Do ticks get sick? A look at ticks and their simple “immune systems”

Jun 26

“Pathogen” vs “Disease”

If you have interacted with TickReport lab staff through our email support, website chat feature, or at one of many community and online outreach events, you know that we are always careful to distinguish between pathogens and diseases. Rather than saying that “this tick has Lyme disease,” we might note that “this tick contained Borrelia burgdorferi” or perhaps “this tick carries Lyme bacteria.” We sometimes get feedback that we’re splitting pedantic hairs, but highlighting that difference is a core goal of our educational mission. These words have specific meanings, and when you are looking at the results of tick testing, it’s crucial that we understand the distinction between a tick containing a pathogen and that tick-borne pathogen causing illness in a person (or pet or other animal).

Pathogen: a microorganism that can cause disease in a host. These might include bacteria, viruses, protozoa, or other microbes.
Disease: a structural or functional disorder in animal or plants. In the tick world, we would be referring specifically to infectious diseases that are caused by pathogens (“germs”) that enter and cause infection as they grow and multiply in a host.

Why does the pathogen/disease distinction matter?

Most importantly, distinguishing between pathogens and diseases helps us to communicate the difference between diagnostic tests carried out by your doctor to determine the cause of symptoms and exposure tests like well water testing, lead paint or asbestos testing, or tick testing. When diagnostic and exposure testing get conflated, we run into problems that the CDC warns about and that we’ve echoed in an earlier post. It’s also useful to know that the tick that bit you wasn’t “sick,” it was just a carrier of a pathogen that can infect other organisms.

Tick immune systems: a multilayered defense

We’ll stick to a bird’s eye view with some recommendations for in-depth reading at the bottom of the post, but we can break defenses into two simple categories: physical barriers and competing microbes.

Physical barriers that protect ticks from infection

Just like humans, a tick’s first line of defense against infection is its outer “skin” that prevents microbes from accessing the tick’s internal organs. This multi-layered system of protein and chitin (plus a few other secreted outer layers depending on the type of tick) prevents entry by most harmful pathogens. Compared to humans and other mammals, tick have very few openings in that protective layer, which helps to prevent infection from outside and also aids ticks in minimizing water loss and desiccation. Some of these openings are further protected by tiny gaps that are too small for most pathogens to enter.

As with human skin, injury to this outer layer creates opportunities for microbes to enter the tick’s body and contend with the tick’s internal defenses.

Internal defenses against infection 1: hemocytes and related processes

The tick circulatory system uses hemolymph (the invertebrate version of blood) to transport materials throughout the tick’s body. This hemolymph can coagulate at the site of an injury just as blood would clot to seal and protect a wound on a person. Elements of the hemolymph can also recognize, surround, and kill certain foreign bodies.

Internal defenses against infection 2: gut flora.

The tick’s midgut is filled with a variety of microbes that are passed to the egg from an adult tick, or are picked up through a blood meal during a tick’s lifetime. This microbiome will vary from species to species according to typical behaviors and host preferences (e.g. tick species that usually parasitize birds may have a very different gut microbiome compared to ticks that prefer medium to large mammals). Individuals within species can also vary, particularly in the microbes they pick up from blood meals from the hosts they happen to encounter in their lifetime.

Just like our own gut microbiome and some of the flora we might try to encourage with probiotics, ticks have plenty of microbial friends that assist the tick in exchange for a home and nutrition. These symbiotic organisms establish themselves in the tick’s gut and don’t like to share resources with newcomers, so they do their best to consume all available nutrients and block or attack foreign microbes. We don’t always understand how or why these processes work, but researchers are working to clarify many of these functions.

How tick immune systems benefit us as humans

A combination of a tick’s species characteristics and symbionts picked up from preferred hosts can actually help protect us from getting sick as a result of a tick bite. Although ticks are often described as “nature’s dirty needle,” the reality is that this needle fights back against many pathogens it encounters. This combination of host preference and internal defenses are the main reasons that certain species of ticks are associated with some tick-borne pathogens but not others. If you find a tick on yourself or a family member, accurate identification of the tick species is the first step in estimating risk from that bite.

Order pathogen testing through the TickReport lab or email a photo to support@TickReport.com for free identification by our experts.