Posted by Kasra
Giardia is a very successful parasite. It’s highly durable cysts enter the body via contaminated food. Once inside the small intestine, the cysts hatch and the trophozoites start swimming with their multiple flagella. They attach to the intestine’s surface and enjoy the nutrient rich environment of the small intestine. Shortly after, they start producing cysts that leave the body via feces. Their presence may or may not cause severe symptoms. In many cases people carrying Giardia in their body – thus shedding cysts – might not feel anything. On the other hand, Giardia infection can lead to severe diarrhea that could stick around for weeks if untreated. Our immune system usually manages to control the infection and get rid of the parasite. But in any case, the parasite can come, live and go undetected.
Similar to many other parasitic diseases, since Giardia infection does not kill a significant number of its victims, is not rampant in industrialized countries and is more or less readily treatable, it is not funded and studied by many researchers. Unfortunately, despite unique biological features such as lack of mitochondria, presence of two nuclei and an anti-inflammatory host-pathogen interaction, Giardia remains largely understudied. We don’t fully understand the host and pathogen factors that could lead to disease or just sub-clinical infection. Nor we know much about how Giardia gets detected by the immune system and what is the nature of the immune response that kicks the parasite out of the body.
One of the strong incentives for studying Giardia is its higher prevalence in children. Infectious diarrhea is still the most common of cause of child death (Cryptosporidium is another understudied parasite and causative agent of diarrhea in children, which I discussed in another post). A recent study by Bartelt et al. published in Journal of Clinical Investigation, presents a new model of Giardia infection, focusing on malnutrition and young age. They argue that many children in areas where Giardia infection is common are undernourished. This malnutrition could contribute to development of a persistent Giardia infection with severe symptoms rather than a shorter non-symptomatic infection. To study the effect of children malnutrition on Giardia infection, they set their model on 3-week old recently weaned (taken away from mother, eating solid food) mice. They show that although healthy mice are able to clear the infection, Giardia parasites manage to stay longer in the small intestine of malnourished mice and also cause more growth impairment. It can be thought that this is a vicious cycle, where infectious diarrhea causes further weakening of the individual and thus further difficulty in fighting the infection, leading to severe weight loss and persistence of the parasite. Interestingly, the load of parasite in the intestine remains unchanged when comparing healthy and malnurished mice. However, the authors describe their model by pointing to other differences such as immune response and small intestine pathology. More studies on this model can help us better understand and hopefully better treat Giardia infection in children.
Bartelt, L., Roche, J., Kolling, G., Bolick, D., Noronha, F., Naylor, C., Hoffman, P., Warren, C., Singer, S., & Guerrant, R. (2013). Persistent G. lamblia impairs growth in a murine malnutrition model Journal of Clinical Investigation, 123 (6), 2672-2684 DOI: 10.1172/JCI67294