Red, swollen testicles in a dog almost always signal inflammation, infection, or injury that needs veterinary attention. The medical term is orchitis (inflammation of the testicle) or epididymitis (inflammation of the tube that stores sperm beside the testicle), and the two frequently occur together. While some causes are straightforward and treatable, others can be serious or even life-threatening, so understanding what you’re looking at matters.
Most Common Causes
Bacterial infection is the leading reason a dog’s testicles become red and swollen. Bacteria can travel up from the urethra or prostate and settle in the epididymis or testicle itself. The resulting inflammation causes visible redness, heat, swelling, and obvious discomfort when the area is touched. Dogs with bacterial orchitis often lick the area obsessively, which can make the skin look even more irritated.
Trauma is another frequent trigger. A blow to the groin during rough play, jumping a fence, or a fight with another dog can cause bruising and swelling that looks alarming. Even without a visible wound, internal bruising in the scrotal tissue can produce significant redness and puffiness.
Less commonly, fungal infections can cause similar symptoms. And in some cases, an autoimmune reaction where the dog’s own immune system attacks testicular tissue is responsible, though this is rare compared to bacterial causes.
Brucellosis: A Serious Bacterial Infection
One bacterial infection worth knowing about specifically is canine brucellosis, caused by the bacterium Brucella canis. This is a contagious disease that spreads between dogs through reproductive fluids, urine, and even saliva. In male dogs, it primarily targets the epididymis, causing it to become enlarged and firm. The scrotal skin can develop dermatitis, and over time, the testicles may actually shrink as chronic inflammation destroys tissue.
Brucellosis is particularly concerning because it can spread to humans, making it a public health issue. Diagnosis requires blood tests, since the symptoms alone look identical to other infections. Dogs with brucellosis also frequently develop swollen lymph nodes, especially under the jaw and behind the throat. Treatment typically involves long courses of antibiotics lasting one to two months, but the infection is notoriously difficult to clear completely. In some cases, especially in breeding facilities, euthanasia is recommended due to the risk of ongoing transmission to other dogs and people.
Testicular Torsion
Testicular torsion happens when the spermatic cord twists, cutting off blood supply to the testicle. This is an emergency. A dog with torsion may show sudden abdominal pain, refuse food, vomit, walk stiffly, or seem lame in the hind legs. The affected testicle will be swollen and extremely painful. Some cases of mild torsion can be surprisingly subtle, with few obvious signs, but severe torsion causes tissue death quickly.
Dogs with undescended testicles (one or both testicles retained inside the abdomen) are at higher risk for torsion. If the testicle is still viable when caught early, a vet can untwist it and secure it in place surgically. If the testicle has been without blood flow too long, or if it was undescended, removal is the standard approach.
Testicular Tumors
In older intact male dogs, testicular cancer is surprisingly common. The three most frequent types are seminomas, Sertoli cell tumors, and Leydig cell tumors. A tumor may cause one testicle to swell noticeably larger than the other, sometimes with changes in skin color or texture. Not all testicular tumors cause redness, but some do, especially if they grow large enough to stretch the skin or trigger secondary inflammation.
Sertoli cell tumors are particularly notable because they can produce estrogen, leading to unusual signs like hair loss along the flanks, enlarged nipples, or attraction from other male dogs. If you notice asymmetric swelling where one testicle is significantly larger, a tumor should be on the list of possibilities your vet investigates.
What Your Vet Will Do
Expect the visit to start with a physical exam, including gentle palpation of the testicles, epididymis, and prostate. Your vet will check for pain responses, asymmetry, firmness, and heat. From there, the most common next step is an ultrasound, which reveals the internal structure of the testicle and can identify tumors, abscesses, fluid buildup, or signs of torsion.
If infection is suspected, your vet may collect a semen sample to look for bacteria and abnormal white blood cell counts. Blood tests can screen for brucellosis and check overall inflammation markers. In cases where a mass is found, fine-needle aspiration (inserting a thin needle to collect cells) is a quick, minimally invasive way to determine whether a growth is cancerous without major surgery.
Treatment Options
For straightforward bacterial infections, antibiotics are the first line of treatment. The specific antibiotic and duration depend on the bacteria involved. Mild cases may improve within a week, while more stubborn infections, particularly brucellosis, can require combination antibiotics for one to two months. Your vet may start with one antibiotic and switch based on culture results.
For chronic or severe infections that don’t respond to medication, neutering (removing both testicles) is often the most effective solution. It eliminates the source of infection entirely and prevents recurrence. The same is true for testicular tumors: surgical removal is both diagnostic and curative in most cases, since the majority of canine testicular cancers are benign or slow to spread.
If your dog isn’t being used for breeding, your vet will likely recommend neutering as part of the treatment plan regardless of the underlying cause. It resolves the immediate problem and prevents future episodes of orchitis, epididymitis, and testicular cancer entirely.
What You Can Do at Home Before the Vet Visit
Apply a cold compress (a cloth-wrapped ice pack or bag of frozen peas) to the swollen area for 10 to 15 minutes at a time. This helps reduce swelling, heat, and pain. Don’t apply ice directly to the skin.
Restrict your dog’s activity. Running, jumping, and rough play will worsen inflammation and increase discomfort. Keep walks short and calm. If your dog is licking the area excessively, put on an Elizabethan collar (the plastic cone). Constant licking introduces more bacteria and can break down irritated skin, making infection worse.
These steps are comfort measures, not treatment. Red, swollen testicles don’t resolve on their own in most cases, and delaying veterinary care risks complications like abscess formation, chronic damage, or fertility loss if you plan to breed your dog.
Signs That Suggest an Emergency
Certain symptoms alongside swollen testicles indicate you should get to a vet urgently rather than waiting for a routine appointment. Sudden onset of severe pain, especially if your dog is vomiting, refusing food, or walking with a stiff or hunched gait, suggests torsion or a rapidly progressing infection. A testicle that feels extremely hard or has changed dramatically in size over just a few hours also warrants immediate evaluation. Fever, lethargy, and discharge from the penis alongside scrotal swelling point to a systemic infection that can worsen quickly without treatment.