This week's co-guest blogger is Zoonotica!  She is a 1st year PhD student whose main interests lie in disease transmission, public health and science communication.  She blogs about amazingly cool scientific research that is going on at the moment; current topics in public health and zoonotic diseases; and a little bit about life as a PhD student.  You can find more from Zoonotica on her blog or by following her on twitter.

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What suckered me into starting a PhD was learning about zoonotic diseases.  I think they’re just so  fascinating – they’re complex and dangerous and everywhere!  According to the literature they’re hiding in our forests, our parks and gardens, they’re even lurking in our houses!  But what the heck are they?

Well, a zoonotic disease, or zoonosis, according to the World Health Organisation is

‘any disease or infection that is naturally transmissible from vertebrate animals to humans and vice-versa’

and there are a large and ever-growing number of them.  (In fact, it’s been estimated that 3 out of every 4 emerging human disease comes from animals!)

I’m even fairly confident that you – yes, you, reading this post right now – will have come across a zoonosis at some point with you or a family member or friend suffering from one (obviously hopefully one of the less dangerous ones!)

Engraving of Little Red Riding Hood by Gustave Doré (1832-1883)

A large proportion of these diseases are foodborne, with a classic example being salmonellosis caused by eating eggs contaminated or infected with the bacteria Salmonella Enteritidis (SE).  SE lives in the chicken gut and is excreted in its poo.  Eggs can be contaminated if they come into contact with infected poo (and given where eggs come from contact with poo does not seem that unlikely an event) and if there are cracks in the shell the bacteria can colonise the inside of the egg.  Eggs can also be contaminated at a much earlier stage:  eggs are released from the chicken’s ovary and pass down a tube called the oviduct.  This is where the albumin (the egg ‘white’) and eventually the shell are laid down.  SE can infect the chicken oviduct and so the egg can become infected before it is fully formed and so before the protective barrier that is the shell is in place.  In countries that vaccinate their poultry flocks against SE this disease has been greatly reduced but as new strains evolve the efficacy of the vaccine will have to be reassessed.  This is certainly not a problem that has gone away.

Another way we can catch these diseases is by coming into contact with wild animals or the messes they leave behind.  Hantaviruses are carried by rodents and are excreted in both urine and faeces and the main way humans get infected is by breathing in aerosolised virus.  The symptoms we suffer if infected by hantaviruses depend on the species of virus which in part depends on where in the world we are.  In the south west USA the Sin Nombre hantavirus affects the heart and respiratory tract causing Hantavirus (Cardio)Pulmonary Syndrome (H(C)PS) whereas elsewhere in the world the general picture of infection is a haemorrhagic fever with kidney damage.

Even if we avoid all contaminated foods and somehow keep well away from wildlife we can still come across these diseases: they can be lurking inside our houses carried by our family pets.  Toxocara canis, a roundworm that lives in the digestive tracts of dogs and foxes can cause toxocariasis in us.  The roundworm lays its eggs which come out in the dog’s poo.  If we eat the eggs they can hatch out into larvae in our digestive tracts.  The symptoms in humans may be vague – such as abdominal pain, a cough, headaches – or there can be more nasty consequences when larvae migrate away from the gut into places it really shouldn’t go, like the eye.  This is mainly a problem that affects small children (who are obviously at a higher risk of coming into contact with the eggs given their propensity to play in the dirt and then lick their fingers!)  The occurrence of this disease has been much reduced in regions where pet dogs are regularly wormed but it has not yet gone away.

With these and the other myriad examples of animal to human transmission it is easy to forget that within the definition of ‘zoonosis’ is the reminder that these diseases can be transmitted both ways (remember that ‘vice-versa’ bit?).

It’s been recognised that elephants are at risk of catching TB (mainly caused by Mycobacterium tuberculosis) from humans and this is especially the case in zoos where the risk is that the elephants may catch it from their handlers.  They can of course then pass it back to any uninfected handlers.  And this is by no means the only example: many of the diseases that pass from animals to us have been documented as going in the other direction as well (which I guess is kind of logical).

Ultimately I suppose it is not even really all that surprising that there are so many diseases that can affect both us and the animals around us – after all, arguably we share more similarities with our furry/feathery/scaly friends than differences – but it does make researching these bugs and parasites a lot more difficult, with their complicated routes of transmission and the complex strategies they have for avoiding all of the different immune systems that they inevitably encounter.  It also makes learning about them a whole lot more interesting!

I hope I haven’t left you totally terrified and determined never to go near an animal again – learning about all these diseases has never put me off cuddling our two and four-legged friends.  But I think it’s worth knowing the risk exists so that you can take a few more precautions, like washing your hands after the cuddle or taking that extra bit of care over preparing your food.  I also hope I’ve managed to spark your interest in these diseases – there’s plenty more out there to learn about!

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If you want any more information on these diseases then the following links are good places to start:

Salmonellosis  http://archive.defra.gov.uk/foodfarm/farmanimal/diseases/atoz/salmonella/index.htm

Hantavirus infection  http://www.cdc.gov/hantavirus/

Toxocariasis  http://www.cdc.gov/parasites/toxocariasis/

TB in elephants  http://www.elephantcare.org/TBinfo.htm