Читать книгу Parasitology - Alan Gunn - Страница 35

The morbidity (illness) and mortality (death) associated with parasitic diseases causes financial losses to both an individual, their family, and to the wider society. These losses divide into the direct costs and indirect costs, and these are used in ‘cost‐of‐illness’ studies to prioritise healthcare funding decisions (Onukwugha et al. 2016). The direct costs include factors such as the costs of diagnosis and treatment. They are therefore relatively easy to identify and calculate because they consist of purchase costs and wages. By contrast, the indirect costs are much more wide‐ranging and nebulous. For example, they include the costs associated with the infected individual’s inability to work or reduced efficiency/productivity. They also include wider and often unappreciated costs that are borne by the family and/or the community. For example, the death of someone results in their family incurring the funeral costs (which can be considerable), as well as debilitating psychological stress that may impair their ability to work. Because most parasites cause chronic infections that persist for months or even years, the indirect costs associated with them often exceed the direct costs. For example, a study in China found that one case of malaria cost $US 239 (1,691.23 Chinese Yuan) of which the direct costs constituted 43% and the indirect costs 57% (Xia et al. 2016). Furthermore, the costs were equivalent to 11% of a household’s income. Similarly, in southern India, lymphatic filariasis costs in the region of US$ 811 million per year and cause productivity losses as high as 27% in the weaving sector (Ramaiah et al. 2000). Parasitic diseases that cause disfigurement often results in social exclusion that further traps the sufferer in poverty and mental ill health. People suffering from lymphatic filariasis can become so isolated that they will not venture out to seek freely available treatment at government clinics, let alone to look for paid employment (Wijesinghe et al. 2007). Although it is not a financial calculation, experimental studies indicate that for wild animals living communally, it is also the indirect costs of parasitism that impact most upon the group (Granroth‐Wilding et al. 2015).

For domestic animals, there are the direct costs of diagnosis and treatment along with mortalities but the losses that result from lost productivity (e.g., milk yield, live weight gain) and/ or work capacity (e.g., draught oxen, camels, donkeys) are much greater. Unfortunately, the calculation of losses associated with parasites in the agricultural industry is problematic, and there is a lot of variation between individual farms. In addition, published figures can rapidly become out of date through currency fluctuations, changes in farming practices and the value of stock (amongst many other factors). Therefore, we provide just a few figures to illustrate the potential of parasites to cause financial losses. In the United Kingdom, gastrointestinal parasitic infections in lambs are estimated to cost the British sheep industry ~£84 million per year (~USD$ 102.4 million); the costs associated with infections in breeding ewes are not known but the combined figure would obviously be much higher (http://beefandlamb.ahdb.org.uk/wp‐content/uploads/2013/04/Economic‐Impact‐of‐Health‐Welfare‐Final‐Rpt‐170413.pdf). Brazil is a much larger country with a huge cattle industry, and the financial impact of parasitic diseases is correspondingly massive. They are estimated to cause losses of approximately US$13.96 billion per year; gastrointestinal nematodes are responsible for ~51% of these losses and the tick Rhipicephalus microplus a further 23% through direct effects and as a vector of other parasites (Lopes et al. 2015a). In the United States, the protozoan parasite Neospora caninum is estimated to cause in the region of US$ 546 million per annum in the dairy industry alone. The losses it causes in agriculture on a worldwide basis could be as high as US$ 2.38 billion per annum (Reichel et al. 2013). There are no figures for the economic cost of N. caninum infection in dogs, but many dog owners will spend large sums of money on the welfare of their pets and pedigree dogs can sell for hundreds or even thousands of pounds. Consequently, control of the disease in dogs is of concern to owners, as well as a means of preventing its transmission to cattle.

In developing countries, the economic costs of parasitic diseases of livestock can have consequences for the expansion of agriculture and the ability of populations to feed and clothe themselves. For example, in Pakistan, the increasing demand for milk and milk products has seen the import of high‐yielding Holstein‐Friesian breeds. Unfortunately, these are particularly susceptible to the tick‐borne protozoan parasite Theileria annulata (causative agent of Tropical Theileriosis) and the losses it causes can account for 13.8% of a total farm’s costs (Rashid et al. 2018). Similarly, in east, central, and southern Africa, East Coast Fever in cattle caused by Theileria parva results in annual losses of hundreds of millions of pounds/dollars and is one of the reasons many people in the region remain subsistence farmers (Muhanguzi et al. 2014). Although vaccines against both T. annulata and T. parva have been available for many years, there are practical problems associated with their use. Consequently, preventing the transmission of infections is mostly through acaricides that kill the tick vectors. However, because tick populations are increasingly resistant to these, there is a fear that the ticks will spread and consequently so will the diseases.