Notes from our lab

Larval Ticks and Lyme Disease

This photo shows the larval stage of the black-legged tick (Ixodes scapularis), about 1/25 inch long.  A larva has 6 legs, instead of the 8 we see on the nymph and adult stages, but the mouth parts are similar.  A larva must complete a blood meal before it can molt into the nymph stage.  A larva is not able to give Lyme disease to the animal it feeds on.  But it can pick up Lyme bacteria if that host animal is infected with them and then transmit them to the animal it feeds on as a nymph, and again if it survives to become a feeding adult.  Much of the Lyme disease afflicting the human population depends on larvae feeding on infected hosts, even though larvae don't give it to people directly.  The rest of the human Lyme infections come from bites by adult ticks that did not get infected as larvae, but then fed on infected animals as nymphs and molted into adults.

Infected Larvae

Some other serious tick-borne illnesses can already be in a larva before it begins its blood meal.  These pathogens can pass directly from an infected mother tick into the fertilized eggs that she lays.  When the eggs develop into larvae, those viruses or bacteria are able to be transmitted to the hosts when the larvae feed on them.  These other pathogens remain in the tick as long as it lives, and can infect the animals it later feeds on if it survives to be a nymph or adult.  Even though larvae are not vectors of Lyme disease, a bite by a larva poses some health risks for people because of these less common diseases that can be in the tick eggs.

The larva in these photographs developed in our lab from eggs laid by an engorged female tick that had been captured.

The tick shown on the left (purple background) is an Ixodes scapularis larva.  This is hard to tell with the naked eye, but under a microscope you can see that near the bottom end of the tick are structures called setae, which resemble short fine hairs.

The tick to the right (white background) is a Dermacentor variabilis larva.   D. variabilis larvae do not have setae along their bottom end, although they may have them in other areas.  And unlike I. scapularis, D. variabilis have festoons, structures that look like grooves or ridges in the bottom end.  Again, while difficult to see with the naked eye, the festoons are fairly distinctive when viewed with a magnifying glass or microscope.   

Photos by D. Schimpf
Text by D. Schimpf and C. Fisher