Pulmonary Mycobacterium kiyosense Infection Presenting with Elevated Serum Carbohydrate Antigen 19-9: A Case Report

The human lungs are constantly exposed to a vast array of microscopic organisms from the air we breathe. Most of these pass through without causing harm, but a specific group of bacteria known as nontuberculous mycobacteria can sometimes take hold, particularly in people with existing lung conditions. These bacteria are distinct from the one that causes tuberculosis, yet they can still lead to chronic infections that are difficult to diagnose and treat. Because these organisms are so diverse, with over 190 known species, doctors often struggle to identify exactly which one is causing an illness. This identification is crucial because different species require different treatments, and some are far more aggressive than others. Complicating matters further, certain infections can trigger the body to produce high levels of specific proteins in the blood that are typically used to screen for cancer. When a patient shows these elevated markers, the immediate assumption is often a recurrence of a tumor, leading to extensive and sometimes unnecessary searches for malignancy. Understanding how a rare bacterial infection can mimic cancer signals is essential for ensuring patients receive the correct care without delay.

In a recent report from Niigata University, a team of researchers described the case of a 72-year-old woman who arrived at their hospital with a confusing medical puzzle. She had a history of gastric cancer surgery seven years prior and had been managing chronic bronchitis for three years. Recently, her condition had worsened, showing up on scans as a growing patch of inflammation in the lower part of her right lung. More alarmingly, blood tests revealed a sharp rise in a substance called carbohydrate antigen 19-9, or CA19-9. This protein is widely recognized as a warning sign for cancers of the stomach, pancreas, and intestines. Given her history, the medical team immediately suspected that her cancer might have returned. However, a thorough investigation using endoscopy and advanced imaging scans found no evidence of a tumor anywhere in her body, except for the persistent shadow in her lung. The only clue pointing toward a different cause was a slow-growing bacterium found in her lung fluid, which previous tests had failed to identify correctly.

The patient was referred to the university hospital for a deeper look, where the medical team took a different approach to solving the mystery. They collected samples of her sputum and grew the bacteria in a lab. Initial tests suggested the organism was a common, usually harmless type known as Mycobacterium gordonae. However, the researchers suspected this identification was incorrect because the patient's symptoms were getting worse, not better, and the bacteria were growing in a way that did not fit the profile of the harmless species. To get a definitive answer, the team turned to whole-genome sequencing, a technique that reads the complete genetic code of the bacteria. By comparing the patient's bacterial DNA against a massive database of known species, they discovered a precise match with a very rare organism called Mycobacterium kiyosense. This species had only recently been confirmed as a distinct type of bacteria, and very little was known about how to treat infections caused by it. The genetic analysis showed that the bacteria in the patient's lung were 99.83 percent identical to the reference strain of M. kiyosense, confirming that this rare microbe was indeed the culprit behind her illness.

Once the true identity of the bacteria was established, the team could design a targeted treatment plan. They began a combination therapy using three antibiotics: clarithromycin, ethambutol, and rifampicin. This regimen is often used for similar types of slow-growing bacterial infections. The results were clear and encouraging. Over the course of the treatment, the patient's cough and weight loss resolved, and the cloudy patch on her lung scans began to shrink. Remarkably, the levels of the CA19-9 protein in her blood, which had been the source of so much anxiety, dropped in direct parallel with her physical recovery. After eight months of treatment, the bacteria were no longer detectable in her sputum. She continued the medication for a total of two years to ensure the infection was fully cleared. When she was followed up a year after finishing her treatment, there was no sign of the infection returning, and her CA19-9 levels remained stable.

This case offers a significant lesson for the medical community. It demonstrates that Mycobacterium kiyosense, while rare, can cause serious lung disease that mimics cancer, and that standard testing methods can sometimes misidentify it as a harmless bacterium. The use of advanced genetic sequencing proved essential in making the correct diagnosis. Furthermore, the report highlights that high levels of CA19-9 are not exclusive to cancer; they can also be a sign of severe lung inflammation caused by infection. In this instance, the protein acted as a reliable indicator of the infection's severity and its response to treatment. The successful use of a standard antibiotic combination suggests that this rare infection can be cured with the right drugs, provided doctors can identify the specific species causing the problem. For patients with unexplained lung shadows and elevated tumor markers, this story suggests that looking for a rare bacterial infection might be the key to finding the right path to recovery.