Joey Wong Choi-ling (name changed for patient confidentiality reasons), 10, had a runny nose, sore throat and cough for a week that developed into a headache and high fever. Worried, her parents took her to Yan Chai Hospital in Tsuen Wan, where she was admitted for testing. Doctors found an enlarged and tender lymph node on the left side of Choi-ling's neck. After she spent a month in hospital, despite intravenous penicillin and exhaustive investigations including blood and microbiological tests, no underlying cause for her fever was found. Then Choi-ling lost vision in her right eye, and doctors immediately referred her to the ophthalmology unit at Caritas Medical Centre in Sham Shui Po. There, doctors Benjamin Chu and Victor Tam examined her pupillary reaction to light using the swinging flashlight test, which involves shining light into one eye and then the other. A normal reaction is for both pupils to constrict each time light is shone into them, and dilate when the light is in the midst of moving to the other eye. In Choi-ling's case, the left pupil constricted promptly, but the right showed an initial weak constriction to the direct light, followed by dilation. This indicated a dysfunction of the optic nerve. On further examination, she was found to have neuroretinitis, an inflammation of the retina and optic nerve in her right eye. There was swelling of the optic nerve head with a 'macular star', so described because of the abnormal pattern of bright streaks in the centre of the retina. These streaks are made up of lipids, fatty substances normally found in the blood. These findings suggested cat-scratch disease, and this was confirmed by a blood serum test for the Bartonella henselae bacteria that causes this disease. A month before falling ill, the girl had travelled to the mainland, where she had been in close contact with cats and dogs. About 40 per cent of cats carry Bartonella henselae at some time in their lives. Kittens under a year old are likelier to be infected. Although fleas spread the bacteria among cats, there's no evidence to suggest that fleas transmit the disease to humans. The bacteria live in the saliva of infected cats but don't make the animals sick. They're transmitted if the saliva comes in contact with a human via the eye or a bite or scratch. Most people with cat-scratch disease develop an infection at the site of the cat scratch or bite. Within a couple of weeks, one or more of the lymph nodes close to the injury site become swollen and tender - typically those around the head, neck, and upper limbs. The victim may develop fever, headache, fatigue, rash, sore throat and a poor appetite. Most cases, says Chu, clear up within a month or two without any treatment. Choi-ling's case was atypical. Such victims usually experience a lingering high fever, and some get an eye infection, such as neuroretinitis or parinaud oculoglandular syndrome (an eye problem similar to conjunctivitis). Choi-ling was treated with a combination of oral antibiotics: 300 milligrams of rifampicin every day for a week and 100 milligrams of doxycyline twice a day for five weeks. Her fever resolved as soon as treatment was initiated, lymph node swelling gradually subsided, and her vision improved. The macular star took longer to resolve, but three months after diagnosis at a follow up visit to Caritas Medical Centre, no optic nerve damage was found and the symptoms had not recurred. The good news is that having one episode of cat-scratch disease usually makes you immune to it for the rest of your life. To avoid the disease, beware of stray and unfamiliar cats, and avoid playing rough with them to prevent being bitten or scratched. Always wash your hands after handling a cat.