by:By Dr. P. Machwhirter B.V.Sc. (Hons) Highbury Veterinary Clinic.
Birth defects can occur because of genetic causes, adverse conditions during pregnancy or a combination of both.
Puppy development from the time of fertilization can be divided into three phases:
" Preimplantation, day 0 to approximately day 14.
" Embyrogenesis. day 14 to day 30 approximately.
" Foetal growth. Day 30 (approx.) to term.
Sperm can survive in the bitch's reproductive tract for over a week before fertilization occurs. Because of this, there can be an apparent variation of up to a week if these phases are timed from the time of mating, rather than from the time of actual fertilization.
The stage during which each developing tissue or organ is most sensitive to disruption is called the "critical period phase". This corresponds with roughly the second to fourth week of the bitch's pregnancy. If damage to the embryo occurs before this time (during the preimplantation period), the embryo is likely to die and be absorbed. During the foetal phase, (from roughly the thirteenth day of pregnancy onwards), resistance to harmful influences increases.
CRITICAL PERIODS IN PUPPY DEVELOPMENT:
Fertilisation; placentation; brain and spinal cord; cerebelium and cerebrum; vertebrae; tail; head and face; heart; sense organs; vision; limbs; palate; reproductive organs.
POTENTIAL CAUSES OF BIRTH DFECTS:
Heredity. Adverse chemical or physical agents, infections and inappropriate nutririon can all result in birth defects.
All dogs carry some genes as hidden recessives. Foe a recessive gene to appear in a puppy, it must be passed on from both parents. With close inbreeding, the chance of both parents carrying the same genes can be desirable ( such as the gene for gold colour in Golden Retrievers), or they can be for undesirable defects. Undesirable recessaive defects may be immediately apparent as a birth defect, or they may not become apparent until later. Loose inbreeding will also increase the chances of undesirable polygenic characteristics emerging. A polygenic characteristic is one that is controlled by several genes. Examples include: Entropion (the turning in of the eyelids) in breeds such as Rottweillers and Golden retrievers.Blood vessel defects such as patent ductus arteriosis or persistent right aortic arch in German and Belgian Shepherds. Heart valve defects such as subaortic stenosis in Newfoundlands.
To breed top quality show dogs, it is necessary to mate closely related stock. While this technique will increase the chances of producing excellent show stock, it will unavoidably increase the chances of producing dogs with genetic defects. If steps are not taken to stop this, the occurrence of inherited defects will increase in pure-bred dogs this is particularly true. This is particularly true if the breed standard calls for a particular characteristic that encourages the development of a defect but it will also occur, even if this is not the case.
Conditions during pregnancy nay also influence the development of inherited defects. Cleft palate is an example of this point. The hard palate develops in the foetus from a pair of processes that shift from being nearly vertical, extending on either side of the tongue, to being orientated horizontally above the tongue. This occurs during the fourth week of foetal development and is followed immediately by the fusion of the two processes to form the roof of the mouth. Any variation in the timing of the horizontal shift, size of the processes, size or location of the tongue, or the width of the mouth cavity, will increase the likelihood that fusion will not occur, resulting in a cleft palate. Cleft palate is a polygenetic defect that occurs across a wide range of breeds. It is also known to be caused by nutrition (excess Vitamin A) and environmental contaminants such as mercury chloride and drugs such as Griseofulvin. Genetic birth defects tend to affect only occasional puppies in a litter, as not all litter mates receive the same genes. Adverse conditions during pregnancy are more likely to affect an entire litter or the majority of puppies in it. Thwere are exceptions to this rule.
NUTRITION RELATED DISORDERS:
Under - nutrition of the bitch during pregnancy can result in low birth weights, low brain weigfhts, behavioural abnormalities and increased puppy mortality. However, no specific structural defects occur.
Vitamin A deficiency or escess during pregnancy can result in congenital malformation. Excess Vitamin A given to bitches from days 17 to 22 of pregnancy, produced abnormalities and kinked tails. Vitamin A deficiency in pigs, rats and rabbits has been associated with eye, heart and urinary defects.
Vitamin D deficiency during pregnancy can result in skeletal defects and abnormal dentition. Vitamin D excess has been implicated in impaired bone formation and some "swimmer" puppies.
Many other Vitamin and Minerals, if deficient or given in excess during pregnancy, have been shown to cause developmental defects in laboratory animals. Copper deficiency can cause abnormal brain development, abdominal hernias and skeletal abnormalities. Iron deficiency can cause anaemia and deaths in offspring. Manganese deficiency can cause loss of balance, due to failure of the middle ear to develop.
A teratogen is an agent that disrupts the normal development process of the foetus. Examples of teratogens that have been shown to cause damage in dogs include:
Carbaryl, an insecticide in flea powders such as Troy 7-dust is safe in puppies and adults but can cause birth defects when fed to bitches during pregnancy. Defects include short jaws, no tails, extra digits, skeletal malformations and fissures along the abdomen with displacement of intestines and abdominal contents. Dystocia (difficult births) from uterine atony (failure of the uterus to contract) also occurred. Other insecticides such as DDT and dichlorvos have been shown to cause birth defects in laboratory animals but have not yet been shown to cause defects in dogs.
Captan given to bitches during pregnancy has produced puppies with crooked tails, hydrocephalus and failure of the abdominal wall to close. Mercury chloride ( a fungicide and an environmental contaminant in fish and meat) caused cleft palates, open fontanelle, extra digits, extra kidneys and umbilical hernias.
Griseofulvin, an anti-fungal drug, causes mall, weak, short haired puppies with cleft palates. Testosterones and progesterones may cause masculinisation of the clitoris and external genitals. Corticosteroid treatment has been associated with anasarcous ( dead ,swollen) puppies and deformed limbs. Many other drugs have been proven to cause birth defects in other species.
Foetus are particularly sensitive to high maternal body temperature, either due to high environmental temperatures or to fevers. This is because the foetus can not readily dissipate heat back into the mothers circulation. Brain tissue is the most likely site to be damaged. Foetal death, growth retardation or developmental abnormalities also occur.
Parvovirus infections in bitches during pregnancy can cause abnormal heart development in puppies if infection occurs during the critical stage of heart development. Infections in puppies after birth attack the digestive system.
While much has been discovered about the causes of birth defects in puppies, there is still much more to learn. Steps that a dog breeder should take to prevent birth defects vary depending on the different problems.
General precautions should include:
1. Lerarning about different genetic causes of birth defects in particular breeds or family lines, and, where possible, adopting strategies to reduce the occurrence of these genetic problems.
2. Providing adequate, rather than excessive or deficient, levels of nutrients in the bitch's diet.
3. Keeping the bitch healthy and not exposing her to infections, drugs, environmental contaminants or extremes of temperature during pregnancy, particularly during the critical stages of foetal puppy development. i.e., between the 14th and 30th days of pregnancy.