Monday June 15 2015
Cats don't usually show any symptoms of toxoplasmosis
“Scientists have discovered a link between people who own cats and the development of mental illnesses, including schizophrenia, and believe a parasite may be to blame,” The Independent reports.
The researchers suggest that toxoplasma gondii (T. gondii), a type of parasite found on infected cats, may be a cause of developing mental illness in later life. T. gondii was blamed for children’s poor reading skills in a study we analysed earlier this month.
The parasite has also been linked to an increased risk of suicide, as we discussed back in 2012.
This latest study used data from over 2,000 families in the United States to look at the number of people who were living with schizophrenia or schizoaffective disorder and owned a cat in childhood. This data was compared to findings of previous studies, conducted by the same study group, with the aim of confirming a link.
A large proportion of study participants were in contact with a household cat as a child, similar to the results found previously.
This study was unable to prove the link between cats and mental illness, and does not give any definite reasons for their observed links. Therefore, we should not be too concerned about the findings.
Where did the story come from?
The study was carried out by researchers from the Stanley Medical Research Institute and Johns Hopkins University, in the United States. Funding was provided by the Stanley Medical Research Institute. No conflicts of interest were declared by the authors. The study was published in the peer-reviewed medical journal Schizophrenia Research.
This story has been reported by a number of UK media sources; however, describing cat ownership as having a “strong link” to schizophrenia is misleading. In fact, there are reports that owning a pet can be of value for some people, in terms of mental health and quality of life, such as older people and patients recovering from major illness.
What kind of research was this?
This study used data from a cross-sectional study conducted at the National Alliance for the Mentally Ill (NAMI) annual convention in 1982. Analysis of the responses was carried out to assess whether there was a link between cat ownership and schizophrenia. This type of study is unable to prove cause and effect, but it can show possible associations, which can provide a route for further research.
What did the research involve?
The researchers used data from a questionnaire conducted at the NAMI in 1982; participants had a family member with schizophrenia or schizoaffective disorder.
The study included 2,125 questionnaires of families who lived in 46 states and the District of Columbia, and attempted to replicate the findings of their previous research linking cat ownership and mental illness. As no control group was used in the 1982 questionnaire, the researchers used the “middle parents” group from the American Veterinary Medical Association (AMVA), as this population was the most similar to their study group.
Questions included details of pregnancy, childhood and family medical history, and cat and dog ownership up to age 17, including ages of pet exposure.
What were the basic results?
The number who owned a cat when the affected person was between birth and age 13 was 50.6%. This result is similar to those found in previous studies in 1992 (50.9%) and 1997 (51.9%).
Among the “middle parents” control group from the 1992 AMVA, 42.6% owned a cat, which was virtually identical to the controls in the 1997 survey. The difference between the rate of cat ownership in the NAMI families and those in the AVMA control group was significant.
How did the researchers interpret the results?
The researchers suggest that cat ownership in childhood is significantly more common in families where the child later develops a chronic mental condition such as schizophrenia. They suggest this link may be due to the parasite T. gondii found on cats. They go on to say, “It is important to ascertain whether or not cat ownership in childhood is a risk factor for later schizophrenia, since it is a risk factor which could be minimised. We therefore urge our colleagues in other countries to collect data on cat and other pet ownership, and a major goal of this paper is to encourage such research”.
This study aimed to replicate the researchers' previous findings, which suggest that cat ownership in childhood is a possible risk factor for developing schizophrenia in later life. This study is able to draw a link, but cannot prove cause and effect. There is a suggestion that this link may be due to the parasite T. gondii, which is transferred from cats to humans if they come into contact with the faeces of infected cats, or eating or drinking contaminated food or water.
Even if this link between cats and mental illness was proven to be true, contact is unavoidable; children could become infected by playing in a public play area, even if their family did not own a cat.
This is because the T. gondii parasite can survive in soil for several months.
Is has also been suggested that exposure to cats provides risk in terms of other infectious agents shed by cats or by allergic exposures, since increased levels of childhood allergic reactions have been associated with increased risk of schizophrenia in later life.
The sample in the survey was also not representative of the whole population. NAMI members tended to be middle and upper class socioeconomically and their affected family member tended to be more severely affected than average.
To ascertain whether or not cat ownership in childhood is a risk factor for later-life schizophrenia, further research must be conducted that is able to prove cause and effect. Though the gold standard for evidence-based medicine, a randomised controlled trial would not be possible (we hope) for ethical reasons.
It is thought that schizophrenia is a very complex condition that can arise due to a combination of environmental and genetic factors, so simply owning a cat is unlikely to be a major risk factor for the condition.