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The image last week (Figure 1) shows a large distinct chorioretinal atrophic lesion centred around the macula.
The pigmentation suggests it is likely to be longstanding. The central placement and the distinct margins are characteristic of a chorioretinitis event and one cause that tends to affect the macula is infection with Toxocara. Toxocara is a species of nematode worm commonly found in pet faeces. It is most commonly associated with dogs (T canis) but also may be found in cat faeces (T felis).
It affects humans when the larvae are ingested. For this reason, the greatest risk appears to be among children who might come in contact with animal waste (in parks and so on) and fail to wash their hands prior to eating. The larvae enter the blood stream and may trigger inflammatory responses in a variety of tissues including the brain and lung, and also the eye.
The inflammatory response very often occurs at the posterior pole and therefore has a dramatic impact upon central acuity. Figure 2 is an old analogue photo of a patient seen at the City clinic as a youth with bilateral central vision loss related to infection. A sobering reminder, if any were needed, of the importance of good personal hygiene and the need for dog owners to ensure regular worming and avoid soiling public places.
Presumably because of the 'toxo' prefix, many confuse Toxocara with Toxoplasma. Figures 3 and 4 show some examples of toxoplasmosis chorioretinitis scarring which is more widely distributed. The infective agent here is a protozoan and there are different risk groups which will be discussed in a later issue of this feature. ?