Hawaiian monk seals are among the most endangered animals in the world. Endemic to the Hawaiian archipelago and with a total population of about 1,400 seals, this species continues to battle for existence against a multitude of threats.

Habitat loss and food limitations associated with climate change are two significant threats to the survival of monk seals. Additionally, seals frequently become entangled in abandoned fishing nets and other types of marine debris, accidentally ingest fishing hooks, and some are even intentionally killed by humans.

Many of these risks are being addressed through monk seal recovery efforts of the National Oceanic and Atmospheric Administration (NOAA), the federal agency responsible for the management of marine mammals. But there is another threat that no one is quite sure how to handle: feral cats.

While cats are known to negatively impact native bird populations through predation, the link between feral cats and endangered seals is not immediately obvious.

Michelle Barbieri, a veterinarian working with NOAA on monk seal recovery, explained that cats carry a protozoan parasite, Toxoplasma gondii. The oocysts, or eggs, of this parasite shed out in cat feces and then persist in the environment for years.

When other mammals ingest these oocysts, there is a possibility they can cause an infection known as toxoplasmosis. Infrequently, toxoplasmosis can cause severe illness and even death.

So how are monk seals coming into contact with T. gondii oocytsts? Research has shown that oocysts shed into the terrestrial environment are flushed into the ocean with runoff and the seals consume them directly from sea water or indirectly through infected prey.

So far, at least eight monk seals have died from toxoplasmosis and it is suspected that the total number is higher, as some seals die at sea and their bodies are never recovered.

For a critically-endangered species, this is no trivial issue. Considering the number of monk seals in the main Hawaiian island is increasing, and so is the number of feral cats, it is likely that T. gondii infections will harm more seals in the future.

Unfortunately, there is no simple solution to the feral cat problem in Hawaii. Current estimates of the feline population on Oahu are between 50,000 and 300,000 cats, which means many more T. gondii oocysts continually entering the environment, increasing the risk of infection for other mammals.

The most popular approach to dealing with feral cat colonies is the practice of “trap, neuter, release” or TNR. However, as Angela Amlin of NOAA’s monk seal recovery team explained, “TNR is not shown to be effective in reducing cat populations” because efforts cannot keep up with the rate at which cats reproduce.

Lethal removal would be the most effective means of eradicating feral cats in Hawai’i. However, this method is highly controversial. Many community members care for the cats and rampantly oppose their removal. Additionally, state laws made to protect house pets from violence make it impossible to eradicate feral cats through lethal means.

NOAA has recently created an interagency working group through which they hope to develop strategies for combating this issue, and then engage with the community to gain support. But without significant changes in local attitudes, reducing feral cat populations and toxoplasmosis in monk seals is unlikely.