Biology Comics

Lyme Disease

Lyme disease is caused by a spirochete bacterium of the genus Borrelia that lives in some small mammals ((white-footed mice, chipmunks, meadow voles) and some birds (robins, grackles, house wrens) without causing them distress. Such animals are reservoir species for the disease. White tailed deer are not a reservoir species.

However the tick of the genus Ixodes that spreads the spirochete has deer for one of its hosts. So the ticks are called deer ticks.

Eggs laid by mature female ticks hatch into larvae (tiny ticks) in early spring. The larvae feed on a small animal, preferably a white footed mouse, but a chipmunk, a bird, a frog or a human will do. The larvae need a blood meal to mature into the next insect stage, a nymph. Feeding lasts a few days. After its meal, the larvae drops off its host, winters over in the leaf litter and the next spring sheds its cuticle to become a nymph.

Nymphs also need a blood meal to mature and also feed preferentially on white footed mice, but again will feed on any animal with blood. Nymphs, as a result of their first blood meal, are more likely to be infected with the spirochete that causes Lyme disease than larvae. Many climb high enough in the vegetation to bite people. Their activity in southern New England and New York State peaks in early summer.

By fall the nymph sheds its skin to become an adult. Adults are able to climb still higher in the vegetation, from where they attach themselves to any animal that brushes against them. Their preferred (or most common) host is deer. The blood meal from the deer or a human lets them develop sexually and survive until spring, when the female lays her eggs and dies.

Lyme disease was here when the Native Americans inhabited the continent (some Puritans seem to have contracted it) but was uncommon. It also exists (with similar ticks carrying it) in Eurasia. Peter Kalm, a Swedish botanist, remarked on the abundance of ticks in the eighteenth century New Jersey forest. Of course Peter Kalm didn’t wear shorts in the woods.

Deer ticks in the Northeast today prefer young forest and forest edges. They probably became much less common around human habitations as the landscape was cleared and settled. The virtual elimination of white tailed deer in the Northeast by 1900 may also have helped break the transmission of the disease to humans.

As northeastern farms were abandoned in favor of better farmland in California or Ohio and as northeastern industries used up their natural resources or failed during the first half of the twentieth century, the land emptied of people and grew back to young woodland, ideal habitat for ticks and deer. Much of the woods was oakwoods, also ideal, once the trees were large enough to bear acorns, for white-footed mice and chipmunks. As the animals returned, so did the disease causing ticks and spirocetes.

Beginning in the 1960s the second growth woodland started to be subdivided into housing lots. Life under the trees had its charms.

In 1975 Lyme disease was recognized as a disease from a cluster of cases in Lyme, Connecticut, a center of upscale suburbia.

A few years later the spirochete was isolated from the gut of a tick. If the disease is recognized, it is easily treated with antibiotics. A bullseye rash is often diagnostic.

Not everyone gets the rash. Other symptoms of the disease include headache, fever, fatigue and depression. If the disease is not recognized early, it becomes, like syphilis, another disease caused by a spirochete, difficult to treat and can result in joint pain and arthritis (especially in the knees), heart trouble and neurological symptoms such as difficulty concentrating and short term memory loss.

What to do? Reducing the number of deer (to fewer than 8-20 per square mile) helps.

Letting in light and air helps. Dehydration is the enemy of insects. Ticks and small mammals like damp leafy woodland edges, unmortared stone walls, brushy areas. Letting forests mature and cutting brush along forest edges probably helps. A few old trees in a sunny mowed lawn is ideal. Such treatments will also make the landscape much less desirable for many birds, small mammals and butterflies.

Cardboard tubes filled with cotton impregnated with pyrethrum helps reduce the number of ticks on mice, if the mice use the cotton to build their nests.

Baited station where mice are treated with insecticide also work; such stations have also been used for deer. Squirrels also enjoy bait stations for mice. A laborious solution is to capture and vaccinate the mice against the spirochete, every year.

Bait stations probably spread chronic wasting disease in deer. Chronic wasting disease (CWD) is a disease related to mad cow disease that was once endemic but uncommon among elk in the Rockies but is rapidly spreading eastward, probably through the use of feeding stations for deer. At the feeding stations deer come in contact with each other’s saliva, thus spreading the disease.

Lyme disease is a problem of landscape design. The best defense against it is a more diverse ecosystem, in which ticks feed on more animals that are not reservoirs of the disease. Then the incidence of disease in the ticks will be lower. (The chance of getting Lyme disease from a tick bite is currently 1-15%.)

In California, habitats with good fence lizard habitat have fewer infected ticks (ticks feed on lizards and lizards are resistant to the disease).

In the Northeast, small areas of woodland (less than five acres, a huge area in the suburbs) have many more white footed mice, as well as many more ticks, and many many more infected ticks, per square meter. This is probably because most of the woodland is edge, with invasive shrub layers and thick invasive groundcovers, ideal habitat for mice, chipmunks, ticks and deer.

One needs a connected landscape so populations of foxes, coyotes, weasels, owls and hawks can catch mice, and populations of raccoons, opossums, skunks, amphibians, different small birds and foxes can be bitten by ticks. The more young ticks bite animals that don’t carry the spirochete, the less likely humans will get the disease.

The ideal suburban landscape has vegetated bridges over roads connecting small woodlands and amphibian friendly culverts under roads to accommodate migrating frogs and toads (also tick bait).

It has habitat for short tailed weasels and snakes. Mice flee an area at the smell of a snake. Cats that eat snakes are kept indoors.

It has coyotes to help control deer. Coyotes will also control feral or adventurous bird and snake eating cats.

And humans with bows hunting deer. A useful addition to the tool kit would be a curare tipped arrow. Then (as long as the poison enters its bloodstream) the slightest scratch will kill a deer. There are many ways to deliver the poison. Perhaps a groove along the point of a hard plastic disposable arrowhead contains curare, covered with wax (so the hunter doesn’t accidentally kill himself). Or a springloaded pellet propels itself upon impact into the animal’s flesh.

Curare kills by paralyzing the muscles. Respiration stops and the animal dies of asphyxiation. But the curare molecules are too large to pass through the gut wall (curare is harmless if taken orally) so the animal’s meat remains edible.

Since the point is population control as well as sport, does as well as bucks are hunted. Young does taste better anyway.