Have you ever wondered what it is that makes us consider some insects and animals pests (like fleas and rats) and others not? The characteristics of pests are widely discussed amongst pest experts, and although definitions differ slightly, they all contain echoes of the same concept, one I think is the most important in defining a pest: the opportunity to cause some form of injury to humans.
Injury can be defined as injury (damage) to property, crops or wellbeing, or illness. One Department of Health defines a pest as animals or insects which “carry disease-causing microorganisms and parasites”.
So if one of the characteristics of a pest insect or animal is that it is “disease causing”, it’s interesting to note that the word “pest” in the dictionary comes from the latin word “pestis” which means plague, as does the word “pestilence” - meaning a fatal epidemic disease. And in archaic use, the bubonic plague was referred to as “the pest”. Which just goes to show that the association between pests and disease is a long one!
So how are pests and disease linked, besides linguistically?
Throughout human history, there have been a number of recorded pandemics (a pandemic is defined as a disease prevalent over a whole country or the world) including smallpox and tuberculosis. However, the most fatal pandemic in recorded history is arguably the Black Death (also known as the Plague) which killed an estimated 75–200 million people in the 14th century.
Most people have, at some stage, heard of the Black Death and how it made the Middle Ages such a terrifying time to be alive (or not!) but the interesting thing about the plague is that it’s been around for a lot longer than the Middle ages, and it’s directly linked (and not only in the dictionary) to pests and pest control, because it was caused by flea bites from fleas infected with the Yersinia pestis bacteria.
The earliest recorded outbreak of the Plague - the Justinian Plague - began in 541 AD and over the next 200 years, it killed over 25 million people. The next Plague pandemic is the one we are most familiar with: the Great Plague or “Black Death”. This time the plague began in China in 1334 and spread along trade routes to Constantinople and Europe, and is estimated to have killed 60% of the European population.
The last plague pandemic occurred in the 1860s (yes, that recently!) and is possibly less well known. It began in China and spread to India, and was responsible for 10 million deaths over the next 20 years. But let’s take a step back and look at what plague is and how it is transmitted.
What is the plague and what does it have to do with pest control?
The plague is a serious bacterial infection that can be deadly (it certainly was in the Middle ages!). Referred to by a number of names, including the “Black Plague” or “the Black Death”, “the Pestilence” or even just “the pest”, the disease is caused by a bacterial strain called Yersinia pestis. Yersinia pestis is found in animals throughout the world, but is usually transmitted to humans through infected fleas.
There are three different types of plague caused by Yersinia pestis: bubonic plague, septicemic plague and pneumonic plague. Bubonic plague, which is the most common, occurs when the Yersinia pestis bacteria enter through the skin through a flea bite and travel via the lymphatic vessels to a lymph node, causing it to swell.
One to seven days after exposure to the bacteria, flu-like symptoms develop. These symptoms include fever, headaches, and vomiting. Swollen and painful lymph nodes (called buboes in medieval times - hence the name Bubonic plague) occur in the area closest to where the bacteria entered the skin. Black boils would then appear on the victim’s skin, hence the term “Black Death.”
Many people think - erroneously - that rats caused the plague, but rats just provided the transportation method for fleas infected with Yersinia pestis to travel far and wide. Rats were everywhere in Europe, and they and their infected fleas spread throughout the continent via trading routes, using boats as transportation.
Better pest control - particularly on trade route ships - would have meant that rats carrying fleas (carrying plague) would not have been able to spread along the trade routes so quickly. Medieval historian Norman Cantor described the Great Plague of the 14th century as “the greatest biomedical disaster in European and possibly in world history.”
Sadly, we didn’t learn from history and at the end of the 19th century, the bubonic plague killed 10 million people in India. Once again, the disease was carried by fleas and rats on opium ships from Hong Kong and spread easily in the cramped and damp conditions of Bombay. The city was damp, there were heavy monsoons, and an ineffective drainage system meant that there was a lot of stagnant water, a great breeding ground for rodents and in turn the fleas that carried plague.
Plague flares up now and again in various countries around the world, but unlike the 14th century, we now understand how the disease is transmitted, and can get it under control with good pest control and hygiene practices. In 2009 Rentokil was called in by the Ministry for Public Utilities of Libya to deal with rats in its major cities, and in Tobruk a crack team supported the World Health Organisation to tackle an outbreak of the Bubonic plague there.
Another notable pandemic is the 1918 influenza pandemic (Spanish flu), which infected 500 million people–about a third of the world's population at the time–in four successive waves. The death toll may have been anything from 17 million to 50 million, and possibly as high as 100 million, making it the second deadliest pandemics in human history. Spanish Flu was caused by an H1N1 virus with genes of avian origin, but how the virus jumped to humans is still unclear, so for now this is one pandemic we can’t pin directly on pests.
Other pest borne diseases that could (or should) be considered pandemics:
I would like to argue that Malaria is another disease that should be considered a pest-borne pandemic. According to the World Health Organisation, there were an estimated 228 million cases of Malaria worldwide, and 405 000 deaths, in 2018 alone. And what is so upsetting in the 21st century is that the majority of the lives claimed by malaria, the planet’s biggest killer, are children under five.
Malaria is caused by Plasmodium parasites. The parasites are spread to people through the bites of infected female Anopheles mosquitoes. Malaria symptoms usually appear 10–15 days after the infective mosquito bite. The first symptoms – fever, headache, and chills – may be mild and difficult to recognize as malaria. However, if not treated within 24 hours, P. falciparum malaria can progress to severe illness, often leading to death.
For the last eight years Rentokil Initial has supported Malaria No More UK and has so far raised almost £200,000 to help the charity’s vital mission of fighting this deadly global disease. “Rentokil Initial have been protecting and saving lives from malaria since 2011. During that time we’ve been inspired to see colleagues across the business showing their passion and commitment by raising almost £200,000 to help end a disease that despite costing less than £1 to treat still claims the life of one child every two minutes. This support has contributed to a tremendous global health success story, with a 60% decline in malaria deaths since 2000” says James Whiting, Executive Director Malaria No More UK.
Just like rats, mosquitoes can be controlled with proper pest control procedures, and the spread of diseases like Malaria and plague can be prevented.
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