College Record 2013
The wonders of tick-spit by Pat Nuttall (JRF 1977–80, RF 1990–95, GB 1995– 2001; SF 2001–) Have you ever been bitten by a tick? If you have, you probably didn’t feel a thing. Maybe you only noticed the little black speck attached to your skin after it had started feeding, growing fatter as it sucked your blood. So how does a tick manage to feed for long periods – up to two weeks or more – without being ejected by the host on which it is feeding? The picture hanging in my office gives some insight into what the tick is doing. It shows a cartoon of a tick attached to the skin surface with a cut-away through the skin showing the skin epidermis, dermis, and red and white blood cells. To achieve this position, the tick must first locate a host, which it does by picking up chemical signals in the air such as carbon dioxide, sensed through receptors at the tip of its first pair of legs. It then climbs onto the host and finds a suitable spot to attach: usually a site that can’t easily be groomed, such as the ears of a mouse or the rear end of a cow. It then uses a pair of appendages (known as chelicerae) of its intricate mouthparts, to saw through the skin epidermis. Into the resulting cut, the tick inserts its feeding 129
tube or hypostome (shown orange in the centre of the picture), which has backward pointing barbs that help secure it in the skin. Just to make sure the tick mouthparts are firmly attached and do not allow any blood to leak out, the tick secretes a milky fluid which solidifies around the hypostome forming a cement cone (grey in the picture). All of this helps explain why it is so tricky to remove a tick once it’s attached. Imagine, though, that this had been a splinter wedged in your skin. First, you would feel a hurtful prick and then your skin would become inflamed and possibly swollen. Why doesn’t this happen when a tick bites? The reason lies in the tick’s large and complex salivary glands. These produce the cement fluid and hundreds of other molecules (proteins, peptides, and small molecules), which are secreted in tick saliva while the tick attaches and then feeds on blood. Saliva molecules have different activities that help make ticks invisible to the host’s protective mechanisms, and keep the blood flowing so they can suck it up. Saliva molecules include anaesthetics, anti-inflammatories, anticoagulants, and immunomodulators. It’s not surprising ticks have been called sophisticated pharmacologists. And it’s all in their spit! 130
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The wonders of tick-spit<br />
by Pat Nuttall (JRF 1977–80, RF 1990–95, GB 1995–<br />
2001; SF 2001–)<br />
Have you ever been bitten by a tick? If you have, you probably didn’t feel a thing.<br />
Maybe you only noticed the little black speck attached to your skin after it had<br />
started feeding, growing fatter as it sucked your blood.<br />
So how does a tick manage to feed for long periods – up to two weeks or more<br />
– without being ejected by the host on which it is feeding? The picture hanging<br />
in my office gives some insight into what the tick is doing. It shows a cartoon of<br />
a tick attached to the skin surface with a cut-away through the skin showing the<br />
skin epidermis, dermis, and red and white blood cells. To achieve this position, the<br />
tick must first locate a host, which it does by picking up chemical signals in the air<br />
such as carbon dioxide, sensed through receptors at the tip of its first pair of legs.<br />
It then climbs onto the host and finds a suitable spot to attach: usually a site that<br />
can’t easily be groomed, such as the ears of a mouse or the rear end of a cow. It<br />
then uses a pair of appendages (known as chelicerae) of its intricate mouthparts, to<br />
saw through the skin epidermis. Into the resulting cut, the tick inserts its feeding<br />
129
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