Why does my dog have a hole on her back and white stuff comes out when you push it?

It appears that your dog may have a skin condition known as a cutaneous fistula, which is more colloquially called a "hole" in the skin. Fistulas present as small (sometimes large), round or oval openings on the skin from which some types of bodily fluid, such as pus or other excretions, drain from a deeper structure to the skin.

A fistula that exudes white substance when pressed is likely to be an abnormal tract or channel within the skin that connects to an underlying bodily structure. In veterinary practice, the most common cause of skin fistulas is infection. Skin trauma and foreign body penetration are the most common causes of infection-related fistulas. These types of fistulas are commonly encountered as draining bite wounds, abscesses, or infected wounds or foreign body reactions within the skin. When deep infection occurs within skin or soft tissues, the body attempts to drain the infectious fluid to the outside through a "path-of-least-resistance", which in most cases ends up on the skin surface. While the infectious material eventually drains out through these "holes", the infection commonly progresses deeper into the tissues as well. This creates a cycle where more infected material keeps accumulating and draining out. Fistula formation is the body's way of trying to heal and rid the body of the infection by attempting to drain the infectious pockets within the skin to allow the external skin surface to heal over the lesion from the outside.

Other than infection, some internal diseases (e.g. abdominal organ rupture, certain tumors) can also cause fistula formation. In such cases, the fistula may drain out digestive juices, urine or other bodily fluids in addition to white substances.

To identify the exact cause of the skin fistula in your dog, veterinary care is highly recommended. The vet will take a medical history, perform skin examinations and possibly run some diagnostic tests to identify the underlying cause and treat it accordingly. It may be necessary to drain off any deep accumulations of pus, cleanse the internal fistula tract and flush it with antiseptic solutions. In some cases, surgery may be required to permanently remove or repair the affected tissue or structure. Antibiotics and pain medications are additionally prescribed as necessary.

In summary, if you notice a hole on your dog's back that exudes white substance when pressed, it is recommended that you seek veterinary attention as soon as possible for appropriate diagnosis and treatment.