First and foremost, I am NOT an expert on international racing. But I know enough to be dangerous.
From my understanding, there is very little genetic difference in thoroughbreds in other part of the world anymore. Horses shuttle back and forth quite easily, and popular US bloodlines have influenced TBs most everywhere. International buyers are present at US-based sales, just like US buyers travel overseas for sales as well. No longer are countries' bloodlines isolated.
Most differences are found in the lifestyle, conditioning, and types of racing the horses campaign in. Racing in Europe and Australia is predominately on the grass. The races are longer and slower when compared to the most common US distances. Decreased speed = decreased risk of injury.
Also, US racing is most frequently found in populated, urban areas. Horses living on the racetrack in US cities don't have the natural lifestyle as racehorses in say, Europe, where tracks are often located in the countryside with turnout, etc. And while they also have 2 year old races in Europe and start horses at about the same age, there are many more opportunities for older horses to be competitive in Europe than there are in the US. A little less pressure to have them run as frequently right off the bat.
And these days, Canadian racing is pretty much identical to US racing.
Don't fall for a girl who fell for a horse just to be number two in her world... ~EFO
Are there substantial differences in TBs from the UK, Ireland, New Zealand, Australia, Canada and South America, that make them heartier than the US bred TBs?
In this year of our lord Two Thousand and Eleven, no.
As recently as 10-15 years ago you would have found TBs in eastern Europe with very unique bloodlines, but even there they have had wholesale importation of foreign (US and western European) lines. Don't know that they were necessarily any "heartier" though.
There was a time when National Hunt bred horses, esp in Ireland, had a distinct look to them, and some would say they still do to a degree, but that may have as much to do with nurture (raising) than nature (breeding).
I didn't know that there was such a thing as an "american" TB vs. "other" TBs....how is this distinction made?
In other words, our OTTB IS an international mix. As I thought MOST TBs were.... He comes from Tetrarch(Irish) stock through Mumtaz Mahal, and has Ambiorix (French) in his family tree three times, and Ribot (Italian) on his dam's side, Nearco (Italy), Petition (british), My Aid (Irish), Arctic Star (British), Northern Dancer (Canadian). So, Is our boy an "american" TB then, despite having so many international ancestors or what?
Horses are bred across international borders....and are often a mix of bloodlines from other countries, at least ours is...So what you are asking, in our TBs case, doesn't really make sense.....
BTW, our OTTB has great feet also, our farrier said he must have got them from his grandsire....Danzig.
Technically speaking there is no difference between one TB and another - they do all trace back to one of three stallions after all! Buuuut there are subtle differences due to breeding the original stock to native breeds.
Apparently Irish TBs are a lot stockier than other European TBs
Where in this wide world can man find nobility without pride, friendship without envy, or beauty without vanity? - The horse. (R.Duncan)
Also, US racing is most frequently found in populated, urban areas. Horses living on the racetrack in US cities don't have the natural lifestyle as racehorses in say, Europe, where tracks are often located in the countryside with turnout, etc.
Horses don't generally live at the track in Europe.
To illustrate what others have already said, this kind of pedigree would be "typical" for a U.S. TB:
One of our broodmares is by Storm Bird who was foaled in Canada, raced in England and Ireland, and stood at stud in the U.S. We bred that mare to Street Cry, who was foaled in Ireland (and whose pedigree is a combination of U.S., French, and GB breeding.) Street Cry raced in both the U.S. and the United Arab Emirates and stands at stud in both America and Australia.
The filly that resulted from that breeding, when she retired from racing, was bred to Candy Ride who was bred in Argentina and who is a combination of Argentine, French, and American breeding. He raced in both SA and NA and stands at stud in the U.S.
Not only are there not substantial differences, family and pedigree-wise, between U.S. TBs and those from other countries, in many cases the bloodlines have become so universal that there are few differences at all.