"I felt like I'd been stabbed," he said. The pain soon passed, however, and he went back to sleep. "I didn't think much of it."
But that would not be the end of his ordeal.
In the weeks that followed, the stabbing pain didn't return, but it was replaced with night sweats, shortness of breath, coughing and weight loss. "I started deteriorating quite quickly," he said.
He was soon diagnosed with pneumonia and promptly given antibiotics. But the drugs didn't work -- and his long road to a true diagnosis began.
An animal source
A few months earlier, Cranston had been working with a private veterinary organization in the town of Nelspruit, located in the province of Mpumalanga, South Africa. He declined to give the name of the group.
Cranston had worked with the organization for a few weeks each year, part of a team passionate about improving the health of the rhinoceros and other wild animals in that region.
"We worked with everything from rhinos to lions to giraffes. The complete range," he said, adding that this was far removed his regular role working with domestic animals in the UK.
On the 2013 trip, one group of animals had dominated his time. "We were predominantly doing work with wildebeest," he recalled. His team had been monitoring the animals' vital signs to learn how they cope in an enclosure.
Cranston had, in fact, spent large quantities of time head-to-head with one particular wildebeest that would later become known -- in his head, at least -- as patient zero.
He suspects that this animal was infected with bovine tuberculosis, caused by the bacteria Mycobacterium bovis, because Cranston himself was diagnosed with this infection months later.
Bovine TB, also known as zoonotic TB, can infect any animal. Its symptoms resemble those of the more regular form of TB seen in humans, which is caused by Mycobacterium tuberculosis.
In 2015, there were an estimated 149,000 cases of humans infected with zoonotic TB, mostly in Africa and South Asia, and more than 13,000 deaths.
'I knew something was wrong'
"Thinking back, that trip I was predominantly at the head end of the wildebeest," he said, recoiling a little at the memory. His hand had been in and out of the animal's mouth to ensure a clear airway when it was anesthetized. This intimate contact exposed him to particles in the animal's respiratory tract, its bloodstream and many other bodily fluids and cavities that could harbor infections of all forms.
When he got sick in 2013 and doctors initially failed to figure out what was wrong with him, this vision plagued his mind.
"To all these doctors, I was an anomaly," he said. "But I knew something (else) was wrong."
After being incorrectly treated for pneumonia, he collapsed at work and was admitted into the hospital.
"By the time I was in hospital, I was pretty sure it was TB," he said, though none of his doctors thought the same. "If it wasn't TB, I was confused what it could be."