Ticks: Lyme disease and life cycles
Lyme Disease is transmitted to humans when we’re bitten by ticks. These are small, blood-sucking parasites that are members of the spider family found across the world. Eva Higginbotham has been learning a bit more about them and how they transmit Lyme disease...
Thomas - Most people think of them as rather disgusting and nefarious, sneaky, crafty... and most importantly they can transmit germs that cause disease.
Eva - That's Tom Mather, a self-described tick collector from the University of Rhode Island, describing the critters he has devoted his life to studying: ticks. Ranging from around 1-5 millimetres in size, depending on life stage and species, ticks are parasitic arachnids that feed on the blood of various host animals.
Thomas - Ticks come in four life stages, three that are active and one is the egg stage. Eggs hatch into little, tiny, six-legged larvae. The next stage after a larva is the nymph: so the larvae grab a host, just like all ticks do; they take a blood meal, and they use the blood meal to grow generally about 10 times their size. Which sounds like a lot, but when you start pretty microscopic then 10 times bigger isn't that much bigger. Nymphs do the same thing: they grab a host that increases their weight about a hundred fold, and those nymphs then transform into the adult stage, either a male or a female.
Eva - Although all ticks feed on blood, they have different preferences for which hosts they want to feed on. Some are very picky, and as a result, pose less Lyme disease risk to humans. But some are generalists, like the American blacklegged tick or the UK castor bean or sheep tick. But how do they get onto their hosts in order to feed?
Thomas - Ticks don't jump. They don't have wings, so they can't fly. What they do is wait, low in the leaf litter and the leaf duff where it's a little bit more humid. The adult stage tick will climb up vegetation just a little bit. They want to optimise where they're going to be in case the right host comes by.
Eva - And if a tick makes it onto your skin...
Thomas - Ticks have a fairly sophisticated cutting mechanism. They have a multi-piece mouth part, a little like a Swiss army knife I suppose. First it can cut a hole in your skin, and then it cuts that hole a little bit bigger and bigger, and then it inserts another part of its mouthpart into this hole that has backward pointing barbs. Once a tick finds a host, it doesn't really want to lose it, so it's specialised to hold on. And that's why people notice that it's hard to pull them out sometimes.
Eva - The tick inserts it's mini-saw mouth part deeper and deeper into the skin until it's fully embedded.
Thomas - Some of the earliest things that it does with its saliva - that it secretes into the host - is secrete a cement substance to form this glue-like matrix that helps hold the tick in place with the tissues. It starts secreting more saliva that has this magical property of suppressing the immune system of the host, keeping blood clotting from happening. So it creates a pool of blood that its mouthpart - basically a straw - is sticking into.
Eva - The thing is it's not the blood drinking that's the Lyme disease risk - it's the saliva that's the key. If the tick is infected, the Lyme bacteria is essentially spat out in the saliva of the tick if it's attached long enough. But how does the tick end up picking up the Lyme bacteria in the first place?
Thomas - We know that the larval stage ticks hatch out of eggs don't carry the germs. So they have to pick it up someplace. And so they pick it up from a reservoir host - an animal that's not only infected, but infective as well.
Eva - To be infective, the animal has to be able to pass along the bacteria. Other animals, like the white footed mice in the USA, seem to have evolved to tolerate the bacteria. They don't get sick, but they also don't seem to eradicate the bacteria from their bodies. This is why they are a reservoir. Any tick that then bites that mouse is likely to pick up the Lyme bacteria from them.
Thomas - About one in four nymphal stage ticks in the United States is infected with the Lyme disease germ. That's exceptionally high. When we think about mosquito-borne viruses, for instance, we're talking about rates of infection of one in 1-5 million mosquitoes carrying a virus. So we're talking about a vector here that has a tremendously high infection rate. As incredible as that is, the adult infection rate is almost 50% or even more. You would say, "oh, well then the adult stage ticks are riskier," but they're not - because they're a little bit larger, more easily seen and more easily found and removed before they've been attached long enough to transmit an infectious dose. And so most cases of disease occur just after and during the season of the year, which is May, June, July when the nymphal ticks are active.
Eva - Importantly, though, that 50% infection rate is the average across the States and will vary greatly depending on precise location and the types of animals that live nearby. And the rate is much lower in the UK. But if you've been out and about and realise you've been bitten by a tick, how do you get it out?
Tom - In order to remove a tick safely, there are all kinds of strategies. We want to dispel right away some of the folklore that people have of touching it with a hot object. My favourite and most reliable strategy is to have a pointy tweezer and you don't want to grab the back end of a tick. Think of it as a sack of germs attached to your skin with a straw. So you want to grab it as close to the skin as possible and just pull it out. It may be a little bit challenging because of those backward pointing barbs we talked about, but it will come out. Sometimes the mouth part breaks off depending on your grip and everything. And that's okay because the germs are in the back part of the tick, and as long as you haven't squeezed the back part, you should be fine.
Eva - And of course the best way to prevent catching Lyme disease is to not get bitten in the first place. Tom advises sticking to the middle of hiking trails where you can, tucking your shirt into your trousers and your trousers into your socks, and if you're regularly outside in a higher Lyme area, you could invest in a can of permethrin; an effective tick repellent, which you can use to treat your clothes. That is unless you're Tom.
Tom - Back in my younger days, I actually wanted to teach people the proper way to remove ticks. So I would grow pathogen-free ticks in the laboratory. And before I would go do an outreach activity, I would let one or two attach to myself in an easy spot so I could show people the best way, the safest way to remove ticks from themselves as well.
Eva - I think I'll leave that to the experts.