Background
Around the world the three major components of climate change already evident and
escalating in magnitude and significance are; 1) warming; 2) altered patterns of
precipitation; and 3) an increased incidence of extreme climatic events [1]. For
the structure and function of ecosystems, impacts of climate change vary with place
and with time, and among the key outcomes are shifting boundaries for many components
and processes within the systems. Among these components are pathogens and infectious
diseases, including those caused by helminth, arthropod and protozoan parasites in
people, domestic animals, and wildlife [2].
For host-parasite assemblages, boundaries potentially vulnerable to climate change
include those for spatial and temporal distributions of hosts and parasites, for parasite
survival and development in hosts and in the environment, for risks of transmission
to hosts at critical points in parasite webs, and for health effects on hosts, including
the emergence or resurgence of disease. The often complex and obscure linkages and
inter-relationships among components of an ecosystem, coupled with the uncertain and
variable trajectories for climate change, make it difficult to identify all these
vulnerabilities, particularly in the medium to long term. Also, faced with non-overwhelming
“stress” most ecosystems display a degree of resilience that may mitigate some of
the consequences of climate change [3,4], and in some circumstances the significance
of parasites remains essentially unchanged. Finally, some recent shifts in disease
occurrence that intuition might suggest are associated with climate change have proved
likely to be wholly or partly the result of other factors [5,6].
The primary aim of this paper is to provide a framework for thinking about the critical
potential connections between climate change, parasites, people, and wildlife in the
circumpolar North, and between these host groups, climate change, parasites and domestic
animals in other areas of the world.
Approaches
Much of the information currently available on climate change and infectious disease
relates to people and is based on retrospective analyses of associations between components
of climate involved in climate change and the occurrence of disease in human populations
[7,8]. In other instances, features of parasite ecology have been linked to model-based
scenarios for future climate change to generate medium to long-term projections for
parasite and disease distribution and occurrence [9,10]. Underlying these approaches
are observational and experimental studies in a range of systems exploring, on a more
intimate scale, the relationships between climate and parasite, and sometimes host,
ecology [11-13]. All these lines of enquiry are increasing understanding of the mechanisms
generating boundary shifts for parasites and diseases resulting from climate change,
and are assisting proper targeting of measures to minimize their impacts on human
and animal health. Encouragingly, effective climate-based forecasting, developed
decades ago for ruminant fascioliasis [14], is now a reality for some epidemic human
malaria in Africa [15] and is being evaluated for other human parasitic diseases,
for example human fascioliasis [16] and leishmaniasis [17] in South America. Exploration
of the effects of climate change on infectious disease ecology presents many opportunities
for valuable comparisons across pathogen and host groups, and across ecosystems.
Central to understanding these climate change-host-parasite linkages is the ability
to detect and measure shifts in key features of parasites and hosts and to assemble
data unequivocally establishing or refuting links to climate change. Given relevant
meteorological data, although monitoring and surveillance of parasitic infections
and diseases may be possible to some extent in people and domestic animals, even in
remote areas with limited infrastructure, it is usually more difficult in wildlife
[18]. A particular issue for this host group, especially in Arctic and the North
and other relatively isolated areas, is the currently limited understanding of the
parasite fauna, including species diversity and distribution, and its health significance,
especially in the absence of obvious disease or mortality [19,20]. A recently initiated
and very promising approach in northern Canada and elsewhere is to recruit, train
and fund northerners, particularly harvesters who have frequent contact with wildlife,
as health monitors. This program is greatly enhanced in the longer term where wildlife
and wildlife health are introduced into curricula for schools in northern communities
(see http://www.ccwhc.ca/Sahtu/index.php).
The fragile North
The North is among areas of the world where climate change is already having significant
and obvious effects and is impacting northerners and the animal and plant resources
vital to their health and well-being [21,22]. For example, at risk on land are keystone
wildlife species, including caribou, reindeer, moose, thinhorn sheep and muskoxen,
waterfowl, and fish, together with berries and other foods of plant origin. In the
surrounding oceans, polar bears, seals, walrus, seabirds and fish are all vulnerable.
Among the elements of climate change threatening the health and sustainability of
people and wildlife in the North, perhaps the most significant is warming, which is
shifting boundaries for animals and plants [23], and for sea ice, permafrost, snow
cover, and hydrology, as well as local and regional infrastructure [22]. Warming
is also a cause of rising sea levels and the consequent erosion and flooding of coastal
areas and disruption of coastal ecosystems and settlements [22].
People and animals that inhabit the North are beset by an array of helminth, arthropod
and protozoan parasites. Most of these are restricted to one of the two host groups,
but several – the zoonoses – are transmissible from animals to people, often through
foods integral to traditional local cultures [18]. These zoonoses include (in North
America) Trichinella, Anisakis, Diphyllobothrium, Echinococcus, and Toxoplasma, and
perhaps Cryptosporidium and Giardia. All of these can cause obvious clinical disease
in people, but not in everyone who is infected.
Host and parasite vulnerabilities
Many aspects of host and parasite ecology in the North and elsewhere have been identified
as potentially vulnerable to climate change. Among possible consequences are boundary
shifts that can alter the structure and function of host-parasite assemblages [24,25].
The speed and extent of these shifts vary with place and with time. For example,
those linked to extreme climatic events may be rapid and localized, whereas those
resulting from warming may be more gradual and widespread. For definitive and intermediate
hosts, including arthropod vectors, these shifts include: 1) geographic distributions
– expansion into new areas and/or loss from old areas and, in some cases, local to
regional extinctions, together with shifts in migration routes; 2) faunal structure
– qualitative changes in the composition of multi-species host communities, including
shifts in opportunities for contacts between wildlife and domestic animals; 3) trophic
linkages - including predator-prey relationships important for parasite transmission,
especially for several zoonoses [26]; 4) phenology - especially the timing of breeding
seasons and migrations, and the synchronization of the need for and availability of
food; 5) level of nutrition – determined by the composition, availability, accessibility
and quality of food and water; 6) health and wellbeing – including patterns of disease
occurrence, and possible detrimental synergies between parasites, other infectious
agents and other diseases; 7) host abundance – possibly affecting host density and
thus parasite transmission dynamics; 8) behavioural patterns – influencing exposure
to parasite and in some cases subsequent environmental contamination with parasites;
and 9) parasite evolution [27] - likely to be detected first among protozoans. For
people dependent to some extent on wildlife, as many northerners are, parasites may
be one of the means by which climate change results in shifts in the availability
and quality, or perceived quality, of their food and other key products (e.g. hides
and pelts) of wildlife origin, and in the role of wildlife in their cultural and economic
wellbeing and in the sustainability of northern communities.
For parasites, some potential boundary shifts are similar to those for hosts. For
example, as distributions and faunal structures for hosts shift, so too will those
for parasites. In some ecosystems, as a result of host switching, both immigrant
(or invasive) and endemic hosts may experience new parasites, and these may be especially
pathogenic for naïve hosts and may result in emergent or resurgent diseases. Shifts
in parasite faunal structure may also result from altered trophic linkages, and the
levels of nutrition, health and wellbeing of hosts will influence their susceptibility
to parasites and other diseases and may lead to shifts in the role of parasites in
ecosystem dynamics. Outside their mammalian and avian hosts, many parasites have
life cycle stages in the environment or in ectothermic intermediate hosts and vectors
that are exposed directly to climate. Key potential boundary shifts here are in parasite
survival and development rates [12] and, for some species, in amplification rates
for parasites developing in ectothermic hosts [11]. If warming from climate change
enhances these rates, lengthens the summers vital for the transmission of many northern
parasites, and shortens and softens the winters then, simplistically, more infective
stages of parasites could be available sooner and the transmission period could be
extended. In some instances, these shifts have the potential to generate greater
parasite abundance in the definitive hosts and to increase their health impacts.
Some case studies
Despite our currently relatively limited understanding of the ecology of host-parasite
assemblages in the Arctic and the North, it is possible to speculate how some might
be influenced by climate change. Although evidence transforming this speculation
to certainty remains sparse, it is important to consider these issues and especially
to identify potential high-risk scenarios for the emergence of significant parasitic
disease in people and in wildlife.
Trichostrongyles of Ungulates
Trichostrongyles (e.g. Ostertagia gruehneri andTeladorsagia boreoarcticus) are non-zoonotic
nematodes that as adults parasitise the abomasum or intestines. They have direct
life cycles involving the development of eggs deposited in the feces to free-living,
infective larvae in the environment. Infection of ungulate hosts is by ingestion
of these larvae. Climate change, as well as its positive or negative effects on the
hosts, may shift patterns of development for the parasites’ free-living stages. For
example, assuming adequate moisture, longer, warmer summers may increase survival
and development rates for the free-living stages leading perhaps to shorter generation
times and to greater abundance and increased longevity for infective larvae in the
environment. This in turn may increase the infection pressure and parasite loads
for hosts and lead to greater adverse impacts on host health (e.g. weight loss and
reduced conception rates) [28,29] and, for species important as food for northerners,
on human health. In addition, altered summer transmission dynamics and fall climate
may shift patterns of larval inhibition in the gastro-intestinal mucosa, an important
mechanism for overwinter survival by some trichostrongyles in other areas of the world.
A useful preliminary glimpse of the links between climate change and altered ecology
for trichostrongyles can be derived from basic information about pre-patent periods
and the relationships between environmental temperatures and larval survival and development
rates as determined in the laboratory and in the field. Data are plentiful on these
aspects of trichostrongyles of domestic animals in several areas of the world [12],
but caution is required when attempting to extrapolate these data to the species of
parasites infecting free-ranging hosts, particularly in the Arctic and the North.
Protostrongylids of Ungulates
Protostrongylid nematodes (e.g. Umingmakstrongylus pallikuukensis, Parelaphostrongylus
odocoiei and P. andersoni) are non-zoonotic and live as adults in the airways, lung
parenchyma or skeletal musculature. Their life cycles are indirect, involving development
of first-stage larvae deposited in feces to infective larvae in gastropod intermediate
hosts. Infection of ungulates is by ingestion of infected gastropods or of infective
larvae spontaneously emerged from the gastropods.
The life cycle stages of these parasites outside the hosts have vulnerabilities to
climate change generally similar to those of the trichostrongyles but it is possible
that gastropod mobility and avoidance of extreme habitat conditions may protect the
larvae from some of the effects of a changed climate [30].
For U. pallikuukensis, an empirical model derived from laboratory and field studies
demonstrated that warming in the North probably has already shortened larval development
times in gastropods and shifted transmission dynamics from a two-year to a one-year
adult-to-adult cycle [31]. A similar model for P. odocoilei indicated that temperature
constraints affecting larval development rates in gastropods may define the northern
limits of the parasite’s distribution, and that warming may remove these and lead
to an expanded parasite distribution [10]. Also, for U. pallikuukensis, attempted
experimental infections indicated that thinhorn sheep, potentially newly sympatric
with muskoxen as a result of shifts in host geographic distributions perhaps associated
with climate change, are not susceptible to the parasite [32].
Trichinella nativa
Trichinella is a genus of zoonotic nematode containing species that infect a range
of vertebrates, including people, in many parts of the world. Trichinella nativa
is the primary northern species. Adult Trichinella live in the small intestine, and
the larvae produced by the female parasites migrate to skeletal muscle and sometimes
other tissues. These larvae are the parasite’s infective stage and transmission is
by carnivorism, including feeding on carrion. In the North many host species are
infected, and of special concern are those consumed by people, especially polar and
black bear, walrus, and seal. Other than in carrion, life cycle stages of Trichinella
are not exposed to the environment and any effects of climate change are likely to
result primarily from shifts in host faunal structure and trophic linkages [26].
Outside the North, the ecology of Trichinella may be modified by climate-induced shifts
in contacts between wildlife and domestic animals, and perhaps through behavioural
shifts in the utilization of infected hosts as food for people.
Cryptosporidium and Giardia
Among the several species and genotypes currently established for each of these two
genera of protozoans some are zoonotic and infect a range of hosts but most seem restricted
to a single host species [33]. Although some species/genotypes are shared between
people and domestic animals, the significance of wildlife as sources of human infections,
and of people as a source of the parasites for wildlife, remain uncertain and unexplored.
Both parasites live primarily in the small intestine and the life cycles are direct.
Infection is by ingestion of infective oocysts (Cryptosporidium) or cysts (Giardia)
from the environment or from contaminated food or water. Climate change has the potential
to alter survival rates for the cysts and oocysts (which are infective when voided
by the hosts) and, because both parasites are found in surface water, shifts in local
and regional hydrology may alter parasite distributions and the risks of human and
animal exposure. In human settlements altered patterns of precipitation and extreme
climatic events may disrupt the integrity of the infrastructure, particularly water
supplies and sewage disposal, increasing the risk of human infection. In addition,
these elements of the climate change may result in increased run-off and contamination
of water with animal feces, and increased risk of zoonotic transmission.
Priorities for action
For people, domestic animals and especially wildlife, in many situations around the
world it is difficult to identify all the causes of detectable shifts in disease occurrence
and, correctly, efforts are directed principally at mitigation of the disease and
at effective control. Additionally, for all host groups, it may be difficult to tease
parasites from among other potential contributors to disease, and to determine the
role of climate in shifts in disease ecology and host health [34]. For wildlife,
the detection of these shifts may also be hampered by a lack of baseline data for
the occurrence and significance of pathogens and diseases. In exploring climate change
as a cause of new patterns of disease, however, much can be learned from the many
data-derived relationships between key climatic factors and host, parasite and disease
ecology, and the integration of these with projections for climate change trajectories.
This capability, coupled with an integrative, multidisciplinary and ecological approach,
makes possible the identification of parasitic infections and diseases likely to be
particularly susceptible to climate change and, with adjustments for regional variations,
the exploration of some of the possible consequences of accelerating climate change
for the occurrence of these diseases and for animal and human health. This is a very
urgent need, and without such an attempt to anticipate the possible, society is likely
to be a more or less impotent spectator to the certainty of continual ecological calamities.