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:
1. What is the most likely diagnosis?
This patient has a diagnosis of ocular toxoplasmosis. Toxoplasmosis is caused by the protozoan organism Toxoplasma gondii. Infections in healthy adults usually cause no symptoms but occasionally can cause a flu-like illness and lymphadenopathy.
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
2. How can this diagnosis be confirmed?
The diagnosis can often be made clinically on the basis of the appearance of the characteristic features on fundoscopy:
- 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.
3. How is this condition treated?
Small extramacular lesions that are not threatening vision can be observed without treatment. Larger, sight-threatening lesions are are treated for 4-6 weeks with classic triple therapy consisting of pyrimethamine, sulfadiazine and folinic acid.
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.
4. What measures can be taken to prevent the transmission or contraction of this condition?
Toxoplasma gondii is usually spread via ingestion of poorly cooked food containing cysts of Toxoplasma gondii or due to exposure to infected cat faeces.
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.