Differential Diagnosis of Influenza
1. Common cold: Caused by various viruses, mostly sporadic, with a slower onset. Upper respiratory tract symptoms are prominent, while systemic symptoms are milder. The common cold, also known as a cold or acute rhinitis/upper respiratory catarrh, primarily manifests with catarrhal symptoms of the nasopharynx. In adults, it is most often caused by rhinoviruses, followed by parainfluenza viruses, respiratory syncytial virus, echoviruses, and coxsackieviruses. The onset is relatively acute, with initial symptoms of dryness, itchiness, or a burning sensation in the throat. Concurrently or within a few hours, sneezing, nasal congestion, and a clear runny nose may appear, which thickens after 2-3 days. It can be accompanied by a sore throat. Sometimes, hearing may be reduced due to Eustachian tube inflammation, and symptoms such as tearing, diminished taste, shortness of breath, hoarseness, and a slight cough may also occur. There is usually no fever or systemic symptoms, or only low-grade fever, malaise, mild chills, and headache. Examination reveals nasal mucosal congestion, edema, and secretions, with mild pharyngeal congestion. Without complications, recovery typically occurs in 5-7 days.
2. Leptospirosis (influenza-typhoid type): More common in summer and autumn, with a history of exposure to contaminated water. Clinically, besides fever, there is tenderness of the calf muscles, and swollen, tender inguinal lymph nodes. Laboratory diagnosis can be made through the microscopic agglutination test (MAT) to detect antibodies. An antibody titer rising above 1:400 is suggestive of the disease, and diagnosis can be confirmed by blood culture.
3. Streptococcal pharyngitis: This condition is characterized by redness and swelling of the pharynx, enlarged tonsils with purulent exudate, and swollen submandibular lymph nodes. There is an increase in white blood cells (WBC) and neutrophils, and blood culture is negative.
4. Other viral respiratory infections: Such as those caused by parainfluenza virus and adenovirus, must be differentiated through etiological examination.
5. Mycoplasmal pneumonia: Also requires differentiation through etiological examination.
Diagnostic Criteria
1. Suspected Case
① A recent significant increase in "upper respiratory infection" (URI) cases in the local or neighboring area.
② Acute onset of chills, high fever, headache, body aches, and fatigue (systemic toxic symptoms), accompanied by upper respiratory catarrhal symptoms.
③ Onset of nausea, vomiting, and diarrhea, which is acute with rapid recovery, accompanied by upper respiratory catarrhal symptoms.
④ Patients with "URI" during an influenza epidemic. Individuals meeting criteria ① and either ②, ③, or ④ are considered suspected cases.
2. Confirmed Case
① Influenza virus is isolated from the patient's nasopharyngeal secretions.
② A ≥4-fold increase in serum antibody titer in the convalescent phase compared to the acute phase.