Toxoplasmosis Threatens Endangered Seal Species

July 30, 2018
Amanda Carrozza

Amanda Carrozza is a freelance writer and editor in New Jersey.

Toxoplasmosis has emerged as the leading cause of disease-related deaths in Hawaiian monk seals, one of the country’s most endangered mammals.

Officials in Hawaii continue to investigate the deaths of nearly a dozen Hawaiian monk seals— and they’re pointing the finger at the single-celled parasite Toxoplasma gondii that causes toxoplasmosis. The National Oceanographic and Atmospheric Administration (NOAA) in Hawaii has recorded 11 Hawaiian monk seal deaths due to toxoplasmosis. The first was confirmed in 2001, but 3 seals died in Oahu earlier this year—2 adult females and 1 premature pup.

This number is significant because the total population of monk seals is estimated at just 1400, with about 1100 seals in the Northwestern Hawaiian Islands and 300 in the main Hawaiian Islands. The estimate of 11 deaths is likely a low approximantion because more seals die each year than are found dead or are examined.


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Experts believe the parasite has become widespread among marine life because of the growing number of free-roaming cats in Hawaii, particularly living near watersheds where rain washes T gondii oocysts into rivers and streams that eventually drain into the ocean.

Toxoplasmosis is now the leading cause of disease-related deaths for Hawaiian monk seals and has been regarded as one of the "big three" threats to the species (the others being trauma and fishing gear). Toxoplasmosis affects species differently. In monk seals, the disease can weaken the immune system and cause vital organ failure.

Historically, toxoplasmosis is believed to have been partly responsible for the extinction of the Hawaiian crow in the wild and is also known to have killed Hawaii's endangered state bird, the nēnē goose, according to NOAA.

NOAA believes an important step in preventing more deaths is to educate Hawaii’s residents about how the seals become infected. In addition to hosting public forums led by NOAA faculty, the administration also released an infographic and fact sheet detailing the route from the mountains to the oceans.

“We simply cannot afford to lose even 1 of these critically endangered mammals to a disease that is preventable," Suzanne Case, chair for the Department of Land and Natural Resources said. "We hope people will provide as much love to our few very special seals as they do to the hundreds of thousands of feral cats around our islands."