Native Mitral Valve Endocarditis Due to Neisseria elongata Following an Untreated Dental Abscess

Dear Editor, we report a new case of Neisseria elongata endocarditis to educate clinicians about this rare but serious disease. We also made a review of previously published cases.

A 60-year-old man suffering from mitral valve prolapse presented with a 4-day history of fever and diarrhea. He had no other medical history and had never taken drugs. A month earlier he had endured transient dental pain that disappeared spontaneously, reason why he didn’t consult a dentist about it. On admission his temperature was 38.7 °C, blood pressure 118/59 mmHg, heart rate 91 bpm, and oxygen saturation was normal. Physical examination detected a previously known systolic murmur at the left sternal border, without any sign of heart failure. The patient had neither pain nor skin lesions suggestive of septic emboli, but examination of the oral cavity showed a dental abscess. Laboratory tests mainly revealed thrombopenia (57 x 10⁹/L), and elevation of both creatinine (111 µmol/L) and CRP (335 mg/L). Chest X-ray was normal, total body CT scan only showed splenomegaly, and brain MRI was unremarkable. Empirical therapy with amoxicillin/clavulanate (2 g/200 mg, 4-hourly) plus gentamicin (3 mg/kg daily) was started due to suspicion of endocarditis. The next day transesophageal echocardiography confirmed the diagnosis, showing a 10 mm vegetation on the mitral valve and a severe mitral regurgitation. On the third day, several blood cultures grew Gram-negative rod-shaped bacteria (incubation at 35 °C, Bact/ALERT VIRTUO^® BioMérieux, Marcy l’Etoile, France), which is why the antibiotics were changed to ceftriaxone (2 g intravenously daily). Under ceftriaxone the patient became apyretic and CRP decreased but given the mobile nature of the vegetation, the severe mitral regurgitation, and a worsening ejection fraction, he was transferred to a cardiothoracic center. There, he was operated on his dental abscess, and then underwent a surgical repair of his mitral valve on day thirteen of hospitalization.

Blood subcultures quickly became positive at 35±1 °C on usual culture media (Columbia agar + 5% sheep blood, Chocolate agar + PolyViteX, BioMérieux) except on Drigalski media, which was not suggestive of HACEK-group bacteria. Otherwise the colonies were moist, mucous and had a slight yellow pigmentation, which is usual for Neisseria spp. The colonies were non-motile, tested positive for oxidase and negative for catalase reaction. Biochemical identification was impossible because of poor discrimination between Kingella denitrificans, N. cinerea and N. elongata (VITEK^®, NH card, BioMérieux). The rapid API^® NH identification (BioMérieux) was also inconclusive, but MALDI-TOF mass spectrometry finally identified N. elongata (Bruker Daltonik GmbH, Bremen, Germany). Drug-susceptibility testing was performed with gradient diffusion method (minimum inhibitory concentration – MIC) test strip, Liofilchem, Roseto degli Abruzzi, Italy), and MICs were as follows: penicillin G 0.25 mg/L, amoxicillin 0.125 mg/L, ceftriaxone 0.032 mg/L, ciprofloxacin 0.003 mg/L. After the surgery the patient received an additional 4 weeks of ceftriaxone with a favorable outcome, and the six-month follow-up was uneventful.

Although belonging to the family of Neisseriaceae, N. elongata are not cocci but rod-shaped Gram-negative bacteria. They are saprophytes of the human oropharynx but are sometimes responsible for severe infections, such as osteomyelitis or endocarditis.

Three subspecies of N. elongata have been described: N. elongata nitroreducens, N. elongata glycolytica and N. elongata elongata. The reason why N. elongata nitroreducens are those isolated most often during invasive infections remains unclear. The combination of molecular approach and biochemical profile is usually required for the definite identification of subspecies as they are genetically very close. In this case the identification of the subspecies was inconclusive, but the negative test for catalase reaction was rather not suggestive of N. elongata glycolytica. Fortunately, we believe that the inability to determine precisely the subspecies involved had no consequences for the patient.

Although risk factors for N. elongata endocarditis are fairly common (recent dental care or infection, predisposing valvular disease), these infections seem rare since only 37 cases (including ours) have been published. The main features of these cases are summarized in Table 1 [1,2,3,4,5]. In our case, the presence of two predisposing factors prompted us to quickly seek endocarditis, which was confirmed by echocardiography. However, a review of the literature showed that N. elongata endocarditis can also occur in the absence of risk factors, which should encourage clinicians to be vigilant when dealing with N. elongata bacteremia [1,2,3,4,5].

We noted other peculiarities of N. elongata endocarditis. First of all, they seem to affect people under 40 in almost half of the cases, including in the absence of underlying disease. Secondly, right-sided N. elongata endocarditis remained exceptional until now, with only one case reported [5].

There are no specific recommendations for the treatment of N. elongata endocarditis. With regard to antibiotic regimen, European guidelines suggest treating endocarditis due to non-HACEK Gram-negative bacteria for at least 6 weeks with a combination of beta-lactams and aminoglycosides. Quinolones may also be added if necessary. In previously published cases we observed that many different antibiotic therapies had been prescribed, and with variable durations, ranging from 3 to 9 weeks. In our case the combination of amoxicillin/clavulanate and gentamicin was stopped on the third day, and changed to ceftriaxone monotherapy. Ceftriaxone was prescribed for only 6 weeks since all intraoperative samples remained negative. In combination with mitral valve repair this allowed a full recovery.

Regarding surgical treatment, it was required in more than 40% of published cases of N. elongata endocarditis, because these bacteria regularly induce large vegetations, valve destructions, or systemic embolizations [1,2,3,4]. Furthermore, European guidelines emphasize its importance in the management of endocarditis due to non-HACEK Gram-negative bacilli. Follow-up by echocardiogram should be careful during N. elongata endocarditis, especially since cardiac complications can occur quickly. In our case, despite a satisfactory clinical response to ceftriaxone, the patient had to undergo valve repair because of a severe mitral regurgitation responsible for a degradation of the ejection fraction.

To conclude, we think clinicians should be aware that N. elongata endocarditis may also affect young adults or healthy people, and that close echocardiographic follow-up is required because of potentially severe valvular or perivalvular damage.