As far as demodex goes.....All dogs actually carry the demodectic mites, but most will never have an outbreak. When put in extremely stressful situations, or malnourished, or in any case where the immune system is comprimised, a demodex outbreak can occur.
Demodex is the most common of the different types of mange.
In the bulldog world, blue coated dogs are especially known for having issues with demodex outbreaks. If a young dog breaks out in demodex for reasons not directly related to stress or illness, it is highly frowned upon to breed that dog, because the chances of his/her offspring of breaking out are high.
X2...stress can bring it on but blue dogs and white dogs tend to be more suseptable and like she stated...it is a genetic issue so those dogs that get the mange should not be bred...
