A female health professional injects a vaccine into an adult man
Fightback: a Lyme disease vaccine trial in Pennsylvania © Gary Baranec/AP

In November 1975, health officials in Connecticut received a series of strange and troubling reports about a cluster of children in the small town of Lyme who had apparently developed arthritis.

The officials sent experts to investigate, and soon identified more than 50 residents in the area with recurrent pain and swelling in their joints — some of whom had initially reported unusual skin markings and fever-like symptoms.

“The seasonal and geographic distribution of cases and the association with a skin lesion suggest that a virus carried by a biting insect may be responsible for this disease,” the officials cautioned in a memo to their peers. “Any meaningful effort to control this problem and prevent new cases must depend on finding new information about its cause.”

Nearly half a century later, a lot more is known about the disease to which Lyme gave its name, and which has since spread to other parts of the US and the world. But further research is needed on the detection and treatment of the bacteria Borrelia burgdorferi, which is transmitted to humans by the bite of infected Ixodes ticks after carriage in “reservoirs”, such as deer and mice.

Every year, nearly 500,000 people in the US, alone, are estimated to be diagnosed and treated for Lyme disease infection, imposing medical and related costs of nearly $1bn. That trend is rising, with cases reported far beyond the north-east of the US. Climate change is among the reasons.

According to the Fifth National Climate Assessment produced last year by the US Global Change Research Program, “climate change has profound negative effects on human health” — ranging from the harmful impact of greater heat and increased exposure to poor air quality, to higher rates of pulmonary, neurological, and cardiovascular diseases, and worsening mental health.

Climate is also “a significant contributing factor” to the rise in a range of vector-borne diseases in the country over the past two decades (those transmitted by blood-feeding arthropods, such as ticks). These include not only Lyme disease — which accounts for four-fifths of vector-borne infections reported in the US — but also Rocky Mountain Spotted Fever and the multiple pathogens transmitted by the Lone Star and Gulf Coast ticks. In addition, there is a rising threat linked to mosquitoes — of dengue, Zika, chikungunya, and West Nile virus.

Ben Beard, deputy director of the Division of Vector-Borne Diseases at the US Centers for Disease Control and Prevention (CDC), and a co-author of the assessment, says: “We’ve seen a huge increase in tick-borne diseases.”

He points to factors including milder winters and longer summers that have encouraged the spread of ticks beyond the north-east of the US, compounded by changes in human activity. Longer summers have helped increase contact, while historic shifts from an agricultural to an industrialised economy, and the growth of suburban sprawl, have increased the likelihood of exposure. Reforestation and the resurgence of deer populations in some areas have added to the scope for interactions.

“People in new areas are being exposed to new diseases they were not exposed to before,” Beard says.

A complication is the range of different vectors and hosts across the US. The mosquitoes carrying West Nile virus in the west of the US are different to those in the east, for instance. The former breed in drainage ditches, the latter in small containers — with rain and temperature differently affecting their reproduction.

If human activity is compounding climate change, other factors also help explain the recent rise in cases of Lyme disease. One is purely definitional: CDC changed its system for reporting cases in 2022 with a simpler classification.

However, the recent coronavirus pandemic — despite all the other the harm it caused — has brought two positive developments.

One is the development of an experimental vaccine at the University of Pennsylvania, which draws on the mRNA approach used for certain Covid-19 vaccines. A late-stage clinical trial using a different approach is now under way by Pfizer and Valneva, which is expected to report in 2025.  

A second development is fresh recognition in the wake of “long Covid” of the risks that, in up to 10 per cent of Lyme disease infections, symptoms may persist despite treatment and even when antibodies cannot be detected.

Prof Nicole Baumgarth, director of the Lyme and Tickborne Diseases Research and Education Institute at the Johns Hopkins Bloomberg School of Public Health, says: “The idea of long Covid was embraced in a way the equivalent with Lyme disease was not. Now, even sceptics consider it’s perhaps normal for many different infectious diseases to have long term consequences.”

Baumgarth stresses that, “in most cases”, patients will fully recover if they see their doctor swiftly once symptoms appear — typically within three weeks — and receive the standard treatment of antibiotics. But symptoms may persist in a small proportion of cases.

Prevention efforts, today, focus on educating walkers to exercise caution when straying off trails and into undergrowth; use of insect repellent; inspecting for ticks, showering, and placing clothes into a hot dryer at the end of the day. The problem, she says, is: “We have no elimination strategies for ticks.”

While there may be some signs of greater scientific understanding and new tools to respond to Lyme disease, climate change seems likely to contribute to its continued growth — and that of other infections — in the years ahead.

This article has been amended to reflect precisely the climate conditions aiding the spread of ticks and the breeding locations of mosquitoes carrying West Nile virus.

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