Bretaigne Jones, DVM
Scientific Communications
Royal Canin, USA

One of the most challenging issues for a breeder and veterinarian is determining the cause of birth defects, and preventing them. Due to the number of possible causes, and difficulty in diagnosing the exact cause, we can’t always find enough definitive information to affectively prevent future occurrence.

Often times the birth defect is the result of more than one event. The range of possible defects can go from the kinked tip of a tail to major life-threatening problems. There is no way to absolutely evade all birth defects. In a breeding environment, some level of defects will be present.

The first issue considered when a birth defect, also referred to as a congenital defect, occurs is genetics. This evaluation will include not only the effected kitten, but the parents, grandparents and further descendents as well. When a kitten is developing during pregnancy, there are millions of episodes of genetic activity as DNA is generating copies of itself for cellular divisions, and also doing its work to provide the blue-prints for all the cellular activities necessary for life. Accidents do happen, and sometimes a glitch occurs during this activity that results in deformity or even death of the embryo.

Heritable issues derived from parents on back in the pedigree, involve specific gene pools and probability of increased expression of detrimental genes. Vigorous inbreeding can exacerbate this problem. There is a fine balance between benefiting from closer familial breeding by concentrating desirable genes, and increasing the occurrence of less favorable genes.

Genetic influence doesn’t work in a vacuum. Environmental variables play an equally important role, not only as an isolated event, but also in conjunction with the specific genetic makeup of the embryos. Environmental issues can include infectious agents (bacteria, viruses and parasites), nutrition, trauma, maternal metabolic factors (diabetes), and exposure to drugs and chemicals.

All these threats are present from the time of fertilization to delivery. Fertilization typically occurs in the oviducts, and this is where the first few cellular divisions take place. Within 12 hours from fertilization, cellular division commences, and repeats at roughly the same interval. This forming mass is called a zygote as it undergoes the many cell divisions on its way to the uterus. By the time it leaves the oviduct, the zygote is approximately 16 cells. Upon its entrance into the uterus, it becomes an embryo. While a zygote, the cells have the same potential to become any organ system or specialized cell-type. With the embryo stage, the cells are destined to become more specific tissue types.

The embryo stage lasts about 30 days, during which the cells begin to differentiate into one of three primordial cell layers, then progressing to form rudimentary organ systems, and finally the specialized cells that characterize individual organs.

The initial three primordial cell layers are identified as endoderm, mesoderm and ectoderm. The endoderm layer is destined to become mucosal membranes (lining the mouth, eyelids, nostrils etc), and the glands of the respiratory and digestive systems. The mesoderm layer will develop into muscles, connective tissue, bone, circulatory system, urinary system and genital system. The ectoderm cells will differentiate to become the outer most layers of the skin with hair follicles and glands, the nervous system and sensory organs.

When the embryo settles into the uterine tissues, it triggers the construction of the placenta. This organ will grow both from the maternal uterine tissues and also from the embryonic tissues. Each develops a fine network of blood vessels and capillaries that will interconnect without ever combining the blood of either. The mesh of capillaries that communicate between the mother and baby are so close that nutrients, oxygen and metabolic waste materials can cross back and forth. It is equally important that the placenta anchors the embryo in one place as protection for its rapid growth.

The embryo will quickly grow past the ability of the cells to derive the necessary nutrients from osmosis, and so a rudimentary heart develops. The umbilical vessels connecting the embryo to the placenta provide the bridge for accelerated provision of nutrients and oxygen.

The embryo stage will evolve into the fetal stage about 4 weeks into gestation, which will continue until the birth of the kitten. This phase is marked by the growth and early functioning of organs, and the almost exponential growth of the fetus overall in the last three weeks of gestation.

The nervous system is one of the first to begin development of specialized cell types, and is also one of the last to complete its formation. Carnivores are born with an incomplete nervous system that requires an additional 6 weeks to mature.

The legs form from the shoulder and hip regions first, going on to create the forelegs and thighs, then the lower legs, and finally the paws. The paws are initially constructed like paddles, with the individual toes becoming evident as the tissue between the toes is broken down. It is very important that the fetus move about and use the joints that are forming to prevent these joints from becoming fixed in place. Some congenital limb deformities can result and introduce the potential for birthing problems.

When the head develops, the facial structure grows from a different origin than that of the cranium (encases the brain). Because of this, the two parts of the skull can be influenced separately by genetics or affects of drugs or chemicals.  It also explains how brachycephalic breeds developed.

There are anatomic structures that develop in embryos and fetuses to protect the lungs and liver, since they are not functional during gestation. These structures are the foramen ovale in the heart, the ductus arteriosus between the pulmonary arteries and the aorta, and the ductus venosus which shunts most of the blood flow around the liver. These organs need lower pressure to develop than what would be present without these bypass structures.

Generally within a few days after birth, these structures normally shut down and normal circulatory routes operate. Occasionally, a fetal structure will persist. The results of which may range from mild to life-threatening. The abnormal structures are referred to as patent, since they are still open and functioning as in a fetus.

When genetic problems occur, they are usually due to mutations or too intensive inbreeding. Mutations can result from inaccurate or incomplete copying of the original DNA in preparation for cell division, or when the DNA is functioning to direct the production of proteins to carry on normal metabolism. Mutations can also happen if an incomplete division of chromosomes in the egg or sperm leads to additional genetic material being present in the zygote. Aging and exposure to free-radicals (by-products of cellular metabolism, exposure to UV light, environmental pollutants such as cigarette smoke, etc) can trigger DNA mutations, too.

Inbreeding purposely decreases the genetic pool of future generations in the hopes of concentrating desirable traits genetics. However, it also serves to increase the prevalence of undesirable traits. There is a much greater opportunity for recessive traits to exhibit when the number of potential gene pairs contain more recessive genes. Inbred individuals lose hybrid vigor.

Geneticists developed a mathematical formula to reflect the degree of inbreeding in an individual. It is called an inbreeding coefficient. The number for an animal that is not inbred at all is represented by zero, while that of an animal that is completely inbred will be one.

When one breeds two cats from different breeds, the hybrid kittens have an average neonatal mortality rate of 3% or less. However, when two cats within a breed are mated, that number increases to an average of 7%, with some breeds significantly higher.

Structural birth defects are caused by a primary error in the formation of a body part. Something actively interferes with the mechanics of building the structure. This may be caused by trauma, excessive temperatures, etc.

Environmental causes of birth defects can be exposure to a virus, bacteria, or parasites during the critical stage in development of that particular body system. Drugs and chemicals can interfere with the normal processes as the embryo forms. All of these causes are called teratogenic, meaning they disrupt normal fetal development. The health of the mother is considered an environmental factor as well, and in the presence of some metabolic diseases such as diabetes, birth defects can form.

Another class of environmental teratogens is nutrients. Research has demonstrated how several nutrients are necessary for normal formation of embryonic tissues such as vitamins A, E, C and B12, folate and nicotinic acid, and the minerals calcium, zinc and iron. In some cases, such as vitamin A, problems can occur not only in a deficiency situation, but also if excessive levels are present. This can be a real problem if breeders are supplementing the dam’s diet with liver. Vitamin A is stored in the liver. So when that organ tissue is fed to cats, they are getting a super-concentrated dose, which then is stored in their own liver. It is not only toxic to the embryos, but also to the dam. As long as every food substance given to the cat is complete and balanced (providing all the necessary nutrients in necessary amounts), no problems should occur in a normal healthy cat. This is a prime reason why having access to a diet formulated specifically for breeding is so important.

Unique to cats is the need for elevated taurine levels, arginine and arachidonic acid. Taurine and arginine are amino acids. For a non-breeding cat, borderline deficiencies can often go unnoticed. With the stress associated with pregnancy and the tremendous increase for all nutrients to meet the needs of fetal development while still meeting the needs of the dam, even borderline deficiencies can cost in fetal death or malformations. Arachidonic acid is a fatty acid. Deficiency of arachidonic acid can lead to infertility or subfertility issues in the queen, and if occurring during gestation can influence development abnormalities and fetal death.

Each of these potential causes can work alone, or in conjunction with other problems. Some genetic defects are amplified in the presence of infectious pathogens or nutritional imbalance. It can be quite frustrating to try to ferret out a primary reason for a rash of birth defects. Good record keeping practices can sometimes shed light on the issue by revealing crosses that have higher number of problems, medications that were administered in pregnancy, or exposure to viral, bacterial or parasitic infections present in the cattery during gestation. The breeder will never achieve a 100% defect free production. But it is reasonable and achievable to maintain a very low level over time.

Prevention and good planning are the strongest weapons. Make sure the cats are healthy, current on vaccinations and dewormings, and test free of the typical viruses. Maintain a steady level of biosecurity through effective cleaning and disinfection, using quarantine and isolation areas, and reducing the exposure to outside visitors. Keep the cats on a quality food appropriate for their lifestages, and work in partnership with a veterinarian to plan the optimal vaccination and deworming program for your facility.