SQUIRREL PLAGUE! Or “Don’t hug a dead squirrel”


During my PhD I worked on the morphological evolution of New World monkeys, phyllostomid bats, Australasian possums, and ground squirrels. For some reason the only part anyone outside academia (and some people in academia) remembers is that I like squirrels. This means that whenever there is a squirrel-related news story, around half a dozen people send me a link and/or want my opinion.

So this July I got sent a lot of copies of this story: “Squirrel infected with the BUBONIC PLAGUE closes major US campgrounds” (the Daily Mail clearly decided the words bubonic plague weren’t shocking enough so they put it in capital letters; sophisticated journalism at its best).  This was less shocking to me than most people expected, because I have an interest in zoonotic diseases (diseases transmitted from animals to humans) and was already aware of the continued existence of plague. However, in light of the over excitement in the media, I thought I’d write a quick blog post about plague and try to clear up some of the confusion.

When most people think of bubonic plague they think of the Black Death in the 14th Century. This is definitely the most famous and most devastating outbreak of plague – it wiped out 30% of the population of Europe and contributed to major changes in society and agricultural practices across the continent. Other famous outbreaks include the Great Plague of London in the mid 1600s which only ended after the Great Fire of London in 1666. Improvements in sanitation have all but eliminated dramatic outbreaks of plague in the developed world, but it is still is a significant problem in Africa (particularly in Madagascar and the Democratic Republic of Congo), Asia and South America, which report around 2000 cases every year with a global fatality rate of 5% to 15%.

There are three major forms of plague: bubonic, septicaemic, and pneumonic. They are all caused by the bacterium Yersinia pestis but have different symptoms, fatality rates and transmission modes. Bubonic plague is the “traditional” Black Death version of the plague with the black necrotic swellings which lead to blood infections, bleeding, shock, organ failure and eventually death. Septicaemic plague has all the same symptoms but without the initial swellings. In both bubonic and septicaemic plague the bacteria are passed from host to host via flea bites, and both are fatal around 50% of the time if no antibiotic treatment is given. Pneumonic plague on the other hand, has an almost 100% fatality rate if untreated, is transmitted by inhalation of airborne droplets, and mainly affects the respiratory system. So really nasty stuff! Luckily most strains of Yersinia pestis respond well to antibiotics.

Although it’s not a major problem because outbreaks are rare and plague can be treated with antibiotics, the disease is still endemic to the southwestern states of the USA. This means the disease is naturally found there. The natural reservoir population for the disease consists of ground squirrels and other small rodents. Often outbreaks occur in humans after a major outbreak in the squirrels results in humans handling dead or dying squirrels. A few years ago a case in a little girl in the USA was due to her finding a dead squirrel and giving it a decent funeral! So unless you go around playing with dead squirrels, there is very little chance of you catching the plague. Before you panic about that dead squirrel you picked up last week, note that when I say “squirrel” I’m referring to a group of rodents (technically called the Sciuridae) that contains just under 300 species (depending on how you count them!), two of which are the red (Sciurus vulgaris) and grey (Sciurus carolinensis) squirrels we have here in Ireland. Mostly the squirrels infected by plague are ground squirrels, not the tree squirrels we’re all most familiar with. Ground squirrels, unsurprisingly, live on the ground and in burrow systems and include things like marmots, prairie dogs and chipmunks. This means that unless you’re handling some dead exotic migrant squirrel, in Europe at least, you’re probably fine! In summary, all fine, totally natural, totally treatable by antibiotics, nothing to see here, DON’T PANIC!

This post was meant to be all about making everyone feel better but then I ended up reading the World Health Organisation (WHO) website. After all this positivity, here’s the scary bit! Naturally multiple-drug-resistant strains of the bacteria have been identified in Madagascar, and antibiotic resistance appears to be easily transferred between bacteria via plasmids in the flea mid-gut. So unfortunately the age of easily treating plague with antibiotics may soon be over. I was also a bit puzzled by the WHO website because although there are quite a lot of diseases out there, WHO has vaccine development programmes for only a few. One of these diseases is the plague. It turns out that bubonic plague is a big worry because it could be used as an agent of biological warfare. In fact, historians suggest that plague was one of the first biological weapons; soldiers would throw plague-infected corpses over castle walls during sieges in the 14th century.  More recently, the Japanese forces near the end of World War II had devised “Operation Cherry Blossoms at Night” to send kamikaze bombers with the plague to infect San Diego, but were stopped by the atomic bomb being dropped on Hiroshima just a month before the planned attacks. The USA and USSR also experimented with aerosolized plague in the 1950s and 1960s, before the Biological Weapons Convention (BWC) was passed in 1972 forbidding offensive research into biological weapons. However, not all countries or organisations believe in the goals of the BWC or the Geneva Convention, and aerosolized, antibiotic resistant, pneumonic plague, which as I mentioned above is almost 100% fatal if untreated, remains one of the most frightening and deadly, potential biological weapons around, particularly given the high mortality rate and ease of rapid person-to-person transmission. Something to think about next time you try and hug a dead squirrel…


Natalie Cooper: ncooper[at]tcd.ie


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