Can Sharks get Rabies?

Last Friday, September 14, we had another Science on Main event on Main Street in Mesa, AZ. This month's theme was Pirates' Block Party, which conjures images of the sea, Jolly Roger flags, and colorful men and women who defied the laws of the powerful Navies of the day. In honor of this theme, the next few blog articles will be on topics related to the ocean and all its mysteries.

One of the best parts of being part of Science on Main is when I hear a question I've never before considered. This week's blog topic is a question that we were asked by a young scientist a few months ago, and it's one that definitely took me by surprise. Can sharks get rabies?

Jim Abernethy, National Geographic Creative

Rabies is a viral disease that is transmitted between animals and people via bodily fluids, usually through bites but rarely through scratches. According to the World Heath Organization (WHO), up to 99% of all rabies infections come from bites from domestic dogs, especially in countries like India, where there are many stray dogs. In the Americas and some southeast Asian countries, bats are the most common carriers. Most rabies deaths today (95%) occur in Africa and Asia, and up to 40% are in children under the age of 15, who may not recognize the signs of a rabid animal. These signs include hyperactivity, a fear of water or drafts, and partial paralysis, especially around the infection site.

Rabies is almost always fatal, and was responsible for many deaths up until the late 1800s. Thankfully, Louis Pasteur and Émile Roux developed the first rabies vaccine in 1885. If a human was bitten by a rabid animal and received treatment within 10 days, that person was very likely to survive. According to WHO, over 15 million people per year receive a post-bite rabies vaccination, which saves an estimated hundreds of thousands of lives. More vaccines have been developed since and are used to prevent infection in the first place. If you have a pet dog in the United States, chances are high you've had to get this vaccine for your pet from the vet!

Dog bites are the #1 cause of rabies in humans.
Picture from Anoir Chafik via Unsplash

Now that we understand what rabies is, we can approach the original question regarding sharks. One interesting fact is that rabies is a disease that occurs only in mammals. No animal other than mammals has ever been reported as rabid. Birds were artificially infected with rabies in 1884, but they developed few or no symptoms, and always recovered. A study in 1988 confirmed that blood from infected birds contained "no significant antibody titers," and therefore supported the conclusion that wild birds are not a significant source for rabies. Curiously, though rats and rodents can be vectors for other serious diseases, rodents are rarely found to be infected with rabies, and no cases of rodents transmitting rabies to humans have been reported.

Sharks are not mammals, so it is not possible for them to get rabies. It is hard to imagine a shark getting bitten by a dog anyway! But there are many mammals that live in the sea. Could they get rabies?

Simply put, the answer is yes, but it is very, very rare. In the Arctic regions in northern Canada and Alaska, the Artic fox is the main rabies carrier, as one of the only canid species in the area. In 1980, rabies was detected in 12 foxes, 3 reindeer, and one ringed seal in Svalbard, an archipelago near the North Pole. The seal that had contracted rabies was found 200 m away from a blow hole, where a seal will come up for air while swimming under the thick ice. A subsequent rabies screening of hundreds of animals all came up negative. This may be the first known case of rabies in a marine mammal.

Luna, an Arctic fox at the Maryland Zoo in Baltimore.

Foxes are known to prey on young seal and walrus pups, which may be where they come into contact with the adults. If an infected fox bites a seal, the seal can develop rabies. But this is a very rare and usually accidental event, and "there is no indication that rabies is endemic in" Arctic fox, seal, and reindeer populations.

There is one case of rabies reported in a polar bear. In 1989, this bear was found dragging its back legs 1.2 km inland in northern Canada, and was shot dead by a group on Inuit hunters, who left the meat untouched because of the unusual circumstances of its death. Scientists tested its spinal fluid and found evidence for partial paralysis and rabies antibodies, confirming the bear was indeed rabid. No other polar bear has ever been reported to be carrying rabies.

People should always be careful and wary of all wild animals, whose behavior is unpredictable. Animals who are acting unusually should absolutely be avoided and reported to local wildlife authorities. That being said, the risk of contracting rabies is especially low, and should never stop you from enjoying the outdoor world. What are you waiting for?

Sources: 
http://www.who.int/news-room/fact-sheets/detail/rabies
http://www.jwildlifedis.org/doi/abs/10.7589/0090-3558-24.2.264?code=wdas-site
https://www.researchgate.net/profile/Morten_Tryland/publication/292257698_Zoonoses_of_arctic_marine_mammals/links/5707a27108aea66081331319/Zoonoses-of-arctic-marine-mammals.pdf
http://www.jwildlifedis.org/doi/abs/10.7589/0090-3558-27.2.337?code=wdas-site
https://www.jstor.org/stable/3560403?origin=crossref&seq=1#metadata_info_tab_contents

Movie Science: Alpha

I saw the movie Alpha with friends last Saturday night. When my friends and I go see movies, we whisper to each other the whole time, pointing out flaws in both the plot and the science it implies. Alpha was a rare treat for me: a movie that highlighted large-scale terrain and large, extinct animals. I knew I would like it.

The movie begins with a sequence of landscape shots, some with animals. The first animals we see are woolly mammoths, traveling in a characteristic herd. A few seconds later, a small group of woolly rhinoceros move through what appears to be a glacier-scarred landscape. Hyena and wolf packs prowl and vocalize. Words appear on the screen informing us that the setting is Europe, 20,000 years ago.

With a date and general location, it is now straightforward to examine the science behind the plot. 20,000 years ago puts the movie near the end of the Pleistocene period, otherwise known by its more famous alias, the "Ice Age." There's a lot to say about the science behind the many ice ages of Earth, so we'll save that for another blog post. This one will focus on the animals seen in the movie, and whether or not they belong there.

First up is identifying the animals seen in the opening sequence. Elasmotherium could be the woolly rhinoceros, as it lived in Europe in the Pleistocene, but the youngest fossils date from 29,000 years ago, slightly before the movie's setting. Another genus of animals, Stephanorhinos, also lived in continental Europe around the same time, but it's not as woolly as the ones in the movie, and their horns are a bit short. My guess is that the animals are Elasmotherium, and maybe they are the last of their kind alive. In any case, it checks out.

American Mammoth skeleton on display at the George C. Page Museum. Photo credit: J. Noviello

The most iconic animal of the Pleistocene is the mammoth, and it's no surprise those animals appear early in the film (though it's a shame that they aren't seen again). The oldest woolly mammoth fossils are found in east Asia. From there mammoths migrated out, eventually evolving into a few different species. Europe was definitely one of those places, as their bones are fairly common in many countries, and mammoths are seen in cave paintings made by ancient humans. While most mammoths were extinct by 10,000 years ago, small populations of isolated animals may have survived until as late as 4,000 years ago. One quick note is that the mastodon is a totally different animal from the mammoth, and only found (so far) in north and central America. This one too checks out.

The humans in the movie (which are very likely true Homo sapiens, even though Neanderthals certainly inhabited the area) are first seen hunting buffalo. This checks out as well, as there are many different species of bison in the Pleistocene fossil record. The length and curvature of the horns, as well as the size of the bison itself, at first made me think of Bison latifrons, but that animal only lived in North America. A better choice for the buffalo hunted in the movie is the steppe bison, Bison priscus, as it was a widespread species and definitely lived in Europe; it also makes many appearances in cave paintings, especially at the Cave of Lascaux in France. This checks out too.

From: https://www.lascaux.fr/en/prepare-your-visit/visit-lascaux/lascaux-2

Wolves and hyenas also check out, though the animals seen in the movies are their modern-day counterparts. Cave hyenas were massive, about twice the size of extant hyenas. Judging from the presence of bones in different caves, it appears that cave hyenas and humans actively hunted each other, or at least scavenged from each other's kills. These hyenas lived across Europe and Asia until about 20,000 years ago, after which their populations severely declined. In any case, this too checks out. Of course wolves check out because we know that dogs are descended from wolves, so the question becomes what kind of wolves these are. While I initially thought they could be dire wolves, they are only found in North America. The wolves in the movie are probably simple cave wolves.

Finally, there is the closest thing to a villain in the movie: the saber-tooth cat. While we only are ever shown a shadowy outline of the beast that kills a hunter early in the film, its profile is unmistakable. Sabre-tooth cats (the most famous of which is Smilodon fatalis, though there are a few) were fearsome predators that went extinct about 10,000 years ago, so the timeline checks out. Now onto where they lived. The most famous location for their bones is the La Brea Tar Pits in Los Angeles, California, but they are found as far south as Argentina. While saber-tooth cats were thought to be extinct in Europe as early as 300,000 years ago, a new jawbone discovered in the Netherlands has been dated to as late as 28,000 years ago, significantly later than what scientists originally thought! Therefore, while it is far more likely that saber-tooth cats lived in the Americas around the time of the movie, it is not impossible they were also in Europe. This checks out, albeit tenuously.

Maybe it's because I'm a scientist at heart, and that prehistoric animals are very close to my heart, but while watching this movie, I automatically evaluated it. I try to remember that it's been made for entertainment purposes, not for education. Even so I can appreciate attention to detail, and I think Alpha did a good job overall representing the paleobiology of Pleistocene Europe. And, if I may say so myself, the movie itself was good too.

Sources:
https://www.nature.com/news/sabre-toothed-cats-prowled-europe-200-000-years-after-supposedly-going-extinct-1.22861
https://en.wikipedia.org/wiki/Cave_hyena
https://en.wikipedia.org/wiki/Steppe_bison
https://en.wikipedia.org/wiki/Megafaunal_wolf#cite_note-goldfuss1823-4
https://en.wikipedia.org/wiki/Woolly_mammoth
https://en.wikipedia.org/wiki/Elasmotherium