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Thursday, Oct. 17, 2002
Ticks: playing a waiting game to gorge on blood
By MARK BRAZIL
Being in the field for several months each year in search of wildlife to study, photograph and write about may sound wonderful, and it certainly does make for an exciting life. There is a downside, though, because there's also wildlife out there looking for me. Well, not me specifically, but warm-blooded mammalian prey or hosts like me.
I have been within striking range of a tiger in India (though thankfully I avoided the terminal tiger-interaction that did for a friend). I have been in crushing range of a charging elephant. I have had my innards purged by various waterborne microbes and my blood tapped by various creatures, including horseflies with mouthparts like drill bits, mosquitoes with hypodermic mouthparts and painful monstrous freshwater leeches. I have narrowly missed a venomous snakebite in Taiwan, have slept where vampire bats frequently feed in Guyana and been snapped at by caiman in Brazil -- but my longest-lasting wildlife injury came from a minuscule creature in Argentina.
I was searching for seriemas in El Rey National Park in northern Argentina. These stately birds of the open grasslands are South America's equivalent of Africa's secretary birds. Wherever grassland animals are common, parasites seem particularly abundant; and wherever there are sheep or their relatives, or woodlands with deer, ticks will be there as well. After a day searching for seriemas, a shower was obligatory to remove grime and dust, but in the shower I discovered to my horror the "bonus" I'd collected from the tall grass -- I was covered with ticks, dozens of them, so many in fact that I lost count in my rush to get rid of them.
Most brushed off easily, some required more diligent pursuit with tweezers, but one had already gone to work, burying its mouthparts in the skin just below my navel in the belief that I was a suitable warm-blooded host. A big mistake. Persistent tugging on my part served to kill it and break it in two. A sorry end for it, and frustrating for me.
I now had a very itchy wound still concealing the tiny mouthparts. Swiss Army Knives are handy tools at the worst of times, and this was just such. So, with a sharp blade I dug, staunching the wound with alcohol.
That was October, and I was absolutely sure that I had dug out all of the tick -- until the itching returned a few days later. It returned again and again. Whatever was causing it eluded my every self-surgery. The wound eventually closed and it finally ceased to itch the following June -- eight months after a creature the size of a match head had tried to drink my blood. I count myself lucky, as none of the other ticks that latched on to me that day survived long enough to benefit from my blood.
Blood is what makes a tick tick. Ticks are tough tenacious freeloaders, and waiting is their game. Some can survive for up to six years without feeding, and during that time they are able to absorb the moisture they need for survival from the air. Tough as nails (unfed ticks can even survive a hammer blow, because they are so flat and so hard) but unable to fly, run or jump, they spend most of their lives simply waiting for a host to happen by. All they can do is find a high grass stem or plant tip, and hang on and wait.
The 850 species of ticks worldwide belong to the Acari, a subclass of the Arachnida, containing the mites and ticks. Within the Acari, there are three tick families, the Ixodidae (hard ticks), the Argasidae (soft ticks) and the one African species of the Nuttalliellidae.
Whichever sort, though, ticks detect potential hosts from their exhaled carbon dioxide, or by their odor (butyric acid if it's a mammal). Vibration is also a clue. Once triggered, a tick either drops from its perch onto its host, or it grips its perch with its rear legs and reaches out to snag the pelt of its prey. Then it moves toward a spot where the fur or skin is thin, commonly around the face, ears, or abdomen.
However, the tick still has hurdles to overcome before it can feed. Not least are the host's defences -- scratching and rubbing, and the inflammation caused by the host's immune system. But a tick has some nifty glands producing a fluid that doubles as cement by helping to glue the tick to its host, while also providing numerous other specialized services -- saliva.
The tick's super-saliva contains prostaglandin -- a fatty-acid derivative that serves to suppress the host's immune response, thus reducing inflammation. It also dilates the host's blood vessels so that more of the red nectar flows to the tick, and contains an anti-clotting agent to keep the vital flow coming until the tick is full. Think of that, a liquid meal delivered by capillary seepage straight into the tick's expandable body -- no pumping required.
A bloated tick looks gross, like an oversoaked sultana, with eight tiny legs and protuberant mouthparts. Being bloated makes movement almost impossible, and when sated a full tick merely extracts its mouthparts from beneath the epidermis and falls off its host.
Ticks concentrate blood, excreting the non-nutritious fluids to leave themselves with a thickened soup of digestible compounds -- yummy for a tick, loaded with "yuck" factor for us. Crushing one at this stage makes an awful mess, believe me!
One major blood meal provides the energy a female tick needs to reproduce. Some lay one large batch of 10,000 eggs or more, while others lay 20 to 50 eggs after each meal. Depending on temperature and humidity, eggs hatch after two weeks to several months, and the tiny six-legged larvae (called seed ticks) emerge . . . and their waiting game starts.
Tiny ticks need an initial blood meal in order to proceed to the next stage of their life cycle. After the tiny larval ticks have fallen from their host, they molt and emerge as eight-legged nymphs. These closely resemble adult ticks, now with eight legs, although they lack a genital opening. Like the tick larvae, nymph ticks also need a blood meal before they can develop into adults, so they, too, can survive for long periods without feeding. Typically, however, ticks are thought to take about three years to reach maturity.
You may spot a tick on an animal, perhaps even a pet, appearing first as a small gray dot that gradually enlarges. A retired pathologist friend remarked to me that doctors had sent her any number of supposed samples of skin cancers to examine -- samples that were actually firmly embedded ticks. At other times they may be mistaken for warts or lumps.
However, ticks are not only blood-sucking parasites but also vectors for a number of diseases affecting other animals and humans.
Perhaps the most famous of these is Lyme disease, which starts with a rash and can cause arthritis and heart damage, and is now a significant concern in North America, where it is carried by deer ticks. (Does any reader know whether similar ticks in Japan carry the disease?) The best cure is, no doubt, prevention -- by avoiding getting ticked in the first place.
I have heard of many traditional ways of removing ticks, from pouring on oil to spreading on toothpaste, but the only way I have succeeded has been with tweezers and patience. If you are unfortunate enough to fall prey to one of these creatures, I recommend you have them removed by a trained medic (avoid do-it-yourself surgery, if at all possible).
An ectoparasite's life sounds great -- free rides, free meals -- the downside is that they have to produce enormous numbers of young for even one representative of their particular gene set to survive. On balance, I think I will count myself lucky to have to pay my own way and make my own meals.