Learning Module | Health THREATS
Chapter One
1. ZOONOTIC DISEASES
OVERVIEW:
Zoonoses are infectious diseases that are transmissible from vertebrate animals to humans under natural conditions, either directly or via insect vectors. Causal agents include viruses, bacteria, parasites, fungi and prions. Approximately 60% of all known infectious diseases are zoonotic. [2]
Largely, zoonotic infections can be divided into enteric (those that impact the gastrointestinal system), and non-enteric (those that do not).
Enteric Zoonoses
Examples of enteric zoonoses include Salmonellosis, Campylobacter, and Giardia.
These are often more common during the summer months. Increased temperature and flooding events are expected to contribute to changes in disease incidence in the future. Outbreaks of E. coli, Campylobacter, and Cryptosporidium have been linked to summer weather. Outbreaks of Vibrio parahaemolyticus in BC have been associated with above-average ocean temperature. [3]
Non-enteric Zoonoses
Examples includes vector-borne diseases (Lyme disease, west Nile virus), directly transmitted zoonoses (Brucellosis, rabies, influenza), and environmentally mediated (anthrax, leptospirosis).
Vector-borne diseases (VBD) are those transmitted via vectors (mainly arthropods such as mosquitos, ticks and fleas), capable of transmitting infectious pathogens between hosts.
In this chapter, we will largely focus on VBD and climate change.
ZOONOTIC DISEASES are greatly affected by our changing climate and are impacted in both direct and indirect ways. [1]
DIRECT IMPACTS
The changing climate creates environments more suitable for vector and pathogen development in a variety of ways:
Climate change & rising temperatures increase the development and reproduction of pathogens. This can be partially accounted for by the fact arthropod vectors are largely exothermic, meaning they rely on external temperature to regulate their internal conditions. In addition, the larval development stage generally requires increased humidity and temperature.
Biting rates of vectors tend to increase with increasing temperature (to an upper limit). [4,5]
INDIRECT IMPACTS
Climate change stands to impact the transmission of VBD by increasing the spread of infectious diseases, with population shifts and changes in land use creating increased opportunities for transmission.
Shifts in the population often result from climate change causing a reduction in suitable living conditions and increased conflict over resources, leading to climate refugees (those forced to relocate due to climate change). As populations shift, there are changes in land use, via both urbanization and agricultural expansion. These factors influence exposure to vectors and thus the transmission of pathogens. [6]
GLOBAL SCALE
Globally, diseases of concern include malaria, dengue, yellow fever, chikungunya and zika. [7] Despite advancements in public health and disease prevention, globally, vector-borne diseases remain an increasing public health burden and stand to be further impacted by climate change. For example, dengue fever has increased in incidence by 30-fold over the last 50 years, and recently outbreaks of dengue fever have been reported in the United States, Madeira, Croatia, China and Japan. [8]
Physicians are also likely to encounter greater numbers of other mosquito-borne diseases including Chikungunya, Dengue, and Zika in travellers. These diseases are not present in Canada but stand to increase their current geographic range due to:
- an increase in exotic pathogens and their vectors outside of Canada
- an increase in travel and global trade,
- increased introduction of pathogens and vectors and increased climate suitability in Canada.
Though these diseases may not expand their range to BC, they will increase their range into areas that Canadian travellers visit, including the Caribbean, Latin America, and Asia. The number of dengue cases reported to WHO increased over 8 fold over the last two decades. [8]
VBD IN CANADA
Although traditionally in Canada, tropical VBDs have not been of concern, increasing temperatures and an overall milder climate may lead to both the introduction of and increased cases of VBD. A review in 2015 identified emerging vector-borne zoonotic diseases of public health importance in Canada, which identified tick-borne diseases including Lyme disease, WNV and other mosquito-borne diseases as a high priority. [9]
Lyme disease.
This tick-borne disease is the most common VBD affecting humans in the temperate Northern Hemisphere, with an increase seen in Canada. [4] The causative agent – bacteria Borrelia burgdorferi - transmitted by ticks.
The incidence of Lyme disease in Canada has risen by over 18 fold from 2009 to 2019 in Canada, believed to be related to changes in climate, that facilitate the geographical spread of the disease. [10] The increase in cases has been primarily seen in central and eastern Canada, and the Northeastern United States.
West Nile virus.
West Nile Virus (WNV) has been present in Canada since 2002 and presents in a “boom and bust” pattern, with years of having few cases, followed by spikes in outbreaks. Transmission is mainly from mosquitos that have fed on infected birds (crows, blue jays).
Recorded case numbers are thought to be underestimated, as most cases are asymptomatic. The first case detected in British Columbia was in 2009. [11] Climate suitability for the vectors of WNV is predicted to increase in the coming years. As treatment is supportive, the focus should be shifted to prevention. [6]
Canada-wide differences in Lyme Disease
In contrast, BC has seen lower and more stable numbers of Lyme disease cases in comparison to central and eastern Canada. Despite expected greater tick numbers in BC the number of people exposed to Lyme disease is not expected to greatly increase.
The reasons for this are multifactorial, in part due to differences in tick species, with the Western Blacklegged tick found in B.C, (I. Pacificus) carrying a relatively low risk of Lyme disease in comparison to the eastern black-legged tick (I. scapularis), found in central and eastern Canada. [6] Other factors include differences in tick diet and regional differences in vegetation and climate. [12]
Adaptation Strategies
AWARENESS
Ticks can be found year-round, most likely to bite in the spring (March to June) [12]
Populations at higher risk for tick-borne disease include:
- Those involved in outdoor activities (hunting, fishing, hiking, camping, gardening, forestry, farming)
- Those who live in known endemic areas
- Those ages 5-9 or older (age 55+) [13]
Encourage patients to identify the risk of Lyme Disease in their area
ONE HEALTH APPROACH
Currently, the “One Health” approach is being used, in which multiple sectors collaborate to create solutions. For example, to manage infectious disease, communication between leaders in agriculture, global trade and healthcare will allow for a coordinated approach. Focus will be placed on the interaction between humans, animals, and the evolving environment. [5]For example, read more about the E-tick app, a way to record tick encounters here.
CITY LEVEL CHANGE
City design centred around increasing green space is key in reducing urban heat island effects and decreasing areas prone to stagnant water buildup. [5]Have an idea for a green space project in Vancouver? Apply for a grant here.
REPORTING
Physicians play an important role in surveillance by reporting cases to public health. A study by Dr. Bonnie Henry found that only 62% of B.C. physicians were aware that Lyme disease was a reportable condition in BC, highlighting the importance of physician education on procedure to ensure disease incidence is reported. [14]Familiarize yourself with the current risk of vector borne diseases in your area, and which diseases are reportable.
Provide guidance to patients about risk and prevention strategies. [15]
To learn more, check out the following resources:
Encourage patients to identify their Lyme Disease Risk Area