On March 22, British Columbia’s First Nations Health Authority and the Vancouver Island Health Authority issued a public warning after four confirmed cases of cholera were reported on Vancouver Island, Canada. The WHO reports only 7 cases throughout all of Canada’s provinces over the last 5 years. Often, these cases are from individuals returning from areas where cholera is endemic; however, this outbreak is believed to have been caused by the consumption of contaminated herring eggs harvested off the island’s coast. How these herring eggs became contaminated remains under active investigation. Below, we highlight some potential factors that may have contributed to the onset of this unique outbreak.

What is cholera?

Cholera, the topic of several previous Outbreak Thursday posts, is a waterborne disease typically acquired by ingesting food or water contaminated by Vibrio cholerae bacteria. The WHO estimates that between 1.3 and 4 million cases of cholera occur worldwide each year, resulting in 21,000 to 143,000 deaths. Prior to the advent of modern sanitation methods, cholera was fairly common around the world. Canada, for example, had six large cholera epidemics during the 1800’s, which killed more than 20,000 people in total. Today, however, cholera outbreaks occur predominantly in developing countries that lack access to clean water and effective sanitation systems.

Possible Contributing Factors

A 2015 report by the Canadian Broadcasting Center showed that 12.7% percent of wastewater in the British Columbia province—in which Vancouver Island is located—went untreated. Additionally, the same report stated that more than 41 billion liters of untreated sewage/wastewater was dumped in nearby waters, including nearly 17 billion liters off the coast of Vancouver, BC, which is directly across the Strait of Georgia from Vancouver Island. The prevailing tidal currents around Vancouver Island would carry all of this wastewater, potentially containing fecally-contaminated sewage, along the coastal area where the contaminated herring eggs were harvested. The spread of fecally-contaminated water is one of the most common routes of cholera transmission, and this could potentially be a factor in contaminating the herring eggs suspected to be the source of the outbreak. For example, several studies have attributed sewage dumping as a contributing factor in cholera outbreaks in the Dominican Republic, Haiti, and Uganda, but it could potentially also be a factor in developed countries like Canada if wastewater is not properly treated.

Additionally, Vancouver has experienced unusually heavy rains recently, with a total of 25 days of rain during the month of January. One study in Haiti found that increased rainfall was “correlated with increased cholera incidence 4 to 7 days later.” Heavy rains could potentially overwhelm water treatment facilities that handle combined wastewater and storm drain systems. When these systems are unable to process the excess volume of water, untreated water, including wastewater, is purged into the local environment. Currently, British Columbia is working to separate sewage and rainwater systems, but this is not expected to be completed until the year 2050.

Another potential contributing factor could be a changing climate. The association of V. cholerae with chitinaceous zooplankton and shellfish has been well documented. Colonized zooplankton blooms can contain more than enough V. cholerae to cause human infections. Recently, a Vancouver sport fishing site mentioned witnessing minor plankton blooms earlier this month. Typically, these plankton blooms are seen between the months of April and May when the temperature increases, but this particular bloom is earlier in the season than normal. Ocean and tidal currents sweeping along coastal areas during plankton blooms could potentially facilitate the spread of V. cholerae. Dr. Shannon Waters, a medical health officer with Island Health, told reporters “[Our oceans] are under pressure from sewage, from boat traffic and from rising temperatures. Our health is connected to the oceans and I think this [outbreak] is a sign of that.”


Canada’s Department of Fisheries and Oceans has issued a temporary closure of herring egg harvest in the area where the illnesses have been reported—a 20-kilometer stretch of coastal waters from French Creek to Qualicum Bay. Island Health, in collaboration with the British Columbia Center for Disease, are testing marine water, leftover food, and clinical samples to gain insight into how this outbreak reached the area. Further genetic analysis to identify the particular strain of V. cholerae is also underway. Knowledge of the particular strain may provide some clues as to the origin of the outbreak as well as whether this strain exhibits any notable mutations.


While these issues play a role in cholera outbreaks in developing countries, it is important to remember that outbreaks in these environments usually occur because cholera is already endemic in the area. Places like Vancouver Island, where the disease is not endemic, would require the introduction of the bacteria from elsewhere. While health officials have focused their attention on contaminated herring eggs as the direct source of infection for the identified cases, further investigation is required to determine how V. cholerae was introduced into the area. This outbreak serves as a reminder that the same factors that contribute to cholera outbreaks in developing countries (eg, untreated water, heavy rains, and climate change) can also impact developed countries as well.


Outbreak Observatory aims to collect information on challenges and solutions associated with outbreak response and share it broadly in near-real time to allow others to learn from these experiences in order to improve global outbreak response capabilities.

Photo: Vancouver Island coastline (British Columbia, Canada); courtesy of Pixabay.