Neutrophils

Neutrophils are the most numerous type of white blood cell and serve as the "infantry" of the immune system. They are the first cells to arrive at the site of any new infection or significant inflammation. In cats with Chronic Kidney Disease, the neutrophil count is a primary indicator used to detect active bacterial infections, such as a hidden kidney infection (pyelonephritis).

The First Line of Defense

• Phagocytosis: Neutrophils work by "eating" and neutralizing bacteria and cellular debris.

• The Bone Marrow Reserve: The body keeps a large reserve of mature neutrophils in the bone marrow. When a "call to arms" is sent out due to infection, these cells are released into the bloodstream immediately.

High Neutrophils (Neutrophilia)

A high neutrophil count in a CKD cat is usually caused by one of three things:

• Bacterial Infection: This is the most critical cause. If a cat has an infection in the bladder or kidneys, the neutrophil count will often spike.

• The "Left Shift": If the infection is severe, the bone marrow runs out of mature neutrophils and starts sending out "immature" versions called Band Neutrophils. Finding "Bands" on a lab report is a major red flag that the infection is serious and potentially overwhelming.

• Inflammation: Non-infectious inflammation, such as a flare-up of pancreatitis or severe tissue damage within the kidneys, will also draw neutrophils into the blood.

• Stress Response: As discussed with lymphocytes, high levels of cortisol (the stress hormone) can cause neutrophils to "detach" from the walls of the blood vessels and circulate in the blood. This is often called a "Mature Neutrophilia" because there are no immature band cells present.

Low Neutrophils (Neutropenia)

A low neutrophil count is rare in stable CKD but is a grave sign if it occurs:

• Sepsis: In overwhelming infections, the bacteria may destroy neutrophils faster than the bone marrow can replace them.

• Bone Marrow Exhaustion: Severe uremic poisoning in end-stage CKD can sometimes suppress the bone marrow’s ability to produce these cells.

• Risk: A cat with low neutrophils has almost no defense against bacteria, making even a minor infection potentially fatal.

The Impact on the Kidneys

Neutrophils are necessary for defense, but they are "messy" fighters. When they attack bacteria in the delicate tubules of the kidney, they release enzymes and inflammatory chemicals that can cause "collateral damage" to healthy kidney tissue. This is why a prolonged high neutrophil count can actually accelerate the progression of CKD.

Interpreting the Results

• Mature Neutrophilia: High count but all cells are mature. Usually indicates stress or mild-to-moderate inflammation.

• Neutrophilia with a Left Shift: High count with immature "bands." Indicates an active, likely bacterial, infection.

• Toxic Changes: Sometimes the lab will note "toxic neutrophils." This means the cells themselves look damaged, which usually happens when the cat is fighting a very severe infection or systemic toxicity.

Management

• Urine Culture: If the neutrophil count is high and a "left shift" is present, a urine culture is the most important next step to see if the kidneys are under attack.

• Antibiotics: Successful treatment of an infection should see the neutrophil count return to the normal range within a few days.

• Hydration: Maintaining blood flow to the kidneys helps neutrophils reach the site of infection more efficiently and helps "flush" out the debris they leave behind.