Hirudo orientalis has two heads, two sets of reproductive organs and nine pairs of testes. Hiding deep in Cambodia’s jungles, it waits for unsuspecting animals to sink its teeth into, using sensors to detect heat and movement and anaesthetic to mask the bite. Jaws in place, hirudo orientalis sends an anticoagulant into its host’s bloodstream and begins feeding, consuming up to five times its body mass in blood. Today, it’s mine it is sucking.

Four hirudo orientalis and their eight heads have already gnawed under my skin today and I see two more in my path, swaying back and forth between the wet leaves, waiting to flick-flack their way up to a blood buffet. The path I’m following has become a sandy stream which I march up, intermittently darting to the side to avoid the wriggling creatures. Fat drops of monsoon rain fall from a humid sky above my head, beating down on jungle leaf drums, rallying a leech army.

I scour the terrain, clutching a fistful of salt, pausing every few minutes to yank up my trousers and check for leeches that have found a way in. A sprinkle of salt and their dark brown bodies begin to twist, regurgitating their stomach contents into my wound before dropping to the jungle floor. “Keep moving!” Iain shouts, flicking one off the edge of my raincoat with a bottle, and we continue up the river – the “path,” our guide Sina calls it – wearing a layer of jungle detritus over our water-logged clothes. The wind gives a rustle-whoosh-hiss and, as if responding to an order, the rain pelts down harder. “Maybe it’ll stop after this shower,” I call back to Sina who is lagging behind. “Maybe no,” he replies, and we trudge deeper into the jungle.

It’s been raining since dawn. Walking in this deluge was a choice, I remind myself. I’ve left my rather smug-looking mother in Chi Phat Village, reading, warm and dry in our guesthouse. A Spanish-French couple who’d booked a trek along a different trail told the staff at Chi Phat’s ecotourism centre it was far too wet to spend the night in the jungle. But not us; we weren’t backing out. “We’re getting a taste of what it was like being a marine in Vietnam,” says Iain, sloshing through the water beside me on our river-path. I’m not so sure. I don’t feel like a marine, but in a vague and ridiculous way I might resemble one. I have unwittingly dressed myself in a khaki-coloured top and green and khaki trousers. I’m wearing a dark-green raincoat and a khaki backpack, stuffed with an army-issue hammock, all of them rented. The only thing that isn’t green or khaki is my rainbow-striped socks, pulled up over my ankles, encrusted with salt to try and keep the leeches out. It isn’t working. I seem only to have seasoned myself.

Five or so kilometres into the Southern Cardamom Forest, the path narrows and we scratch our way past branches and brush, onward, onward to the vision of a dry village hut where we’ll stop for lunch. The rain is so heavy I can barely see, but we tear through the dense jungle, running away from a throng of leeches, letting adrenalin carry us along. A canopy of leaves casts a dim, green shadow on the gushing water around my shoes and, eyes on the ground, I follow an imaginary rhythm, left-right, left-right. My body gives a sudden jolt as my brain wills it to stop: a glistening spider web is a foot away from my face. On slanted, silver threads adjoining a tree branch an orange and black spider stretches eight legs, flaunting its poisonous beauty. We push on through the mud.

I focus my thoughts on lunch. Arriving at the village halfway will make this wet slog worthwhile, I tell myself. We’ll savour a feeling of satisfaction exactly proportionate to the discomfort that preceded it. Our arrival in Chi Phat Village two days before brought that same sense of gratification. Twelve hours spent in speedboats, tuk tuks, buses and crossing the Cambodian border; another bus, a longtail boat in drizzle, gliding further and further from Koh Krong Province’s roads and then, elation – or relief in my mother’s case – when we glimpsed Chi Phat around a bend in the Phipot River. Unsure what we’d find in the village when we arrived, everything was a pleasant surprise: large beds, a clean bathroom, a mosquito net, tasty noodles – even cold beer. Our lunch stop will be heavenly after this rainy trek, I think. Half an hour under a roof away from leeches is all I need.

We reach a clearing in the trees, where the pebbly ground is bronze in the damp air. The rain has slowed to a drizzle. “Eat lunch?” Sina says. “I thought we were eating lunch at the village halfway?” I say, confused. “No village,” he replies, taking three packets of instant noodles out his bag. I try not to let the disappointment sink in. Can’t be far to the village, Can’t be far to the village has been my mantra for the past two hours. Now what? I think.

“Ground is very wet for cooking,” adds Sina, and hands us each a pack of dry noodles. The three of us crunch away, looking down at our feet for leeches, shuffling back and forth so they can’t clamber aboard. Sina is in flip flops with his trousers rolled up to the knee. He spots one of the near-black creatures almost immediately against his bare skin and tries to flick it off. He’s not fast enough; it’s already in too deep. Shaking his head at my salt, he burns it with a cigarette. We shuffle and munch, shuffle and munch until we hear the rain getting louder, first in the distance falling against unseen trees. We’re off again, raincoats on, hoods up, back into the murky green jungle.

Chi Phat is about seven kilometres behind us, Sina tells us; it’s another seven to the campsite. We’re only halfway through our rained-out trek – an incredible experience, we remind ourselves, all things considered. Why else would we still be walking though a storm in leech-infested jungle? “Think how hot it would’ve been yesterday…” “Much more fun to be in the jungle in a storm…”

I visualise the campsite at the end of today’s walk: a fire and a candlelit evening watching the rain from under a blanket in our cosy wooden hut, safely looking out at a jungle expanse. Sina is striding ahead in the pouring rain. Iain glances back at mud-spattered me, my face frozen in concentration and we laugh at ourselves. “Uh oh,” he says suddenly, looking under his sock where – sure enough – a blood-fat leech is feeding. “Where’s the salt?” he calls, hopping on a patch of mud. I pause beside him while he smears it on, the two of us scuttling from side to side, laughing uneasily. A few metres on and I feel a sudden pain in my arm; it’s when they start squirming their way out that it hurts. “Ow!” I shout grumpily, loud enough for Iain to stop up ahead. A skinny black leech takes the opportunity to flick-flack its way up his shoe. “Don’t stop!” he calls. “If you don’t stop they can’t get you!”

“It’s too late!” I shout back, bounding through the water to where Iain’s trotting on the spot. “Get it!” I say, looking straight at the leech I can feel in my elbow, both heads buried in my skin. Iain grabs a bottle of mosquito repellent and squirts it at my arm, but not before taking a photo, ignoring my protestations. “Try not to think about them,” he says, but it’s easier said than done. Within seconds he thinks he feels one on his thigh. “Urg…” he groans, “Where is it?” Then, pale with horror, “Where’s it going?” As much as we’d like to, we just can’t strip to investigate – but not out of any sense of decorum or shyness. We’d only be killing one at the cost of gaining… how many more? We tear through the downpour, groaning in tortured unison, “Oh, aarg, eeee… What was that? Where is it? I think I’ve got one… Ow, ow! Noooo!”

An hour later, our manic retreat is taking its toll on my muscles. I imagine I feel at least three leeches on different parts of my body, but we’ve given up trying to get them off and are plodding toward the campsite, dying for this to be over. “Thank goodness we chose the trail with huts, not tents,” I say to Iain. “Mmm,” he replies, eyes on the path ahead. Sina’s kept a steady pace all day, and continues silently leading us: two defeated, mud-brown tourists. “Ten minute,” he announces, and we summon all our energy and charge forward to the dry reward at the end of this absurdity.

The path narrows and the sound of a river grows louder until it is thunderous in the silent forest. Sina slows to a halt and Iain and I stop suddenly behind him. “This campsite,” he says. I scan the foliage, seeing nothing but jungle until my eyes settle on a structure of tree branches: a timber platform and a roof of fronds, with a few thick branches tied together to form steps. Iain and I look at each other, fling off our raincoats and silently leap up the steps, allowing the panic to take over completely.

“Let’s check each other’s bodies!” I shout. “Make a screen!” We hang our raincoats over two level branches and strip. Sina has wandered off, but looking around frantically, I call out, “We’re just changing,” hopping on one foot trying to peel my soaking trousers off. I notice a smear of blood on my bra and call out to Iain. “Help me get it off! Help me check for more!” trying to hand him the bag of salt, watching him spin around, inspecting his own body. Satisfied that there are no leeches on any vital parts, he comes over to help me. I have already salted the breast-dweller and discovered another leech alarmingly low down on my waist. “Oh my god…” he says slowly.

“What is it, what is it?” I shriek.

“Oh… my…god…” and goes to get the camera.

“Where is it, where is it?” I shout, lifting up limbs and folds of flesh.

“On your neck,” he finally says, with a certain glee.

“Get it OFF!” I scream, adrenalin mixed with anger as he aims the camera.

“Just one second,” he says softly. “Okay… I’m nearly done…” After a torturous few seconds, photograph taken, he reaches for a pinch of salt and dabs it on one, two and then three leeches, happily feeding on my neck.

The hunt gradually slows down until there’s a pile of bloody leeches on the jungle floor beneath us. I make a kamikaze mission to the hole-in-the-ground and then the river, where I collect water for rinsing off the blood, mud and debris we’re covered in. All along the river bank, leeches are waving to and fro: blood-crazed enemies. One makes it into my shoe and – I later discover – my sock and my toe. A cold water rinse, some dry clothes, a bowl of (cooked) noodles and I’m ready to stay in my hammock until morning – safely above any leeches that might have made it onto the wooden platform of our shelter. Sina climbs into his hammock an hour or so later and Iain and I read until the candle burns out. It must be seven o’clock when I fall asleep to the roaring of the river and the pitter-patter of the rain above my head. I wake in the night and listen to rustles and chirps I can’t identify. Louder than them all is the river and the rain. The wind picks up and, trying to fall back asleep in the curve of my hammock, I hear a thump: the sound of Iain’s hammock falling onto the platform below. Slowly he gets off the floor with a growl and ties it back up. We lie there unable to toss and turn, trying to sleep. We both know we’ll need a good night’s rest before heading back into the typhoon tomorrow.