Abstract
The prevalence of polycystic kidney disease (PKD) has been estimated in the USA, Australia, UK, and Germany, but no data are available to date in France. The purpose of this study was to determine prevalence of PKD in Persian and Persian related breeds of cats in France. Medical records of all healthy cats presented for ultrasonographic screening of PKD between December 2000 and April 2002 were analysed from two centres (ENVL and ENVA). Cats were classified as positive when at least one anechoic cavity was found in at least one kidney. Prevalence of PKD was compared between the two screening centres, between different breeds evaluated, and between male and female using Chi-square test. A total of 310 cats were examined, including 92 at ENVL (57 Persians, 22 Exotic Shorthairs, 7 Chartreux, 4 Norwegian Forest Cats, and 2 Abyssins) and 218 at ENVA (163 Persians, 42 Exotic Shorthairs, 4 Chartreux, 4 British Shorthairs, 2 American Whirehairs, 2 Norwegian Forest Cats, and 1 American Shorthair). Prevalence of PKD was 41.8% in Persian cats and 39.1% in Exotic Shorthair. No PKD was detected in cats from other breeds. There was no significant difference between prevalence of PKD found in ENVL and ENVA, between prevalence of PKD in Persians and in Exotic Shorthairs, and prevalence of PKD in male and in female. Prevalence of PKD in Persians and Exotic Shorthair cats in France is currently high but is similar to prevalence in other parts of the world. Selection based on ultrasonographic detection of cysts should decrease prevalence of PKD in the future.
Feline polycystic kidney disease (PKD) is an inherited autosomal dominant disease that has been identified in Persian and Persian related breeds of cats (Biller et al 1990, Bosje et al 1998, DiBartola 2000). The disease, first identified in the late 1960s as sporadic case reports (Silvestro 1967, Battershell and Garcia 1969), has been described more comprehensively in the 1990s with identification of its mode of inheritance and similarity with human autosomal dominant PKD (Biller et al 1990, 1996, Eaton et al 1997, DiBartola 2000). The disease is progressive and may be responsible for irreversible renal failure in affected cats (Biller et al 1990, 1996, DiBartola 2000). Eradication of the disease is possible through active selection of cats free of PKD (Cannon et al 2000). Ultrasonography has proven to be the most cost-effective modality for large scale phenotypical evaluation of potentially affected individuals (Biller et al 1996, Cannon et al 2000, Barrs et al 2001, Beck and Lavelle 2001). Cysts can be identified as anechoic cavities within renal parenchyma (Biller et al 1990, Reichle et al 2002). Sensitivity of ultrasonography for detection of cysts has been determined to be 91% with a specificity of 100% at 36 weeks (Biller et al 1990) and it has been recommended to screen cats older than 10 months (Cannon et al 2000, Barrs et al 2001).
The prevalence of the disease in Persian cats has been determined in the US (DiBartola 2000), Germany (Gerwing et al 1999), Australia (Barrset al 2001, Beck and Lavelle 2001), and in the UK (Cannon et al 2001) and range between 40% and 50%. The prevalence of PKD in other breeds of cats is less known. Knowing the prevalence of the disease is essential to justify a screening programme and to evaluate its success. It was the purpose of this study to estimate the prevalence of PKD in France.
Materials and methods
Medical records of all healthy cats presented for ultrasonographic screening of PKD at the Ecole Nationale Vétérinaire de Lyon (ENVL) and the Ecole Nationale Vétérinaire d'Alfort (ENVA) between December 2000 and April 2002 were analysed. Cats presented with evidence of renal failure were excluded from the study. Breed, age, and gender were collected. Ultrasonographic examination was performed in all cats by an ACVR or ECVDI diplomate using a VingMed CFM 810 with a 5–9 MHz mechanical transducer (ENVL) and an ATL HDI 3500 with a sectorial 5–8 MHz and a linear 7–12 MHz transducer (ENVA). Cats were not sedated. Hair clipping was absent or reduced to a small window on each side of the abdomen. Both kidneys were scanned for evidence of anechoic cavities within the renal parenchyma. The remainder of the abdomen was not examined. Cats were classified as positive when at least one anechoic cavity was found in at least one kidney (Beck and Lavelle 2001, Cannon et al 2001). The Chi-square tests were used to compare prevalence of PKD between the two French screening centres, between different breeds evaluated, and between male and female. In addition, prevalence of PKD in Persian cats was compared to prevalence reported in the USA, UK, Australia, and Germany using published data (Gerwing et al 1999, Barrs et al 2001, Beck and Lavelle 2001, Cannon et al 2001). Hypotheses tested were accepted for P-value less than 0.05.
Results
Three hundred and ten cats were examined, including 92 at ENVL (57 Persians, 22 Exotic Shorthairs, 7 Chartreux, 4 Norwegian Forest Cats, and 2 Abyssinians) and 218 at ENVA (163 Persians, 42 Exotic Shorthairs, 4 Chartreux, 4 British Shorthairs, 2 American Whirehairs, 2 Norwegian Forest Cats, and 1 American Shorthair). Prevalence of PKD was 41.8% in Persian cats and 39.1% in Exotic Shorthairs (Table 1). No PKD was detected in cats from other breeds. There was no significant difference between prevalence of PKD found in ENVL and ENVA, between prevalence of PKD in Persians and in Exotic Shorthairs, and prevalence of PKD in male and in female cats. No significant difference was identified between prevalence of PKD in Persian cats in France and in the USA, UK, Australia, and Germany.
Ultrasonographic findings in 310 cats tested for PKD
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Discussion
The prevalence of PKD in Persian cats found in the present study is high and is similar to the prevalence of PKD in Australia and otherEuropean and North-American countries where similar studies have been conducted (Gerwinget al 1999, Beck and Lavelle 2001, Cannon et al 2001). The prevalence of PKD in Exotic Shorthair cats has been estimated in Australia on a relatively small population sample (Barrs et al 2001, Beck and Lavelle 2001) and no other data could be found in the literature for other countries. The present study demonstrates a similar prevalence of PKD in Exotic Shorthairs as in Persians in France on a larger population sample. Prevalence of the disease is likely to be similar in all countries involved in international exchange of cats for breeding purposes. The high prevalence of the disease and the progressive nature of the renal cysts leading to irreversible renal failure justify the growing interest in this disease by veterinarians and breeders (Beck and Lavelle 2001).
Cats were considered as positive for PKD when at least one cyst was identified in at least one kidney. This criterion, suggested from humanliterature (Ravine et al 1994), has been used in other similar studies (Beck and Lavelle 2001, Cannon et al 2001). It is justified by the progressive nature of the disease (Biller et al 1996) and the low prevalence of solitary cysts not related to autosomal dominant PKD in the cat (Gerwing et al 1999). It was supposed that cats with a solitary cyst in one kidney may have had other small undetected cysts or would havedeveloped multiple cysts later in their life. This is supported by findings of the Australian study (Barrs et al 2001) where ‘equivocal’ cats (ie less than three cysts or cysts in only one kidney) were classified as positive on repeat examination 6 months later. In addition, disqualifying a false positive cat with a solitary nonautosomal dominant PKD cyst has less consequence than breeding from a false negative cat.
Eradication of the disease should be achieved quickly through active screening programmes considering the mode of inheritance, the high heritability, and the high sensitivity and specificity of the screening procedure. Concerns regarding the effect of ultrasonographic equipment and ultrasonographer's training and experience on accuracy of the procedure have been raised leading to recommendations on minimal ultrasound frequency to be used for adequate detection of cysts (Gerwing et al 1999, Beck and Lavelle 2001) and accreditation of the ultrasonographer and the equipment by the Feline Advisory Bureau (FAB) in the UK (Cannon et al 2000). False negative results may be due to insufficient spatial and contrast resolution of the equipment, near field artefact when using mechanical and electronic sector transducers, incomplete examination of the kidneys, and lack of expertise of the ultrasonographer. It has been reported that cysts may be more difficult to detect in the medulla than in the cortex because the echogenicity of the medulla may be close to the echogenicity of the cysts (Barrs et al 2001). In addition, the hypoechoic nature of the medulla may be misleading and yield false positive results.
Estimating the prevalence of the disease is essential before starting a screening programme in order to be able to document the efficiency of the eradication process. Monitoring the prevalence of the disease during the selection programme, however, may be biased by including only cats from catteries involved in screening programmes and give a false impression of decreasing prevalence. No progress reports are available to date on success of screening programme for eradication of PKD in cats.