A 34-year-old man with a known history of HIV presents with blurring of his vision and ‘floaters’ in his right eye that have gradually worsened over the past 10 days. He is experiencing no eye pain and there is no redness or injection of the eye. He states that he had a ‘flu-like’ illness a few weeks earlier but is otherwise well. He has no other past medical history of note and keeps two cats at home as pets. His visual acuity has been reduced to counting fingers in the right eye but is 6/6 in the left eye. Your fundoscopy findings are shown below:
Image courtesy of the National Intitutes of health via Wikimedia Commons
Toxoplasmosis is one of the commonest causes of infectious chorioretinitis. This most commonly occurs as a congenital infection but can also occur in patients with a weakened immune system, as has occurred in this case.
The commonest presenting features of ocular toxoplasmosis are unilateral reduced visual acuity and floaters. The condition is usually painless.
Image courtesy of the National Cancer Institute
- Exudative ‘cotton balls’ (focal white atrophic areas)
- Overlying inflammation of the vitreous humour
- Old dark chorioretinal scarring often also present
Because of the high prevalence of positive toxoplasma titres in many populations, serology is generally only useful to ‘rule out’ the diagnosis. Completely negative IgG titres can rule out the diagnosis in an immunocompetent individual. The development of PCR testing for toxoplasmosis has proved to be useful in assisting diagnosis in atypical or difficult cases. Detection of Toxoplasma gondii DNA by PCR in both the aqueous and vitreous fluid is both sensitive and specific.
Corticosteroids, such as prednisolone, can be used for 3-6 weeks to reduce optic nerve and macular inflammation. Corticosteriods should not be used without concurrent antibiotic therapy or in immunocompromised patients (as in this case) due to the risk of exacerbating the condition.
Toxoplasmosis can therefore be prevented by:
- Using gloves and meticulous hand hygiene when changing cat litter
- Not feeding raw or undercooked meats to cats
- Cooking meats to a safe temperature
- Cleaning kitchen preparation surfaces that have contacted raw meats
Due to the potentially devastating effects of congenital toxoplasmosis it is advised that pregnant women should completely avoid the handling of cat litter.