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Quarantine inspection dog - Alert and always inquisitive, an Australian quarantine inspection dog sniffs passenger's bags in the arrivals hall at Sydney's international airport, Australia.

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Regimental mascot - Catherine, Duchess of Cambridge, presents a sprig of shamrock to the mascot of the 1st Battalion Irish Guards, an Irish Wolfhound called Domhnall, during a visit with her husband, Prince William, to a St Patrick's Day Parade at Mons Barracks in Aldershot, England.

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In the army - An army dog stands up as retiring soldiers salute their guard post before retirement in Suqian, Jiangsu province, China.

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Sniffer dog - A French elite police officer checks with a sniffer dog in one of the meeting rooms at the venue of the G8 summit in Deauville, northern France.

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Medical response dog - Macie, a small medical alert and response dog, pauses during her handler's shopping spree. These dogs are trained to help ease their handlers' symptoms, such as bringing medications or a telephone if the patient cannot reach one. (Photo: Wikimedia/CC BY-SA 4.0)

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Diabetes alert dogs - Diabetes alert dogs like Shirley (left) that are partnered with diabetic people may be trained to detect when the handler's blood sugar becomes too high or low. The Labrador Retriever is pictured with sniffer dog Jake.

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Psychiatric service dog - A psychiatric service dog pictured during training. These dogs are trained to assist their handler with a psychiatric disability or a mental disability. (Photo: Wikimedia/CC BY-SA 3.0)

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Canine companion - A trained service dog sleeps at the feet of a retired US Army soldier who was injured in Afghanistan.

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Helping people who have disabilities - Another former soldier, who was shot and paralyzed during the Iraq war, sits with his service dog Yazmin while training at the Canine Companions for Independence training center in Santa Rosa, California, USA.

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A friend at your side - A US army veteran who suffers from post traumatic stress disorder sits in his bedroom with his service dog Panzer at his side in Hanover Park, Illinois, USA. 

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Military training - An agile Chinese soldier trains a dog by skipping rope at a military base in Nanjing, in east China's Jiangsu province.

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Police dog - A police officer and her police dog stand to attention at the Metropolitan Police Service Passing Out Parade in Hendon, London, England.

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Multipurpose canine - Pronto, a Special Operations Forces Multipurpose Canine, successfully jumps out of a CH-47 Chinook helicopter during a training exercise off the coast of Florida, USA.

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Good at his game - A police sniffer dog is used to check for explosives on Centre Court during security checks ahead of the annual Wimbledon tennis tournament near London, England.

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Army dog - An army dog carries flowers as a soldier marches during the pre-celebrations of Vijay Diwas, in the eastern Indian city of Kolkata.

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Therapy dog - Therapy dog Hank, a Boston Terrier, visits the Combat Outpost Nangalam in the Pech River Valley of Afghanistan's Kunar province.

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Sheepdog - A shepherd guard dog looks over a flock of sheep at a farm near Canberra, Australia.

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Non-smokers - French Customs Labradors Clop (left) and Chic (right) hold a carton of contraband cigarettes during a successful swoop on a smuggling operation.

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Guard dog - A dog is often called to guard all sorts of things. Here, a vigilant canine is watching over arabica coffee beans laid out to dry in Hua La village, Vietnam.

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Trained to search for explosives - A Dutch police officer and a sniffer dog trained to search for explosives patrol inside the World Forum ahead of the Nuclear Security Summit at The Hague.

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The good shepherd dog - A Great Pyrenees dog surveys a flock of sheep near Les Diablerets. Swiss farmers use Great Pyrenees dogs to protect their cattle from wolves.

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Brave canine - Afghan sappers—with the help of a highly trained sniffer dog—search for landmines in Kabul. 

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Search and rescue dog - Rescuers pull out a sniffer dog at the site of a collapsed residential building on the outskirts of Mumbai, India.

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Anti-drug tasks - A Colombian policeman holds the leash of a dog smelling packages of marijuana seized by anti-drugs policemen near Cali city.

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First responders - A rescue worker searches for victims with a sniffer dog at the scene of a collapsed building that was undergoing construction, in São Paulo, Brazil.

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Extra security -

Silhouetted against encroaching twilight, a dog guards the lighthouse of Berlenga Island near Peniche, Portugal. 

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Attack dog - A trained German Shepherd dog chases a mock intruder during an Indian National Security Guard display in Manesar near New Delhi, India. 

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Helicopter hounds - A pair of US Coast Guard Maritime Safety & Security Team Belgian Malinois canines take part in helicopter proficiency training with the California Air National Guard in San Francisco Bay, USA. 

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A very furry friend - A kennel owner walks one of his Komondors, a traditional Hungarian guard dog, in Bodony, Hungary. 

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A nose for the job - Mona, a Colombian drug-sniffer dog, stands guard next to packs of cocaine at a police station in the country's capital, Bogota.  

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Running sheep - A sheepdog runs sheep towards a pen at the British National Sheep Dog Trials in York, England. 

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Mascot - A dog mascot of the Italian elite military unit Cuirassiers' Regiment is seen during the Republic Day military parade in Rome, Italy. 

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Faithful friend - Sully, a yellow Labrador service dog for former President George H. W. Bush, sits near the casket of the late former President as he lies in state at the US Capitol, Washington, D.C., USA.

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