Towards evaluating the economic impact of bovine neosporosis

Abstract

In spite of the global importance of neosporosis as a cause of bovine abortion, there is very little information about its economic consequences. The economic costs are a product of estimations of the quantity of the effects attributable to Neospora infection, and the particular unit costs of those effects. In this brief review, which arose from a workshop on the economics of coccidiosis held at the COST 820 meeting, Toledo 1998, we discuss the possible effects of neosporosis which are of economic significance and summarise the available estimates of their magnitude to provide a basis for further economic analysis. Neospora infection has been associated with abortion, increased culling and reduced milk yield. In addition, it has been diagnosed in cases of stillbirth and neonatal mortality, it is likely to contribute to early foetal death and resorption and it is responsible for a reduction in the value of female breeding cattle. In quantifying the role of Neospora, it is important that epidemiologically based, case-controlled studies are conducted because, given the extreme efficiency with which bovine Neospora infection is vertically transmitted, demonstration of prevalence of infection in affected animals (including foetuses) is not a true indicator of the significance of this disease. Relatively few epidemiological studies have been conducted, but in investigations in the USA, Holland and Britain, infected cows have been shown to be about three times more likely to abort than non-infected cattle. In the UK this approach has been used to estimate the proportion of abortions in the national dairy population which may be attributable to Neospora caninum.

Introduction

Neospora caninum[1], the causal agent of neosporosis, is a newly recognised protozoan parasite, closely related to Toxoplasma gondii. Originally reported by Bjërkas et al. in 1984[2]in the brains of dogs suffering from meningoencephalitis, N. caninum-like organisms were first associated with bovine abortion in the USA3, 4. Since these initial observations, N. caninum parasites have been reported to be associated with bovine abortion in an increasing number of countries worldwide, including the USA, Europe, Australia, New Zealand, Africa and Japan[5]. Characterisation of the organisms in dogs and cattle have shown them to be indistinguishable[6], and recently the dog has been described as a definitive host of N. caninum[7]. There have been a few reports of natural infections with N. caninum parasites in other species of animals, but such infections appear to be rare[5]. Despite the fact that the parasite is very closely related to T. gondii, a known zoonotic pathogen, there is no convincing evidence to date that N. caninum can infect humans. It is clear from the accumulating data that the vast majority of neosporosis cases occur in cattle and dogs. Although pathogenic in dogs, N. caninum is not associated with canine abortion. This article will confine itself to considering the economic impact of neosporosis in cattle, and particularly dairy cattle. A number of well-documented cases have shown that bovine neosporosis can be associated with considerable and potentially economically devastating losses in the form of abortion outbreaks (see[5]). However, these outbreaks do not necessarily give a good picture of the overall impact of N. caninum infection, and there is an urgent need to assess just how important the disease is to cattle production at both national and international levels.

Very crudely, calculation of the economic losses due to neosporosis in cattle requires: (i) quantification of the effects which are directly attributable to infection with N. caninum, e.g. number of abortions in a population, and (ii) the direct and indirect costs of those effects, e.g. unit cost per abortion. Currently there are substantial difficulties in gathering data for either of these. Firstly, there are few quantitative and reliable estimates of the significance of bovine neosporosis, and then only for some of its economic consequences. Secondly, computing the economics of the effects is complex, requires a specialist approach, and will vary substantially from country to country. Here we attempt to address the first question only, and to provide a framework to identify the key variables that need to be determined for economic cost calculations.