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1- Aggravating factor: modern transport
Long before planes, there were ships and their ballast tanks – those big tanks that optimize navigation. Wooden ships used dry ballast – tons of sand, earth and stone – but metal ships, since the 19th century, have used ballast filled with water, which is emptied as soon as it reaches its destination. Microbes travel there more easily. Historically, we owe them several cholera epidemics, as recently as 1992 in Peru. An international treaty on ballast cleaning came into force in 2017.
But with air travel, germs can now spread to more than one continent in a matter of days. This was the case with SARS in 2003. Viruses that are transmitted by a mosquito, such as West Nile virus, are unlikely to survive air travel, but respiratory viruses benefit from it, as we have seen over the last couple of years.
2- Aggravating factor: more excrement
Historically, human excreta have been a means of disease transmission. Before it was understood in the 19th century that water could contain bugs invisible to the naked eye, many epidemics were caused by bacteria present in our faeces and traveling in running water. In several countries, it remains a problem: in Haiti, in 2010, a cholera epidemic, brought by Nepalese soldiers from a United Nations contingent, worsened considerably because only 19% of the Haitian population had access to toilets or latrines.
But humans are not the only problem. Farm animals also produce large amounts of manure: 130 times more in the United States, according to a 1999 estimate. In Canada, in 1996, this represented an average of 361 million kilograms of manure per day, half from cattle, and about 500 million tons in 2006. The proximity of different species of livestock and the fact that part of their excrement ends up in the surrounding streams creates new opportunities for pathogens, thanks to the ability of microbes to exchange genetic material when they meet (“transfer horizontal of genes”).
And its spread may be favored by global trade: this is what happened in May 2011, when a batch of fenugreek seeds, contaminated in Egypt by a hitherto unknown strain of E. coli, caused an epidemic in Europe (mainly in Germany): at least 4,000 cases. On a smaller scale, it is contamination withE. coli which explains the removal of romaine and cauliflower from supermarket shelves in Canada in January 2019.
3- Aggravating factor: overcrowding of animals and humans
The growth of cities was one of the reasons why the 2014 Ebola epidemic was so deadly (more than 11,000 deaths): the previous ones always occurred in rural areas or relatively small cities, so in 2014 the virus hit three metropolises. And unlike the previous 21 outbreaks that were contained within a few months, the 2014 outbreak lasted nearly two and a half years. Journalist and author Sonia Shah wrote prophetically in 2016 in her book Pandemic “The same can happen with smallpox.”
This risk has been observed even longer with the flu, in which the crowding of animals, including poultry and swine, has facilitated the creation of new, more virulent strains.
It must be remembered that influenza viruses originated from wild birds. Occasionally, some arrive at the poultry farms. And the most virulent, such as the one currently underway, are called “high pathogenic avian influenza”. In 1997, one such flu, H5N1, proved capable of infecting humans. Although cases remain very rare, since then more than half of those infected have died. H5N1 has spread to farms across the world and continues to evolve.
Of all viruses, influenza viruses are among the most worrisome, due to the high risk of mutations and contact with humans through very large poultry farms, particularly in China. It should be added to this that certain strains can also be transmitted to other livestock mammals, including pigs.
4- Aggravating factor: global warming
A warmer climate favors the northward movement of insects such as those that carry West Nile virus, dengue and even malaria, potentially including in Canada.
More precipitation also gives a boost, for example to ticks that cause Lyme disease. A 2015 study estimated that heat and rain would start tick season two weeks early in North America.
By increasing the flow of runoff water, rainfall also promotes the spread of pathogens. So, across the planet, so-called “waterborne” diseases – linked to contaminated water such as cholera – are responsible for 40% of climate-related health emergencies since the early 2000s, according to a report. recent. WHO.
In 2007, French researchers established a correlation between the emergence of cholera epidemics and climate data in five West African countries, indicating at the same time that climate change and variations in the volume of precipitation had an impact on the emergence of infectious outbreaks, an impact that can last for years.
More recently, an international team estimated that “more than half” of infectious diseases would sooner or later be “aggravated” by climate change.
The heaviest rains would have preceded the appearance of two-thirds of the waterborne diseases that occurred between 1948 and 1994 in the United States. More recently, researchers have linked a heavier-than-average rainy season in 2021 in the southwestern United States to an outbreak of 1,600 West Nile virus cases in Arizona — the largest outbreak in Joe Biden’s country’s history.
5- Aggravating: sociology?
In 2016, Sonia Shah was also prophetic about social behavior: “Unlike acts of war or catastrophic storms,” she wrote four years before COVID began, pandemics “do not build trust or favor cooperative defenses. On the contrary, they are more likely to generate suspicion and distrust among us, destroying social bonds as surely as they destroy bodies. »
It could be because the enemy is invisible, or it could be because of our feeling of helplessness in the face of that enemy. But in any case, if she is right, we will have to follow how different populations will react, in the coming months, to the evolution of smallpox, or to a possible advance of poliomyelitis in North America, after the first cases detected this summer near New York. .
Pascal Lapointe and Melody Alasset
Image: Scientific Photo Library