Other than the strong odor and scent itself...per Pets MD....
"Pet guardians should be aware that pure tea tree oil (as well as other pure essential oils) is considered toxic for pets and should never be used in its pure undiluted or “raw” form.
Tea tree oil is a natural essential oil with curative properties that help soothe and protect dogs’ skin. When used correctly and in a diluted form on pets, tea tree oil can be very effective in treating and preventing topical bacterial and yeast/fungal infections. Tea tree oil is an excellent anti-inflammatory, so it helps reduce skin inflammation that can lead to itchiness and irritation. It also has great antibacterial, anti-viral, and anti-fungal properties that can help prevent and treat a whole host of skin problems.
Due to its effectiveness, tea tree oil is found in many pet grooming products in varying diluted concentrations.
How should I administer tea tree oil to my dog?
Tea tree oil must not be ingested, and should be applied externally only, and only in its diluted form. Products such as Be Soothed Spray and Be Soothed Shampoo contain a diluted amount of tea tree oil. While the percentage of tea tree oil in these products is proprietary, rest assured they are well below the level considered toxic to dogs and puppies (over 8 weeks of age.) As with any new product, always seek the advice of your veterinarian before applying to your dog.
Tea tree oil should not be used on cats. Use caution when applying tea tree oil if you have cats in the house."
"Tea tree oil is applied to the skin (used topically) for infections such as acne, fungal infections of the nail (onychomycosis), lice, scabies, athlete’s foot (tinea pedis), and ringworm. It is also used topically as a local antiseptic for cuts and abrasions, for burns, insect bites and stings, boils, vaginal infections, recurrent herpes labialis, toothache, infections of the mouth and nose, sore throat, and for ear infections such as otitis media and otitis externa.
Some people add it to bath water to treat cough, bronchial congestion, and pulmonary inflammation.
How does it work?
The chemicals in tea tree oil may kill bacteria and fungus, and reduce allergic skin reactions." I dab a bit on the dogs wounds, and a q-tip dab on various spots on both dogs (away from the head, eyes, sensitive nose)keeps fleas completely away!