Pododermatitis is a spongy, moist, inflamed callous on one or both heels, often infected. It occurs almost exclusively in heavy, elderly bucks.
The condition has traditionally been considered to be extremely hard to treat, but recently I have discovered that Calendula (marigold) extract works wonders. I used an infusion of Calendula in a base-oil, but plain Calendula infusion or ointment would probably work just as well. It's early days yet, but judging from my experience so far, I would go so far as to say that anyone who has elderly bucks should keep a supply of Calendula ready, and check their rats' feet about once a week for signs of callouses developing.
For a small, new callous which is only just forming, a single application of Calendula may be enough to get rid of it. For callouses which are already large and established, I wrap the rat's foot in a length of cotton-wool or soft paper soaked in Calendula oil, and then hold the rat for 20 minutes or so to give the oil time to soak in, every night until the callous shrinks.
It's certainly best to catch these callouses early. If they get very big, shrinking them will still leave a sort of ulcerated patch which will require treatment (with more Calendula and/or tea-tree oil) until it heals, which will probably take weeks.
These callouses tend to recur, even if surgically removed. However, a further application of Calendula should see them off again.
Encouraging the rat to paddle in a dilute solution of tea-tree oil is also said to bring some relief from both swelling and infection.
Another effective treatment for inflammation and for open, surface infection is a poultice made from:
3 tablespoons of pure unscented talc or kaolin powder
2 teaspoons of mustard powder
¼ teaspoon of garlic powder (not fresh garlic)
This should be mixed to a thick paste with a little warm water, smeared on a strip of bandage or paper, wrapped round the inflamed foot and held in place for about 20 minutes every night. [Thanks to herbalist Damien Suil-Levanne for this recipe.]
In extreme cases the condition can lead to infection inside the capsules of the hocks, which is painful and very difficult to treat. Topical (external) treatment can't get at it, and even systemic (injected or ingested) antibiotics have trouble reaching the infection, because it is so sealed-off from the blood-supply. I have know it take six weeks on Genticin (super-powerful antibiotic which can - and did - damage a rat's hearing if used long-term like this) to shift such an infection, so it is important to shrink the callous with Calendula before the problem spreads to the hock.
Pododermatitis in rats appears basically to be a sort of pressure-sore. Wire-floored cages and/or wire-mesh shelves which are so placed that the rat has to climb on them to get around the cage are not a good idea, as these are believed to increase the risk of developing such sores: though wire shelves which are merely used for sitting on occasionally don't seem to cause problems. Very soft floor-covering, especially VetBed (a type of absorbent, machine-washable fur-fabric sold by some pet-shops), would probably reduce the risk of developing sores.
However, Peter Gurney of the Cambridge Cavy Trust has found that similar heel-sores occurring in guinea-pigs often seem to be, or at least involve, a fungal infection, and improve if the patient is treated with the anti-fungal drug Griseofulvin. This might at least be worth a try in rats.