The Centers for Disease Control and Prevention estimates that 40 million Americans have varying levels of Toxoplasma infection, although most cases are asymptomatic. Commonly passed to humans via exposure to farm animals, infected cat litter, or undercooked meat, the parasitic infection causes unnoticeable or mild, to flu-like symptoms in most healthy people.
But for a small number of patients, these microscopic parasites hunker down inside of neurons, causing signaling errors that can result in seizures, personality and mood disorders, vision changes, and even schizophrenia.
“After the initial infection, humans will enter a phase of chronic infection. We wanted to examine how the brain circuitry changes in these later stages of parasitic cyst infection,"
said Michael Fox, a professor at the Fralin Biomedical Research Institute and lead author of a new paper describing how the common Toxoplasma gondii parasite prompts the loss of inhibitory signaling in the brain by altering the behavior of nearby cells called microglia.
The parasite forms microscopic cysts tucked inside of individual neurons.
“The theory is that neurons are a great place to hide because they fail to produce some molecules that could attract cells of the immune system,"
Fox and his collaborator, Ira Blader, recently reported that long-term Toxoplasma infections redistribute levels of a key enzyme needed in inhibitory neurons to generate GABA, a neurotransmitter released at the specialized connection between two neurons, called a synapse.
Building on that discovery, the scientists have now revealed that persistent parasitic infection causes a loss of inhibitory synapses, and they also observed that cell bodies of neurons became ensheathed by other brain cells, microglia. These microglia appear to prevent inhibitory interneurons from signaling to the ensheathed neurons.
“In neuropsychiatric disorders, similar patterns of inhibitory synapse loss have been reported, therefore these results could explain why some people develop these disorders post-infection,"
Fox said the inspiration for this study started years ago when he met Blader, a collaborating author and professor of microbiology and immunology at the University at Buffalo Jacobs School of Medicine and Biomedical Sciences, after he delivered a seminar at Virginia Tech. Blader studied Toxoplasma gondii and wanted to understand how specific strands of the parasite impacted the retina in mouse models.
Working together, the two labs found that while the retina showed no remarkable changes, inhibitory interneurons in the brain were clearly impacted by the infection. Mice — similar to humans — exhibit unusual behavioral changes after Toxoplasma infection. One hallmark symptom in infected mice is their tendency to approach known predators, such as cats, displaying a lack of fear, survival instincts, or situational processing.
“Even though a lot of neuroscientists study Toxoplasma infection as a model for immune response in the brain, we want to understand what this parasite does to rewire the brain, leading to these dramatic shifts in behavior,"
Future studies will focus on further describing how microglia are involved in the brain’s response to the parasite.
 Carrillo, GL, Ballard, VA, Glausen, T, et al. Toxoplasma infection induces microglia‐neuron contact and the loss of perisomatic inhibitory synapses. Glia. 2020; 1– 19. https://doi.org/10.1002/glia.23816
 Justin M. Brooks, Gabriela L. Carrillo, Jianmin Su, David S. Lindsay, Michael A. Fox, Ira J. Blader. Toxoplasma gondii Infections Alter GABAergic Synapses and Signaling in the Central Nervous System. mBio Oct 2015, 6 (6) e01428-15; DOI: 10.1128/mBio.01428-15