S. suis is Gram-positive bacterium with a strong pathogenic effect that commonly persists in the mucous membrane, nasal cavity, and gastrointestinal tract of pigs. It may infect humans through damaged wounds, mucous membranes, the digestive tract, and other pathways[4]. Patients often engage in close contact with pigs or eat infected or raw pork. However, according to the literature, less than half of S. suis patients in Asian countries reported exposure to pigs or pork[8]. In our case, the patient had not engaged in close contact with pigs and denied eating raw or infected pork. Moreover, other people who ate with the patient had no similar symptoms. However, S. suis is not an opportunistic pathogen despite its strong pathogenicity. From this, we hypothesize that the patient had an inconspicuous wound that may have been the pathway for infection before the onset of the disease.
S. suis infection can cause purulent meningitis, sepsis, osteoarthritis, endocarditis, endophthalmitis, endophthalmitis, and other diseases. Meningitis is the most common clinical manifestation[2]. Common pathogenic microorganisms may lead to local tissue injury, whereas S. suis infection causes extensive tissue disruption in multiple tissues or organs. The incidence of sequelae, particularly hearing loss, in patients with S. suis infection is significantly greater than that in patients with other bacterial infections. One study from Vietnam showed that 66.4% of patients with S. suis meningitis developed hearing loss during hospital admission[9]. This finding was also confirmed in a meta-analysis based on 24 studies with a total of 913 patients, in which the rate of mortality due to S. suis meningitis was low, but hearing loss was common[2]. An animal study showed that due to the dissemination of the exotoxin, S. suis in the blood is believed to enter the perilymph via the cochlear aqueduct, causing suppurative inflammation[10]. This eventually lead to hearing loss in the cochlea. However, this theory has not been confirmed in humans to date. A previous study [11] stated that some genes within S. suis promote adhesion to the ocular epithelium. This may explain why S. suis infections are more likely to cause endophthalmitis than other bacterial infections, such as pneumococcal, Listeria, tuberculosis, and cryptococcal infections. It was thereby inferred that S. suis infection should be highly suspected if the patient suffers from meningitis with impaired vision and/or hearing.
The diagnosis of S. suis meningitis is based on clinical signs and typical CSF findings. Isolation of S. suis from a sterile site (such as the blood or CSF) or the lesion site results in a definitive diagnosis[4]. However, in a previous report on S. suis meningitis, 66.7% of patients had S. suis isolated from the CSF, 50% had S. suis isolated from the blood, and 25% had S. suis isolated from both fluids[12]. It is more difficult to detect S. suis in patients with antibiotic use before treatment. In this case, the symptoms of visual impairment and hearing loss were prominent in the early stage of the disease, but the meningitis symptoms were easy to ignore. Lumbar puncture was performed after realizing that S. suis infection was a possibility. However, the blood culture, CSF culture, and properties of the CSF of the patient were not sufficient to support a diagnosis of S. suis infection. S. suis meningitis was eventually definitively diagnosed in the patient based on the results of mNGS. mNGS is a high-throughput approach that can directly detect the genetic material of pathogens in clinical samples, yielding almost complete coverage of the pathogens causing infections, including those that could not be cultured[7]. This technique can support the early and rapid diagnosis of infectious diseases even if use antibiotics before testing.There is no doubt that the mNGS approach was the key to rapidly detect the cause of infection, additionally it could have revealed potential mutation to antibitic target genes, associated with resistance, with no need of isolation and culture of the organism for conventional susceptibility tests. Although there have been few reports of infections diagnosed based on mNGS, the future of mNGS is promising. Using mNGS can facilitate the achievement of an early and accurate diagnosis and avoid misdiagnoses and missed diagnoses, thereby improving outcomes.
The principles for treating S. suis meningitis include mainly etiological therapy, similar to those for treating most other types of bacterial meningitis[9]. The early use of sensitive antibiotics is vital. Ceftriaxone is the antibiotic that is most commonly used clinically through the intravenous administration of 2 g ceftriaxone every 12 h for at least 10 days. If the treatment effect is unsatisfactory after 1 week, the antibiotics should be adjusted in a timely manner. The literature has pointed out that a long course of treatment should be required for recrudescent S. suis meningitis[13]. In addition, a previous report showed that approximately 66.4% of patients with S. suis meningitis suffered from hearing loss during hospitalization; the hearing loss remained in 44.7% of patients after discharge, and the early use of hormones was sufficient to reduce hearing loss[14]. In our case, the patient spent only three days from onset to vision and hearing loss. With effective anti-infection treatment and appropriate hormone treatment, his vision and hearing loss were slightly improved. However, two months after discharge, the patient still experienced severe hearing loss. This result is consistent with those in previous studies.
S. suis infection is uncommon in clinical settings, and its diversified symptoms and low specificity make clinical diagnosis difficult. Patients may present with meningitis, vision loss, and/or hearing loss. S. suis infection should be highly suspected regardless of the patient's occupational background. As a convenient and promising diagnostic method, mNGS has excellent diagnostic value to determine which was the etiological agent for infectious diseases. It is hoped that this case report can improve clinicians' knowledge of S. suis infection and reduce missed diagnoses and misdiagnoses.