*The Truth about Cardiac Screening*
Dr. Bonnie Lefbom, DVM, DACVIM-Cardiology,
http://www.cvcavets.com/doc%20bios.html
Cardiac screening prior to breeding is crucial to maintain the health of current
and future breeding populations. Large breed dogs such as the
Beauceron are at risk for a variety of heritable cardiac conditions. The most
common of these conditions are tricuspid valve dysplasia (TVD),
subaortic stenosis (SAS) and dilated cardiomyopathy (DCM). Dogs are born with
TVD and SAS as anatomic defects (congenital diseases), although SAS can
worsen with age. DCM is an acquired disease in that dogs are not born with the
disease, but born with the genes to develop the problem over time.
Veterinary scientists have defined the precise heritability of several cardiac
conditions in dogs. SAS is inherited in the Newfoundland either as
an autosomal dominant trait with modifiers or a polygenic disease.
Cardiomyopathy in Boxers is inherited as an autosomal dominant trait with
variable penetrance. Great Danes inherit DCM as an X-linked recessive trait,
while Portuguese water dogs develop juvenile onset DCM with an
autosomal recessive inheritance pattern. A fantastic amount of research and time
is required to define heritability of disease in all species.
Therefore, the precise mode of inheritance of disease in each and every breed or
family line is likely to take generations of investigators.
(Please see the medical dictionary appearing at the end of this article for
definitions of the genetic terms used).
The following information describes the various heritable cardiac
conditions noted thus far in Beauceron dogs:
Tricuspid valve dysplasia (TVD) is a result of a congenital malformation of the
tricuspid valve. The tricuspid valve is located within the right side of
the heart. The most common variant of TVD creates right heart enlargement and
the risk for right sided heart failure including fluid accumulation
around the abdominal organs or lungs.
Subaortic stenosis (SAS) is the result of a defect at or below the level of the
aortic valve. This valve is located within the left heart and a defect
here creates risk for left heart failure with coughing, exercise intolerance,
and potentially sudden death from arrhythmia (irregular heart
rhythm).
Dilated cardiomyopathy (DCM) typically develops after 2- 4 years of age and can
result in sudden death, signs of left sided failure with coughing and
breathing difficulties or right sided failure with abdominal distention and
exercise intolerance. Appropriate cardiac screening is crucial for any
breeding dog to prevent future generations from developing these conditions and
to avoid potential amplification (worsening) of these defects in
offspring.
The current Orthopedic Foundation for Animals (OFA) cardiac screening is an
attempt to define the presence or absence of congenital cardiac defects in
breeding dogs. A dog who is found to be “OFA clear” by a veterinary cardiologist
has no evidence of cardiac disease based on physical
examination, i.e., no murmur or abnormal heart sounds were heard. OFA clearance
by a veterinary cardiologist screens for congenital defects
(problems dogs are born with) such as subaortic stenosis, tricuspid or other
valvular dysplasia and various anatomical defects. Subaortic stenosis (SAS)
is a bit unique in that the severity of this defect can worsen up to two years
of age. The potential for disease worsening is why OFA clearance
should be performed when dogs are two years of age or older.
An OFA clearance is NOT a tool that can be used to predict
or define the presence
or development of acquired disease (diseases that develop with age) such as
degenerative valve disease or dilated cardiomyopathy. The ACVIM College
of
Cardiology is currently developing guidelines to improve cardiac screening in
all breeds.
Although the exact mode of inheritance in Beauceron dogs is not currently
defined (one can’t predict DCM likelihood based on ancestry, at this time),
screening for DCM should be performed yearly in at-risk breeds including
Beaucerons. An echocardiogram by a board certified veterinary cardiologist
will define the heart size and function and help identify affected dogs.
Many dogs with abnormal hearts appear normal at home. These dogs have
occult disease (i.e., are developing dilated cardiomyopathy (DCM), but do not
yet have clinical signs), and should not be used as breeding dogs
because they are likely to pass this trait on to their offspring.
An OFA clearance at two years of age has no bearing on the
development of DCM with
age. Currently, only echocardiogram screenings can identify dogs affected with
DCM.
All breeding screening evaluations should be performed by a board-certified
veterinary cardiologist. These individuals have three to five years of
additional training after completion of veterinary school and have passed
intense examinations in veterinary cardiology. Their credentials include
Diplomate of the ACVIM- College of Cardiology within the American College of
Veterinary Internal Medicine. A complete list of veterinary cardiologists
is available on the ACVIM website – www.acvim.org
.
*Medical Dictionary*
arrhythmia - abnormal heart rhythm
autosomal dominance – a form of inheritance in which only one copy of a gene
coding for a disease need be present for that disease to be expressed
meaning, if either parent has the disease, an offspring has a 50% change of
inheriting the disease
autosomal recessive - a pattern of inheritance in which both copies of an
autosomal gene must be abnormal for a genetic condition or disease to occur.
An autosomal gene is a gene that is located on one of the autosomes or non-sex
chromosomes. When both parents have one abnormal copy of the same
gene, they have a 25% chance with each pregnancy that their offspring will have
the disorder.
penetrance - a term used in genetics that describes the extent to which the
properties controlled by a gene, its phenotype, will be expressed. A highly
penetrant gene will express itself almost regardless of the effects of
environment, whereas a gene with low penetrance will only sometimes produce
the symptom or trait with which it has been associated. Penetrance and
heritability appear closely related at first glance, but in fact it is
possible to carry a huge number of inherited genes with low penetrance and not
be aware of them. The opposite is not the case however, if you carry a
higher penetrant gene, you will know (assuming the gene has a noticeable effect,
many do not).
polygenic - (genetics) of or relating to an inheritable character that is
controlled by several genes at once
occult – in medicine, applied to a condition that is not readily detectable by
signs and symptoms, or which is detectable only from microscopic
examination or chemical analysis.
x-linked – related to, or characterized by genes located on the X-chromosome
(sex chromosome associated with female characteristics in mammals, occurring
paired in the female – XX, and single in the male - XY). The X chromosome
occupies an exceptional place in the mammalian genome. Together with the Y
chromosome, the X chromosome differentiates the sexes. Males have one X
chromosome and a Y chromosome and females have two X chromosomes. Because of
this fundamental genetic difference, diseases caused by genes located on the X
chromosome affect males and females differently and thus present unusual
inheritance patterns.
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