Catherine Marley, MD
Between the years of 1964 and the present, 24 University Animal Hospitals have participated in a national database listing the diagnoses for all animals brought for examination. Since these are University clinics, we have to assume that a good portion of the patients seen there were seen on referral from local Vets. This means that the population is pre-selected for a higher incidence of pathology than the local Veterinarian's office would be. Aside from the selection for a higher incidence of pathology, these patients probably represent a good cross section of the Lhasas in the United States and Canada.
From 1964 to 1997 there were 10,697 Lhasa Apsos examined. 31 cases of PRA were found over the entire 33 year period. This represents a total of 0.29% for 33 years, or an average case discovery of less than 1 case per 10,000 population per year. Compare this with the situation in England and Scandinavia, with 17 cases in about 1 year, in a test sample of around 500. This amounts to about 350 per 10,000 per year.
This data were obtained from the Veterinary Medical Data Base at Perdue University. Participating institutions include: Michigan State, U of Missouri, U of Minnesota, Iowa State, Cornell, Ontario Veterinary College, Purdue University, U of Georgia, U of California, Ohio State, Kansas State, U of Illinois, U of Saskatchewan, Colorado State, Auburn University, Texas A&M, U of Tennessee, Louisiana State, Virginia-Maryland Regional U., U of Wisconsin, U of Pennsylvania, Tuskegee University, Oklahoma State, and U of Florida.
How good is this data? There is virtually no way to verify the validity of this data. The major problem is a lack of reporting outside of the 24 cooperating University clinics. All testing in The USA is completely voluntary, and there is no record kept of the number of dogs tested or the number of positive cases. Most are just seen in the vet's office, diagnosed, and gone. In the USA, breeders are simply not going to share negative information with anybody. The only motivation we have is that those who do test have a CERF certificate and number, which may act as an economic advantage in selling puppies or promoting a stud dog. So statistics from the USA are hard to come by. I think the Perdue stats are about the best we are going to get - at least in the forseeable future.
Other sources have have been examined to attempt to verify the Perdue data. CERF has a record of 276 Lhasa Apsos seen since 1991(119 male and 157 female). Only one definite case of PRA in Lhasas was found in that period. Reporting of cases to CERF has been admittedly poor, since CERF charges a substantial fee for its form, and most vets don't want to buy them or to do the paperwork. However, the concurrance with the Perdue study is quite striking at 0.36%.
Several Veterinary Ophthalmologists with very large practices have been contacted. One, whose practice covers Southern New York State and all of Connecticut, has never seen a case in 35 years. Another, a professor at Cornell University, in a busy practice for 30 years in Northern New York State, has seen "not more than 3 or 4". A third practitioner, who covers all of New Jersey, has seen "a few" over his 25 years of practice. A fourth, a Pennsylvania practitioner who wrote the book on PRA, told me that he has seen fewer than 10 Lhasas with PRA in his 25 years of practice, and that many of these cases were in elderly dogs with end stage retinas. Other conditions besides PRA, such as old age, renal disease, diabetes etc. can also cause retinal degeneration. In the end stage of retinal degeneration, the retina is obliterated, and the classic signs of PRA are no longer recognizable. He could not be sure that these old retinal degeneration cases were not due to aging or other diseases. All of these men see many Lhasa Apsos because of their many other eye problems - "dry eye", "cherry eye", cataract, corneal abrasion and ulceration etc. This anecdotal data again tends to support the Perdue data.
Geographically, the Perdue cases were mainly from the Mid-Atlantic and Southern States, and from the Midwest. The tendency to find most cases in the midwestern and southern states follows the geographic frequency of puppy-mill operations, where accidental cross-breeding is a common occurrence. There may be a significant contribution to the "Lhasa" gene pool from puppy mill sources of dubious parentage. In this sample, there were no cases from California or from the Northeast in the past 33 years.
There is no significant change in the incidence of PRA over the 33 year period. This indicates that the gene frequency is maintained a constant, very low level in the North American population. And remember that a portion of the sampled population was referred from local practitioners, who evidently had some suspicion of a problem. The true incidence rate and carrier rate may be somewhat smaller. On the basis of this data, Lhasa Apsos from North America can be considered relatively clear of the gene for PRA.
The presence of PRA in the USA is not surprising. No-one ever doubted the gene existed in American "Lhasas" (we don't know how many of these were really Lhasas), but it seems that the numbers have always been low. The numbers from English sources are growing steadily. And this, too, is understandable. Genetic problems tend to accumulate in isolated populations such as in a quarantine country. The gene has always been there at a very low level, but the over-use of a few pre-eminent dogs, who unfortunately had carried that gene, is what has concentrated the problem.
Since North America does not appear to be a hotbed of Lhasa Apso PRA, with only 31 cases in 33 years and 10,000 Lhasa Apsos, I think that we can safely conclude from these data that North America is probably not the source of the current PRA cluster in Northern Europe. Furthermore, North American sources can be used for breeding with a certain amount of confidence. This is extremely good news for certain European countries now experiencing a high incidence of PRA. Importation from North American sources can help breeders in Europe rejuvenate their gene pool, and breed their way out of their present predicament. Anyone who still feels that the numbers are insignificant in Europe, should consider the following:
USA and Canada = 30 cases / 10000 dogs / 30 years or 1 case per 10,000 per year
United Kingdom = 7 cases / 130 dogs / 1 year or 540 cases per 10,000 per year
Norway and Sweden have PRA test statisitcs similar to those in UK. Denmark has the same bloodlines, but as yet has released no results.
If you knew you had a 540 times greater than normal chance to go blind if you ate spinach, I am sure you would never touch it again, the bottom would drop out of the spinach market, and the Government would probably pass a law against it. I think untested dogs from the affected countries are in just that sort of "spinach" position.
Despite all the evidence for a very low incidence in North America, we still need to test for PRA. Yes the North American population is "cleaner" from PRA than the European. But this is only relative. Ideally, any prospective export should be of a mature age and clear of PRA, and both its parents should be checked for PRA as well. If possible, ERG should be done on any dog exported to Northwestern Europe which might be used at stud. The potential harm certainly outweighs the expense. What a tragedy it would be to import the same problem all over again. Only testing can make this a remote possibility.
Catherine Marley, MD
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