Impact of climate change on emerging infectious diseases: Implications for Canada

It is now accepted that our planet is warming and that seasonal stability has given way to unpredictable and irreversible violent hydrometeorological events (e.g., floods, heat waves, and droughts) (1). Canada is not spared by climate change, and this has led to the adoption of the Pan-Canadian Framework on Clean Growth and Climate Change by the Government of Canada in 2016 (2). This plan includes an approach to pricing carbon pollution and measures to achieve reductions across all sectors of the economy. It also includes actions to advance climate change adaptation and build resilience to climate impacts across the country.

Many infectious diseases are either strongly influenced by short-term weather conditions or show seasonal changes suggesting that they are influenced by long-term climate changes (3). Changes in the long-term average values of daily weather parameters, including temperature and precipitation, are most likely to affect microorganisms that spend the majority of their life cycle in outside hosts (such as ticks or mosquitoes) and those with environmental reservoirs (e.g., waterborne infections and endemic fungi). It is likely that climate change will affect the incidence and severity of several infectious diseases in Canada, through territorial expansion of vectors, the emergence of new pathogens with environmental reservoirs, and the rise of infectious diseases associated with natural disasters.

Vector-borne diseases in Canada

Tick-borne diseases

The number of Lyme disease cases acquired in Canada has increased significantly over the last decade, and, while Ontario and Nova Scotia are the two provinces with the highest incidence of Lyme disease, all Canadian provinces have reported cases, although not all are autochthonous to the province of reporting, with 917 cases documented across the country in 2016 (4). The rise of Lyme disease in Canada has been correlated with the geographic expansion of its vector Ixodes scapularis. It has been shown that temperature is the most important determinant of environmental suitability for tick-population establishment in Canada. Models predict that the proportion of the human population in eastern Canada inhabiting areas with established tick populations will increase from 18% in 2010 to over 80% in 2020 (5). Besides Ixodes scapularis, the combined annual proportion of submitted Ixodes cookei (a vector of Powassan encephalitis), Dermacentor variabilis (a vector of Rocky Mountain spotted fever [RMSF] and tularemia), Rhipicephalus sanguineus (a vector of RMSF), and Amblyomma americanum (a vector of human ehrlichiosis and tularemia) in passive surveillance in Québec rose from 6.1% in 2007 to 16.0% in 2015; moreover, an annual growing trend has been observed for each tick species (6). Indeed, an expected consequence of the ticks’ expansion is an increase in the risk of acquiring the diseases they carry.

Mosquito-borne diseases

There have been several outbreaks of dengue and chikungunya fever in Italy and France since 2010 (7,8). The emergence of these infectious diseases has been associated with the establishment of the associated vectors Aedes albopictus and Aedes aegypti (9). In Canada, recent modelling data have shown that the risk for autochthonous transmission in the current climate status is very low, with all the regions of Canada classified as either unsuitable or rather unsuitable for transmission (10). However, small areas of southern coastal British Columbia might become suitable based on short- and long-term projected changes in the climate. On the other hand, the vector of the West Nile virus, Culex spp, is now well-established and has been expanding its territory in Canada (11); this is consistent with the steady transmission rate of West Nile virus infections in many Canadian provinces for over a decade (12).

Fungal infections with environmental reservoirs in Canada: Cryptococcus gattii and Blastomyces dermatitidis

Before emerging on Vancouver Island in 1999, Cryptococcus gattii was considered primarily a tropical and subtropical pathogen (13). Historically, infection with C. gattii has been associated with exposure to two species of Australian eucalyptus trees. On Vancouver Island, the fungus has been isolated on a variety of non-eucalypt trees such as the Douglas fir and Western red cedar (13). It has been suggested that warming conditions over the past decade have created an optimal environment for the establishment and spread of the fungal pathogen. Recently, Uejio et al have shown that changes in climate have systematically influenced C. gattii concentrations in environmental samples from Vancouver Island and that the highest airborne C. gattii concentrations have occurred on sunny days with moderately windy conditions (14). Moreover, an increasing number of cases have been observed in patients living in temperate regions in Europe, including those without recent travel history to disease-endemic areas (15). Similarly, two cases of C. gattii infections have also been recently reported in Québec in patients who had not travelled outside the province (16). This paper suggested a possible newly established endemicity in Québec. Altogether, these data suggest that the organism is expanding its geographical range into previously uncolonized areas because of the warming trend.

The ecology of Blastomyces dermatitidis is still incompletely understood, in part because its reservoir has been difficult to identify based on culture isolation of the organism from environmental samples. Klein et al have, nonetheless, demonstrated the existence of this fungal pathogen in rich, moist organic soils (17,18). Therefore, environmental exposure may increase the risk of contracting the infection. Current epidemiological data exploring risk factors for blastomycosis acquisition have primarily come from surveys conducted in certain endemic areas around rivers such as the Mississippi, the Ohio, and the St. Lawrence, as well as in the Great Lakes area (19). Retrospective studies have indicated an increased incidence of blastomycosis in Canada and the United States (19–22). The St. Lawrence River Valley has been widely cited in the literature as an endemic area. A recent study has reported an increased incidence of the disease in Québec between 1988 and 2011 (23). Similar findings have been observed in several parts of the United States (24). Some authors have suggested a possible relationship between the increasing incidence and certain climate factors, such as total precipitation and higher maximum temperature (20,25). Research presented at IDWeek 2018 showed that the severity of blastomycosis cases observed in Québec over the past 30 years has increased (26). In this study, severe cases were defined as patients needing mechanical ventilation, patients who developed acute respiratory distress syndrome, and those who developed septic shock. The proportion of severe cases rose from 3% between 1988 and 1997 to 21% between 2008 and 2017. This increase in disease severity may be partially explained by an increase in the number of immunosuppressed patients; however, changes in climate may also lead to inhalation of higher loads of fungal spores which, in turn, may translate to higher disease severity.

Impact of severe hydrometeorological events on infectious diseases in Canada: The great unknown

Floods are one of the most common natural disasters occurring worldwide. Over the past 100 years, the number of floods occurring across the country has increased, and extreme floods are becoming more common; moreover, the five most destructive floods in Canadian history have all occurred since 2010 (27). There is a strong link between flood occurrence and the acquisition of infectious diseases. These risks are numerous and very well documented in developing countries, primarily those in tropical areas. Flood-linked infections have been shown to include skin, soft tissue, and gastrointestinal infections (28–31). The spread of vector-borne diseases such as malaria can also be promoted by the proliferation of vectors associated with stagnant water (32). The occurrence of floods could even promote the dispersal of microorganisms resistant to antibiotics into the environment. Given the significant use of antibiotics in agriculture and the presence of antibiotic-resistant microorganisms in solid and liquid waste by-products (33), there is a risk of spreading these resistant microorganisms, which could result in the direct transfer of resistance genes to human pathogens.

The risks of acquiring infectious diseases following flood disasters are much less well documented in the North American context, apart from specific hydrometeorological events such as Hurricane Sandy in New York in 2012 (34) or the major floods that occurred in Colorado in 2013 (35). There is even more limited literature documenting the impact of floods on the acquisition of infectious diseases in Canada (36). To develop appropriate preventive measures and emergency responses, we need a better understanding of the impact of flooding disasters on the transmission of microorganisms in our country, and there are significant gaps in the research in this field; this will also help public health authorities to assess whether the methods of risk mitigation currently used are appropriate (37).

Conclusions

Changes in climate have both direct and indirect effects on the incidence and severity of infectious diseases. Despite efforts to minimize global warming, it is likely that climate-driven emerging infections will become an increasing threat to public health in Canada. Thus, it is important to increase our capacity to respond to the rising demands posed by these infections and to improve our resiliency to the health impacts of these infectious diseases through surveillance and monitoring. Finally, notwithstanding all of the interest and discussion surrounding climate change, there has been little success in changing human behaviour with a reduction of the carbon footprint of households. Perhaps the explosion in the incidence of some infectious diseases will be the trigger for a major human change?

Competing Interests:

The authors have nothing to disclose.

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Funding:

No funding was received for this work.

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This article has been peer reviewed.