Female reproductive rates and pup survivorship are generally higher in undisturbed areas with abundant food resources. There are a variety of pathogens and parasites that inflict otter populations. Most studies have focused on the more problematic southern populations. One of the main causes of death in California populations is Acanthocephalan peritonitis. This is caused by small worm-like parasites that enter the body with food, perforate the intestinal wall, resulting in infections of the abdominal cavity.
Protozoal encephalitis is a disease that is also relatively common in otters.
This disease involves swelling of the brain which has been found to be caused by several genera of protozoa. Coccidioidomycosis is an infection derived from a soil-borne fungus that is common in dry interior valleys. Cases of this type of infection are limited to populations in San Louis Obispo county. The nature of propagation of the disease is not well understood. Bacterial infection is another significant cause of otter mortality.