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This is a list of items I keep on hand at all times. Some aren't really first aid items, but health and grooming, but these are all things I believe guinea pig owners should have in their cupboards! Guinea pigs don't show they are ill until they are very sick, and having the first aid items at home can prevent a midnight run to the store -- and hopefully bring relief to your little one. This is NOT to replace going to the vet, many times you will need to go to the vet for antibiotics, but this is to help see you through until you can get to the vet -- or to supplement what the vet is doing.
Guinea pigs need their nails trimmed on a regular basis, but it needs to also be done safely because guinea pigs do bleed through their nails. A good pair of nail clippers is always great to have on hand. It is highly important to be careful when trimming nails, but one time I saw a vet take blood through the nail and then add cornstarch to the nail to help the bleeding stop. (It may need to be added multiple times, never leave a bleeding pig alone.)
When guinea pigs get gas, the bloat is a potential killer. It is highly important to keep baby gas drops with simethicone on hand. It is an inexpensive item that can save a guinea pig's life.
I also keep a bottle of unflavored Pedialyte on hand. When a pig is very sick, on antibiotics, they may not be eating as well. This can help give them the extra fluid with electrolytes they need.
If a guinea pig has an upper respiratory infection, they need to be seen by a vet immediately as they need antibiotics. Once on antibiotics, I have found that allowing them to SMELL (not to put it on them) Vick's Vapor Rub can help clear their sinuses.
Another common household item with great guinea pig uses is coconut oil. Guinea pigs' feet are susceptable to Bumble Foot, a potentially deadly disease, so when I see my guinea pigs' feet are dry or inflamed, out comes the coconut oil! Also, it is a great scrub for the grease gland on boars. I keep my coconut oil in a small condiment container with a lid that I got from a fast food place and cleaned it up. That way I can keep it right by the cage, and know it's for guinea pig use only.
I cannot stress the importance of having Critical Care on hand at all times. This is a feeding supplement for herbivores, and if your pig isn't eating, it's absolutely necessary to hand feed them, as their body can start to shut down within 4-6 hours of no food or water. If a pig is sick enough to be on Critical Care, he should also be seen by a vet. With my pigs they usually start showing signs of illness late at night, and having this on hand can help me get them fed before the vet opens in the morning. I always keep large plastic eye droppers on hand -- which I wash well after each feeding as well as syringes without needles which are disposable for feedings on the go. (One time I was taking my guinea pig to the vet, and had to pull over at a gas station to do a hand feeding of Critical Care.) I sometimes add a little Pedialyte to the water when mixing up the Critical Care.
If your pig is prescribed antibiotics, keeping Bene-Bac on the ready is a good idea. Antibiotics can mess up the bacteria in the gut, just like in humans. Being able to give probiotics will help your pig feel better! Although this comes in both gel and powder, I like the powder because I can make it using Pedialyte.
If your pig absolutely won't eat, and you have tried Critical care different times, then an absolute last case scenerio is organic baby food in your pig's favorite food (blueberry, pear, etc.) When I hear a guinea pig parent suggest I try this, I know the pig's time is nearing and it's either a 'make him comfortable' or last ditch effort. Baby food has too much sugar to be given at any other time, but if your pig won't make it without then is the time to try this. Even better than baby food is canned pumpkin. Again, this isn't an everyday food, but a smidgen when you are baking a pumpkin pie wouldn't hurt unless your pig is obese.
One thing you should do each week is weigh your pig. I have this kitchen scale. In the unlikely event your pig is too big to fit comfortably on the scale, you can add something like a plate BEFORE you turn the scale and it will turn on to 0. Any fluctuation in weight should be a cause for concern. It is highly recommended to weigh them and keep their weight tabulated in a notebook. You can even use a guinea pig design notebook for this purpose if you would like.
Guinea pigs are a great pet, and it's important to keep them happy and healthy. I hope this list helps you know what to do in an emergency.