Invasive species, infectious diseases and some new ticks in Litchfield County

When we hear ‘invasive species’ around here we think of Oriental Bittersweet strangling trees, Japanese Knotweed clogging riverbanks, Purple Loosestrife choking wetlands, and Emerald ash borer beetle decimating our ash trees. These are threats to our environment that are visible from almost every road, path or hiking trail across the northwest corner of Connecticut. These are introduced species that adapted to our local climate, out-competed our native flora and fauna, and occupied the ecological niche formerly home to local native species.

What is invisible from your car window as you pass across the region is another invasion, one with big health consequences. It is an invasion of infectious disease driven by arthropods that act as disease vectors arriving in our county from elsewhere. Some of the most closely watched arrivals are the new tick vectors that have spread to southern Connecticut in the last couple of years. Most of us are familiar with the black-legged deer tick which has been in Connecticut for millennia and transmits Lyme Disease, Anaplasma, Babesia and Powassan Virus in mice, birds, deer and humans. What few realize is that there are three new tick species that have arrived in the state enabled by the rising global temperatures, movement of host species carrying tick passengers, and human disruption of habitat allowing the new arrivals to become established.

Lone Star Tick, Gulf Coast Tick and Asian Longhorned Tick have all been detected as breeding populations in the southeastern part of Connecticut and if the black-legged tick’s rapid move north into Canada over the last 20 years is a guide, these new ticks can be expected to move northwards at 15-40 miles per year. They transmit a whole new selection of protozoa, bacteria and viruses: Rickettsia parkeri, Heartland Virus, Dabie Bandavirus, Tularemia, Southern Tick Associated Rash Illness, Bourbon Virus, Relapsing Fevers, Tick Borne Encephalitis and Rocky Mountain Spotted Fever to name a few. Some of these infections are often mild but some are more severe and some can be life-threatening. We are poorly prepared for these invaders with few diagnostic tests and not much in the way of treatment options, other than the fortunate efficacy of doxycycline, used widely for Lyme, against some of the new bacterial infections.

Ticks are not the only invasive arthropod vectors taking advantage of the changing climate and human-disrupted environments. Mosquitoes are also moving here. There have been Asian bush mosquitoes such as Aedes japonicus and A. albopictus spreading in the state for some years but the acceleration of warming will likely bring new invaders adapting to human-made habitat. The daytime human biter Aedes aegypti thrives in urban environments and its global spread has driven the explosion in Dengue Fever cases over the last couple of years. It is so well adapted to towns and cities that it can breed in the drops of condensation from an air conditioner. Viral infections like Yellow Fever, Dengue, Zika and Chikungunya are transmitted by Aedes species. Across Africa a new invasive malaria-carrying mosquito, Anopheles stephensi, is setting back hard won gains in malaria control. It’s another daytime biter that lives happily in towns and cities and is changing malaria from a rural infection of farms and villages to an urban plague like Dengue Fever. Could it invade here? Why not?

Global factors, mainly warming winter temperatures, are clearly driving these range expansions. To slow the spread of these invaders we need to slow the heating up of our planet by burning less fossil fuel and storing more carbon. But there are local factors too. Losses of native biodiversity due to human activity such as habitat fragmentation, intensive agriculture, and pollution open up habitat for invaders. Range expansion of new species is a natural process that happens in native ecosystems but it is reasonable to assume that an abundant and thriving mix of local species controls the expansion of newcomers through competition. We are losing these resilient ecosystems and the health consequences are unfolding as we watch.

According to Homegrown National Park [homegrownnationalpark.org] about 80% of U.S. land is privately held. A commitment to manage our property for native biodiversity will go a long way to restoring resilient habitat that can dilute the expansion of disease-carrying invaders. Certainly new infectious diseases will arrive in Litchfield County given the planetary changes already set in motion, but the level of impact they will have is still in our hands.

James Shepherd, Section of Infectious Disease at theYale University School of Medicine, lives on Smokedown Farm in Sharon.