Can Larval Ticks Transmit Disease? Borrelia miyamotoi Study
Larval ticks can bite humans and animals.
Unlike Lyme disease, Borrelia miyamotoi can be passed from infected female ticks to their offspring.
This unique form of transmission may increase the importance of larval ticks in human disease.
Can larval ticks transmit disease? Yes—but the answer depends on the pathogen. Larval black-legged ticks are generally not considered an important source of Lyme disease because they typically hatch free of Borrelia burgdorferi, the bacterium that causes Lyme disease. In contrast, research shows that larval ticks may transmit Borrelia miyamotoi, an emerging tick-borne pathogen, because infected female ticks can pass the bacteria directly to their offspring before the larvae ever take their first blood meal.
This process, known as vertical transmission or transovarial transmission, allows infected female ticks to transmit Borrelia miyamotoi directly to their eggs. As a result, newly hatched larvae may already carry the bacteria, unlike larvae carrying Borrelia burgdorferi, which generally become infected only after feeding on an infected reservoir host such as a white-footed mouse.
Because larval ticks are extremely small, abundant, and active during late summer, this difference may have important implications for human exposure to Borrelia miyamotoi.
Why Borrelia miyamotoi Is Different
Unlike Borrelia burgdorferi, the bacterium that causes Lyme disease, female ticks infected with Borrelia miyamotoi can pass the bacteria directly to their offspring.
In the study Vertical transmission rates of Borrelia miyamotoi in Ixodes scapularis collected from white-tailed deer, Han and colleagues demonstrated how efficiently infected female black-legged ticks transmitted the organism to their eggs and larvae.
The investigators wrote, “Therefore, in addition to nymphs and adults, larvae can vector B. miyamotoi to wildlife and human hosts.”
This distinction helps explain an important difference between Lyme disease and Borrelia miyamotoi disease. Most Lyme disease cases occur in late spring and early summer, when infected nymphal black-legged ticks are most active. However, studies suggest that many cases of Borrelia miyamotoi disease occur later in the summer—from July through September—during the larval questing period.
Human infection with Borrelia miyamotoi has now been documented in the northeastern United States, highlighting the importance of recognizing this emerging tick-borne disease in patients with compatible symptoms and tick exposure.
How the Study Was Conducted
Han and colleagues designed the study to better understand how Borrelia miyamotoi persists in nature and whether infected larval ticks could contribute to human disease.
The authors sought “to understand better how B. miyamotoi is maintained in nature and to help predict disease risk from B. miyamotoi-infected larvae.”
Hunters brought freshly harvested white-tailed deer to check stations located in Maine, New Hampshire, Tennessee, and Wisconsin during the fall hunting seasons of 2015 and 2016.
Researchers collected engorged female Ixodes scapularis (black-legged ticks) from the deer and later tested the adult ticks, their eggs, and the newly hatched larvae for evidence of Borrelia miyamotoi infection.
Among the 1,263 adult ticks collected, 33 tested positive for Borrelia miyamotoi. Most infected ticks came from Wisconsin, with smaller numbers identified in Maine, New Hampshire, and Tennessee.
Although the investigators did not report the total number of deer examined, the study provided important evidence that infected female ticks can efficiently transmit Borrelia miyamotoi to the next generation.
Study Findings
The study demonstrated that Borrelia miyamotoi is transmitted from infected female ticks to their offspring with remarkable efficiency.
Among the 33 infected adult ticks, the investigators found:
- The overall infection rate among collected adult ticks was 2.6%, with site-specific rates ranging from 0% to 7%.
- Among the 11 infected female ticks that laid eggs, the rate of transmission from mother to eggs was 90.9%.
- Once the eggs were infected, approximately 84.8% of the larvae also carried Borrelia miyamotoi.
- The proportion of infected ticks was expected to increase further after larvae and nymphs obtained blood meals from infected wildlife reservoirs.
These findings demonstrate that many larval ticks can emerge already infected—without ever feeding on an infected animal.
The authors concluded that:
“While nymphs pose the greatest epidemiological risk for several Ixodes scapularis-borne pathogens, larvae may pose an important, and possibly the greatest risk for Borrelia miyamotoi disease due to their high abundance and tiny size.”
Clinical Implications
Most clinicians have traditionally considered larval black-legged ticks relatively harmless because they generally do not transmit Lyme disease. This study challenges that assumption for Borrelia miyamotoi.
Larval ticks are extremely small—often compared to grains of sand or poppy seeds—and may go unnoticed after biting humans. Because thousands of larvae may emerge in a relatively small area during late summer, even a modest infection rate could result in human exposure.
Unlike Lyme disease, where larvae typically hatch free of Borrelia burgdorferi, infected larval ticks carrying Borrelia miyamotoi are capable of transmitting infection during their very first blood meal.
This helps explain why clinicians should consider Borrelia miyamotoi disease even when patients report exposure only to tiny larval ticks or cannot recall a typical nymphal tick bite.
Patients with Borrelia miyamotoi disease often present with fever, chills, headache, fatigue, muscle aches, and recurrent febrile episodes. Unlike Lyme disease, an erythema migrans rash is uncommon.
Recognition of this distinction is becoming increasingly important as awareness of Borrelia miyamotoi disease continues to grow.
What Later Studies Have Shown
Subsequent investigations have strengthened the evidence that larval ticks contribute to the ecology of Borrelia miyamotoi.
Field studies have confirmed that infected larval ticks are present throughout endemic regions and that vertical transmission plays a critical role in maintaining the organism in nature. Reviews of Borrelia miyamotoi disease likewise recognize transovarial transmission as one of the defining biologic differences separating this relapsing fever spirochete from Borrelia burgdorferi.
Although nymphs and adult ticks remain important vectors, clinicians should recognize that larval ticks are not necessarily harmless when Borrelia miyamotoi is involved.
Clinical Perspective
Patients frequently ask whether tiny “seed ticks” or larval ticks can transmit disease. For Lyme disease, the answer is usually no because larvae generally hatch uninfected with Borrelia burgdorferi. However, this study demonstrates why that rule does not apply to Borrelia miyamotoi.
Because infected female ticks efficiently transmit the organism to their offspring, larval ticks may already carry Borrelia miyamotoi before taking their first blood meal. Clinicians evaluating patients with summer tick exposure should therefore consider Borrelia miyamotoi disease even when exposure involves only tiny larval ticks.
Related Articles
- Larval Ticks May Be a Threat After All: Insights From Borrelia miyamotoi
- Move Over Nymphal Ticks: Larval Deer Ticks May Also Pose a Threat
- Doctors Face Challenges Diagnosing Borrelia miyamotoi Disease
- Blood Smear Not Reliable for Diagnosing Borrelia miyamotoi Disease
References
- Han S, Lubelczyk C, Hickling GJ, Belperron AA, Bockenstedt LK, Tsao JI. Vertical transmission rates of Borrelia miyamotoi in Ixodes scapularis collected from white-tailed deer. Ticks Tick Borne Dis. 2019;10(3):682-689.
- Molloy PJ, Telford SR 3rd, Chowdri HR, et al. Borrelia miyamotoi disease in the northeastern United States: a case series. Ann Intern Med. 2015;163(2):91-98.
- Krause PJ, Fish D, Narasimhan S, Barbour AG. Borrelia miyamotoi infection in nature and in humans. Clin Microbiol Infect. 2015;21(7):631-639.
- Cutler SJ, Vayssier-Taussat M, Estrada-Peña A, Potkonjak A, Mihalca AD, Zeller H. A new Borrelia on the block: Borrelia miyamotoi – a human health risk? Eurosurveillance. 2019;24(18):1800170.
Dr. Daniel Cameron, MD, MPH
Lyme disease clinician with over 30 years of experience and past president of ILADS.
Symptoms • Testing • Coinfections • Recovery • Pediatric • Prevention
