments, dogs shed only a few oocysts [11,12]. Whether other canids or carnivores can excrete N. caninum oocysts needs to be investigated. The prevalence and the survival of N. caninum oocysts in the environment are currently unknown. The search for endogenous stages leading to oocyst formation in the intestine of dogs and understanding the mechanism of immunity to oocyst reshedding will be necessary to develop strategies for prevention of environmental contamination with Neospora. Nothing is known at present concern- Marsh et al. [34] 18. Dogs identi®ed as a de®nitive host for N. caninum.
McAllister et al. [11] a Modi®ed from Dubey and Lindsay [28] ing antigenic variation in various isolates of Neospora, or the mechanism of immunity or lack of it in cattle infected with N. caninum. The incidence of repeat abortion due to N. caninum is also not known. Answers to these questions will be necessary for a rational approach to the development of a vaccine to prevent N. caninum-associated bovine abortion.