Persistent Bartonella quintana in Captive Japanese Macaques Points to Long-Term Host Adaptation
A cluster of Bartonella quintana infections in Japanese macaques at a primate research institute in Japan is complicating the familiar story of trench fever. Investigators isolated the bacterium from 3 wild-caught macaques that had been brought into one institute in 2009, and whole-genome comparison tied those isolates more closely to a Japanese macaque–derived strain than to strains from rhesus macaques or humans. The molecular pattern matters because it argues against a simple spillover event from people to animals and instead supports a lineage that may be maintaining itself in macaques over time.
The infected animals had undergone periodic ivermectin treatment every 1 to 2 years to control blood-feeding arthropods. Even with that management, bacteremia was still detectable years after the macaques entered captivity. In practical terms, a parasite that resides inside erythrocytes can evade casual observation for long stretches, especially when the host shows no obvious illness and the infection unfolds without dramatic clinical signs.
A Bloodstream Pathogen That Can Hide in Plain Sight
is the causative agent of trench fever and is classically transmitted by the human body louse. In humans, the infection can produce relapsing fever, headache, dizziness, and persistent tibial pain, while immunocompromised patients may develop bacillary angiomatosis or infective endocarditis. Yet the organism often behaves differently when it settles into its long-term niche: it can persist asymptomatically in erythrocytes, circulating quietly while remaining difficult to detect unless blood is specifically screened.
That biology fits the macaque observations closely. Earlier work in homeless populations showed prolonged human bacteremia lasting weeks to many months, and older historical and experimental reports placed chronic carriage anywhere from several weeks to roughly 8 years. The Japanese macaque findings extend that same logic into a nonhuman primate host. The implication is straightforward: prolonged bacteremia is not an oddity of human infection alone, but a feature of the organism’s broader host ecology.
Persistent bacteremia was the central concern here, not acute syndrome.
Genomic Signatures Favor a Japanese Macaque Lineage
Whole-genome analysis sharpened the case. The three isolates recovered from the captive macaques had higher homology to a Japanese macaque–derived strain than to either rhesus macaque–derived or human-derived strains. In all three isolates, the bepA locus was absent. The investigators also classified the trwL locus as structural variant C, which matches known Japanese macaque–derived strains.
Those loci are biologically meaningful. bepA is known for anti-apoptotic effects in vascular endothelial cells, while trwL is implicated in erythrocyte adhesion and host specificity. A bacterium that lacks one endothelial-survival locus but retains a macaque-patterned erythrocyte adhesion structure suggests a lineage shaped by adaptation to a specific primate host rather than a recent acquisition from humans. The genomic profile therefore supports a resident animal reservoir, not an accidental import from a human carrier.
Why the Captive Setting Did Not Break Transmission
The animals were not left unmanaged. Wild-caught macaques received ivermectin at arrival, and both wild-caught and captive-bred groups underwent periodic treatment during routine examinations. At the two institutes, husbandry differed: one site kept macaques outdoors in social groups according to geographic origin, while the other housed animals indoors in individual cages and used medicated baths with organophosphorus ectoparasiticide at 2-year intervals. Even with those interventions, bacteremia persisted in animals that had been in captivity for 7 to 8 years.
That duration is the key clinical and epidemiologic clue. Long persistence under human management makes a one-off accidental introduction less persuasive, especially when paired with a strain that genetically resembles previously described Japanese macaque isolates. Captivity did not erase the organism’s foothold. Instead, the bacterium appears to have remained viable within the host population long after the animals were removed from their original environment.
The Bedside and Biosafety Meaning of an Apparently Silent Infection
For clinicians and laboratory personnel working with nonhuman primates, the findings raise a practical question: how often does an apparently healthy macaque carry a persistent erythrocyte-associated pathogen without any external sign of infection? The answer cannot be inferred from this study alone, but the data clearly show that absence of symptoms does not equal absence of bacteremia. Routine health assessment would miss a carrier state unless blood testing specifically targets Bartonella DNA or viable organisms.
The result also has a biosafety edge. Because B. quintana is a recognized human pathogen, the possibility of exposure in research settings cannot be ignored. The genomic data, however, cut in a specific direction: they do not support the idea that humans recently passed the bacterium back into these macaques. Instead, the strains look aligned with a macaque-associated lineage, which points toward long-standing circulation among primates rather than a fresh human-to-animal transfer.
What This Adds to Bartonella Ecology
Macaques have now joined the list of recognized hosts for B. quintana. Earlier reports had already identified infection in captive-bred cynomolgus macaques, rhesus macaques, and wild-caught Japanese macaques. This study adds a different layer by showing that infection can persist for years in macaques living under institute supervision, even when ectoparasite control is built into the care routine. The pathogen’s ability to remain inside erythrocytes likely explains how such long carriage can continue without overt disease.
For public health and comparative infectious disease work, the message is less about a single outbreak than about reservoir biology. A bacterium associated with trench fever in humans also appears capable of settling into a primate host and staying there for many years. The open question now is not whether macaques can carry it, but how widely such persistent carriage extends across managed primate populations and how often routine screening is missing it.