Treatment Plan

The Spectrum of Toxoplasmosis

By Leo Semes, OD, FAAO

Toxoplasmosis infection has worldwide distribution (London et al, 2011). Estimates suggest that up to 20 percent of the world's population may be infected. While this may sound like an epidemic, the ocular manifestations are much lower in frequency. The organism involved is Toxoplasmosis gondii, an obligate intracellular parasite. A recent review of the literature has offered guidelines for recommending treatment in the event of ocular involvement (Commodaro et al, 2009). This column will discuss ocular toxoplasmosis presentation and management as well as risks for its development.

Risks

A central toxoplasmosis laboratory has been established at Stanford University. Its most recent publication has discussed newly discovered risks for developing toxoplasmosis infection. In addition to the well-known ones of eating raw or undercooked meat and proximity to cat, rodent, or farm animal feces, the newly enumerated risks include:

• Eating locally produced cured, dried, or smoked meat.
• Working with meat.
• Drinking unpasteurized goat's milk.
• Owning three or more kittens.

Eating raw oysters, clams, or mussels was significant in a separate model among persons asked this question (Jones et al, 2009).

Given this list, it may seem that exposure to T. gondii is almost inescapable. The purpose is not to instill fear in our patients but to enumerate the risks for the purpose of minimizing exposure and the possibility of infection.

Presentation

Ocular involvement rests on recognition of an active lesion. Ocular presentations often involve reactivation of previous lesions. Inflammation secondary to T. gondii ocular infection is the most frequent cause of posterior uveitis, in 30 percent to 50 percent of cases by some estimates (Commodaro et al, 2009; Da-la-Torre et al, 2007). The fundus may be obscured by the overlying uveitis/vitritis.

When a teenager presents with reactivation of a lesion, the possibility of HIV infection or AIDS should be ruled out (Kramer et al, 2003). Retinal involvement primarily involves the inner retina. Understand that the inflammatory response can extend to the outer retina as well as to the choroid (Monnet et al, 2009).

Treatment is indicated when the posterior pole is involved. Specifically this would include macular involvement or threat by proximity to the optic nerve (Eckert et al, 2007).

Interpreting studies of reactivation is confusing. On the surface, the longer that patients go with an interval free from reactivation, the lower the chances of that occurring. That optimism, however, is somewhat counterbalanced by increasing risk of reactivation with age (Holland et al, 2008; Holland, 2009).

Management

The classic treatment includes pyrimethamine and sulfadiazine with folic acid supplementation. A contemporary treatment of choice is Bactrim (sulfamethoxazole + trimethoprim). Alternatives include clindmycin and azithromycin, which are both available as generics. Yet controversy remains regarding the balance of efficacy, ease of dosing and cost (Soheilian et al, 2005; Holland, 2005). So, the jury is still out on oral management in immunocompetent patients.

Intravitreal injection of clindamycin plus dexamethasone may emerge as the optimal treatment strategy (Soheilian et al, 2011). Finally, two Internet resources that may be useful to clinicians to share with patients include information from the Centers for Disease Control and Prevention (www.cdc.gov/parasites/toxoplasmosis, United States) and the Feline Advisory Board (www.fabcats.org, United Kingdom). CLS

For references, please visit www.clspectrum.com/references.asp and click on document #186.

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Dr. Semes is a professor of optometry at the UAB School of Optometry. He is also a consultant or advisor to Alcon, Allergan, Optovue, and Zeiss.