The notion of hybrid vigor is worth understanding if you're looking for a Maltese Shih Tzu. Hybrid vigor isn't necessarily characteristic of mixed breeds; it occurs when new blood is brought in from outside the usual breeding circle--it's the opposite of inbreeding.

However, there is a general misconception that hybrid vigor automatically applies to mixed breeds. If the genetic pool for the mixed breed remains the same over time, the offspring won't have hybrid vigor. And if a purebred breeder brings in a dog from a different line, those puppies will have hybrid vigor, even though they're purebred.

Maltese Shih Tzus are generally healthy, but like all breeds, they're prone to certain health conditions. Not all Maltese Shih Tzus will get any or all of these diseases, but it's important to be aware of them if you're considering this breed.

If you're buying a puppy, find a good breeder who will show you health clearances for both your puppy's parents. Health clearances prove that a dog has been tested for and cleared of a particular condition.

Before you bring home your Maltese Shih Tzu, find out if he's from a first-generation or multigenerational breeding (though multigenerational breedings are rare for this mix). If he's a first-generation dog, research the health concerns that occur in both the Maltese and the Shih Tzu. Regardless of generation, both parents should have applicable health clearances. Some disorders are caused by recessive genes that may not appear for generations.

In Maltese Shih Tzus, you should expect to see health clearances from the Orthopedic Foundation for Animals (OFA) for hip dysplasia (with a score of fair or better), elbow dysplasia, hypothyroidism, and von Willebrand's disease; from Auburn University for thrombopathia; and from the Canine Eye Registry Foundation (CERF) certifying that eyes are normal. You can confirm health clearances by checking the OFA web site (offa.org).

• Patellar Luxation: also known as slipped stifles, this is a common problem in small dogs. The patella is the kneecap. Luxation means dislocation of an anatomical part (as a bone at a joint). Patellar luxation is when the knee joint (often of a hind leg) slides in and out of place, causing pain. This can be crippling, although many dogs lead relatively normal lives with this condition.
• White Shaker Syndrome: This affects young to middle-aged dogs. The disease is seen in both the Shih Tzu and Maltese and has been seen in any crosses produced by either breed. Symptoms are uncontrollable shaking and an inability to walk. An episode can last all day. Treatment is usually corticosteroids for three to six months, after which time most dogs don't need additional treatment (although some may require low doses every other day to keep the condition under control).

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