Rabbits and guinea pigs differ from most other mammals in that their teeth grow throughout their life.

Their diet of fibrous plant material requires lots of chewing, which causes wear and tear on the teeth, but, because they're constantly growing, the teeth are not ground down too far, nor do they grow too long.

"This continued dental growth works well in the wild, to make sure the teeth are always well-formed and ready for use. But it is, unfortunately, common to see pets that are fed an inadequate diet, resulting in reduced use of the teeth," says veterinarian David Gething, of Creature Comforts (www.creaturecomforts.com.hk).

With the constant growing and reduced use, the teeth eventually become much longer than normal, a condition called dental malocclusion. "This affects their ability to eat properly and, in severe cases, can prevent the pet [from] properly closing the mouth, resulting in discomfort, pain and, if severe, starvation."

The clearest sign of dental malocclusion is overgrown teeth. To check for this, Gething says, hold the pet rabbit or guinea pig gently and lift the lip - if the front teeth look long and spindly and, in some cases, are bent or not properly meeting up, the teeth are overgrown. "Quite often, they will have wet lips and chin from saliva staining and drooling, and there may be a bad smell from the mouth."

Once the pet cannot eat properly, there may be noticeable weight loss and lethargy, and, Gething says, in severe cases, the teeth can press back into the gums and jaw, resulting in swelling and facial pain, and sometimes a watery eye discharge.

Dental malocclusion is painful and it affects a pet's quality of life, but it can be easily prevented.

The first step is proper nutrition. The vet says owners often make the mistake of providing a diet of only commercial pellets, believing they are a complete source of nutrition. "These diets are good in that they usually contain vitamins, minerals, protein and energy, but they should only make up 10 per cent of a rabbit or guinea pig's diet at most," Gething says. He advises owners to think of pellets like protein bars or cookies - fine in small quantities but not something to be eaten all day.

The majority of the diet (about 75 per cent) should be hay, with 15 per cent vegetables and the occasional piece of fruit, and less than 10 per cent pellets. There are different types of hay, but the best for rabbits and guinea pigs, Gething says, is Timothy grass, orchard or oat hay. Alfalfa hay can be fed in small amounts, but is much higher in calories.

"Hay should always be available in your rabbit or guinea pig's pen, and you'll notice that they spend a lot of the day chewing. In addition to grinding their teeth down and keeping their mouth healthy, hay is excellent roughage that aids digestion, promotes healthy intestinal bacteria and nourishes the gut," Gething says.

Gething suggests raw green, leafy vegetables such as lettuce, spinach, parsley and kale, with some raw carrots, along with smaller amounts of broccoli and cauliflower. Occasionally, a small piece of apple, mango or banana can be given, the vet advises, but too much can cause stomach upsets. Rabbits and guinea pigs should never be fed onions, garlic, rhubarb, peas, potatoes or beans, or any seeds that could get stuck.

Owners who are concerned that their rabbit or guinea pig may have overgrown teeth are advised to speak to a vet familiar with these animals. In mild cases, a change in diet may be enough, the vet says, but if the teeth are too overgrown, they may need to be trimmed back before the rabbit or guinea pig can properly chew. "This isn't as difficult as it sounds in most cases, but it does need to be done by someone with expertise in the area."

Gething says that dental malocclusion is probably the single most common issue he sees in pet guinea pigs and rabbits. "It can significantly affect their quality of life, to the point of being life-threatening, but is easily prevented with a little care and knowledge." 

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