tear ducts cause epiphora, an overflow of tears onto the face, which is common but primarily cosmetic. Entropion, the inward folding of the eyelids, causes the eyelashes to rub against the cornea, and can lead to tearing, pain, infection and cornea damage. This condition is not uncommon in Persians, and usually involves the medial aspect of the lower eyelid. Similarly, in upper eyelid trichiasis or nasal fold trichiasis, eyelashes/hair from the eyelid and hair from the nose fold near the eye grow in a way which rubs against the cornea. Dystocia, an abnormal or difficult labor, is relatively common in Persians. Consequently, stillbirth rate is higher than normal, ranging from 16.1% to 22.1%, and one 1973 study puts kitten mortality rate (including stillborns) at 29.2%. A veterinary study in 2010 documented the serious health problems caused by the brachycephalic head. As a consequence of the BBC program Pedigree Dogs Exposed, cat breeders have also come under pressure from veterinary and animal welfare associations, with the Persian singled out as one of the breeds most affected by health problems. Animal welfare proponents have suggested changes to breed standards to prevent diseases caused by over- or ultra-typing, and prohibiting the breeding of animals outside the set limits. Apart from the GCCF standard that limits high noses, TICA and FIFe standards require nostrils to be open, with FIFe stating that nostrils should allow "free and easy passage of air." Germany's Animal Welfare Act too prohibits the breeding of brachycephalic cats in which the tip of the nose is higher than the lower eyelids. Polycystic kidney disease (PKD) which causes kidney failure in affected adult cats has an incidence rate of 36–49% in the Persian breed. The breed – and derived ones, like the British Longhair and Himalayan – are especially prone to autosomal dominant polycystic kidney disease (ADPKD). Cysts develop and grow in the kidney over time, replacing kidney tissues and enlarging the kidney. Kidney failure develops later in life, at an average age of 7 years old (ranging from 3 to 10 years old). Symptoms include excessive drinking and urination, reduced appetite, weight loss and depression. The disease is autosomal dominant and DNA screening is the preferred method of eliminating the gene in the breed. Because of DNA testing, most responsible Persian breeders now have cats that no longer carry the PKD gene, hence their offspring also do not have the gene.