by Ticks can be a hospitable environment for a wide range of microorganisms that harm people and their pets. The question is, when are these pathogens a risk to people and animals? Does the microbiome of the tick gut have a different effect on disease risk than the microbiome of its saliva?
The invention of PCR and high-throughput DNA sequencing has allowed researchers to better understand the relative populations of these visitors found in different parts of the tick. This information combined with innovative laboratory experiments is leading to a better understanding of pathogen transmission.
A tour of the tick will reveal some points of weakness in pathogen transmission that researchers are exploring.
The “Head”
As most people know, ticks feed on the blood of a host by attaching with their “head”. The host is typically a mammal, like a deer or mouse, but studies have shown that ticks can acquire a blood meal from reptiles, like snakes, as well. The “head” of the tick is referred to as the capitulum and is composed of mouth parts that each serve a specific purpose during feeding.
The chelicerae, two serrated appendages connected to either side of the mouth, allow the tick to tear into the skin of the host. The length and width of these appendages vary depending on the species of the tick. Once an opening is created, the tick can attach and anchor itself by inserting the hypostome. This narrow, barbed projection allows the tick to stay put while ingesting blood for the next 3-10 days. The structures on the hypostome point away from the opening, making it difficult to remove the tick from the skin. While the tick is feeding, the palps move laterally and do not enter the skin of the host.
The Saliva
Like other animals, ticks have salivary glands that produce saliva when feeding. The saliva contains components that aid in the feeding process. Anti-coagulants keep blood from clotting, and anti-inflammatories help numb pain during feeding. In addition, research suggests that tick saliva contains immunosuppressive and activation factors that can aid in successful pathogen transmission to a host.
The salivary glands also harbor pathogens. Pathogen abundance and diversity within the salivary glands varies greatly between species and location. Researchers using PCR on salivary samples from Ixodes ricinus in France found that 73% of males and 59% of females were infected by a least one pathogen. Anaplasma phagocytophilum was the most common among all ticks, but about 25% were co-infected with other pathogens such as Rickettsia and Borrelia species. Likewise, PCR results of hard-bodied ticks in Japan yielded a higher percentage of Rickettsia.
The mode of transmission and amount of time it takes for pathogens to be transmitted via saliva is not well understood at this time. Pathogens are thought to migrate through the skin or enter directly into the bloodstream, but some research shows that a bite from an infected tick does not always lead to infection.
In a recent study of Lyme Borrelia species, researchers used PCR on skin biopsies to find DNA in the skin of mice as early as 24 hours after attachment. Intriguingly, the infection was no longer present 4 weeks later in some of the mice. How are some mice able to evade systemic infection and what can this teach researchers about protecting humans? This is an exciting avenue of research.
The Gut
By the time an adult tick is finished feeding, it can be engorged up to ¼ inch in diameter. Some of the blood it has consumed, mixed with the prior contents of the gut, regurgitates back into the host. This process is considered the likely cause of Alpha-Gal allergy, by which a human becomes allergic to an animal protein found in red meat.
If the host that the tick fed on was infected by tick-borne pathogens, like Ehrlichia or Babesia species, then those are ingested as well. However, not every pathogen remains viable in a tick’s gut until the next attachment.
Numerous studies have been conducted to understand what is present in the gut of ticks when they are starved, after they feed, and after biting a new host. Recently, researchers found that 56.3% of ticks tested in New York and Connecticut had Borrelia burgdorferi in their gut. They also found about 14 viruses found and a filarial nematode.
What evolutionary changes are required for a pathogen to be able to survive the tick gut? These changes may create weaknesses in the pathogen’s ability to survive in a mammal. Exploring these weaknesses is an avenue of research into better treatments.
Conclusion
The blacklegged tick, Ixodes scapularis, is often considered one of the most important species because it transmits pathogenic agents that cause over 90% of reported tick-borne diseases. Lyme borreliosis, ehrlichiosis, and babesiosis cases in the United States can all be attributed to a bite from this vector.
However, research shows that other tick species, including the lone star tick (Amblyomma americanum) and the American dog tick (Dermacentor variabilis), can harbor complex microbiomes that consist of bacteria, protozoans, and viruses.
Why is the blacklegged tick so much more likely to transmit pathogens? Understanding more about the microbiome in each part of the tick will help to answer that question. If scientists discover specific factors in the salivary glands or the gut, better methods of prevention and treatment may be the result.
References
Tokarz, R. et al. (2019). Microbiome analysis of Ixodes scapularis ticks from New York and Connecticut. Ticks and Tick-borne Diseases, 10(4), 894-900. https://doi.org/10.1016/jttbdis.2019.04.011 https://www.sciencedirect.com/science/article/pii/S1877959X19300330?via%3Dihub
Thapa, S. et al. (2019). Effects of temperature on bacterial microbiome composition in Ixodes scapularis ticks. MicrobiologyOpen, 8(5), e00719. doi:10.1002/mbo3.719 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6528569/
Carylsue. (2013). How a tick does its dirty work [Blog post]. NationalGeographic.com. Retrieved from: https://blog.education.nationalgeographic.org/2013/10/30/how-a-tick-does-its-dirty-work/
Pospisilova, T. et al. (2019). Tracking of Borrelia afzelii transmission from infected Ixodes ricinus nymphs to mice. Infection and Immunity, 87(6), e00896-18. https://doi.org/10.1128/IAI.00896-18 Retrieved from: https://iai.asm.org/content/87/6/e00896-18.long
Ŝimo, L. et al. (2017). The essential role of tick salivary glands and saliva in tick feeding and pathogen transmission. Frontiers in Cellular and Infection Microbiology, 7, 281. doi:10.3389/fcimb.2017.00281 Retrieved from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5479950/#B203
Lejal, E. et al. (2019). Tick-borne pathogen detection in midgut and salivary glands of adult Ixodes ricinus. Parasites & Vectors, 12, 152. doi:10.1186/s13071-019-3418-7 Retrieved from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6444572/
Phelan, J. P. et al. (2019). Genome-wide screen identifies novel genes required for Borrelia burgdorferi survival in its Ixodes tick vector. PLoS Pathogens, 15(5), e1007655. https://doi.org/10.1371/journal.ppat.1007644 https://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1007644
Velasquez-Manoff, M. (2018, July 24). What the mystery of the tick-borne meat allergy could reveal. The New York Times Magazine. Retrieved from: https://www.nytimes.com/2018/07/24/magazine/what-the-mystery-of-the-tick-borne-meat-allergy-could-reveal.html