The first study that analyzes the prevalence was carried out in the United States, where the prevalence of PKD in Persian cats was around 38% [11]. Later, other authors studied the prevalence in this breed in Australia (50%), the United Kingdom (49.2%), France (40.45%), Italy (41%), Slovenia (36%), Taiwan (15.7%), Iran (36.38%), Japan (46%) and Brazil (5%) [6,12,13,14,15,16,17,18,19]. In these studies, no statistically significant differences were documented between males and females, suggesting that the inheritance of the disease is not sex-linked [14,20]. It has been studied less in other feline breeds such as Neva Masquerade cats or Siberian cats [9,21], although some authors suggest that it could be present in all feline breeds, since around 80% of all current feline breeds have had some type of cross with the Persian breed, so they could have inherited the mutation that provokes the disease [22]. In fact, Lyons et al. (2004) concluded that PKD affects 6% of the total feline population around the world. This would represent that this pathology is the most prevalent genetic disease in cats [5]. This high prevalence and the lethal nature of the disease justify the increasing interest of veterinarians and breeders in this disease. Thus, in 2001, the English organization Feline Advisory Bureau (FAB), now International Cat Care (ICC), developed a research program with selective purpose in the United Kingdom. The main aim was to identify affected cats, create a registry of animals with the genetic tests, and thus allow breeders to select healthy animals in their breeding lines [23]. More recently, a study in Brazil indicates a significant decrease in feline PKD prevalence (5% in Persian cats), which could be related to the first success of genetic counseling [19]. However, more studies should be carried out to evaluate the repeatability of the data obtained and eliminate possible mistakes. Although this is the first study to analyze the results of genetic counseling in reducing the prevalence of the disease, there is no doubt that good advice from veterinarians in the genetic selection of cats can greatly reduce this severe pathology.

Since the 1970s, several cases of feline PKD have been described in the literature. In 1990, Biller et al. hypothesized for the first time a hereditary nature for the disease, after studying a 6-year-old female Persian cat that had been crossed with a healthy male Persian cat [24]. The female was referred for hematuria and polyuria-polydipsia, whose diagnosis of PKD was made by ultrasound and confirmed by anatomical-pathological examination. This female had given birth to five kittens divided into two litters. Ultrasound examination of four kittens showed that two males and one female were affected by PKD [24]. In 1996, the same authors identified the type of inherited transmission of PKD in a study carried out in a colony of cats. For this study, the authors created an experimental pedigree of affected cats from the case previously studied in 1990 (Figure 1). To create the colony, two litters obtained by crossing this female with a healthy Persian male were used. All affected cats in this family were identified by ultrasounds by anatomical-pathological analysis, or both [25].

The results of this study made it possible to identify the autosomal dominant mode of transmission of PKD. The statistical analysis showed proportions that corresponded to an autosomal dominant inheritance with complete penetrance. The animals suffered from the disease only with the presence of a defective allele in their genotype and with a penetrance of 100% (it occurred in all individuals) [25]. To date, no homozygous animals have been found for the mutation of PKD1 gene, which reinforces the idea that it is lethal in utero [5,8,17].

Feline PKD was first identified in 1969 in some sporadic cases [26], and it was described in more detail in the 1990s, with the identification of the hereditary transmission and its similarity to humans ADPKD [25]. The following studies made it possible to discover the implicated gene called PKD1 and the mutation that provokes the disease [5,27]. The PKD1 and PKD2 genes code for the polycystin-1 and 2 proteins. Mutations in these genes are responsible of 85% and 15% of human ADPKD cases, respectively, and affect the kidney and the bile duct [28]. The presence of cysts depends on animal age, so 90% of presumed gene carriers will have cysts that can be identified based on the age of the animal [2,29]. For example, a clinical case was described in six kittens that died at seven weeks of age, in which the necropsy revealed the presence of renal and bile duct cysts, undiagnosed by the young age of the animals [30]. Recent studies in human medicine reveal high genetic and clinical heterogeneity of this disease [31]. Thus, although now only the PKD1 gene has been identified as the cause of the disease in cats, several studies found cases of animals with kidney cysts and a wild-type genotype of the PKD1 gene. This raises the existence of other responsible mutations and, therefore, a possible genetic heterogeneity of feline PKD [3,8,32].

4. Pathophysiology and Clinical Features