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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
HIV/AIDS	57.6	54.6–61.0	570,000–1.1 million (2018)	https://www.unaids.org/en/resources/fact‐sheet
Measles	5.72	2.15–12.26	73,400 (2015)	Wang et al. (2016)
Ebola	0.0003	0.0002–0.001	5,500 (2015) 33 (2018)	Wang et al. (2016) https://www.afro.who.int/health‐topics/ebola‐virus‐disease
Diarrhoeal diseases	74.41	63.4–93.4	1.65 million (2016)	Troeger et al. (2018)
Syphilis	9.42	5.47–14.60	107,000 (2015)	Wang et al. (2016)
Road injuries	71.40	67.52–76.13	1.35 million (2018)	https://www.who.int/violence:injury_prevention/road_safety_status/2018/en/

DALY and DALY range data were derived from Hay et al. (2017). The mortality data were derived from the most recent year available at the time of writing and from various sources.

Although some workers have attempted to use economic costings for wildlife diseases, it is a controversial approach: how much is a blackbird worth? Indeed, there has been a tendency for parasitologists to view wildlife mainly from the perspective of their potential as reservoirs of disease for human infections or those of our domestic animals (Thompson et al. 2010). This has sometimes led to widespread culling of wildlife. For example, in parts of Africa it was once common practice to kill antelopes and other large game animals to prevent them acting as a reservoir of Trypanosoma brucei infection. Similarly, at the time of writing, the practice of culling badgers in the United Kingdom to prevent the spread of TB in cattle is proving hugely controversial and expensive. Its effectiveness is also debateable.

The rate of extinctions amongst animals and plants is proceeding at an alarming rate and with it the realisation that we need to do more to conserve them. This is not just an ethical issue, but it also has economic implications since wildlife tourism is big business in some countries. Any attempt at conserving an organism must consider the diseases it suffers from. In addition to natural infections, wild animals are also afflicted by parasites introduced to their habitat by humans. It would be wrong to consider natural infections as invariably benign and those introduced by humans as invariably malign. For example, until the introduction of the New World screwworm fly (Cochliomyia hominivorax) eradication campaign in the USA, one estimate suggested that it killed up to 80% of white‐tailed deer fawns in the southern states every year (Fuller 1962). The screwworm fly was present naturally and the eradication campaign was solely to prevent infections in cattle and other domestic animals, but the result was beneficial to wildlife too. More commonly, a parasite colonises a new area through contamination (e.g., in soil or ship ballast water) or through infections of us and our domestic animals. The consequences then depend upon whether the invading species finds other suitable hosts and, if it needs one, a suitable vector or intermediate host. The exposure of any naïve animal (or human) to an agent capable of establishing an infection in them often ends badly and if that agent can complete its life cycle in the area, then the consequences for the local population of new hosts is equally dire. For example, on the Galapagos Islands, the populations of several of the species of Darwin’s finches have been devastated following the arrival of the fly Philornis downsi. It probably came to the islands in the 1960s among imported fruit and vegetables. The adult flies are free living, but their blood‐feeding larvae are ectoparasitic on nestling birds and cause high mortalities (McNew and Clayton 2018). Wildlife tourism brings in hundreds of millions of dollars per year to the Galapagos Islands (https://www.galapagos.org/wp‐content/uploads/2012/01/TourismReport2.pdf). Although most people do not visit the Galapagos Islands to spot Darwin’s finches, the loss of iconic species such as the Giant Tortoises to introduced parasitic infections would undoubtedly have serious implications for the tourist industry.