Veterinarians are working around the clock to save elephant seals afflicted with otostrongylus (Cailfornia, USA)

Basco, a young elephant seal weaner, was badly infected with otostrongylus and died. © The Marine Mammal Center

May 3^rd, 2013. As you may have heard, 2013 has been an extremely busy year for our veterinary team and animal care volunteers due to a sea lion crisis. As they continue to care for over 140 patients on site, including many emaciated sea lion pups, they are now also caring for a large group of elephant seals, many of which have a parasite infection known as otostrongylus. Sadly, so far six of the 13 elephant seals diagnosed with this infection have died.

What is particularly interesting about this disease is that it is most commonly seen in harbor seals – a species which has adapted and is able fend off the infection. Unfortunately elephant seals do not have a strong resistance to the infection, so for them it can be fatal.

What is Oto ?

Otostrongylus circumlitus is commonly known as harbor seal lungworm. In elephant seals, the parasitic worms causing the disease are usually found in the heart. There they often plug up pulmonary arteries, resulting in so many clots in the lungs that the animal exhausts its clotting mechanism. This is called Disseminated Intravascular Coagulation, or DIC for short. Some veterinarians refer to this as “Death Is Coming”… that gives you an idea of how serious this parasite is for elephant seals!

All in a Life Cycle !

No one knows for sure, but we believe this parasite, like many other worms, requires an intermediate host, in this case probably a fish. In fact, scientists have found larval (immature) forms of this worm in fish. Scientists believe the life cycle goes like this: an infected harbor seal passes worm eggs in its feces. The eggs pass into the water and are either eaten by a fish or a small creature which is then eaten by a fish. The parasite then hatches. Once inside the small creature or fish, they are in the “intermediate host” and they molt into another immature worm larvae that, if ingested by a mammal, can infect the host.

When the intermediate host is eaten by a seal, the larvae migrate into the intestinal wall and start their journey toward the heart. They travel through the veins to the intestines, connect to the portal vein, taking them to the liver, migrate through the liver and end up in the right ventricle of the heart via the caudal vena cava. From there they head to the pulmonary arteries and the lungs where they remain until they bore their way into the airways to molt and become adult worms. They deposit eggs which get coughed up, swallowed and then passed through the feces. And with that, the cycle repeats again. Most harbor seals have developed an immunity against this parasite. You may remember from Science class that a parasite that kills its host is not a successful parasite, since if kills its most, it has no home and loses protection from the elements and its food supply.

Why is it a Problem for Elephant Seals ?

When a parasite that evolved with one host gets into a different one, it can cause real problems. The new host, in this case elephant seals, does not have resistance to Otostrongylus circumlitus, leaving it vulnerable to infection. In fact, in some elephant seals it causes a massive inflammation response, like an allergic reaction in the blood vessels.

The big question is why? One possibility is that this parasite is relatively new to the elephant seal population. The current population of approximately 127,000 northern elephant seals descended from a mere 20-100 lucky survivors after they were hunted almost to extinction in the late 19th century. This population bottleneck leaves them particularly vulnerable to disease.

As elephant seals have come back from near extinction and have started hauling out and having babies on the mainland, they are now overlapping with harbor seal habitat, which puts them into contact with the intermediate hosts of harbor seal parasites. So the seriousness of this new disease may simply be an early phase of their reaction to this parasite and they may eventually evolve and develop resistance to it.

The lungworm Otostrongylus circumlitus under the microscope. © The Marine Mammal Center

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