By disabling a cell receptor, geneticists have produced pigs resistant to a disease that has killed millions of pigs and cost pork industries billions of dollars.
Three modified pigs challenged intramuscularly and intranasally with an isolate of porcine reproductive and respiratory syndrome virus showed no clinical signs of illness, whereas seven unmodified pen mates also challenged with the virus had respiratory disease signs and fever, according to correspondence published online Dec. 7, 2015, by Nature Biotechnology. Researchers with the U.K.-based animal genetics company Genus, the University of Missouri, and Kansas State University developed and tested the modification.
Dr. Bill Christianson, chief operating officer at the Genus subsidiary Pig Improvement Company, said the modification has produced exciting results but is in the early stages of development, and estimating when it could be available would require speculation. But he acknowledged that company officials had made public statements that sales probably were five years away.
These pigs were used in a study that indicated deleting genetic material connected with a certain cell receptor could provide protection against the porcine reproductive and respiratory syndrome virus. (Courtesy of Nic Benner/University of Missouri)
Where the pigs would be sold will depend on disease prevalence, regulations, and market demand, he said. Officials with Genus have started discussions with regulatory agencies, including the U.S. Food and Drug Administration, he said.
In November, the FDA granted approval for the first genetically engineered animal intended for sale as food: an Atlantic salmon modified by AquaBounty Technologies to grow to market size in about half the time of conventional Atlantic salmon. The agency has determined that under current regulations, recombinant DNA constructs intended to affect the structure or function of bodies of engineered animals qualify as drugs.
Dr. Christianson said that, while the AquaBounty salmon were modified through addition of genetic material from other species, the PRRS-resistant pigs are modified only to remove a gene connected with production of a protein needed for PRRS infection. He expressed hope the agency would decide case by case how to regulate animals engineered through gene editing without addition of genes from other species.
Raymond “Bob” Rowland, PhD, one of the researchers who helped develop the PRRS-resistant pigs and a professor of diagnostic medicine and pathobiology at the Kansas State University College of Veterinary Medicine, said the genetically modified pigs are the first in his 20 years of research on PRRS that remained free of infection when challenged with the PRRS virus. The experiments had shown the genetic change could eliminate PRRS, he said.
He described PRRS as a stealthy virus that kills some pigs, increases others’ vulnerability to other pathogens, causes untold animal suffering, and is the biggest “bugaboo” in modern pork production.
Dr. Rowland expects further refinements will lead to modifying only part of the protein so that the modified protein will no longer allow the virus to bind to cells yet still will remain functional, although he noted the pigs developed so far have lacked any known abnormalities. The protein, known as CD163, removes excess hemoglobin from pigs’ blood and belongs to a group known as scavenger receptors, he said.
Dr. Tom Burkgren, executive director of the American Association of Swine Veterinarians, said that, if the pigs truly are resistant to all PRRS viral strains, their development is monumental. And, because breeding pigs today are more often bought, rather than raised within the same company, he expects pig production companies would accept use of proprietary breeding pigs from Genus.
Dr. Derald Holtkamp, an assistant professor of veterinary diagnostic and production animal medicine at the Iowa State University College of Veterinary Medicine, noted that his National Pork Board–funded study conducted in 2010 and 2011 indicates almost 10 million pigs and $664 million are lost each year through a combination of deaths and failures to reproduce related to PRRS. Those losses are part of the market of about 110 million pigs slaughtered in the U.S. each year.
The pork industry has made incremental progress in controlling PRRS, and transmission of the virus has declined since the industry also began fighting porcine epidemic diarrhea in spring 2013, Dr. Burkgren said. The prevailing theory holds that PRRS transmission declined as farms increased biological controls during the PED outbreak, he said, yet he noted that this view remains unproven.
But a PRRS isolate that has spread in the past few years has been destructive, killing pigs and causing abortion storms, Dr. Burkgren said. The losses are severe, yet not out of the ordinary, a stark reality of life with PRRS.
Dr. Holtkamp said he would not want to speculate how much impact the introduction of PRRS-resistant pigs could have. But, depending how quickly they were added, their introduction could result in dramatic short-term price declines for pigs, he said.
The emergence and elimination or control of diseases have set precedents for rises and falls, respectively, in pork market prices, he said. Examples are the devastation caused by porcine circovirus type 2 and development of a vaccine as well as the more recent deaths of millions of neonatal pigs from PED and the implementation of biological controls that have reduced its spread.
While Dr. Holtkamp also cautioned against assuming PRRS would be eliminated within the next decade, he said the gene-editing technology used in the modification has great potential.