Jeff De Young served in Afghanistan with a bomb-detection dog named Cena N641, a black Labrador. In the intense atmosphere of war the two developed an unbreakable bond. This is the story of how Cena helped Jeff survive not only war, but also life after war.

The day I turned 18 I started Marine Corps boot camp, and 15 months later I went to Afghanistan. It was 2009 and I was absolutely terrified.

You could hear the rounds snap overhead, and then when the round went past you, you heard a zing almost like a whistle

They paired us with the dogs based on our personalities. Cena was a slightly goofy, quiet dog, and I was a slightly goofy, quiet kid, so it made sense for us to be with each other.

Together we were known as Kid and Chicken. Chicken was one of those nicknames that you don't remember where it came from, it just kinda stuck. And although I was 19 by this stage, I looked like I was about 12, I didn't even have any facial hair. As a joke, the Marines mailed a permission slip home for my mom to sign because I looked so young they didn't believe that I was allowed to be over there.

I would operate Cena using hand and arm commands and a whistle. I'd be in front of the patrol and Cena would be further ahead again, so if either of us walked on an improvised explosive device, although we would have been hurt, the rest of the patrol would be safe. I'd never been faced with a situation like that before and it felt like a crash course in adulthood, responsibility, and survival.

Cena had been a champion bird dog. When waterfowl falls from the sky there is no scent trail to follow like there would be with a rabbit or a deer, so the dog has to investigate the area and find the scent on the wind, it's amazing.

Dogs' noses are so much more powerful than ours. We smell cookies, but they smell the flour, the nutmeg, the butter, the eggs, the milk - they can dissect everything and they can detect smells that we don't even know exist.

He'd been trained to detect more than 300 different types of explosives and if he smelled something interesting on patrol he would lie down and notify me, and then I'd call in an explosives technician.

We had to trust each other - we would have a dozen, two dozen marines behind us and any mistake could have been fatal.

Media playback is unsupported on your device Media caption Listen: Jeff describes how Cena supported him during his darkest hours serving in Afghanistan

The battle of Marjah was a turning point in my life. We approached the town before the sun came up, no-one was talking, no-one was joking. It was very tense. You could hear the rounds snap overhead, and then when the round went past you, you heard a zing almost like a whistle.

I was so worried about getting Cena to safety, I even had to lie on top of him to protect him from gunfire. Another time I carried him through a freezing cold, flooded river on my shoulders like a hunter would a deer.

It got so cold in the fighting holes that even Cena's body heat didn't help, so one day I offered an Afghan soldier the entire contents of my wallet for his scratchy, olive, drab wool army blanket. I had $100 (£80) in my wallet. I was either going to burn the money or get the blanket, that's how cold I was. I still have that blanket.

The first week inside Marjah I lost a couple of very good friends. One of them was a former room-mate I'd trained with, Lance Corporal Alejandro Yazzie. He was 23, a Navajo, and an all-round good guy. His grandfather had been a wind talker [code talker] in World War Two. When I found out it was Yazzie I was devastated. I held on to Cena and cried into him.

Yazzie was the first of seven friends I lost in Afghanistan. I carried a flag inside my helmet and whenever a friend would pass away I'd add their name to it.

Eventually I just couldn't cope any more. I grabbed my military rifle and went to the latrine area. I remember sitting there trying to prepare my mind and make peace, and then Cena peeked around the corner. His ears went up like in the cartoons and he opened his mouth like he was smiling. His tail started spinning so hard that his whole body was rocking back and forth like he was excited by a piece of bacon.

I started laughing, and I laughed so much that I just broke down crying. I realised then that I couldn't leave Cena because I didn't know if his next handler would love him the way I did. He really was the only person in my life that I had a deep relationship with at that time. I left the latrine, put my rifle back and focused on work.

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Jeff De Young spoke to The Documentary: It's a dog's life on the BBC World Service

It's really hard to explain what it's like, psychologically, coming back from war. Even the drive home was strange. New music was out, new cars were on the roads, there were new stores. It felt like when you leave the cinema to get popcorn and then miss the best part of the film.

I got married three days after returning and I was so busy doing all this happy stuff, it was like a Band-Aid over Afghanistan. But I wasn't really taking care of myself and dealing with what had happened over there.

Aside from my children being born and the day I was married, that was the happiest day of my life, it was like all of my Christmases rolled into one

A couple of weeks after coming home the post-traumatic stress (PTSD) and separation anxiety from being away from Cena really hit me. I'd always understood that I wouldn't have him forever but I'd had no idea how being apart from him would affect me. I felt like a stranger at home and I didn't feel comfortable unless I was with my battalion members or other veterans. I had nightmares and spent many nights crying in the bedroom corner or talking out loud to my fallen friends.

Over the next four years Cena was always on my mind, but as time went on it became hard to keep up hope that we would be together again.

Then one day, when I was in college, I got a call. The woman on the phone said: "Mr De Young? My name is Mrs Godfrey, would you like to adopt your bomb dog?" Without even thinking I said, "Heck, yes!" That was 24 April 2014, one day shy of four years since Cena and I had been separated.

It was just a turmoil of emotions on the car ride there. When Cena came down the aisle I very awkwardly - like a guy crossing a high school dance floor - ran up, kneeled down and started hugging him. He leaned into me like, "Hey man, what's up?" and started licking my face.

Aside from my children being born and the day I was married, that was the happiest day of my life. It was like all of my Christmases rolled into one.

I'd been married for four years by the time I got Cena back. Unfortunately, my inability to recognise that I had issues as a result of being in Afghanistan ultimately led to my divorce. Cena was helping me with healing and support but the damage to my relationship was already done. On 5 June 2015 I ended my marriage.

I have three daughters, they are six, five and two-and-a-half. Cena took to them instantly, and they love him back - they try to paint his nails and put bows on him. Before getting Cena back, the sound of a child crying would trigger a panic attack in me, as a result of an incident in Afghanistan, and it was tough knowing that I couldn't help my kids because my brain couldn't process that memory.

The military teaches us how to put the uniform on, but it doesn't teach us how to take it off, metaphorically speaking

With Cena, if my daughters cried I would sit on the couch, put my forehead to his, scratch his ears and just breathe. Gradually, Cena would only need to be beside me and I could cope.

By the time my third daughter was born I was able to do a lot of the diaper changes and bottle feeding even if she was crying, and to finally be able to help my daughter felt like being released from jail, it was freedom.

I'm a military ambassador for the American Humane Association now and I travel around the country raising awareness about how important it is to reunite service dogs with their handlers, and how the dogs can be a vital form of treatment for veterans with PTSD. My work is most definitely therapy for me, too. The military teaches us how to put the uniform on, but it doesn't teach us how to take it off, metaphorically speaking. I've lost count of how many friends I've lost now, who've taken their lives - four just last year alone.

I couldn't even think about talking about what I saw in Afghanistan four or five years ago, but slowly, by opening up to other veterans, by putting myself out there and airing everything that happened it's becoming so much easier.

I've recently found out that I have a heart condition called tachycardia. The doctors say it was probably triggered by an explosion or something that happened in Afghanistan. When I'm stressed my heart rate goes up to 200 beats per minute, high enough for a heart attack, so I'm having an implantable cardioverter defibrillator (ICD) fitted in my chest. I'm still mentally processing the idea that soon I'm going to have an electronic box in my chest to keep my heart in check.

Cena is in OK health, although his front wrist bothers him and his hips are pretty bad. He'd been back to Afghanistan, and I tracked down two of his other handlers through Facebook. I keep them up to date with how he is doing and I hope to get them to come to Michigan to see him - it's been years since they've seen Cena too.

Cena was retired after his third deployment because of a hip injury and there's no doubt in my mind that he has PTSD. I think he has memories of things that he saw that he doesn't like. He has nightmares, he'll whimper, he'll run around in his sleep and his teeth will snarl. But he's always by my side - we go to the gym together, we go to college together - my college even wants to get him his own cap and gown for when I graduate.

Cena's nine-and-a-half now. Dogs tend to live to 11 or 12, so I've started making peace with the fact that he may pass away soon. I've been preparing my mind for that.

Jeff De Young was interviewed by Sarah McDermott and Rose de Larrabeiti.

All images courtesy of Jeff De Young.

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