Blood infection is a severe complication and a leading cause of hospitalization and death in cancer patients. The term generally refers to bacteremia (bacteria in the bloodstream) or the more serious condition called sepsis. Sepsis is the body’s life-threatening, dysregulated response to an existing infection, causing injury to tissues and organs. About one in five sepsis hospitalizations are cancer-related, highlighting the substantial risk for oncology patients. This vulnerability stems from cancer treatments, the physical presence of the disease, and entry points for common microbes.
Immunosuppression Caused by Cancer Treatment
Anti-cancer therapies are the primary cause of increased infection risk due to their effect on the immune system. Chemotherapy targets and destroys rapidly dividing cells, including cancer cells. Unfortunately, this action also affects healthy, rapidly dividing cells, most notably the blood-forming cells in the bone marrow.
This collateral damage leads to neutropenia, a significant reduction in circulating neutrophils, a type of white blood cell. Neutrophils are the immune system’s first responders, responsible for engulfing and destroying bacteria and fungi. Neutropenia is diagnosed when the absolute neutrophil count (ANC) drops below 500 cells per microliter of blood.
The depth and duration of neutropenia directly correspond to the risk of developing a severe infection. The risk of infection-related hospitalization increases significantly for every additional day a patient experiences low neutrophil counts. The ANC typically reaches its lowest point, or nadir, 7 to 12 days after chemotherapy, marking the period of highest risk.
Radiation therapy can also contribute to a compromised immune system, especially when directed at large areas of the body containing bone marrow. The resulting inability to mount an adequate defense means that even common, low-virulence microbes can quickly overwhelm the body and spread to the bloodstream. Without the typical inflammatory response, a fever may be the only initial sign of a rapidly progressing, life-threatening infection.
Physical Entry Points for Pathogens
While a weakened immune system is the internal vulnerability, physical breaches provide entry points for pathogens. One common breach is mucositis, the inflammation and ulceration of mucous membranes lining the gastrointestinal tract. Chemotherapy and radiation damage these cells, creating open sores and microscopic breaks in the mucosal lining.
These ulcers act as direct portals for the bacteria that normally reside in the mouth and gut to enter the bloodstream. When a patient has both mucositis and neutropenia, the risk of a blood infection is more than four times greater than in patients with neutropenia alone. This is a major pathway for the patient’s own bacterial flora to cause systemic infection.
Another major pathway for pathogen entry is through necessary medical hardware, specifically central venous catheters (CVCs), such as ports and PICC lines. These devices are essential for administering chemotherapy, fluids, and nutrition. However, they provide a direct highway for skin bacteria to reach the central circulation, increasing the risk of central line-associated bloodstream infections (CLABSIs), especially during neutropenia.
The presence of a foreign body like a catheter allows bacteria to create a protective biofilm on the device surface, making the infection difficult to clear without removing the line. The risk of infection can be two to six times higher in patients with a long-term CVC compared to those without a catheter. Handling the catheter and administering parenteral nutrition are also risk factors for CVC-related infections.
Cancer-Specific Disease Factors
Beyond the consequences of treatment, the cancer itself can directly cause conditions that lead to severe localized infections that then spread to the blood. One such mechanism is the physical obstruction of hollow organs or natural passageways. Tumors can grow large enough to compress or block structures like the bile ducts, urinary tract, or the bowel.
This blockage causes fluids, such as urine or bile, to back up, creating a stagnant environment where bacteria can multiply rapidly. For example, a malignant bowel obstruction can lead to localized infection that may progress to a systemic blood infection. This mechanical blockage can be caused by the primary tumor, metastatic disease, or scar tissue from prior surgery or radiation therapy.
Furthermore, large or rapidly growing tumors can sometimes outgrow their blood supply, leading to the death of internal tumor tissue, a process known as necrosis. This necrotic tissue provides a perfect, nutrient-rich medium for bacteria to colonize. Infections originating from these sources are often more difficult to treat because the blood supply to the infected area may be compromised, limiting the delivery of antibiotics.
The Common Infectious Agents
The microbes responsible for blood infections are classified into three major groups: bacteria, fungi, and sometimes viruses. The majority of these infections are endogenous, meaning they originate from the patient’s own microbial flora rather than the outside environment. When the body’s barriers break down or the immune system fails, these commensal organisms become opportunistic pathogens.
Bacterial pathogens are the most common culprits, categorized as Gram-positive or Gram-negative organisms. Gram-positive bacteria, such as coagulase-negative staphylococci and Staphylococcus aureus, frequently cause infections related to the skin and indwelling medical devices like CVCs. The viridans group streptococci is also associated with infections occurring after chemotherapy-induced mucositis.
Gram-negative bacteria, including Escherichia coli, Klebsiella species, and Pseudomonas aeruginosa, are highly prevalent and often originate from the gut. These organisms cause rapidly progressing and severe infections; Pseudomonas aeruginosa is concerning due to its high mortality. Translocation of these gut bacteria across a damaged intestinal lining is a major route for Gram-negative bloodstream infections.
Fungal pathogens, though less frequent than bacteria, pose a significant risk, especially in patients with prolonged neutropenia or those who have received extensive broad-spectrum antibiotics. The yeast Candida species are the most common fungal cause of blood infection. These fungi often exploit the breakdown in the mucosal barrier, particularly gastrointestinal mucositis, to gain entry into the systemic circulation.