Science1 publisher3 min readPublished
Countries with more cats show less Toxoplasma exposure in a 93-country happiness analysis
Researchers comparing 93 countries in PLOS One found more cats went with higher national happiness, and more Toxoplasma exposure with lower. Cat-dense countries also had less of the parasite, so pet and wellbeing studies need to measure how cats are kept before crediting or blaming them.
The Scientist · Science desk
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What happened
- Toxoplasma gondii, the parasite whose primary host is the domestic cat, infects as many as one in three people.
- Infection is usually symptomless, but it can harm vulnerable groups and has been linked to anxiety, depression and schizophrenia.
- National wealth was also part of the analysis, as the factor most often reached for to explain why some populations are happier than others.
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Why it matters
- contradiction ScienceAlert's write-up says 'now we know' cats could boost contentment, while the study's authors say their country-level data cannot show that cats make anyone happier.
- constraint Because each data point is a whole country, the study cannot say whether an individual cat owner is happier or whether infection lowers a given person's mood.
- decision Pet and wellbeing studies that treat cat ownership as parasite exposure will have to record indoor keeping, vet care and sanitation, since national cat numbers ran opposite to infection.
The sample size in this study is 93. Each nation enters the analysis as one row: a happiness score, a figure for domestic cat density, a Toxoplasma infection rate and a measure of wealth [1][4]. With that few points, a handful of unusual countries can move a correlation, and the researchers say some may have [6]. China is their example, with relatively high happiness and fewer cats than most nations [6]. The findings are reported as directions of association [5]. The coverage does not include effect sizes, or how much accounting for wealth changed them.
The result that complicates the pet-and-parasite picture is the third correlation. Countries with more cats tended to have lower T. gondii exposure [7]. Cats are the parasite's primary host [2], though not the only route of human infection [14], so the expected sign was the opposite. "This goes against the simple assumption that having more cats necessarily means greater parasite exposure and suggests that factors such as pets management, sanitation, and socioeconomic conditions may be important," said Javier I. Borraz-Leon, a biologist at the National Institute of Psychiatry Ramon de la Fuente Muniz in Mexico [10].
ScienceAlert offers one account of how that could happen. Wealthier countries may have more cat owners and better veterinary care, and their cats are more likely to be kept indoors, checked by a vet and fed commercial food [9]. Wealth was in the analysis [4]. A national wealth figure, though, cannot show whether a particular household keeps its cat indoors.
For studies of pets and wellbeing, that changes what has to be measured. If exposure tracks pet management and sanitation, as Borraz-Leon suggests [10], then counting cats may tell a researcher little about parasite risk. I think a study comparing owners with non-owners would need to record how the animal is kept, and ideally test people for infection, before attributing any mood difference to the cat. The parasite belongs in that comparison because it has been linked to anxiety and depression, the outcomes a wellbeing study is built to detect, as well as schizophrenia [3].
The authors are careful about what their design can show. "Of course, these are associations at the country level, so we cannot say that having more cats makes people happier or that Toxoplasma makes people less happy, but the patterns are interesting enough to deserve further investigation," they said [8]. ScienceAlert's own write-up went further: "Now we know that having a cat around could lead to a boost in contentment and well-being" [11]. Country-level correlations cannot support that claim.
The study that could test it would track cat owners and non-owners individually [13]. The authors wrote that future national studies "should integrate both socioeconomic and biological perspectives and include longitudinal and interdisciplinary approaches to elucidate the underlying mechanisms and processes" [12].
What to watch
- An individual-level study that follows cat owners and non-owners over time, with infection testing, would show whether the national pattern holds for people.
- A reanalysis that drops influential countries such as China would show whether the cat-happiness association survives without them.
- Effect sizes and wealth-adjusted estimates from the PLOS One paper itself would show how large these associations are.