It is not uncommon for mice to be affected by infestations of external parasites such as fur mites. Clinical signs of infestation include bald patches, thinning of hair (especially in areas which they find hard to reach and groom like the head and trunk), a greasy appearance to their hair/coat, and self-induced trauma as the affected mice scratch or chew themselves.
These parasites are too small to be seen with the naked eye, so a trip to the veterinarian will usually be required. Your veterinarian may need to look at hair samples and/or skin scrapings under the microscope to be able to tell you what is affecting your mice.
Treatment with appropriate topical ‘spot-on’ treatments is very successful. Talk to your veterinarian about this and what products are appropriate and safe to use. Avoid treatments such as washing with flea shampoos as this is very distressing to the mouse, and the new smell after washing could lead to social isolation.
Although internal parasites (e.g., pinworms and protozoa) are reported to be relatively common in mice, these are generally not symptomatic infections. If mice have a symptomatic internal parasite infection, they may show signs such as weight loss, diarrhoea, faecal impaction, or other gastrointestinal disturbances. Mice showing any of these signs or who are not eating well should be taken to the veterinarian promptly for diagnosis and treatment. If your mice have been diagnosed with pinworms, it is very important to deep clean and disinfect the environment thoroughly under the direction of your veterinarian as well as treating the affected animals, as pinworm eggs have been shown to persist in the environment and could cause reinfection.
As symptomatic infections with internal parasites are not common in mice, regular routine worming for your mice is not generally necessary, unless recommended by your veterinarian.
Frohlich J (2020) Rats and mice. In: Quesenberry KE, Orcutt CJ, Mans C, Carpenter JW (eds) Ferrets, rabbits and rodents, 4th ed. W.B. Saunders, pp 345–367