As a vet I'll kick in my $0.02. The honest answer is because of the potential of BSE or Mad Cow Disease. The disease causing organism (prion) has been found in lung tissues in addition to brain, nerves, and bones in infected cows. Sheep do not get or carry Mad Cow Disease but do get Scrapie, a frighteningly similar disease. There are some in the veterinary research field that believe these two diseases are the same thing. The bad thing is that you cannot diagnose or identify infected sheep without running very specific tissue tests that take several weeks to get results. Not very cost effective. For this reason the Food Inspection Service considers lung tissue inedible for human consumption and will only allow it to be used in pet food manufacture (check the label of a can of Alpo).
The next question is if lung is potentially infected with BSE or Scrapie, what about my pet eating it. For one thing, lung tissue is usually not a major ingredient in pet foods (I would not feed my dog an all cow or sheep lung diet!). It's found mostly in canned foods and most pets eat mainly dry food. Yes, the lung tissue is usually denatured and cooked, but Scrapie can still be found to be active in tissues that were incinerated. At this time, no pets (meat eaters) have contracted the disease. Not because it isn't there but because they don't live long enough to develop the disease (takes years for the prion to establish itself then cause damage and symptoms). Humans on the other hand live much longer than pets and can develop the human form of BSE or Creutzfeldt-Jakob's disease.
So the moral of the story - I would watch my haggis ingestion to an occasional pleasurable indulgence instead of a regular dietary ingredient!
by Miriam C. » Thu Nov 01, 2007 5:42 am 
Actually the website says foot N mouth. 
