CONGENITAL heart disease (CHD) in cats is uncommon compared with its acquired counterpart. Reports on the prevalence of feline CHD vary from as little as 0.2 per cent to as much as 15 per cent (Bolton and Liu 1977, MacDonald 2006, Schrope 2015, Tidholm and others 2015). Based on these studies, it appears that the most common CHD in cats is ventricular septal defect probably followed by tricuspid dysplasia, pulmonic stenosis, atrial septal defect and patent ductus arteriosus (PDA), but not necessarily in this order. The apparent disagreement between studies on the occurrence of feline CHD in general, and PDA in particular, is related to the difficulty of acquiring accurate data to obtain reliable population statistics. Ideally, prospective and systemised large-scale data collection should be the way to gain this knowledge, but this becomes particularly difficult when studies focus on diseases that are rare or difficult to diagnose. The paper by Wustefeld-Janssens and others (2016) summarised on p 17 of this issue of Veterinary Record , illustrates the difficulty. This retrospective study comprises a total of 19 feline PDA cases obtained during a period of nine years from three busy referral cardiology services in the UK; this is less than one case per centre per year on average. Similarly, the prevalence of PDA in cats has been estimated to be as low as 0.2 cases per 1000 cats (Jones and Buchanan 1981). This represents only a small number of cats indeed, …