Animal & Veterinary
A Primer on Cloning and Its Use in Livestock Operations
by Siobhan DeLancey, Consumer Safety Officer, and Dr. Larisa Rudenko, Senior Advisor for Biotechnology, Office of New Animal Drug Evaluation; and John Matheson, Senior Regulatory Review Scientist, Office of Surveillance and Compliance
FDA Veterinarian Newsletter 2006 Volume XXI, No V
Imagine the perfect dairy cow. For eight years she has gotten pregnant on the first try, given birth easily, and produced gallon upon gallon of the best milk. Even when others in the herd got sick, she stayed healthy. She is ideally suited to the climate in which she lives. The farmer has depended on this cow and her daughters in lean times to carry the farm through, but now she is at the end of her reproductive life.
Although the farmer may have this cow’s daughters to carry on the line, he also has another alternative: copying her. Biological copying is referred to as cloning. By cloning his prize cow, breeding the clones, and keeping their offspring, the farmer can introduce the natural positive characteristics into the herd quickly. It would take several more years to achieve these same improvements by conventional breeding.
Farmers can also clone animals to produce more uniform quality meat. Take, for example, a male pig (boar) who time after time sires piglets that mature quickly and provide lean meat. If a farmer has several of these boars he could quickly produce an entire herd with consistent, high quality meat.
Researchers have been cloning livestock since 1996. In 2001, when it became apparent that cloning could become a commercial venture, the Food and Drug Administration’s (FDA) Center for Veterinary Medicine (CVM) asked that food from clones and their offspring be voluntarily kept out of the food chain. FDA then began an intensive evaluation that included examining the safety of food from these animals.
What is cloning, really?
Cloning is a complex process that lets one exactly copy the genetic, or inherited, traits of an animal (the donor). Livestock species that scientists have successfully cloned are cattle, pig, sheep, and goats. Scientists have also cloned mice, rats, rabbits, cats, mules, horses, and one dog. Chickens and other poultry have not been cloned.
Most people think of livestock breeding taking place through traditional mating, in which males and females physically get together to reproduce. In fact, this type of mating is not often the case.
Traditional mating is not that efficient, if the goal is to produce as many offspring as possible. For example, a male has enough sperm to produce many more offspring than would be possible by traditional mating. Traditional mating also has certain risks: one or both of the animals may be injured in the proc-ess of mating. The female may be hurt by the male because he is often so much larger, or an unwilling female may injure the male. There is also a risk of infection or transmission of venereal disease during traditional mating.
Because of these factors, many farmers use assisted reproductive technologies for breeding. These include artificial insemination, embryo transfer, and in vitro fertilization. Artificial insemination was first documented in the breeding of horses in the 14th century. The first successful embryo transfer of a cow was in 1951, and the first in vitro fertilization (IVF)-derived animal was a rabbit born in 1959. Livestock production in the United States now uses all these methods regularly. For example, most dairy farms don’t have bulls, so more than 70% of the Holstein cows bred in the United States are artificially inseminated. The frozen semen can come from a bull many miles, or even many States, away.
Cloning is a more advanced form of these assisted reproductive technologies. Much of the public perception of cloning likely comes from science fiction books and movies. Some people incorrectly believe that clones spring forth fully formed, or are grown in test tubes. This is just not the case. Clones are born just like other animals. They are similar to identical twins, only born at different times. Just as twins share the same DNA, clones have the same genes as the donor animal. A clone is not a mutant, nor is it a weaker version of the original animal. It’s just a copy.
In all of the other assisted reproductive technologies, the male and female parents each contribute half of their genes to their offspring. Farmers have worked for years to choose animals with the best traits and breed them together, which increases the chance these good traits will be passed on and become more common in livestock herds. Even though farmers have been able to improve their herds over time, they still can’t absolute predict the characteristics of the offspring, not even their sex. Cloning gives the farmer complete control over the offspring’s inherited traits. Thus, a farmer who clones an especially desirable, but aging or injured animal knows in advance that the clone will have the genetic potential to be an -especially good, younger animal. He can then use that animal to further reproduce by traditional mating or other assisted breeding.
How do you make clones?
Most cloning today uses a process called somatic cell nuclear transfer (SCNT). Just as with in vitro fertilization, scientists take an immature egg from a female animal (often from ovaries obtained at the slaughterhouse). But instead of combining it with sperm, they remove the nucleus (which contains the egg’s genes). This leaves behind the other components necessary for an embryo to develop. Scientists then add the nucleus containing the desirable traits from the animal the farmer wishes to copy. After a few other steps, the donor nucleus and egg fuse, start dividing, and an embryo begins to form. The embryo is then implanted in the uterus of a surrogate dam (again the same as with in vitro fertilization), which carries it to term. (“Dam” is a term that livestock breeders use to refer to the female parent of an animal). The clone is delivered just like any other baby animal.
What can go wrong with cloning?
There are no complications that are unique to cloning. These problems are also seen in animals born from natural mating or assisted reproductive technologies. They seem to happen more often in clones for a number of reasons that probably have to do with parts of the procedure that occur outside the body. The embryo may fail to develop properly during the in vitro stage or early on after transfer to the surrogate and may be flushed out of the uterus. If it does develop, the embryo may not implant properly into the uterus of the surrogate dam. Alternatively, the placenta may not form properly, and the developing animal won’t get the nourishment it needs.
Large Offspring Syndrome (LOS) is seen in pregnancies of cattle and sheep that come from both assisted reproductive technologies and cloning. With LOS, the fetus grows too large in the uterus, making problems for the animal and its surrogate dam. LOS has not been observed in goats and swine.
As a group, livestock clones tend to have more health problems at birth, and may die more often right after birth than conventionally bred animals. If clones survive the first few days after birth, they seem to become just as healthy and strong as other animals of the same age. By the time clones are young adults, it’s not possible to tell them apart from other animals of the same age, even if you conduct a detailed examination. Scientists at FDA and research institutions have looked at blood work for clones that’s similar to what people get when they have physicals. These results show that the clones are perfectly healthy, and walk, wean, grow, mature, and behave just like conventional animals.
The main use of clones is to produce breeding stock, not food. Clones allow farmers to upgrade the overall quality of their herds by providing more copies of the best animals in the herd. These animals are then used for conventional breeding, and the sexually reproduced offspring become the food-producing animals. Just as farmers wouldn’t use their best conventionally bred breeding animals as sources of food, they are equally unlikely to do so for clones.
Some examples of desirable characteristics in livestock that breeders might want in their herds include the following:
- Disease resistance: Sick animals are expensive for farmers. Veterinary bills add up, and unhealthy animals don’t produce as much meat or milk. A herd that is resistant to disease is extremely valuable because it doesn’t lose any production time to illness, and doesn’t cost the farmer extra money for veterinary treatment.
- Suitability to climate: Different types of livestock grow well in different climates. Some of this is natural and some results from selective breeding. For instance, Brahma cattle can cope with the heat and humidity of weather in the southwestern United States, but they often do not produce very high grades of meat. Cloning could allow breeders to select those cattle that can produce high quality meat or milk and thrive in extreme climates and use them to breed more cattle to be used for food production. Similarly, pork production has traditionally been centered in the eastern United States, but is moving to different regions of the United States (e.g., Utah). Cloning could allow breeders to select those pigs that naturally do well in the new climate, and use them to breed more pigs to be used for food production.
- Quality body type: Farmers naturally want an animal whose body is well suited to its production function. For example, a dairy cow should have a large, well-attached udder so that she can produce lots of milk. She should also be able to carry and deliver calves easily. For animals that produce meat, farmers breed for strong, heavy-muscled, quick-maturing animals that will yield high quality meat in the shortest time possible. The most desirable bulls produce offspring that are relatively small at birth (so that they are easier for the female to carry and deliver), but that grow rapidly and are healthy after birth.
- Fertility: Quality dairy cows should be very fertile, because a cow that doesn’t get pregnant and bear calves won’t produce milk. Male fertility is just important as that of the female. The more sperm he can produce, the more females a bull can inseminate, and the more animals will be born. Beef cattle or other meat-producing animals such as pigs need to have high fertility rates in order to replace animals that are sent to slaughter. Cloning allows farmers and breeders to clone those animals with high fertility rates so that they could bear offspring that would also tend to be very fertile.
- Market preference: Farmers or ranchers may also want to breed livestock to meet the changing tastes of consumers. The traits the producers are looking for include leanness, tenderness, color, and size of various cuts. Preferences also vary by culture, and cloning may help tailor products to the preferences of various international markets and ethnic groups.
How does cloning help get these characteristics into the herd more quickly? As we’ve previously said, cloning allows the breeder to increase the number of breeding animals available to make the actual food production animals. So, if a producer wanted to introduce disease resistance into a herd rapidly, cloning could be used to produce a number of breeding animals that carry the gene for disease resistance, rather than just one. Likewise, if a breeder wants to pass on the genes of a female animal, cloning could result in multiples of that female to breed, rather than just one.
Is it safe to eat food from clones?
It’s important to remember that the purpose of clones is for breeding, not eating. Dairy, beef, or pork clones will make up a tiny fraction of the total number of food producing animals in the United States. Instead, their offspring will be the animals actually producing meat or milk for the food supply.
Dairy clones will produce milk after they give birth, and the dairy farmers will want to be able to drink that milk or put it in the food supply. Once clones used for breeding meat-producing animals can no longer reproduce, their breeders will also want to be able to put them into the food supply.
In order to determine whether there would be any risk involved in eating meat or milk from clones or their offspring, in 1999 FDA asked the National Academy of Sciences (NAS) to identify science-based concerns associated with animal biotechnology, including cloning. The NAS gathered an independent group of top, peer-selected scientists from across the country to conduct this study. The scientists delivered their report in the fall of 2002. That report stated that theoretically there were no concerns for the safety of meat or milk from clones. On the other hand, the report expressed a low level of concern due to a lack of information on the clones at that time, and not for any specific scientific reasons. The report also stated that the meat and milk from the offspring of clones posed no unique food safety concerns.
Meanwhile, FDA itself began the most comprehensive examination of the health of livestock clones that has been conducted. The evaluation has taken more than four years. This examination formed the basis of a Draft Risk Assessment to determine whether cloning posed a risk to animal health or to humans eating food from clones or their offspring (http://www.fda.gov/cvm/cloning.htm). FDA conducted a thorough search of the scientific literature on clones, and identified hundreds of peer-reviewed scientific journal articles, which it then reviewed. They were also able to obtain health records and blood samples from almost all of the cattle clones that have been produced in the United States and data from clones produced in other countries. FDA compared these health records, and the independently analyzed blood results with similar samples from conventional animals of the same age and breed that were raised on the same farms.
After reviewing all this information, FDA found that it could not tell a healthy clone from a healthy conventionally bred animal. All of the blood values, overall health records, and behaviors were in the same range for clones and conventional animals of the same breed raised on the same farms. FDA also saw that milk from dairy clones does not differ significantly in composition from milk from conventionally bred animals.
In the Draft Risk Assessment, FDA concluded that meat and milk from cattle, swine, and goat clones would be as safe as food we eat from those species now. It did not have enough information to make a decision on the safety of food from sheep clones.
For another study similar to the one conducted on cow clones, the Agency also evaluated the health of offspring sexually derived from swine clones, as well as the composition of their meat. After reviewing this very large data set, the Agency concluded that all of the blood values, overall health records, and meat composition profiles of the progeny of clones were in the same range as for very closely genetically related conventionally bred swine. Based on these results, other studies from scientific journals, and our understanding of the biological proc-esses involved in cloning, the Agency agreed with NAS that food from the sexually reproduced offspring of clones is as safe as food that we eat every day. These offspring animals will produce almost all of the food from the overall cloning/breeding process.
FDA’s Draft Risk Assessment includes data collected or published before early 2006. FDA will continue to closely monitor the development of clones and their progeny as a source for food as further data become available.
FDA encourages public comments on the Draft Risk Assessment, Proposed Risk Management Plan, and Draft Guidance for Industry. We will review these comments and evaluate any additional data that may be shared with us
during the comment period. We will then issue a Final Risk Assessment, Risk Management Plan, and Guidance for Industry.
Comments and suggestions regarding any of these documents should be sent to the Division of Dockets Management (HFA-305), Food and Drug Administration, 5630 Fishers Lane, Room 1061, Rockville, MD 20852. Comments may also be submitted electronically on the Internet at http://www.fda.gov/dockets/ecomments. All written comments should be identified with Docket No. 2003N-0573. Please specify which document your comments address.
Copies of the Draft Risk Assessment may be requested from the Communications Staff (HFV-12), Center for Veterinary Medicine, Food and Drug Administration, 7519 Standish Place, Rockville, MD 20855, and may be viewed on the Internet at http://www.fda.gov/cvm/cloning.htm