Single-cell organism could have killed some tyrannosaurs
New research indicates that tyrannosaurid dinosaurs may have been afflicted with a disease similar to one that causes lesions in the jaws of modern birds of prey.
Ewan D.S. Wolff, PhD, a vertebrate paleontologist and third-year veterinary student at the University of Wisconsin-Madison, is the lead author for a report suggesting that smooth-edge, erosive lesions often found on Tyrannosaurus rex mandibles were caused by a Trichomonas gallinae-like protozoan.
"We found that, in fact, modern birds may be very commonly affected by a dinosaur disease," Dr. Wolff said.
He and his co-authors found the lesions on 10 of the 61 tyrannosaur mandibles examined during their study, and had found two more specimens with such lesions by late October. The co-authors are Steven W. Salisbury, PhD; John R. Horner, who has honorary doctorates of science from the University of Montana and Pennsylvania State University; and David J. Varricchio, PhD. Their report, "Common avian infection plagued the tyrant dinosaurs," was published in late September through PLoS One, the online journal of the nonprofit Public Library of Science.
"There were a bunch of jaws that had holes in them that hadn't really been explained," Dr. Wolff said. "People thought maybe it was some sort of artifact from preservation of the bones, maybe bite wounds, or something else."
Mandibular lesions in Sue— the largest and most complete tyrannosaur skeleton, an exhibit of the Field Museum in Chicago— had been attributed to bite wounds or actinomycosis, a bacterial bone infection. But the recent study indicates the dinosaur likely starved to death as a result of a trichomoniasis-like disease.
"This finding represents the first evidence for the ancient evolutionary origin of an avian transmissible disease in non-avian theropod dinosaurs," the report states.
Trichomonas gallinae is a protozoan parasite that affects falcons, hawks, and owls, and is responsible for the disease known by falconers as frounce, according to the 2002 edition of "Birds of prey: health and disease." Pigeons are the natural hosts.
According to this source, infection with T gallinae in birds of prey often begins as a stomatitis. Early signs typically are mild, and may include difficulty swallowing. Later signs include loss of appetite, dehydration, and lesions on the ears, larynx, respiratory tract, and internal organs.
Dr. Wolff's study indicates tyrannosaurids were commonly infected with a similar T gallinae-like protozoan, and the disease likely became endemic and spread through bites, feeding, and possibly cannibalism.
In birds, the disease is transmitted by eating prey or by feeding young, Dr. Wolff said. There are no preserved tyrannosaur young, but bite marks on the dinosaurs' skulls provide evidence of possible salivary transmission.
About 45 percent of tyrannosaur skulls contain evidence the dinosaurs had healing bite wounds, suggesting that tyrannosaurs commonly engaged in aggressive face biting, Dr. Wolff said. About a third of specimens with holes related to trichomoniasis also had evidence of bite wounds elsewhere on their skulls.
"My hope is that this will open up a new front in the way we study disease in past animals," Dr. Wolff said. "People haven't done much to look at the history of infectious disease in dinosaurs, in part because people haven't taken into account the strong connection between the immune system that must have been present in dinosaurs and (the immune systems in) their current relatives."
Dr. Wolff suspects further examination will reveal a rich history of co-evolution of pathogens with their hosts.
It is possible other dinosaurs—including many more tyrannosaurs—were affected by the trichomoniasis-like disease without developing similar large mandibular lesions, Dr. Wolff said. He noted that T gallinae causes substantial harm before holes develop in the jaws of modern birds.
Dr. Wolff said he is fascinated by ancient diseases and interested in current medicine, and he sees a benefit to having an evolutionary biology background when treating animals.
"I like treating animals and studying diseases in both the living and the dead," Dr. Wolff said.
He is currently focusing on treatment of small animals, and is interested in specializing in orthopedic surgery.