2020 updates by Cathy Bickel
Respiratory infections are most common in pups around weaning time, when they are 3-4 weeks old. They can also develop in elderly gerbils and gerbils of any age.
The most obvious symptom is a "clicking" noise made by the gerbil as it breathes. It sounds like someone breathing through a stuffy nose. Other symptoms include labored breathing, listlessness, puffy fur, and reddened skin. You may notice rapid breathing, with the sides heaving more noticeably. Activity will fall off as movement becomes exhausting. As breathing becomes more difficult, gerbils may sleep in strange positions, with their heads propped up on the edge of a nest or box, or jamming themselves nose-up into a corner. They may sleep crouching rather than lying down. Toward the end, the gerbil may try to breathe through its mouth; at this point, the condition is life-threatening. One underlying cause of respiratory infections is fluid buildup, such as from an internal tumor or heart disease. Respiratory infections can be fatal.
The recommended approach is to see your vet for an exam and antibiotics. Additionally, warm one corner of the tank, make sure the tank is clean and odor-free, provide extra bedding material, and watch to be sure your gerbil is still eating and drinking.If the respiratory infection responds to treatment but recurs when treatment ends, an underlying condition is most likely present, and euthanasia may be required as the gerbil's condition fails.