Toxoplasmosis and Vision Impairment

Toxoplasmosis is a parasitic disease, which occurs as a consequence of the Toxoplasma gondi (T.gondii) infection, one of the most common parasites in the world. It is an obligate intracellular parasite estimated to have infected over one billion people throughout the world (Reynolds, Falco, Schechtman, & Pizzimenti, 2012). In humans, T.gondii can be acquired as well as congenital, the latter transmitted to the placenta through maternal infection. Congenital infection can result in spontaneous abortion, stillbirth, neurological disorders, and various ophthalmic disorders. There have also been several studies which have reported an association between the Toxoplasma gondi infection and the increased risk of schizophrenia (Niebuhr, Millikan, Cowan, Yolken, & Li, 2008; Selten & Kahn, 2002).

The focus of this literature review is to examine current research relating to ocular toxoplasmosis, including its implication for orientation and mobility (O&M), with an emphasis on the following aspects of the disease:

The toxoplasmosis parasite infects animals and birds although the sexual cycle of the parasite occurs in the small bowel of cats and as such cats play a major role in the spreading of infection in nature. The research presented reveals the presence of cats is significantly important in the transmission of the parasite (Brown, 2011; Mustafa, Yusof, Norma, Muthusamy, & Shoib, 2012). Cats transmit the parasite through their faeces which contain oocysts (eggs) that can continue to be active for up to 18 months in moist soil. As such a repository of the parasite can exist in the environment from which other animals can be infected.

Humans can consume water or food that has been directly contaminated or eat undercooked meat from previously infected animals (Brown, 2011; Ng & McCluskey, 2002). A concise outline of the way T.gondii infects humans is provided by McLeod et al. (2006), whereby it is asserted that three main routes can transmit the parasite namely: (i) consuming infected or undercooked meat; (ii) ingesting the oocysts (eggs) of the parasite that cats pass on in their faeces, with contamination coming from cat litter or soil (the latter including eating unwashed vegetables and fruit, contaminated water, working in the garden); and (iii) a mother acquiring toxoplasmosis during a pregnancy and the infection flowing to the foetus.

Congenitally acquired toxoplasmosis is mostly not identified in pregnant women, especially if they are immunocompetent (have strong immune systems) as the presence of the infection in these cases is generally asymptomatic i.e., no apparent symptoms. Dr Rima McLeod, Professor of Opthalmology and Visual Science and Paediatrics at the University of Chicago, Director of the Toxoplasmosis Centre and her cohort have conducted extensive research into the treatment of ocular toxoplasmosis, with reference also to studies in France, which are at the forefront of treatment of the disease in utero (McLeod et al., 2006; McLeod, Kieffer, Sautter, Hosten, & Pelloux, 2009; Phan et al., 2008).

On the basis of their research, recommendations include that there should be systematic screening for acquired toxoplasmosis for all pregnant women in the USA during prenatal visits. In addition, standard practice should include the testing of all newborn babies for congenital toxoplasmosis and that confirmation of laboratory test results and relevant counselling should be part of the process (McLeod et al., 2006). The physicians who collaborated on the National Collaborative Chicago-based Congenital Toxoplasmosis study identified that signs, symptoms, and recognisable risk factors were absent in more than 50% of the mothers of children with congenital toxoplasmosis. The clinical practise of a doctor asking questions and performing physical examinations would only have revealed half of the mothers who had acquired the disease during pregnancy; the other half would have been identified through blood test screening (McLeod, Boyer, Karrison, Kasza, Swisher, Roizen, Jalbrzikowski, Remington, Heydemann, Noble, Mets, Holfels, Withers, Latkany, & Meier, 2006). McLeod et al. (2006) referred to obstetric care in France and Austria where education about toxoplasmosis and screening for the parasite is part of the regular medical appointments for a mother during her pregnancy. As a result the rates of infection have decreased by more than half, with monthly blood tests assisting the early identification of any infection and timely treatment to help prevent the parasite infecting the foetus. Previously, infection rates in France were quite high as their food culture included eating undercooked meat, such as steak tartare. The authors maintain that an early diagnosis allows for timely treatment, which can decrease the chances of ophthalmologic or neurological damage. Reynolds et al. (2012) consider that there should be a range of serological testing, the primary test being the Sabin-Feldman dye test. They argue that comprehensive tests are required as the basic clinical presentation of ocular toxoplasmosis can be similar to that of other infections, including tuberculosis and syphilis.

In Australia there appears no evidence in literature that advice provided by the Department of Health in the various States and Territories with regard to pre natal testing includes screening for the presence of the toxoplasmosis parasite as part of the recommended standard blood tests. Given the increasing presence of domestic cats and wild cats in urban and regional areas, the collection of water samples from various locations such as dams, rainwater tanks, and soil samples from locations of high food production may provide data useful in increasing our understanding of the way toxoplasmosis parasite is transmitted in Australia.

A diagnosis of toxoplasmosis in infants can often be delayed if its presence is asymptomatic. Also if infants are pre verbal then they are unable to describe their vision. Where symptoms exist they can include eye pain, inflammation, and reduced vision, often described as “headlights in a fog” (Brown, 2011). The clinical features may include retinochoriditis (i.e., creamy white lesions adjacent to scars on the retina), scotomas (i.e., diminished visual acuity) or floaters which are opaque spots floating in the vitreous fluid. Major vision impairment can be evident from lesions on the macular, while lesions in the periphery may not have an obvious impact on vision. Delair et al. (2011) recommended the use of an Amsler Grid on a daily basis for those with retinal scars for early detection of any reactivation of the parasite; although this test may not be useful for detecting new lesions outside of the central vision. A recurrence of ocular toxoplasmosis is common in those who have congenital toxoplasmosis, especially if untreated as an infant, and often occurs during adolescence (Phan et al., 2008). The reactivation of the infection can also be experienced if a person is immunocompromised i.e., their immune system is in a weakened state such as patients with HIV or undergoing chemotherapy.

There appears to be general agreement in the research literature as to the current drug treatment for the parasite when detected early. Further, McLeod et al. (2009) published additional data from their National Congenital Toxoplasmosis Study (McLeod et al., 2006) indicating that treatment during gestation and within the first year of life, reduced the potential damage caused by the parasite and prevented many adverse consequences arising from congenital toxoplasmosis. Ng and McCluskey (2002) contend that the requirement for treatment and its duration needs to be assessed on an individual basis, determined by factors such as the potential threat to vision, state of vision in the other eye, and the state of the patient’s immune system at the time.

A combination of drugs is often used to achieve a rapid result, with the use of pyriemethamine, sulfadiazine, and corticosteroids being a common approach (Delair et al., 2011). Therapy can range from four to six to 12 weeks in patients with a competent immune system, but patients with a compromised immune system, such as those receiving organ transplants or who have HIV infection may require longer term treatment. The use of Bactrim (sulfamethoxazole and trimethoprim) for a four to six week period appears to be a safe yet effective alternative for the common approach described (Reynolds et al., 2012).

The desired clinical result is evident when the retinal scars are no longer white and fluffy in appearance, the vitreous clears, and the existing scars on the retina develop sharp margins (McLeod et al., 2009). Generally, if a person is immunocompetent, i.e., they have a healthy immune system, the toxoplasmosis parasite can be kept in a dormant cyst state, although reactivation can still occur often without apparent symptoms, or symptoms which resemble a flu.