- Parasitic disease - Wikipedia;
- Global change, parasite transmission and disease control: lessons from ecology.
- Parasites inside your body could be protecting you from disease!
Where pharmaceutical interventions have had a clear effect in reducing infectious disease prevalence, the challenge now is to ensure that such success is sustainable in the context of environmental change. River blindness, caused by the nematode Onchocerca volvulus carrying the Wolbachia bacterium [ 12 ], was introduced into South America by the Simulium black fly, which has been treated with periodic ivermectin administration since Elsewhere, problems remain in terms of attributing causality to, or quantifying the success of, MDA programmes [ 8 ].
Second, global climate models reveal an ever-changing pattern of land surface temperature, rainfall and vegetation cover across the surface of the planet [ 15 ]. Thus, contemporaneous environmental changes could potentially confound the effects of MDAs. Third, host range shifts may spread parasites into areas where monitoring and MDA are not being applied. Finally, the programmes themselves may have generated selection pressures, as has been observed in other systems such as malaria [ 16 ], leading potentially to resistance, adaptation and other evolutionary consequences.
In looking to the future sustainability and success of MDA and other interventions, we posit that it is imperative to consider what factors related to global change not only impinge on current efforts, but how global changes, including those brought about by control efforts themselves, might influence the outcome of attempts at control, elimination or eradication of specific infections. Stress—response impacts on parasite control programmes. Online version in colour. We are nonetheless reminded of the value to be drawn from proxy observations from comparable systems [ 17 ].
In terms of parasitology, much of the relevant proxy information has been drawn from wildlife disease ecology, which has tended to pay more attention to the issues of global change than comparable studies on human and domestic animal parasites.
In this paper we demonstrate how ecological studies of parasites in wildlife may be used to enhance our understanding of stressors arising from global change, which are likely to be important in the context of parasitic infections both macro- and microparasites of humans and domestic animals. First we consider the influences of some major abiotic and biotic stressors associated with global change and, second, how these stressors might affect parasite life cycles, transmission and ecology. In doing so, we highlight that the abiotic—biotic distinction is blurred, particularly as many stressors also act simultaneously and indirectly on parasites through their hosts.
Third, we explore how parasites may respond to the evolutionary pressures of such stressors. Finally, we consider how these complex impacts of global change potentially militate against the sustainable control of parasites affecting animals and humans, and make suggestions for improved understanding and control in an uncertain future world. The majority of modern day ecosystem stressors are driven by industrialization combined with human population growth.
These in turn are responsible for increased resource use and generation of waste products, many of which have negative impacts on the environment in complex direct and indirect ways, which may subsequently affect disease risks. For example, the combustion of fossil fuels for energy production and for powering transportation modes significantly contributes to air pollution and carbon dioxide emissions, which promote climate change. Changing land uses, including farming for food, further contribute to climate change and both are considered major drivers of biodiversity loss.
Broad-scale biodiversity loss, latitudinal and altitudinal host range expansions and retractions, reduced wildlife population sizes and more limited habitat connectivity are subsequently affecting host interactions and changes in parasite transmission. The multiple components of climate change, including temperature, precipitation and atmospheric CO 2 , have been extensively studied individually [ 18 — 20 ], but the interactions between these environmental stressors and the consequent effects on parasite transmission are complex.
Thus, there is considerable uncertainty about how future climate variation and change will affect disease dynamics [ 21 — 23 ]. In combination these stressors may counteract each other, such that the overall rate of parasite transmission remains unchanged. Higher temperatures, for example, often increase parasite growth, reproduction and infectivity [ 25 , 26 ], yet can also increase parasite mortality, and as such there is no change in the number of transmitted parasites [ 27 , 28 ].
Likewise, while temperature elevations accelerate the replication of arthropod-borne viruses in their insect or tick vectors, they simultaneously increase vector mortality and decrease biting rate, making the net effect of temperature increase on transmission difficult to predict without detailed knowledge of each component in the system [ 29 ]. In other instances, increased temperatures have more pronounced effects on the host, which may exhibit acclimation, adaptation or be forced to shunt resource investment into various life-history components, resulting in thermal preference shifts.
Poikilothermic hosts are particularly vulnerable to temperature shifts, but also show remarkable adaptations and such responses by the host can be damaging for the parasite. Some fish, for example, exhibit adaptive behavioural traits to reduce transmission risk, by actively selecting thermal conditions that are detrimental to parasites behavioural fever; [ 30 ] or selecting flow conditions that minimize fitness costs of infection and potentially reduce transmission [ 31 , 32 ]. Disentangling anthropogenic environmental change from that of natural variation is problematic, particularly for indirect effects and naturally rare events such as extreme weather conditions or disease outbreaks [ 33 , 34 ].
The relationship between environment and transmission is also complex. Different environmental parameters may have additive, multiplicative or antagonistic and nonlinear effects on transmission, which themselves may be intercorrelated or vary at different spatial or temporal scales, with such effects difficult to measure [ 35 , 36 ]. Such relationships may be a consequence of transmission mode. For example, flooding events can be a key driver of some water-borne disease epidemics [ 37 ], while drought conditions cause hosts to aggregate at sites where water is available, amplifying transmission and triggering outbreaks of vector-borne diseases such as African horse sickness and Rift Valley fever [ 38 , 39 ].
Other environment—transmission relationships are likely to be a result of a host—parasite range shift due to climate warming. This can change the distribution of vector-borne diseases, including malaria [ 40 ] and Rift Valley fever [ 41 ]. However, climate change is not spatially homogeneous and could render previously suitable areas unsuitable for transmission and vice versa [ 42 ].
The effect of range shift can be yet more complex if the degree or rate of change differs between the host and parasite, causing host—parasite interactions to decouple across some or all regions [ 43 ]. For example, tick-borne encephalitis virus TBEV transmission is sustained only when temperatures result in synchronous feeding of larvae and nymphs [ 44 ]. Projected temperature rises might desynchronize feeding and shrink the area within which TBEV persists [ 45 ]. Even the immediate effects of change in temperature and rainfall on parasites are therefore complex and strongly modified by host factors.
However, pollutants also impact parasites themselves, and in aquatic ecosystems, both the infective stages of parasites and their intermediate hosts can be highly sensitive to their effects [ 47 ].
Heavy metals can inhibit the release of trematode cercariae from molluscan hosts, as well as impair their swimming behaviour and longevity [ 48 — 50 ]. Pharmaceutical pollutants are widespread stressors likely to affect host susceptibility to disease. The scale of this threat is increasingly apparent in aquaculture: in Chilean salmon farms alone, hundreds of tonnes of antibiotics are used annually [ 51 ]. Eutrophication—another important stressor of aquatic ecosystems, arising from excessive nutrient input—is associated with elevated intermediate host densities, parasite fecundity and increased prevalence of certain pathogen infections [ 52 ].
However, as yet, there is no overall consensus on its consequences for general patterns of infection [ 53 , 54 ].
Infectious diseases - Symptoms and causes - Mayo Clinic
Other forms of pollution are less well studied with regard to disease transmission. While it is known that light pollution can impact the structure and function of ecosystems via cascading effects [ 55 ], and that natural light cycles govern both relevant parasite life-history traits e. Although the introduction of electricity to socio-economically developing communities has overall human health benefits, night lighting inevitably attracts certain insect vectors and increases human night-time activity. Thus vulnerability to being bitten by a vector is increased, which is implicated in higher incidences of leishmaniasis and malaria in some regions [ 58 ].
Parasites A-Z Index:
In other insect-vectored diseases, artificial lighting may have a less overt effect on transmission dynamics: triatome bugs, the vectors of Chagas' disease, typically avoid well-lit areas and artificial lighting may be driving Chagas transmission towards a sylvatic cycle [ 58 ]. Noise pollution, a known stress-induced modulator of the immune response [ 59 ] that can significantly affect behaviour and predator—prey interactions [ 60 ], has not yet been considered in terms of infectious diseases, even though it could have a major influence on farmed animals.
The gaps in knowledge concerning the impacts of all types of pollution on parasite transmission are considerable, and without this information it is challenging to assess its importance across host—parasite systems. Habitat alteration due to climate change or anthropogenic activity poses a major threat to ecosystems, often leading to substantial loss of biodiversity, ecosystem functioning and services, and reduced resilience to external stressors [ 61 — 65 ].
The effects of habitat change can even have contrasting effects on closely related parasite species infecting the same host; for example, sunbirds in disturbed habitats exhibited increased prevalence of Plasmodium lucens but decreased prevalence of P. Habitat edges often promote increased species diversity i. How the differential effects of edge versus interior sites impacts parasitism varies between host—parasite systems; infections may significantly increase [ 77 ], decrease [ 78 ] or be unaffected [ 80 ].
Although re-establishing connectivity may facilitate initial disease spread [ 81 ], in the long term, a larger host gene pool is likely to decrease vulnerability to disease [ 82 ], while also increasing overall biodiversity. Over the past 50 years, there have been unprecedented changes in farming practices and associated land use [ 83 ].
Growth in crop production and livestock has been driven by the demand for higher yields.
Modern and large-scale farming practices typically rely on concentrating and containing inbred hosts, which can increase host exposure to and facilitate parasite transmission [ 86 , 87 ]. Chronic stress induced by high stocking densities in aquaculture can have important implications for fish immuno-competence [ 89 ], but relationships with infection levels are variable. While high host densities can promote greater parasite population densities, the number of conspecific parasites per host may be reduced [ 90 ].
Positive effects of high host density on transmission can be attenuated by mixing susceptible and resistant hosts in rotational grazing systems [ 92 ], showing the importance of multiple hosts in modifying infection pressure. However, in aquaculture, where hundreds of thousands of hosts are contained together, this is not yet possible [ 93 ], partly because of the need to track farmed fish in the event of an accidental release, and also because of concerns about disease transmission between farmed and wild stocks and vice versa.
While density-dependent transmission of human parasites may be expected to increase with high population densities and ownership of companion animals, decreased human—wildlife contact and better sanitation in cities of developed countries generally point to lower levels of disease transmission among such populations e. Dengue, for example, is more prevalent in urban areas due to the provision of suitable human-created microhabitats for the Aedes mosquito [ 95 ]. Urban environments with high human densities are potentially more vulnerable to water-borne or faeco-orally transmitted parasites if investment in sanitation infrastructure is neglected or disrupted due to socio-economic unrest.
Poverty is an important related factor; a study of the contiguous cities of Laredo USA and Nuevo Laredo Mexico on the USA—Mexican border found that dengue transmission was strongly affected by income, and hence access to technologies such as air conditioning [ 96 ]. In developing countries, human—wildlife conflicts can be a major issue. Most emerging and re-emerging human infectious diseases EIDs are zoonotic, typically with origins in mammalian wildlife [ 97 , 98 ] or interactions between wildlife and domestic animals [ 99 , ]. This might increase further as habitat loss forces the co-occurrence of wildlife and humans, although this could be offset by the greater effects of biodiversity loss see below.
A major factor underpinning urbanization is demographic change. Millions of individuals are also expected to migrate during their lifetimes due to factors associated with the urban—rural cycle, extreme weather events, economic necessity, water and food security, and conflict [ ]. Increased patchiness of wealth associated with urbanization, combined with disrupted social structures has already changed the entire landscape of NTDs. Associated with urbanization is increased road building.
Road building has already increased the risk of some diseases associated with human development e. Such large-scale road building will almost certainly further facilitate bushmeat hunting in the most biodiverse regions of the planet [ ] and change the scale at which people are able to move wild animals out of newly exploited areas and into commodity chains, thereby increasing public health zoonosis risks.
Overall, pathogens likely to thrive as a result of urbanization tend to be either those for which transmission is strongly density-dependent, or those with vectors or reservoirs that are themselves well adapted to urban environments. The net effect on parasite burdens will be highly case-specific and difficult to predict, especially where urbanization is rapid and strong interactions with rural populations persist [ ]. Current extinction rates are estimated to be — times greater than background levels [ ], with biodiversity loss being one of the hallmarks of the Anthropocene [ ].