Family dogs caught up in the Japan earthquake of 2011 and subsequent nuclear disaster at Fukushima showed signs of stress not inconsistent with PTSD long after the events, a new study finds.

The research compared abandoned dogs rescued from Fukushima with non-disaster affected dogs abandoned in 2009 and 2010, before the earthquake. The dogs that lived through the disaster had stress hormone levels five to 10 times higher than the dogs that were simply abandoned or found as strays.

"Long-term care and concern regarding the psychological impact of disasters appears necessary in humans and companion animals," the researchers wrote today (Oct. 11) in the journal Scientific Reports.

As part of a dog-rehabilitation program at Azabu University in Japan, researchers took in eight dogs from shelters in Kanagawa Prefecture and measured their levels of physical stress by monitoring the stress hormone cortisol in the dogs' urine. After the 2011 Tohoku earthquake and tsunami, thousands of refugees were forced to abandon their dogs. Many of the animals lived a semi-feral existence in areas made uninhabitable for humans by the Fukushima nuclear plant meltdown that followed the disaster. [Photos: Japan's Rescued Pets]

In May and November 2011, the Azabu University team took in 17 abandoned dogs collected at shelters and rescue centers in Fukushima. These dogs, like the Kanagawa canines, were rehabilitated and had their cortisol levels monitored daily. All dogs were later adopted by new owners.

When compared with the Kanagawa dogs, the Fukushima dogs were less aggressive toward unfamiliar people but also less attached to caregivers and more difficult to train. The disaster-affected dogs had five to 10 times the cortisol levels of dogs not touched by disaster, a gap that narrowed but did not close even after 10 weeks of loving care in the rehabilitation program.

The Fukushima dogs' handicaps in trainability echo learning problems in human trauma survivors with post-traumatic stress disorder, the researchers wrote. They suggested that similar brain chemicals could be at play in dogs and humans. Trauma-impaired humans can also struggle to bond with others, similar to the Fukushima dogs' lack of attachment to their caregivers.

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The researchers warned that the samples were small and not entirely equivalent, with the Fukushima dogs being older, on average, than the Kanagawa dogs. Nevertheless, they found no evidence that age affected how dogs responded to abandonment, suggesting the disaster was the biggest driver of the dogs' stress.

"Humans affected by the disaster are already recovering and gradually returning to normal life," the researchers wrote. "However, our results suggest the possibility that stress can induce excessive, deep psychosomatic impacts with implicit behavioral manifestations, such as deficits in attachment and learning ability also in dogs."

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