MEDICAL BAG
This month’s casebook focuses on two medical mysteries: how can light cause sneezing, and what is making black bears behave more like dogs?
THE MYSTERY OF PHOTOSNEEZIA
Scientists are attempting to unpick the mystery of the “photic sneeze” – a bizarre reflex that causes people to sternutate, or sneeze, in response to bright light. Also known as “photosneezia”, the phenomenon is thought to affect about a quarter of the UK population, but the mechanism that triggers it is unknown. “It is typically preceded by a tickling sensation, and the sneeze is not immediate. It will take a few seconds,” said Manuel Spitschan, an experimental psychologist at the University of Oxford, who has launched a study. “It also appears that spending some time in darkness prior to the light exposure is necessary.” A particular trigger is stepping from a dark building into the bright outdoors.
In his Book of Problems, Aristotle pondered: Why does the heat of the Sun provoke sneezing? His hypothesis was that sweat in the nose triggered it, but Francis Bacon disproved this theory by facing the Sun with his eyes closed and finding that it didn’t make him sneeze. His theory was that the Sun’s light made the eyes water, causing moisture to trickle down the nose, initiating a sneeze.
Various modern theories have been proposed, but none is easily testable. Sneezes are thought to occur when the layer of cells lining the nose becomes irritated. This triggers the trigeminal nerve, which carries sensory information from the face to the brain, to initiate the sneeze reflex, expelling the offending irritant at speeds of up to 150km/h (93mph). One theory is that the trigeminal nerve can occasionally become cross-activated by signals from the optic nerve, which connects eye to brain, so that bright light hitting the back of the eye may also initiate a sneeze.
Another theory is that optic nerve fibres could accidentally activate neighbouring nerve fibres as they converge in the brain. Curiously, the reflex can also be triggered during eye surgery, meaning needlebearing an æsthetists must be alert to nascent sneezes to avoid damage. The condition is thought to be heritable, so if one of your parents has it the chances are you will too.
A related condition called gustatory rhinitis can prompt certain individuals to sneeze after eating, particularly spicy foods, although the mechanisms for this too remain a mystery.
The goal of the Oxford Photic Sneeze Survey, coordinated
“Infected bears approach people in a peaceful and friendly manner”
by Spitschan, is to better understand what drives the reflex, and maybe find the ideal stimulus. After completing a questionnaire, volunteers are shown different light sources. “We know it’s only bright light that triggers it, but wavelength might also be a factor,” said Spitschan. “Once we know that, we can start to unpick the retinal mechanism, so is it going through the rods or cones or something else?” He is also curious to investigate whether people’s chronotype, their preference for mornings or evenings, influences their propensity to photosneezia. Guardian, 12 Oct 2020.
BEARS TURN INTO ‘FRIENDLY DOGS’
Young Californian bears are becoming dangerously friendly due to a mysterious neurological disease. The brain infection affecting Californian black bears ( Ursus americanus) causes them to behave like pets, approaching humans without fear, seemingly curious and playful. In March 2018 a young female bear walked into a school classroom where she sat down among the children. And in 2019, another infected bear was caught on video trying to climb onto a snowboard, alongside its rider. This young male was eventually rescued after the snowboarder and his friends played with him and fed him sandwiches.
Ann Bryant, executive director of the BEAR (Bear Education Aversion Response) League has been involved in bear rescue. “Infected bears come to our attention because they approach people in a peaceful, friendly and non-aggressive manner,” she explained. “Their behaviour is similar to a dog, not a bear.” Ms Bryant cautions against human interaction with these large, powerful mammals. “Bears approaching people who might then treat the animal as if they are tame could easily present a danger,” she said.
The bears, all around one year old, appear to be suffering from a form of infectious encephalitis, an inflammation of the brain tissue that can be caused by viruses, bacteria, fungi or parasites. It may also be part of an autoimmune
response. Symptoms include a prominent head tilt, lethargic movements, muscle tremors, seizures, walking in circles and being significantly underweight, as well as this uncharacteristic fearlessness towards humans. Veterinarians are currently unsure as to what is causing the problem, or how fast it is spreading among the population. Four individuals were found with the disease in 2020, the highest number of cases since it was first recorded in 2014. This was in Nevada, when veterinarians detected an infection in bears seen around Lake Tahoe. Because the black bear population in California has increased from between 10,000 and 15,000 individuals in 1982 to between 30,000 and 40,000 in 2020, vets aren’t too worried about the impacts of the disease on the overall population.
The most recent case of a bear with a brain infection was discovered in February 2021, lying in the back of a truck parked on a residential property, severely underweight and covered in fleas. The bear had to be put down because of its poor physical condition, and its fate is typical of the majority of infected bears, unable to take care of themselves and apparently having been abandoned by their mothers. Infected bears are unlikely to be able to fully develop into selfdependent adults.
Five new viruses in black bears have been identified, any one of which may be the cause of the encephalitis, but none has yet been confirmed. A research lab is also investigating whether a protozoan parasite is involved, but scientists acknowledge there could be more than one pathogen responsible. Although they do not believe the disease to be transmissible between individual bears, scientists are concerned since the cause of the infection is unknown. “The worst part about this threat to our bears is that we don’t know for sure what causes it,” said Ms Bryant. “The unknown is troubling.” livescience.com, 2 Apr 2021.