Experienced and professional animal trainer provide their insights in answering this question :
A. It is probably an eye infection but could also be due to trauma. I would recommend having it checked out by your vet as soon as possible. Some eye problems can cause irreversible damage if left untreated.

How to Identify Common Pet Problems ?

Our sources include academic articles, blog posts, and personal essays from experienced pet care professionals :

Hyphema simply means blood inside the eye. It can occur for many reasons and it can be a sign of a serious disease process, such as rat poison ingestion, certain infections, cancer, high blood pressure, trauma, and retinal detachment.
There are many causes of a red eye, but all ocular diseases should always be considered an urgent matter, and be checked immediately by a veterinarian.
Hold your dog`s eye open with one hand and apply a steady stream of a sterile eyewash solution, available at a pharmacy, or lukewarm water to flush out your dog`s eye. Do not use medicated eye wash. Catch excess solution with a towel or cloth. Clean up the area and wash your hands after handling your dog.
A red eye is usually nothing to worry about and often gets better on its own. But sometimes it can be serious and you`ll need to get medical help.
Dogs` eyes are as sensitive as humans`, and can be just as vulnerable to irritation, allergies, injury, and disease. If your dog`s eyes look red or swollen, it could be a sign of irritation, mild allergy, or physical trauma.
Broken blood vessel in the eye

Even a strong sneeze or cough can cause a blood vessel to break in the eye. You don`t need to treat it. A subconjunctival hemorrhage may look alarming, but it`s usually a harmless condition that disappears within two weeks or so.

One popular method is using a warm, damp cloth to gently clean and soothe the eye area. Another option is using a saline solution to flush out any irritants.
The third eyelid has a cartilaginous frame and a gland at its base, which is responsible for producing 30-50% of the aqueous tear film. The gland is very loosely attached to the third eyelid and in some dogs it pops up from behind the edge of the third eyelid, looking like a red cherry hence its nickname `Cherry eye`.
Eye conditions often associated with Liver imbalances include: myopia, astigmatism, AMD, retinitis pigmentosa, Presbyopia, dry eyes, floaters, glaucoma, Stargardt`s, cataracts, red and dry eyes. Other eye issues include: photophobia, red and dry eyes, bloodshot eyes, poor night vision, blurry vision, headaches.
Conditions that involve red eye include conjunctivitis, corneal ulcers, dry eye syndrome, and blepharitis. Red or bloodshot eyes occur when small blood vessels on the surface of the eye become enlarged and congested with blood. Red eyes alone are not usually a reason for concern.
If your dog is dehydrated, this can easily occur. Red, dry eyes can be a warning sign of two conditions. First, when your dog isn`t getting enough water, to the point that they are not producing tears, a life-threatening situation can occur.
The red/brown discoloration in tears comes from porphyrin. Porphyrins are iron containing molecules produced when the body breaks down iron. Porphyrins are excreted through the gastrointestinal tract, urine, saliva, and TEARS!
“A pool of blood has collected right underneath the clear layer of the eye, and it shows up as completely red, like the surface of a tomato,” says Dr. Witsberger. Like a bruise, the mark normally fades over days or weeks, changing from red to purple to green and then yellow.
It`s vital that you see a vet if you suspect your dog has conjunctivitis or other eye problems so they can diagnose the problem and start treatment. You should not wait for it to go away on its own or rely on home remedies. In many cases, your dog`s eye infection will not go away without treatment.
Unfortunately, you would be mistaken to think that the same eye drops you use are suitable for dogs. You should not use human eye drops on dogs. If your dog is experiencing eye irritation, contact your vet and take them in to get an eye examination and possibly eye drops that are safe for dogs.
To clean your dog`s eyes, moisten a sterile gauze pad with a sterile saline solution. (Note: Some preparations recommend use of cotton balls for this purpose, but we are not fans of cotton balls, since they can shred, leaving tiny filaments of cotton attached to your dog`s eyelashes that can lead to further problems).
With appropriate treatment, bacterial conjunctivitis is usually fully resolved within 5 to 7 days. Viral conjunctivitis can take up to 3 to 4 weeks for full resolution. Allergic conjunctivitis will persist until the underlying allergen is discovered and eliminated.
In addition, an eye infection can easily be spread to other pets in the home. Untreated eye problems can lead to significant vision loss or blindness if left untreated or if treatment is delayed. This is true even for easily-treated eye conditions such as pink eye.
A cherry eye appears as a red, swollen mass on the lower eyelid near the nose or muzzle, resembling a cherry. The cherry eye may be large and cover a significant portion of the cornea, or it may be small and only periodically.
Signs and symptoms of acute liver failure may include: Yellowing of your skin and eyeballs (jaundice) Pain in your upper right abdomen. A swollen belly (ascites)
A liver that is working poorly cannot get rid of bilirubin, a substance that produces a yellowing of the eyes and skin called jaundice.
In most cases, the conditions that cause eye redness aren`t serious and will clear without medical treatment. Home remedies, such as compresses and artificial tears, can help relieve any symptoms you may be experiencing. If the symptoms persist or include pain or loss of vision, you should seek immediate medical care.

Relevant Questions and Answers :

the most relevant questions and answers related to your specific issue

Q. Yellow lab F, approx 9-10 yrs. Sudden onset of extreme redness in one eye, now seems to have spread to other over 24-48 hrs. Healty, active, good app.
ANSWER : A. This could be a bacterial conjunctivitis. Dogs can pass these infections around, just like people can. The fact that it started in one eye and moved to the other suggests infection, but I can’t rule out something more serious like glaucoma. I feel like anytime a dog’s eye gets acutely red like this it’s cause for alarm, and needs to be checked out as soon as possible. Like I said above, it’s likely a simple eye infection, but get it checked out to be sure.

Read Full Q/A … : Vetinfo

Q. One eye is more red than the other. Last night he was pawing at it,today, he isn’t. Should I bring him to the vet?
ANSWER : A. Sounds possible that he had some irritation to the eye that is at least not itchy anymore. You can do either, having it checked now to confirm mild irritation, potentially due to debris or a topical irritant to the eye; or you can flush the eye with sterile saline eye wash (over the counter) using care not to poke or prod the eye and see if the irritation goes away on its own within the next 24 hrs. It has shown improvement already it appears, however if it is not cleared up and your pet seems irritated by it AT ALL, then it should be looked at by a vet and tested for a possible corneal scratch or ulcer. These can cause redness of the eye, eye discharge and pawing at the face. They are usually treated by prescription only topical eye antibiotic ointment and generally do very well after treatment.

Read Full Q/A … : Eye Problems in Cats

Q. Weak, discharge from eyes, swelling on right eye, does not want to open eyes. What is wrong?
ANSWER : A. If the discharge from the eye is green or yellow in color, it can indicate a secondary infection. Swollen eyes can be caused by a number of things ranging from allergies to infections, to injury to the eye itself or surrounding areas.

It is best to have your pet’s eye examined by your local vet. They will most likely wish to place a stain in the eye that can check for damage such as cuts or scratches. An ointment can then be given to help reduce inflammation, pain and take care of infection.

Until you can get to the vet, be sure to not let your pet scratch or paw at their eye as this can make things worse. You can also use a warm wet washcloth to gently remove any debris and allow the eye to open some, providing relief. However, if symptoms worsen, or the swelling travels to the face, head or neck, it may indicate a serious allergic reaction which should be treated immediately.

Q. My female tabby kitten looks like she has blood in the color part of her eye. Not seeping out but like actually inside the eye.
ANSWER : A. It sounds that she symptom called hyphema which is presence of blood in eye chamber. In cats it usually happens after eye trauma but also may be related to increased blood pressure,clotting disorder, eye infection or even parasites. You should take her to your vet for accurate diagnosis and to get treatment for her

Q. Dogs left eyes waters quite a bit at different times of the day. Clear, look like tears. Is it normal or maybe allergies?
ANSWER : A. It could be allergies or over active tear glands. If the discharge is clear as you say and the white of the eye is not red and the eye looks otherwise normal then I wouldn’t worry. Any swelling, closing of the eye, redness or rubbing at the eye should be checked out sooner rather than later by your vet.

Q. My 14 week old English bulldog has what looks like a small pimple on the outside corner of her eye . It appears to be completely external. What to do.
ANSWER : A. If the pimple is not bothersome it may be best to just keep an eye on it and watch for any changes such as reddening, growth, or itching/pain to your pup. Puppies can sometimes get strange bumps that may go away over time. If the pimple grows in size, becomes very red, affects the eye itself or is bothersome to your pup then it is best to have the vet take a look. They can safely remove the growth if needed, and may also recommend additional treatments to protect the eye. If your pup is bothered, placing an Elizabethan (cone) collar on her to prevent scratching at and possibly hurting the eye is best until she can get into your vet.

Q. Cat isn’t moving much, not vocal, not eating much of favourite food and is limping. Indoor, urine and feces look normal. What could be wrong?
ANSWER : A. Unfortunately there could be a lot of things going on. Some of them are more likely or unlikely depending on your cat’s age, but I’ll give you a general idea. You’re describing two basic problems – lethargy and inappetance, which often go together in cats. Causes can include organ diseases, like kidney or liver problems, heart problems, hormonal diseases like diabetes, or blood disorders that cause anemia (low red blood cells). Your vet will likely recommend blood work and possibly x-rays to get started. But keep in mind it might take some testing to figure out what’s going on, because these are pretty vague symptoms. Good luck.

Q. My pet is suffering eyes discharge, what should I do?
ANSWER : A. Mucus, yellow-green pus, or a watery eye discharge can all be signs of conjunctivitis, which is inflammation of the lining of your dog’s eye.

There is a wide range of causes for conjunctivitis, from allergies, injury, birth defects, and tear duct problems, to foreign bodies, dry eye syndrome, infections or even tumors.

Other signs of conjunctivitis include excessive blinking or keeping the eye closed, squinting and pawing at the eyes.

Treatment of this condition depends on the underlying cause. In most of the cases cleaning, soothing the eye and applying antibiotics eye drops suffice but is some instances further investigation is required to establish the cause of the excessive eye discharge, and this should be performed by a veterinarian.