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.
1.7.1 DALYs: Disability‐Adjusted Life Years
A common means of measuring the consequences of human disease and other causes of morbidity is to calculate disability‐adjusted life years (DALYs). These are derived by summing an estimate of a disease or condition’s potential for reducing lifespan and an estimate of the amount of time a person suffering from the disease/cause is disabled (www.who.int/evidence/bod). One DALY is the equivalent of the person losing a year of healthy life.
For example, a person committing suicide or dying in a traffic accident would suffer premature death, but there would be little or no disability (assuming they died instantly), whilst a person with malaria may suffer prolonged ill health and ultimately die prematurely years later. DALYs facilitate the comparison of morbidity and mortality factors and thereby help prioritize funding and policy decisions and determine the effectiveness of health initiatives. In some studies, the DALY model is refined to place greater value on the life of a young adult than of a child or older person. This version considers young adults more economically beneficial to society and with a longer productive life in front of them than a child or older person. However, the use of age weighting is contentious and the WHO ceased using this approach in 2010.
The use of DALYs began in 1994 and although the WHO and many other organisations employ them, they have always been controversial. For a detailed consideration of the limitations of DALY calculations, see Parks (2014). The use of DALYs to assess the importance of parasitic diseases is particularly difficult because the estimation of the years of life with disability includes a weighting factor that supposedly accounts for the severity of the disease. This can result in wildly different estimations. For example, although some studies suggest that the global burden of human schistosomiasis is ~3 million DALYs, others have put it as high as 70 million (Hotez et al. 2010). Furthermore, coinfections with several parasite species and parasite–pathogen interactions (e.g., Leishmania‐HIV) are common and can have major implications for disease progression and outcome.
A comprehensive study of global health metrics by Hay et al. (2017) provides an insight into the relative importance of various causes of mortality and morbidity. Table 1.2 shows a selection of their data. Except for malaria, many parasitic diseases have comparatively small DALYs compared with other sources of morbidity/mortality – this is because they operate within restricted distributions. For example, car accidents are a common source of morbidity and mortality in all countries, and therefore, it is not surprising that they have high DALY values. Similarly, diarrhoeal diseases, sexually transmitted infections, and measles are serious diseases throughout the world – though many people do not realise that in addition to causing morbidity, many can also be fatal. The accuracy of all statistics depends upon the accuracy with which the data are recorded. For developing countries with few resources and those in the grip of armed conflict, this is extremely difficult. Consequently, the literature often includes huge discrepancies about how many people suffer from a disease and how many people die from it. For example, according to Wang et al. (2016), the nematode Ascaris lumbricoides was responsible for 2,700 deaths in 2015, but a WHO website suggested that around 60,000 people die of the disease every year (https://www.who.int/water_sanitation_health/diseases‐risks/diseases/ascariasis/en/).
Table 1.2 A comparison of global disability adjusted life years (DALYs) and mortality for selected parasites and other factors.
| Factor | All‐age DALY (million) (year = 2016] | DALY range (year = 2016] | Mortality per annum (year) | Reference for mortality data |
|---|---|---|---|---|
| Malaria | 56.2 | 45.8–67.9 | 435,000 (2017) | https://www.who.int/news‐room/fact‐sheets/detail/malaria |
| Visceral leishmaniasis | 0.71 | 0.40–1.21 | 24,200 (2015) | Wang et al. (2016) |
| Cutaneous/mucocutaneous leishmaniasis | 0.27 | 0.18–0.40 | Rarely fatal | |
| African trypanosomiasis | 0.13 | 0.06–0.22 | 3,510 (2015) | Wang et al. (2016) |
| Schistosomiasis | 1.86 | 1.12–3.18 | 4,400 (2015) | Wang et al. (2016) |
| Lymphatic filariasis | 1.19 | 0.59–2.11 | Rarely fatal | |
| Ascariasis | 1.31 | 0.88–1.94 | 2,700 (2015) 60,000 (date not stated, website accessed 2019) | Wang et al. (2016) https://www.who.int/water_sanitation_health/diseases‐risks/diseases/ascariasis/en/ |
| Hookworm | 1.69 | 1.00–2.65 | Rarely fatal |
