Cheetah Genetics: the Debate Over Disease
Thus, a more important question for the cheetah than how it got its low genetic diversity is whether the cat's survival has been compromised by it. Scientists ask, for instance, whether the cheetah's genetic uniformity has indeed made it overly susceptible to disease. To answer that question, they consider a disaster that began in May 1982, when a female cheetah that had been recently brought to Oregon's Wildlife Safari became ill and died from feline infectious peritonitis (FIP), a viral disease that strikes domestic cats but is rarely fatal to them. Within the next year, 18 of the 42 cheetahs at the park died of FIP-related illnesses.
O'Brien suggested that the "catastrophic sensitivity" the cheetahs displayed to the virus was a consequence of their lack of genetic variation. In other words, the genes that code for certain defenses in the cheetah's disease-fighting immune system were so similar among the cats that if FIP could evade the defenses of one cat, the same virus could hit all the cheetahs equally hard.
In research published in 1985, O'Brien and his colleagues tested the amount of genetic variation in the cheetah's immune system by grafting patches of skin between pairs of unrelated cheetahs. Normally, when skin or other tissue is transplanted from one individual to another, the recipient's immune system rejects the foreign tissue, attacking and trying to destroy it. Strong drugs are necessary to prevent this rejection response in human transplant patients, and biologists have found that a house cat usually rejects a skin graft from an unrelated cat within 14 days. In contrast, of the 14 cheetahs that received skin grafts in O'Brien's study, only 3 appeared to reject the grafts, and these rejections took 40 days or more. Thus, the investigators concluded, the cheetahs' immune systems must have been genetically very similar.
Nevertheless, other scientists question whether the cheetahs' vulnerability to FIP resulted from genetic uniformity or simply from the fact that the cats had never before been exposed to the virus. Now that cheetahs have been exposed to FIP, these researchers note, the cats seem to be developing resistance to the disease. Only three captive cheetahs died of FIP between 1987 and 1991, and by 1991 two-thirds of the captive cats in the United States carried antibodies (protective proteins) to FIP in their blood, showing that their immune systems had learned to ward off the virus.