We can't evolve or engineer our diseases away. And besides, the arms race between viruses and our genes may mean health is a moving target, one that would be dangerous to reach.
The Chronicle of Higher Education reports on research suggesting why natural selection hasn't succeeded in eradicating our susceptibility to disease, and why breeding other animals for health or productivity also has a dark side.
You'd think that evolution would weed out the bad seeds. People who carry them get sick and die, while those with good genes thrive and have more children -- shouldn't their genes crowd out the bad ones? But no. Instead, there is an astounding diversity of those genes, which belong to a group called the Major Histocompatibility Complex, or (luckily) MHC for short.
"There are hundreds of varieties of MHC genes in most vertebrate species," says Wayne K. Potts, a professor of biology at the University of Utah. "One gene has 2,300, which holds the record." And since the weaker varieties persist, so does vulnerability to disease.
But when researchers transferred viral particles among strains of mice that had developed different genotypes to resist them, they found that, in each strain, some of the viruses were able to penetrate defenses and infect the mice. If all the strains had developed identical defenses, this could not have happened. But the mice would then be more susceptible to a devastating new supervirus that could wipe out all the strains.
The mouse and human species are thus protected by genetically diverse responses -- but it's imperfect protection. And a continuing arms race between viruses and MCH genes may mean that health is a moving target, and one that would be dangerous to reach even if we could. By breeding cattle, pigs, and other animals to resist disease and administering antibiotics, we are also reducing diversity and increasing the likelihood of diseases that may jump to human beings.
"We could back off the antibiotics if we bred back in some MHC diversity," [Wayne Potts] says. Range cattle still have it, and could be bred into dairy herds. Of course, range-cattle MHCs may be accompanied by genes that don't produce as much milk. Farmers may be reluctant to cut their sales, and consumers may be unwilling to pay higher milk prices. That's another kind of arms race: economics and health.
When I spoke to an agricultural marketers association in the 1990s, the members were talking about PRRS -- Porcine Respiratory and Reproductive Syndrome -- an autoimmune disease of swine that is as awful as it sounds. According to my hosts, it had developed in large farms where pigs were treated with antibiotics for "enhanced health status." It had become such a threat that trucks servicing possibly affected farms had to have their tires disinfected each time they left. According to the UN Food and Agriculture Organization (FAO) PRRS has since become a threat in Asia.