Ocular Toxoplasmosis

Toxoplasmosis is an infection caused by the Toxoplasma gondii protozoan, capable of infecting any nucleated cell. The parasite depends on cats for its reproduction, but later it infects other animals and, indirectly, reaches us. It is typically found in undercooked poultry, beef, or pork. In the peninsula, infection through traditional sausages, chorizo and ham is typical. The ingestion of vegetables contaminated by infected feline feces or the consumption of contaminated water are also valid transmission routes. An estimated rate of 28.3% of Catalonia’s population is infected by this parasite¹.

At the ocular level, toxoplasmosis is the leading cause of posterior uveitis in our environment (approximately 50% of cases in my experience). There are many forms of presentation of the disease: congenital, punctate external retinitis, necrosis… but, of all of them, posterior chorioretinitis is the most frequent. In 2022, the “International Ocular Toxoplasmosis Study Group” (of which I am a part) published a study² on the different presentations and current management of this disease. The study was especially interesting due to the participation of professionals at a global level and the variety of presentations registered. I recommend reading it if you really want to delve into the subject.

Posterior chorioretinitis

As we have already said, it is the most frequent manifestation of ocular toxoplasmosis. Clinically, patients note a marked decrease in progressive visual acuity. It usually begins with a sensation of floaters that rapidly increase to a generalized fog that limits vision. It can be associated with discomfort or slight pain, but it is not usually very intense, compared to other entities.

Fundus exploration is usually difficult to perform due to the presence of inflammation in the vitreous (which causes the perception of fog) but a whitish retinal focus is usually identified. Once the inflammation stabilizes, a deep, whitish, cotton-like retinal lesion appears. The natural evolution is to grow and destroy the underlying retina. In successive relapses, we will see a clear scar with dark edges, with an acute whitish cottony focus nearby… Unfortunately, there is no treatment to eradicate the parasite and we can only fight against relapses. Toxoplasma gondii has a tendency to encyst and acquire a vital form in which it is not amenable to current antibiotics. Only when it breaks out and causes an outbreak, is it sensitive to antibiotic treatment.

Diagnosis is made by the clinical appearance, in the consultation, by an uveitis specializied ophthalmologist. Treatment is started immediately and a confirmatory analysis is requested. On specific occasions, the diagnosis can be delayed while waiting for the analysis. That said, I insist on the diagnosis in consultations by experienced eyes. Delay in starting treatment can lead to a worse visual prognosis. Therefore it is essential to establish the diagnosis as soon as possible and start treatment promptly.

The visual prognosis depends on the place where the focus of the infection is located. If the focus is in the peripheral retina, the prognosis will be excellent; on the other hand, if the focus is in the macular region, the prognosis is variable.

Treatment consists of a combination of oral antibiotics, for a few weeks. Once the parasite is destroyed by the antibiotic, the protozoan dies and releases inflammatory substances into its immediate surroundings. This causes exaggerated inflammation, known as the Jarisch-Herxheimer reaction, and appears to be getting worse when it’s actually getting better. In order to avoid this situation, we usually associate oral cortisone with antibiotic treatment.

Macular involvement

When the macula is affected by Toxoplasma gondii, we are faced with a more delicate situation. We have already mentioned that the natural evolution of the disease is towards the destruction of the retina. If a fragment of the peripheral retina atrophies, the consequences are derisory, but central involvement entails sequelae for the patient’s vision.

That is why when the macula is altered, we have to be more aggressive and ensure prompt treatment of the disease. The fastest way to do this is the injection of the antibiotic directly intraocularly. Conceptually it sounds painful but it is much less terrible than we imagine. In addition, these injections are usually repeated weekly for 2-4 weeks.

Thanks to this very aggressive approach many macular toxoplasmosis are stopped, but some of them inevitably leave a scarring macular lesion that will limit our vision for life. Normally the brain usually adapts to the new visual situation. But this is not always the case if we are talking about patients with only one eye (the other is lazy), squints…

Other forms

Toxoplasmosis can present in many other forms: retinal necrosis in immunosuppressed patients, internal or external punctate retinitis… and congenital toxoplasmosis should be mentioned specifically.

The congenital form affects children of mothers who have been infected for the first time during the first trimester of pregnancy. It is for this reason that pregnant women are prohibited from eating ham or other sausages during pregnancy. This infection can cause brain malformations, among other alterations. At the ocular level, bilateral macular lesions are typical, especially when these patients reach adolescence. Unfortunately, there is no treatment. In our environment, this pathology is in recession and for this reason the withdrawal of the routine screening program for pregnant women has been proposed.

Sources

1. Munoz Batet, C., et al. Toxoplasmosis and pregnancy. Multicenter study of 16,362 pregnant women in Barcelona. Med Clin (Barc), 2004; 123(1): p. 12-6
2. Yogeswaran K, Furtado JM, Bodaghi B, et al. Current practice in the management of ocular toxoplasmosis. British Journal of Ophthalmology Published Online First: 23 February 2022.