Original sentence: Female cats and dogs are seven times more likely to develop mammary tumors if they are not spayed before their first heat cycle.However, certain medical problems are more likely after neutering, such as urinary incontinence in females and prostate cancer in males.In some countries of Europe, spaying or neutering of dogs is less common, and owners are usually instead advised to keep their dogs on leash and under supervision to avoid unwanted matings; drugs are used to prevent pregnancy and abort unwanted litters
Edited sentence: Female dogs spayed before their first heat have a vastly lower incidence of mammary tumors than dogs which are spayed after their first heat or pregnancy (1% verses 22%).In the less developed countries of Europe, spaying or neutering of dogs is less common, and owners are usually instead advised to keep their dogs on leash and under supervision to avoid unwanted matings; drugs are used to prevent pregnancy and abort unwanted litters