Viral Infections with Neutrophilia
Although most viral infections cause lymphocytosis, some can present with an elevated neutrophil count, especially early in the infection or in specific patient populations.
- Etiology/Pathophysiology
- Mechanism: Viruses can induce a systemic inflammatory response, leading to the release of pro-inflammatory cytokines that stimulate the bone marrow to release neutrophils. This is often an early response before the adaptive (lymphocytic) immune system is fully activated.
- Causative Agents:
- Herpes Simplex Virus (HSV)
- Varicella-Zoster Virus (VZV)
- Epstein-Barr Virus (EBV) - can cause neutrophilia, especially in young children.
- Poliomyelitis
- Severe cases of COVID-19 may show neutrophilia, which is often associated with a secondary bacterial infection or a cytokine storm.
- Clinical Presentation & Dx
- Features are those of the underlying viral illness (e.g., vesicular rash in VZV/HSV, pharyngitis and fatigue in EBV).
- Dx: CBC reveals neutrophilic leukocytosis. A “left shift” (increased band forms) might be present, similar to bacterial infections. Diagnosis is confirmed with viral-specific testing (e.g., PCR, serology).
- Key Differentiating Point
- The clinical context is crucial. A patient with a classic viral syndrome (e.g., chickenpox) who has neutrophilia should still be suspected of having the primary viral illness. The neutrophilia can be a transient part of the body’s initial inflammatory reaction.
Bacterial Infections with Lymphocytosis
This is a less common but high-yield exception to the general rule.
- Etiology/Pathophysiology
- Mechanism: Certain bacteria produce toxins or have cellular components that directly stimulate the proliferation and prevent the extravasation of lymphocytes from the blood into lymphoid tissues, leading to a marked increase in their circulating number.
- Causative Agents:
- Bordetella pertussis: The classic example. Pertussis toxin causes lymphocytosis by preventing lymphocytes from entering lymphoid tissues.
- Bartonella henselae: The cause of cat-scratch disease, which can lead to self-limited lymphadenopathy and potentially lymphocytosis.
- Tuberculosis (TB): Chronic infections like TB can be associated with lymphocytosis.
- Brucellosis
- Syphilis
- Clinical Presentation & Dx
- B. pertussis: Presents with paroxysmal coughing fits followed by an inspiratory “whoop” and post-tussive emesis. The CBC finding of marked leukocytosis with an absolute lymphocytosis is a hallmark.
- B. henselae: Typically presents with a tender lymph node (lymphadenopathy) following a cat scratch or bite, often accompanied by fever and malaise.
- Peripheral Smear: In pertussis, lymphocytes are mature but small with cleaved nuclei. In Bartonella infection, lymphocytes can be large and atypical.
- DDx
- The marked lymphocytosis in pertussis can be mistaken for leukemia. However, the presence of preceding upper respiratory symptoms followed by the characteristic cough, along with the patient’s age (typically infants or unimmunized children), points toward the infectious cause.
- Infectious mononucleosis (caused by EBV or CMV) also causes atypical lymphocytosis, but it typically presents with fever, pharyngitis, and lymphadenopathy, and can be distinguished by specific serologic tests (e.g., heterophile antibody test for EBV).