Mouse Bite: ICD‑10 Medical Coding

Understanding Mouse Bites and Their Medical Significance

Clinical Presentation of Mouse Bites

Symptoms and Signs

Rodent‑related injuries require precise documentation to ensure correct classification within the ICD‑10 system. When a patient presents after a bite from a small rodent, the clinical picture typically includes a puncture or laceration at the bite site, often surrounded by erythema and localized swelling. Pain is usually sharp at onset and may persist as a throbbing sensation. Early infection manifests as increased warmth, purulent discharge, and expanding erythema; systemic signs such as fever, chills, and malaise indicate progression to cellulitis or sepsis. Regional lymphadenopathy frequently accompanies deeper tissue involvement, while tenderness and limited range of motion may signal joint or tendon injury.

The appropriate ICD‑10 code for a mouse bite falls under the category for injuries caused by other mammals. The specific code W55.0 (Bitten or stung by other animals, unspecified) is commonly applied, with an additional code for the nature of the injury, such as S51.81 (Open wound of forearm, initial encounter) when the bite occurs on the arm. If infection is evident, an accompanying code from the T14.8 series (Other specified injuries of unspecified body region) may be added, together with a secondary infection code like A49.9 (Bacterial infection, unspecified). Tetanus prophylaxis is recorded with Z23 (Encounter for immunization), and any rabies exposure, though rare for mice, would require Z20.3 (Contact with and exposure to other viral communicable diseases).

Key clinical indicators for coding purposes include:

Visible puncture or laceration at the bite site
Localized erythema, edema, and pain
Purulent discharge or signs of secondary infection
Fever, chills, or systemic inflammatory response
Enlarged regional lymph nodes
Functional impairment of the affected limb

Accurate capture of these signs and the corresponding ICD‑10 identifiers supports reliable epidemiological tracking, appropriate reimbursement, and optimal patient management.

Potential Complications

Mouse bites can lead to a range of medical complications that must be documented accurately for coding and treatment purposes. The primary concerns include local infection, systemic involvement, and injury‑related sequelae.

Bacterial infection – Common pathogens such as Pasteurella multocida, Staphylococcus aureus, and Streptococcus species may cause cellulitis, abscess formation, or wound necrosis. Prompt antimicrobial therapy is essential to prevent progression.
Tetanus risk – Penetrating puncture wounds create an environment for Clostridium tetani spores. Immunization status should be verified and prophylactic tetanus toxoid administered when indicated.
Rabies exposure – Although rodents are rarely vectors, any bite from a wild mouse warrants assessment of rabies risk, especially in regions with documented cases. Post‑exposure prophylaxis may be required.
Septicemia – Uncontrolled local infection can disseminate, leading to bacteremia and septic shock. Monitoring of vital signs and laboratory markers (e.g., white‑blood‑cell count, lactate) is critical.
Osteomyelitis – Deep bites involving bone tissue may result in chronic infection of the skeletal system. Imaging studies and prolonged antibiotic courses are often necessary.
Lymphadenitis – Regional lymph node enlargement indicates spread of infection; drainage or additional antimicrobial coverage may be indicated.
Allergic reaction – Immediate hypersensitivity to mouse saliva or fur can manifest as urticaria, angioedema, or anaphylaxis, requiring antihistamines or epinephrine.
Delayed wound healing – Factors such as poor perfusion, diabetes, or immunosuppression prolong recovery and increase the likelihood of secondary complications.

Accurate assignment of the appropriate ICD‑10 code for a mouse‑related bite (e.g., W55.0XXA for initial encounter) ensures proper documentation of these complications and supports reimbursement and epidemiologic tracking.

Diagnostic Considerations

Medical History

Accurate documentation of a patient’s medical history is essential for assigning the correct ICD‑10 code to injuries caused by rodent bites. The coder must verify that the encounter pertains to a bite from a mouse, distinguish it from other animal injuries, and confirm that the diagnosis reflects the specific nature of the wound. Precise historical information prevents misclassification, supports appropriate reimbursement, and ensures reliable epidemiological data.

Key components of the medical history required for coding a mouse bite injury include:

Patient age and gender, which influence risk assessment and treatment protocols.
Date and time of the incident, establishing the acute nature of the encounter.
Circumstances of exposure, such as location (home, laboratory, occupational setting) and activity at the time of the bite.
Description of the wound: depth, size, presence of puncture marks, signs of infection, and any retained foreign material.
Prior medical conditions that may affect healing, including immunocompromising disorders, diabetes, or chronic skin diseases.
Allergic history, particularly reactions to animal saliva or previous bite injuries.
Initial management steps taken before presentation, such as wound cleaning, tetanus prophylaxis, or antibiotic administration.
Follow‑up plan, including scheduled visits, wound reassessment, and additional interventions if complications arise.

Incorporating these elements into the clinical record creates a comprehensive narrative that aligns with the ICD‑10 classification system and facilitates accurate code selection.

Physical Examination

A thorough physical examination is essential for accurate ICD‑10 classification of mouse‑bite injuries. The examiner must systematically document all findings to support the selected code and to guide clinical management.

Inspect the wound for size, depth, and shape; note whether the bite is punctate, lacerated, or avulsed.
Assess surrounding skin for erythema, edema, induration, or necrosis.
Palpate the area for tenderness, fluctuance, or crepitus, indicating possible infection or deep tissue involvement.
Examine adjacent structures, including tendons, nerves, and vascular supply, to identify functional impairment.
Record any signs of systemic involvement: fever, lymphadenopathy, or signs of sepsis.
Capture patient‑reported symptoms such as pain intensity, timing, and any allergic reactions.

Documentation should include the exact anatomical location using standard terminology (e.g., “right dorsal hand”) and the time elapsed since the bite. When multiple bites are present, each site requires separate description. The presence of foreign material, such as mouse hair or debris, must be noted, as it influences both coding and treatment decisions.

The examiner’s findings directly affect code selection: the primary code reflects the external cause (mouse bite) and the encounter type (initial, subsequent, or sequela). Additional codes may be required for complications such as cellulitis, abscess formation, or neurovascular injury. Accurate, detailed physical examination data ensure that the coding reflects the clinical reality and facilitates appropriate reimbursement and statistical reporting.

Laboratory Tests

Laboratory evaluation of a patient bitten by a mouse focuses on identifying infectious agents, assessing tetanus immunity, and establishing baseline hematologic status. The primary tests include:

Blood culture: detects systemic bacterial infection; documented with ICD‑10 code A48.2 (Other bacterial infections following animal bite).
Wound swab culture: isolates local pathogens such as Staphylococcus aureus or Streptococcus species; coded under T14.30 (Injury of unspecified hand, initial encounter) with an additional infection code if positive.
Serology for hantavirus: indicated when exposure to rodent excreta is suspected; positive result reported with A98.5 (Hantavirus infection).
Rabies virus antibody test: ordered if the mouse is suspected to be rabid; a confirmed case uses A82.9 (Rabies, unspecified).
Tetanus antibody titer: determines need for prophylaxis; lack of immunity leads to administration of tetanus toxoid, coded as Z23 (Encounter for immunization).
Complete blood count (CBC) with differential: screens for leukocytosis or anemia; abnormal findings are captured by the appropriate R codes (e.g., R73.9 for unspecified abnormal glucose, if relevant).

Interpretation of results guides clinical management and determines supplemental ICD‑10 codes for complications. Positive bacterial cultures add infection-specific codes, while negative cultures may still warrant observation under the primary injury code W55.0XXA (Bite of rodent, initial encounter). Documentation must reflect each laboratory finding to ensure accurate reimbursement and epidemiologic tracking.

ICD-10 Medical Coding for Mouse Bites

Core ICD-10 Codes for Mouse Bites

External Cause Codes (W57)

External cause codes in the ICD‑10 system capture the circumstances of injuries such as mouse bites. The primary code for a rodent‑related bite is W57.0, which denotes “bite of rat, mouse or other rodent.” Sub‑categories within the W57 block include:

W57.0 – Bite of rat, mouse, or other rodent
W57.1 – Bite of other mammal (e.g., dog, cat)
W57.9 – Unspecified mammalian bite

When documenting a mouse‑bite injury, the W57.0 code must accompany the appropriate injury code from the S00–S99 range (for example, S60.2 Open wound of hand). Additional external cause codes may be required to describe the setting (Y92), activity (Y93), or intent (Y94) of the incident. Accurate pairing of the W57.0 code with the injury and contextual codes ensures comprehensive reporting and facilitates epidemiological analysis.

Injury Codes (T14)

Mouse bite injuries are classified under the ICD‑10 chapter for external causes of morbidity and mortality, specifically within the injury code block T14. This block captures injuries of unspecified parts of the body when the precise anatomical location cannot be determined, and it includes a subcategory for bites by rodents.

The relevant entry is:

T14.4 – Bite of rodent, unspecified site

This code applies when documentation confirms a mouse bite but lacks detail about the exact body region affected. If the site is known, the coder should select the appropriate specific bite code (e.g., T14.0 for “bite of head and neck”) and then add a secondary code indicating the animal.

When coding a mouse bite, the following steps ensure compliance:

Verify that the clinical record identifies a bite by a mouse or other rodent.
Determine whether the injured body part is documented; if not, assign T14.4.
If the site is documented, use the corresponding specific bite code and add T14.4 as an additional code to denote the rodent source.
Include any relevant external cause codes (e.g., V01–V99) to capture the circumstance of the injury.

Accurate use of T14.4 prevents misclassification and supports reliable data collection for epidemiological analysis and reimbursement processes.

Infection Codes (A00-B99, L00-L08)

Mouse bites are classified under the external causes of injury section of ICD‑10, but the resulting infections are recorded using the infectious disease chapters. Accurate assignment of infection codes ensures proper documentation, reimbursement, and epidemiological tracking.

Relevant infection codes include:

A20–A28: Zoonotic bacterial infections. Examples:
- A20 Plague – rare but possible after rodent exposure.
- A21 Tularemia – may follow a bite from a wild mouse.
- A23 Brucellosis – documented in some laboratory settings.
A30–A49: Other bacterial infections. Notable entries:
- A41 Septicemia, unspecified – used when systemic infection is confirmed.
- A49 Other bacterial infections, not elsewhere classified – covers atypical pathogens from rodent oral flora.
A70–A74: Chlamydial infections – occasionally reported in bite‑related conjunctivitis.
B95–B97: Bacterial infectious agents. Use these supplementary codes to specify the organism (e.g., B95.5 Staphylococcus aureus) when identified.
L00–L08: Skin and subcutaneous tissue infections. Applicable codes:
- L03 Cellulitis – most common presentation after a mouse bite.
- L04 Acute lymphangitis – indicates spread along lymphatic channels.
- L08 Other local infections of skin and subcutaneous tissue – captures abscesses or necrotizing fasciitis when present.

When coding a mouse‑bite encounter, combine the external cause code (e.g., W55.0 Bitten or struck by rodent) with the appropriate infection code from the lists above. If laboratory results identify a specific pathogen, attach the corresponding B95–B97 code as a secondary identifier. This dual‑coding approach captures both the mechanism of injury and the clinical consequence, fulfilling ICD‑10 requirements for comprehensive medical records.

Coding Specificity and Best Practices

Documenting Bite Location

Accurate documentation of the bite site is essential for assigning the correct ICD‑10 code to a mouse‑related injury. The primary external cause code for a rodent bite is W55.0 (Bitten by other rodents). This code must be paired with an appropriate injury code that describes the anatomical location, severity, and laterality of the wound.

When recording the bite location, include the following elements:

Body region (e.g., hand, forearm, foot, cheek).
Specific structure (e.g., skin, subcutaneous tissue, muscle).
Laterality indicator (right, left, or unspecified).
Depth or type of injury (puncture, laceration, avulsion).

The injury code is selected from the S00‑S99 series. For example, a puncture wound on the right index finger is coded as S61.321A (Puncture wound of right index finger, initial encounter). If the wound involves multiple structures, assign separate codes for each affected area, ensuring that each code reflects the most precise description available.

In addition to the external cause and injury codes, document any associated conditions such as infection (e.g., T81.4XXA for infection following a procedure) or allergic reaction (e.g., T78.40XA for unspecified allergic reaction, initial encounter). These secondary codes provide a comprehensive clinical picture and support appropriate reimbursement and epidemiological tracking.

Identifying Causative Agent

Accurate identification of the organism responsible for a rodent‑related bite is essential for proper ICD‑10 classification and reimbursement. The primary diagnosis code reflects the wound type, while the causative agent determines the secondary infection code and any applicable external cause modifiers.

Common pathogens associated with mouse bites include:

Streptobacillus moniliformis (rat‑bite fever)
Hantavirus species
Leptospira spp.
Staphylococcus aureus
Pasteurella spp.

The identification process follows a defined sequence:

Obtain a detailed exposure history, noting the animal species, bite location, and time elapsed.
Perform a physical examination to assess wound depth, signs of inflammation, and systemic symptoms.
Collect specimens for microbiological analysis: swab the wound edges, aspirate any purulent material, and order blood cultures if fever is present.
Request targeted serologic or PCR tests for viral agents such as hantavirus when epidemiologic risk is high.
Review laboratory results and correlate with clinical findings to confirm the infectious agent.

Once the pathogen is confirmed, apply the appropriate ICD‑10 code for the infection (e.g., A25.0 for rat‑bite fever) alongside the injury code (e.g., W54.0XXA for accidental mouse bite). If the bite is work‑related, add the external cause code for occupational exposure. Documentation must include the identified organism, diagnostic method, and any complications to ensure compliance with coding guidelines.

Recording Complications

Accurate capture of complications arising from mouse bites is essential for reliable reimbursement, epidemiologic tracking, and clinical quality assurance. The primary injury is coded with an external cause code for rodent bite (e.g., X10.0), while each subsequent complication receives a separate diagnosis code that reflects the clinical manifestation.

When a bite leads to infection, the coder must assign the appropriate bacterial infection code (e.g., L03.113 for cellulitis of the left arm) and link it to the external cause using the seventh character “A” for initial encounter. If the infection progresses to sepsis, the coder adds a sepsis code (e.g., A41.9) and retains the original bite code as the underlying cause. For allergic reactions, use the appropriate allergy code (e.g., T78.4) and note the association with the bite in the clinical documentation.

Key steps for recording complications:

Verify the bite’s anatomical site and assign the corresponding external cause code.
Identify each clinical complication (infection, abscess, septicemia, allergic response, nerve injury, etc.).
Select the most specific ICD‑10 diagnosis code for each complication.
Apply the correct encounter modifier (A = initial, D = subsequent, S = sequela).
Document the temporal relationship between the bite and the complication in the medical record.

Documentation must include the date of the bite, the onset of the complication, objective findings (e.g., culture results, imaging), and the treatment administered. Explicit statements linking the complication to the mouse bite eliminate ambiguity and support accurate coding.

Adhering to these practices ensures that every complication is reflected in the claim, facilitates data analysis of rodent‑related injuries, and upholds coding integrity.

Modifier Usage in Mouse Bite Coding

Anatomical Site Modifiers

Anatomical site modifiers refine the ICD‑10 classification of mouse‑bite injuries by specifying the exact body region involved, thereby supporting accurate reimbursement and epidemiological reporting. The primary external cause code (W55.0) identifies a bite or strike by a mouse; a seventh character (A = initial, D = subsequent, S = sequela) indicates encounter type. To pinpoint location, coders append a site‑specific code from the injury chapter (S00‑S99) or use the extended external cause series when available. Common modifiers include:

W55.01 – bite to the hand or finger
W55.02 – bite to the foot or toe
W55.03 – bite to the head, face, or neck
W55.04 – bite to the trunk or back
W55.05 – bite to the upper arm or elbow
W55.06 – bite to the lower leg or knee

When multiple regions are affected, each site code is listed separately in addition to the primary cause code. Combining the cause code, encounter character, and appropriate anatomical modifiers yields a complete, searchable record of the mouse‑bite injury.

Laterality Modifiers

Laterality modifiers identify the anatomical side affected by a mouse‑bite injury and are essential for accurate ICD‑10 reporting. The external‑cause code for a mouse bite (W55.0) does not itself contain a laterality element; the side must be captured in the accompanying injury code. When a bite results in an open wound, fracture, or soft‑tissue injury, the coder selects the appropriate injury category (e.g., S51.0 for an open wound of the forearm) and adds the laterality character in the fifth position: “A” for the initial encounter, “D” for a subsequent encounter, and “S” for sequelae, preceded by the side indicator—“L” for left, “R” for right, or “B” for bilateral. For example, a left‑hand open wound caused by a mouse bite would be coded as W55.0 + S61.0XL A, where “L” denotes left and “A” denotes the initial encounter.

Key points for applying laterality modifiers:

Determine the precise anatomic location of the injury.
Choose the injury code that matches the clinical description.
Insert the side indicator (L, R, B) as the fifth character before the encounter/sequence character.
Verify consistency between the external‑cause code and the injury code; both must reference the same episode of care.
Review documentation for any bilateral involvement; if both sides are affected, use the bilateral modifier “B” and ensure separate injury codes are not required unless the injuries differ in type or severity.

Common errors include omitting the laterality character, using an unspecified side when the record specifies left or right, and confusing the encounter character with the laterality indicator. Accurate use of these modifiers improves data quality for epidemiological tracking, reimbursement, and clinical research related to animal‑bite injuries.

Other Relevant Modifiers

When coding a mouse bite injury, additional modifiers clarify circumstances that affect reimbursement, reporting, and clinical interpretation. These modifiers are appended to the primary diagnosis code to capture external cause, encounter type, and any influencing factors.

External cause codes (V, Y, W, X, or Z series).
V01–V99 identify the agent (mouse) and the mechanism (bite). Y92 specifies the location where the bite occurred, such as “home” or “workplace.” W55–W64 denote “bitten or struck by other animals,” with the appropriate subcategory for rodents.
Encounter type modifiers (Z00–Z99).
Z04.1 indicates a “encounter for examination and observation following a bite.” Z51.5 records “encounter for antivenom administration” when relevant.
Place of occurrence modifiers (Y92).
Y92.0 denotes “home,” Y92.1 “school,” and Y92.6 “other specified place,” providing epidemiological detail.
Activity codes (Y93).
Y93.5 captures “playing,” Y93.6 “working,” and Y93.7 “other activity,” linking the bite to a specific action.
Severity or complication modifiers (R series).
R45.0 records “agitation” or “anxiety” that may accompany the injury; R68.2 denotes “unspecified complications of surgical or medical care,” useful when infection develops.
Social and environmental factors (Z55–Z65).
Z59.0 identifies “homelessness,” which can influence treatment planning; Z60.0 records “social environment issues” such as “lack of adequate housing.”

Applying the correct combination of these modifiers ensures that the mouse bite episode is fully described in the health record, supports accurate statistical analysis, and aligns with payer requirements. Each modifier must be validated against the latest coding guidelines to avoid misclassification.