“Sticking it” to Ticks

 Preventing tick bites when spending time outdoors has been a cornerstone for protection from tick-borne diseases.  A new scientific research paper this year added another method for prevention.  The study was done in Scotland with Ixodes ricinus ticks, which are similar to the Ixodes scapularis ticks of the Great Lakes region and transmit some of the same diseases.  The researcher found that swishing the vegetation with a walking stick made many ticks drop off the plants to the ground.  Ticks “quest” for a host animal by climbing a plant, then waiting for an animal to brush against it and be latched onto.  Ticks on the ground are less able to climb aboard a large moving animal like a human.  Swishing the stick so that it strikes a plant at least twice before you closely walk past it knocked off most of the ticks in the Scotland study.  If there is more than one person in a hiking party, only the leader needs to use the stick if everyone walks the same path.  With two or more people in a hiking party, dogs could be given some protection by keeping them leashed and behind the leader who is using the stick.

 

Post by D. Schimpf

Original article citation: 

Wilkie, I.C. (2020). How not to be the perfect host: a simple tactic for avoiding sheep ticks (Ixodes ricinus). The Glasgow Naturalist Online. 27; 2 https://doi.org/10.37208/tgn27209

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

The Trouble with Nymphs

Photo provided by David Schmipf

As ticks molt out of their larval phase, they enter their "teens," gaining their seventh and eight legs and, as you might expect from other teenage species, become much more trouble than their larval stage counterparts.

Nymphal Ixodes scapularis ticks are still very small, about 1-1.5mm, although this is a little larger than a larva.  They tend to look like a very small adult female, having a very dark brown or black scutum that covers half to 2/3rds of their back and a pale tan or red brown posterior end.  To the right we have a photo of three nymphs on a 1mm grid and shown above the ticks is a poppy seed for size reference.  On the left we have an I. scapularis nymph, in the middle is a Dermacentor variabilis (also known as the dog or wood tick) nymph and on the right is an Amblyomma americanum (also known as the lone star tick) nymph.

Ixodes nymphs are a little easier to see than larvae, as they have somewhat darker coloration, but the posterior end of their bodies can still be semi-transparent.  And, as you can surmise from their still tiny size, would hide easily under hair or in areas where you can't see or reach without assistance.  This makes them very problematic, because during its larval blood-meal, a tick can pick up pathogens and is able to spread those pathogens on to its next prey.  The Minnesota Department of Health estimates as many as one in five Ixodes nymphs may be carrying Borrelia burgdorferi, the bacteria responsible for causing Lyme disease.  Because Ixodes nymphs are now potentially capable of spreading disease when they feed, they tend to be less picky about their prey and, if they bite a human, they can be easily overlooked and allowed to feed long enough to increase the risk of pathogen transmission like B. burgdorferi, some sources consider nymphal ticks to be the riskiest stage for bites. 

Ixodes nymph season is typically reported as being mid-May through June and then tapering off in July.  However, peak nymph season may vary depending on weather patterns and geographic location.  Finding nymphs in our active tick collection process is a challenge, because they are so small, and because they tend not to be out questing for food as long in the warm seasons as their adult counterparts.  However, when we find more than adults in a given area year after year, we can conclude that the species has taken hold in that location instead of being accidentally carried in on people or animals as they migrate through on their travels.  When a tick species takes a hold and is seen to be actively reproducing and increasing its population in a new area, this helps us get a picture of how populations change, expand and migrate over time.  Also, this information is crucial to understanding the risk of disease emergence in new regions.

Currently, in northern Minnesota we do not find A. americanum ticks, although there are reports of some being spotted in the southern areas of Minnesota.  While A. americanum and D. variabilis ticks can spread other pathogenic microorganisms to humans and domestic animals, neither are believed to be able to cause Lyme disease by the transmission of B. burgdorferi. 

No matter where you enjoy the great outdoors, the adoption of appropriate protection methods to avoid tick bites and conducting thorough tick checks daily are the best line of defense against the spread of tick-borne diseases.

-C. Fisher

The curious case of larva legs

Photo provided by D. Schimpf

Ticks pass through three life stages, well four if you count the fact they start life as an egg.  After hatching, they emerge from the egg as larvae.  

In the Duluth area, we may expect larvae to hatch starting in June and July, depending on when they were laid.  So, we hope that we will soon begin to document finding larvae in our active tick collection activities.

However, larvae are exceptionally tiny and very hard to see with the naked eye.  Fortunately, we do not believe they pose much risk for disease transmission, particularly as Lyme causing bacteria are not thought to be passed from the female to eggs.

If you do happen to see a larva, and you have a magnifying glass or very good eyesight, you may notice that they only have six legs.  This is a curious fact, as ticks are a member of the class Arachnida, which means they are not true insects.  They are more closely related to spiders, than say mosquitos, and would be expected to have eight legs.  

In this photo, we have three larval ticks.  The green lines in the background form 1mm squares, so you can get a sense of just how tiny they are.  The far left is a Dermacentor variabilis, or wood tick/dog tick.  You can tell by the little lines on the bottom end of the tick body.  These are called festoons, and are not present on Ixodes scapularis ticks.  The tick in the middle is an Ixodes, and you can clearly see that it only has three legs on the left side (likely one of the legs on the right broke off during collection).  The larger larva on the right is engorged from feeding, it was pulled from a migrating White-throated Sparrow captured at Hawk Ridge in autumn 2019.

Had these ticks not been caught and used for scientific purposes, if they were able to successfully feed, they would then molt and become a nymph.  Emerging from their first molt, they would gain two more legs for the arachnid class standard of eight legs.  From there they would look to feed again in order to molt (retaining their eight leg status) one last time into adults, completing their life cycle.

-C. Fisher

WDIO Special Report: Ticks On The Move

WDIO ABC News interviews Dr. Benjamin Clarke, Dr. David Schimpf and Cole Fisher.  Click on images above to read and watch the news story.