Haemophilus influenzae Infection

Etiology

Haemophilus influenzae is characterized as a small (0.3 micrometer to 1 micrometer), facultatively anaerobic, pleomorphic, and capnophilic gram-negative coccobacillus of the family Pasteurellaceae. It requires a medium containing two erythrocyte factors, i.e., factor X (hematin) and factor V (phos­phopyridine nucleotide), which are released after lysis of red blood cells, thereby favoring the growth of this organism on chocolate agar. Some of H. influenzae have a polysaccharide capsule depending upon which they are classified into 6 distinct groups designated as serotypes a, b, c, d, e, and f. These serotypes are detected based on the agglutination reaction to serum specific to them. H. influenzae type b (Hib) is noteworthy because of its polyribosyl ribitol phosphate (PRP) capsule, which accounts for 95% of invasive disease in children and >50% of invasive disease in adults.

The other capsular types are less common causes of infection. Most isolates are non-typeable (NTHi), which means that they lack a polysaccharide capsule, and therefore agglutination with antiserum does not occur. More definitively, tests like polymerase chain reaction (PCR) and matrix-associated laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) are done to determine capsule types. They can also help to differentiate true NTHi from Haemophilus haemolyticus and from those H. influenzae strains that possess a complete or partial capsule focus. Human beings are the sole natural host for this bacteria, and some NTHi strains are considered to be part of the normal flora of the upper and lower respiratory tract, the conjunctivae, and the genital tract.[2][4][5]

Epidemiology

After the introduction of the Hib conjugate vaccine in the United States (US) in 1987 for children, and three years later in 1990 for infants, the annual incidence of invasive Hib disease in the pediatric population of age less than 5 years old has markedly decreased. The majority of invasive H. influenzae disease is now caused by non-typeable H. influenzae (NTHi) in all age groups in the US. Interestingly, the incidence and prevalence of H.influenzae are higher among Alaska native children (incidence rate: 5.4 per 100,000) as compared to other races despite Hib immunization. Due to the inclusion of the Hib conjugate vaccine in routine immunization schedule, it primarily occurs in nonimmunized children and in newborns who have not yet completed their vaccination series.

In 2017, the incidence rate was 0.18 for Hib, 1.7 for non-b H. influenzae, and 1.7 for non-typeable H. influenzae per 100,000 population of children younger than 5 years of age. Among adults 65 years of age and older, the incidence of invasive non-typeable H. influenzae was 6.2 per 100,000. The infectious agent in acute otitis media and sinusitis in children is NTHi in around 40% of cases, and it is also the causative factor in the recurrence of otitis media in that age group. Healthy children colonize H.influenzae in the nasopharynx and throat in developing countries where vaccination is not readily available. The carriage rate is approximately 20% in one-year-olds and increases to more than 50% in children 5 years of age.[6]

Pathophysiology

There are several mechanisms through which H. influenzae adhere to the host cells and exhibit virulence. Encapsulated H. influenzae uses proteins such as protein H and Haemophilus surface fibrils (Hsf), but these features are not present in NTHi subtypes. The capsule confers antiphagocytic properties to the bacteria, and the lack of anti-capsular antibody causes increased bacterial proliferation. NTHi has been widely causing infections after the advent of specific vaccines for capsular H.influenzae.

Therefore, there is ongoing research to investigate the mechanisms of host-pathogen interaction in cases of NTHi. NTHi exerts its final effect through an array of host proteins, both via direct attachment to the surface epithelial cells and accessing the underlying extracellular matrix layer, and also through invading certain serum factors. These binding mechanisms allow NTHi to maintain strong adhesion, mediate colonization, and favors easy entry into the host cells. Through similar interactions, they evade and modify the host complement and immune system responses, and also help in developing a biofilm-like colony.

About half of the adult population aspirates some amount of oropharyngeal secretions while lying down in sleep position, but effective mechanisms like mucociliary clearance, coughing, and humoral and cellular immune processes are in place which protects the lower airways and prevents the occurrence of recurrent infections. That is the reason why silent aspiration is a concern with advancing age and is a significant factor in cases of community-acquired pneumonia (CAP).[7][8]

History and Physical

The exact incubation period of  H.influenzae is still unknown, and it is postulated that it may take as little as few days to cause clinical symptoms. People at increased risk are infants, children younger than 5 years of age, and adults who are 65 or older. It also has a racial distribution in American Indians and Alaska natives, and people with certain medical conditions such as sickle cell disease, human immunodeficiency virus (HIV) infection, asplenia, antibody and complement deficiency states, patients with cancer receiving chemotherapy, radiotherapy, and in post bone marrow transplant states.

There are two types of infections caused by H. influenzae, invasive and non-invasive infections. The invasive Hib infection occurs commonly in children under 5 years of age and mainly includes pneumonia and meningitis. Less commonly, endophthalmitis, urinary tract infections, abscesses, osteomyelitis, and endocarditis can also be the presenting feature of invasive Hib diseases. NTHi is responsible for invasive infections like bacteremia without identifiable focus, bacteremic pneumonia, and meningitis. The majority of non-invasive infections of mucosal surfaces are caused by NTHi in all age groups, such as otitis media in infants and children, sinusitis in children and adults, non-bacteremic pneumonia in the elderly, and acute exacerbation of chronic obstructive pulmonary disease (COPD) in adults and elderly.[9]

Clinical features depend on the organ infected by H. influenzae, and history and physical examination are geared towards positive and negative findings in the system involved. The most common presentation is pneumonia, which presents as high-grade fever, chills, productive purulent cough, shortness of breath, chest pain, lethargy, and generalized body aches. It is often clinically indistinguishable from other causes of bacterial pneumonia and is a frequent underlying pathogen in CAP patients who had a history of complete pneumococcal vaccination or have already diagnosed respiratory illnesses.

If the meninges are infected, then symptoms such as fever, headache, altered consciousness, photophobia, nausea, vomiting, and nuchal rigidity are commonly present. A history of a preceding upper respiratory tract infection is sometimes positive. Signs of meningismus are positive on neurologic examination. Infants usually show nonspecific symptoms such as irritability, drowsiness, poor feeding, and vomiting in addition to the classic symptoms. Epiglottitis presents in unimmunized children, and the affected child appears toxic and adopts a tripod position in an attempt to fully open the airway. On examination, there is fever (57%), stridor (81%), respiratory distress (78%), sore throat (65%), drooling (42%), painful swallowing (49%), tender neck (65%), altered voice (33%), cervical adenopathy (39%), and sometimes cough.[10]

Occult bloodstream infection can occur with or without any specific organ system involvement, and symptoms include fever (> 39-degree Celcius), chills, excessive fatigue, abdominal pain, nausea, vomiting, diarrhea, difficulty breathing, and confusion. NTHi strains that colonize the genital tract in pregnant women can cause antenatal problems like low birth weight, premature birth, premature rupture of membranes, and chorioamnionitis. Infection can also be transmitted vertically, and neonates present with invasive H. influenzae disease within 24 hours of birth. Maternal postpartum infections like endometritis, tubo-ovarian abscess, chronic salpingitis, and sepsis are also associated with NTHi.

NTHi is responsible largely for mucosal infections such as otitis media, conjunctivitis, and sinusitis. Otitis media presents in infants with fever, irritability, and ear tugging, and older patients report typical ear pain. Patients may or may not give a history of viral respiratory infection before the episode. NTHi is well known to cause conjunctivitis outbreaks in daycare centers and swimming pools, and the examination shows bilateral red eyes with purulent discharge. In patients with COPD, NTHi infection can cause exacerbation of baseline disease, and symptoms like a low-grade fever, increasing dyspnea on exertion, worsening cough, and sputum production can occur. Septic arthritis due to H. influenzae can occur rarely and usually affects large joints (knee, ankle, hip, and elbow). A single or multiple joints may be involved, and on examination of the joint, it appears red, warm, tender, and swollen with a decreased range of motion.

Evaluation

Laboratory tests are done on samples obtained from body fluids. The initial test is Gram staining, which reveals pleomorphic gram-negative coccobacilli. Cultures of blood and body fluids confirm the presence of the organism. Its growth is potentiated on chocolate agar and BVCCA, which contains antibiotics (bacitracin, vancomycin, and clindamycin) in addition to chocolate agar constituents. It also determines the presence of NTHi in nasopharyngeal swabs. The sensitivity of BVCCA to grow NTHi from true positive swabs is approximately 10% higher than that for chocolate agar. Agglutination with antiserum or capsular typing with polymerase chain reaction (PCR) is often done for serotyping. For rapid diagnosis, methods like immunoelectrophoresis, latex particle agglutination, and enzyme-linked immunosorbent assay are employed for the identification of PRP polysaccharide, in addition to ordering cultures. False-positive results are seen in urine and serum but are less common in cerebrospinal fluid (CSF). In children infected by invasive NTHi, a workup for immunodeficiency is necessary to establish the precipitating factor for such infections.[4][11]

Investigational methods are based on the location of the infection. If meningitis is the presenting feature, then a lumbar puncture (LP) with CSF examination is done. Hib meningitis often shows a marked pleocytosis with neutrophil predominance. The glucose levels in the CSF are decreased, and protein levels are elevated. The capsular antigen is detected in 90% of patients, and 80% demonstrate a positive CSF gram stain. To ensure a proper diagnosis, the LP is done before the administration of antibiotics. Pneumonia is often diagnosed by ordering a chest x-ray initially, and then sputum gram-stain and cultures are done for definitive diagnosis. A real-time PCR is usually done for capsule serotyping, and it also precisely differentiates NTHi from other pathogens such as H. haemolyticus, which resembles H.influenzae but is not pathogenic.

In pneumonia patients, reverse transcription (RT) PCR of respiratory secretions yields a high sensitivity (75%), specificity (80%), positive predictive value (PPV) (45%), and negative predictive value (NPV) (94%) as compared to other methods. Multiplex PCR is an advanced technique that allows detecting multiple pathogens at the same time and also identifies pathogens even after the initiation of antibiotics resulting in shorter time to diagnosis. A line probe assay (LPA) based on the mechanism of multiplex PCR and reverse hybridization using specific sequence probes can detect bacteria causing meningitis like H. influenzae, S. pneumonia, and N. meningitis. LPA has a sensitivity and specificity of 88% and 96% for Hib infections. A better and cheaper alternative to PCR is the isothermal nucleic acid amplification test (isothermal NAT), as it does not require an expensive thermocycler apparatus. A promising kind is duplex recombinase polymerase amplification (RPA) because it has both sensitivity and specificity of 100% for diagnosing H. influenzae meningitis. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) can aid in quick species identification and has a high sensitivity (100%) and specificity (92%) for capsule identification.[12][13]

Imaging is helpful to get a better view of the extent of the disease process, e.g., a computed tomography (CT) head scan demonstrates complications of meningitis, such as subdural effusion and orbital cellulitis. A chest X-ray is often required in the evaluation of pneumonia and shows characteristic diffuse, patchy, or lobar infiltrates. In some pneumonia cases, invasive procedures, i.e., bronchoscopy and trans-tracheal aspiration, are performed to get samples and establish a diagnosis. It is more common to find pleural and pericardial effusion with H.influenzae than other bacterial infections. The workup of epiglottitis includes a lateral neck radiograph, and the hallmark feature is a thumb-print sign, which signifies a swollen epiglottis.[9]

Treatment / Management

Medical Treatment

The approach to treating H. influenzae infections mainly involves antibiotics and conservative measures. The initial antibiotic choice is a third-generation cephalosporin while waiting for the culture and sensitivity results. Antibiotic resistance is the issue of concern, and it is essential to monitor the response to treatment and alter the antibiotic accordingly. In a study performed in the UK on sputum samples of H. influenzae positive patients with COPD, 67% showed resistance to ampicillin (more than half were beta-lactamase positive), 46% were resistant to macrolides like erythromycin, and none of the specimens demonstrated resistance to fluoroquinolones. Similar results were obtained in a study carried out in China on the pediatric population, with almost 52% isolates resistant to ampicillin. Cefotaxime and ampicillin/sulbactam had the highest susceptibility rates making them the best choices for empiric therapy. The duration of antibiotic treatment depends upon the site of infection and the response to therapy.[14]

In cases of suspected Hib meningitis, antibiotic choices include ceftriaxone, ceftazidime, cefotaxime, ampicillin-sulbactam, fluoroquinolones, and azithromycin given via parenteral route for one week. Antibiotics typically avoided are ampicillin due to resistance caused by plasma-mediated beta-lactamase production, cefuroxime due to delayed sterilization, and chloramphenicol because of the exhausting monitoring of drug levels for side effects such as bone marrow toxicity and idiosyncratic aplastic anemia. Dexamethasone is an important adjunctive treatment as it reduces the cerebral edema associated with inflammation of the meninges and also reduces complications such as hearing loss and other neurological sequelae. For steroids to be beneficial, they must be administered prior to or along with starting parenteral antibiotics. There is an established benefit of a 4-day steroid course given at a dose 0f 0.6 mg/kg/day. Additionally, continuous supportive therapy, and proper management of complications like shock, syndrome of inappropriate antidiuretic hormone secretion (SIADH), seizures, and abscesses are carried out to decrease mortality. A repeat lumbar puncture is needed in complicated cases to check the sterility of CSF after treatment completion.[15][16](A1)

If children come in with respiratory distress from suspected epiglottitis, management includes starting face mask oxygen, nebulized epinephrine in 1:1000 ratio, repeated intravenous steroids, and, most importantly, ensuring patency of the airway by endotracheal tube or tracheostomy in emergency cases. The antibiotic of choice is ceftriaxone 2 grams intravenously (IV) once daily until clinical improvement, and then switched to oral antibiotics, for example, amoxicillin/clavulanate 625 mg three times a day. The antibiotics may be changed depending upon the culture and sensitivity results, and the total duration of treatment varies from 7-10 days. In cases of allergies to the above antibiotics, IV vancomycin plus IV ciprofloxacin followed later by an oral fluoroquinolone for 7-10 days is in practice. The recommended management of occult bacteremia is obtaining repeated cultures, chest radiographs, and CSF analysis to find out the focus of infection. A 5-14 day course of parenteral antibiotics is given subsequently to control infection, similar to the course followed in other gram-negative bacteremias.

Mucosal infections by NTHi are treated with oral antibiotics. The drug of choice is amoxicillin, given in a high dose, i.e., 80-90 mg/kg/day two times a day, and the second line is amoxicillin/clavulanate. If there is an allergy to penicillin, then erythromycin-sulfisoxazole or cefaclor is prescribed. The length of treatment depends upon the age of children and the severity of the disease process. Children less than 2 years with severe infection are treated for a total of 10 days, 2 to 5-year-olds with mild to moderate otitis media are treated for a week, and 5 to 7 days of therapy is enough for children older than 6 years with mild-moderate symptoms. Invasive NTHi disease is treated similarly as invasive Hib by administering parenteral antibiotics.

Surgical Treatment

In cases of complications such as subdural and pleural effusion or empyema, surgical drainage is required. Septic arthritis requires repeated joint fluid aspirations and arthroscopy with the placement of a surgical drain to relieve pressure and reduce the risk of avascular necrosis of the femoral head.

Differential Diagnosis

Meningitis and pneumonia caused by H. influenzae are indistinguishable from other bacterial causes of meningitis and pneumonia. An appropriate workup includes obtaining a sputum specimen and doing a gram-stain followed by a culture. Bronchitis (inflammation of airways) may have similar symptoms as bacterial pneumonia, but the presence of fever and productive cough in H. influenzae pneumonia differentiates between the two. H.influenzae epiglottitis can be differentiated from parainfluenza virus infection (croup) based on the presence of the 'steeple sign' on posteroanterior neck x-ray in the latter. The age group most affected by the parainfluenza virus range from 6 months to 3 years, and classic features are barking seal-like cough and inspiratory stridor.

Prognosis

Early diagnosis and treatment of meningitis ensure a good prognosis. Timely initiation of empiric antibiotics and blood culture specimens to help direct management is the best approach to control the disease. Prognosis also depends upon the age at presentation and capsular polysaccharide concentration in body fluids. Prognosis is relatively better for uncomplicated Hib pneumonia and nonencapsulated H. influenzae pneumonia as compared to the complicated or invasive Hib disease.

Complications

Meningitis is the most serious of all infections and can cause hearing loss, seizures, empyema, cerebral abscess, brain edema, subdural effusion, syndrome of inappropriate antidiuretic hormone, hydrocephalus, and cerebral herniation. Meningitis can sometimes be fatal and can cause hemiparesis, coma, and even death within a few hours if left untreated. H.influenzae pneumonia can progress to sepsis or can give rise to localized empyema and pericarditis. Otitis media and sinusitis can be complicated with mastoiditis and parameningeal abscess, respectively. Epiglottitis is known to cause airway obstruction in untreated cases, warranting endotracheal intubation or tracheostomy to secure the airway and establish air entry.