Arcanobacterium haemolyticum (formerly Corynebacteria haemolyticum) is a gram-positive, facultatively anaerobic, non-spore-forming, non-motile, catalase-negative bacteria (1). It is a rare cause of pharyngitis infections in teenagers and young adults. Although complications such as peritonsillar abscess (2), Lemierre’s syndrome, septicemia (3), orbital necrotizing fasciitis, or osteomyelitis (4) are infrequently reported in the literature in the pediatric population, they might have far-reaching consequences. Herby, we present a case of recurrent pharyngitis complicated by peritonsillar abscesses.

A 16-year-old male patient presented to the emergency department with trismus, sore throat, fever of 39.50, and elevated CRP (187.6 mg/l). The symptoms (fever, sore throat, and swallowing difficulties) started a month before. The patient reported that his pediatrician initially treated him with cefuroxime and the improvement was observed. However, the symptoms returned four days later, after completing the antibiotic therapy. Treatment with amoxicillin and clavulanic acid was initiated, and clindamycin was added after six days due to no improvement. After two days of the modified therapy, the patient experienced abdominal pain, vomiting, and a patchy rash without itching located on the arms, legs, and back (fig. 1a-c). The decision was made to discontinue the antibiotic therapy.

During the physical examination at the Emergency Department, a peritonsillar abscess on the left side was diagnosed and drained with a purulent discharge. Specimens for culture were obtained and the patient was admitted to the hospital. An empiric intravenous antibiotic therapy was initiated with metronidazole, cefuroxime, and fluconazole. On admission, CRP was elevated (16.8 mg/dl), and WBC was 18.39 (norm 4.0-10.0) [10³/mL]). After nephrological and allergology consultations, IgA-related vasculitis was ruled out. The culture was positive for Arcanobacterium haemolyticum which was sensitive to benzylpenicillin and resistant to ciprofloxacin, moxifloxacin, and clindamycin. Crystalline penicillin was administered for 14 days, and the improvement of the patient’s condition was observed. The patient was discharged from the hospital and a four-day treatment with phenoxypenicillin was prescribed.

Three weeks later, the patient presented with a recurrence of symptoms, including a sore throat, fever, enlargement of the left cervical and bilateral submandibular lymph nodes, and trismus with an enlarged left tonsil. The laboratory examination revealed an elevated WBC to 13 (10³/mL) with a neutrophilic smear and a 15 mg/dL CRP level. The strep test was negative. A suspicion of a recurrent left peri-tonsillar abscess was made. An ultrasound examination showed an enlarged left tonsil (34 x 36 x 46 mm) with congestion and a hypoechoic heterogeneous fluid space (29 x 22 x 32 mm). A puncture of the peritonsillar area was performed, with no purulent discharge observed. Intravenous treatment with benzylpenicillin was initiated. After two days, the abscess was drained, and piperacillin with tazobactam was added. An improvement in the patient’s condition was noticed. Benzylpenicillin was discontinued after five days.

The culture revealed the presence of Arcanobacterium haemolyticum, among other oral flora. The antibiotic therapy was continued according to the antibiogram, with the addition of amoxicillin with clavulanic acid. The treatment with piperacillin with tazobactam was continued for up to 10 days, with an observed improvement, while amoxicillin with clavulanic acid was continued for up to 7 days. On the 10^th day of the antibiotic therapy with piperacillin and tazobactam and the 4^th day of the amoxicillin and clavulanic acid treatment, the patient exhibited signs of recurrent infection, including tonsillar enlargement, sore throat with redness, lymphadenopathy, and swallowing difficulty. Laboratory tests revealed leukocytosis with increased inflammatory parameters. PCR identified adenovirus infection. Symptoms resolved after four days. Given the recurrent and severe nature of the infection, immunodeficiency was suspected. Immunoglobulin, BNK T-lymphocyte subpopulation, IgG subclass, and neutrophil oxidative burst assay were ordered. All results were within the normal limits. The patient was qualified for tonsillectomy and phenoxy penicillin was prescribed before the surgery. For days after, the tonsillectomy was performed without complications. Subsequently, the patient was prescribed amoxicillin with clavulanic acid for a 4-day course. The exams revealed that immunoglobulin A, G, and M levels were within the normal limits, while IgE levels were elevated. Molecular test Alex showed sensitization to inhalant allergens. Given the possibility of a correlation with the antibiotics the patient was taking, allergy testing was ordered. With the overall clinical picture and laboratory test results, a correlation between the skin lesions and clindamycin intake was still possible. However, an infection was the most likely cause.

Arcanobacterium haemolyticum was first described and isolated in 1946 by MacLean from American soldiers and natives in the Pacific Islands during nasopharyngeal and skin infections (5). It is a gram-positive facultatively anaerobic, non-spore-forming, non-motile, and catalase-negative bacteria (6). Growth is sparse on ordinary media but is enhanced by blood or serum (2). Although A. haemolyticum is usually not part of the normal oral flora, some studies have described the bacterium as a commensal of the nasopharynx and skin. However, the bacterium can be isolated from healthy and asymptomatic patients (6-8). Arcanobacterium haemolyticum is responsible for 0.5% to 2.5% of bacterial pharyngitis cases, especially among pediatric and adolescent patients (9). According to Mackenzie et al. study, the highest incidence of A. haemolyticum-related pharyngitis was observed in the pediatric and adolescent population, with the maximum incidence (2.5%) in the 15-18-year-old age group (10). In the elderly population, A. haemolyticum is more often associated with soft tissue infection in immunocompromised individuals or those with diabetic mellitus (11). Tan et al. reported that 94% were cultured from patients with soft-tissue infections, and 72% of all patients had concomitant diabetes mellitus (12). It seems that the transmission of the bacterium is through droplet exposure. A. haemolyticum occurs often in polymicrobial infections with pathogens such as streptococci group A or B (5).

Arcanobacterium haemolyticum infection is mostly associated with throat pain (13), followed by cervical lymphadenopathy, tonsillar exudates (14), and rash (6). Pharyngitis often precedes the rash by 1-4 days (16). The characteristics of this rash vary between erythematous, urticarial, macular, maculopapular, or scarlatiniform (1, 11). Frequently described as „goosebumps” or „sandpaper-like”. The rash first appears on the distal extremities and then spreads over 2-3 days covering the neck, chest, and back, sparing the soles, palms, and face (11). The cause of the rash in A. haemolyticum infections is not understood. Rarely are complications reported in clinical case series or case reports in the pediatric young adult population, such as peritonsillar abscess (2), Lemierre’s syndrome, septicemia (3), orbital necrotizing fasciitis, and osteomyelitis (4).

Arcanobacterium haemolyticum is a β-hemolytic pathogen that can have poor hemolytic activity in sheep blood agar and requires the use of human or rabbit blood agar media in 5% CO₂ (1). However, it has been shown, that trypticase soy agar with 5% horse blood in 5% CO₂ is the most optimal media (17). Most clinical laboratories use sheep blood agar, where A. haemolyticum colonies average 0.1 mm in diameter at 24 h with hardly noticeable hemolysis and only 0.5 mm at 48 h with a narrow rim of hemolysis (18). Consequently, plates are often discarded and colonies may be easily missed (4). The reverse CAMP test and rapid alpha-mannosidase test can also aid in diagnosis, both of which are positive for this bacterium. These tests combined with the Gram stain and catalase test are highly beneficial for the rapid identification of A. haemolyticum (19). Moreover, with the appropriate laboratory equipment matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) allows for the swift and effective identification of bacteria at the species level in less than 10 minutes (20). There are 2 types of colonies: the rough-type, which is more frequently associated with respiratory system infections, and the smooth-type which is linked to infections of the skin and soft tissues (21).

Treatment of A. haemolyticum infections should be considered on a case-by-case basis based on the clinical condition, site of infection, and microbiological tests, including the antimicrobial susceptibility profile of individual strains (22). According to in vitro studies, A. haemolyticum is particularly sensitive to penicillin, as well as to macrolides, cephalosporins, clindamycin, ciprofloxacin, and vancomycin but resistant to trimethoprim-sulfamethoxazole (18). -According to Sayad’s review article, penicillin was the most common antibiotic of choice in monotherapy, proving effective in most cases. The second most common and effective drug of choice was erythromycin (9). However, many clinical and microbiological failures have been reported after penicillin therapy (6, 16) despite full sensitivity to penicillin in in vitro tests (23). There are fewer failures after erythromycin treatment than after penicillin treatment possibly due to intracellular invasion of A. haemolyticum (24).

In most cases, treatment for 7-10 days was sufficient (6). Sayad et al. proposed an algorithm for the management of pharyngitis in children and young adults if symptoms suggest Group A Streptococcus pyogenes (GAS) or A. haemolyticum infection and the rapid GAS test is negative, empirical treatment with amoxicillin should be initiated. If the culture confirms A. haemolyticum infection, consider narrowing the treatment to oral penicillin or erythromycin (if penicillin allergy is present), to complete a 10-day course (9).

We have presented a case of a patient with recurrent pharyngitis complicated by peritonsillar abscesses, which illustrates the variety of symptoms caused by A. haemolyticum infection. An antibiogram should be used to guide antibiotic therapy. Penicillins are typically the first-line treatment, but they may not always be effective even if A. haemolyticum shows sensitivity in vitro. In such cases, treatment with macrolides should be considered. When diagnosing A. haemolyticum infection, it’s important to consider if the sheep blood agar culture and the Group A Streptococcus pyogenes rapid test results are negative. In cases of recurrent infections of this nature, culture on suitable media for the diagnosis of A. haemolyticum should be considered. Deciding to stop antibiotic therapy based on a negative result from a rapid GAS test or culture for patients infected with A. haemolyticum would mean leaving them without appropriate treatment.