The owl, which makes its home in California, Oregon and Washington, is classified as threatened under the federal Endangered Species Act.
The new study, co-authored by San Francisco State biologists, compared the presence of blood parasites in spotted owls with 10 other West Coast owl species.
Though many kinds of blood parasites are not considered harmful, their presence is an indication of the owl's overall immune health, according to the researchers.
According to the study, researchers found spotted owls contained more blood parasites than any of the other owl species examined, and also detected for the first time the presence of the potentially harmful avian malaria parasite in the spotted owl.
The study raised the possibility that the spotted owl may have been infected with the avian malaria parasite by mosquitoes that fed on one of its main competitors for food and territory, the barred owl.
The barred owl, which was found to have lower infection rates for avian malaria, may thus hold a competitive advantage over the spotted owl, the study postulated.