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Chiggers are present world-wide in a diversity of hundreds of species. They are the primary vector for scrub typhus, but their potential as a vector for other pathogens including Bartonella and Borrelia species is under-studied. After years of neglect, 2023 saw an increase in research on these tiny arachnids.
What Are Chiggers?
“Chigger” is the US term for the mites that make up the family Trombiculidae. In the UK they might be called “harvest mites.” They are also known as “berry bugs”, “bush-mites” or “scrub-itch mites.” Chiggers are found around the globe in every region that has a moist environment in the mid-70s to mid-80s degrees Fahrenheit (mid-20s to 30 degrees Celsius). About 700 species have been identified worldwide.
They are tiny arachnids (as are spiders and ticks). You might see bright red adults on leaves and grasses, but those aren’t the ones that bite. Only the tiny larvae bite. They are not very strong and have trouble piercing the skin. Consequently, they go for folds and wrinkles, and then they release saliva that dissolves skin into food. Reactions to the saliva can range from small pustules to widespread rashes. Men can develop a serious reaction that can make it difficult to urinate.
Eutrombicula alfreddugesi is the most common biting chigger in the US. Neotrombicula autumnalisis is the most common biting chigger in Europe.
Chigger Bites
Not getting bitten in the first place is the best way to deal with chigger bites. This means avoiding contact with lawns and foliage, especially during the hot moist weather that they prefer. Repellents using permethrin or DEET can keep them off of human hosts, and there are a variety of treatments available for lawns.
If you are bitten, the first step is to wash with soapy water to get the bugs off the skin. Unlike the mites that burrow into the skin and cause scabies, chiggers are easily washed off. You can then treat the bites with anti-itch products. If you have a severe reaction or if the bites become infected, see your doctor for further treatment.
Chiggers can also bite wildlife, pets and livestock. Chigger infestations can be deadly to poultry. Fur is generally protective, so bites on animals such as dogs will be found on exposed skin such as the area around the eyes. Seek advice from an animal health professional if an animal needs treatment for chigger bites.
Don’t want to get bitten? Techniques to prevent tick bite can also protect from chigger bites.
Scrub Typhus
Leptotrombidium delicense is primary species of chigger identified as a vector for scrub typhus, which is caused by the bacteria Orientia tsutsugamushi from the Rickettsiaceae family. Scrub typhus is found in Southeast Asia, China, India, Japan and Australia. A dark scab called an eschar may be found at the chigger bite in about half of cases. Other symptoms are nonspecific, including fever, headache and body ache, neuropsychological symptoms, enlarged lymph nodes and sometimes a rash.
In 2023, researchers in Chile found Orientia genus DNA in six species of mites and in rodents. While scrub typhus is not currently found in South America, this research suggests that the species of mites found in the region may have the capacity to be vectors if the bacteria is introduced. The US also has the risk of chigger species that can carry Orientia species.
To better evaluate the possibility of scrub typhus spreading to chigger species beyond its current range, researchers from Thailand and the UK looked at what is known about the chigger microbiome as of 2023. If other regions have chigger species that can carry Orientia species, what more is needed to carry Orientia tsutsugamushi specifically? Are there additional elements to the chigger microbiome that are necessary to carry the pathogen? At this point in time, very little is known about the microbiome of chiggers, even chiggers that carry scrub typhus.
Other Pathogens
At the end of 2022, researchers from the Department of Entomology and Plant Pathology and the Comparative Medicine Institute at North Carolina State University looked at the status of research on US chiggers. There are almost 250 known chigger species in the US and little is known about their potential as pathogen vectors. Study of this important topic peaked in the 1980s and has been slow since then with a renewed interest in the last couple of years.
Worldwide research has found chiggers carrying Bartonella species including Bartonella tamiae. Where chiggers were found with B. tamiae, three human cases of the infection were also found. Chiggers have also been found to carry Rickettsia species and Borrelia species, including Borrelia burgdorferi s.l. They may be a vector for Dabie bandavirus, a virus first seen in China in 2009 and known to be tick-borne.
Further research is needed to determine if chiggers are competent vectors for any of these pathogens. While they have picked up the pathogen from a previous bite or from their mother, little is known about whether their anatomy and lifecycle allows them to transmit that pathogen to a host.
Alpha-gal Syndrome
It is also possible that chiggers play a role in the development of alpha-gal syndrome (an allergy to a carbohydrate found in red meat that is believed to be most commonly caused by tick bite). Some people who develop alpha-gal syndrome have reported chigger bites before developing the allergy.
One Health Impacts
Chiggers do not need to directly infect humans to impact disease risk. For example, chiggers may play a role in hantavirus by spreading the virus between mice. One Health research looking at the interplay between people, animals and the environment is needed to fully understand the impact of chiggers on human disease risk.
The number of species of chiggers also makes them a unique risk that One Health research is needed to understand. For example, in 2023, researchers found 149 different species of chiggers on oriental house rats (Rattus tanezumi) collected from five areas of southwest China; the terrain of each area impacted the diversity of chigger species on each rat.
Also in 2023, researchers found 120 chigger species in an area of Yunnan Province, China. Those 120 species form a complex network parasitizing overlapping species of hosts. Adding to the complexity, environmental determinants such as elevation affect the prevalence of different species.
If a novel pathogen arose in one host, what network of chigger species could potentially spread it to another region and/or another species? One Health research allows for better preparation for potential future public health threats.
Conclusion
After 40 years of neglect, the potential for chiggers to be a vector for human pathogens is receiving new interest. Although they are already known to be a vector for scrub typhus, more investigation is needed on their potential as a vector for other pathogens. This research is particularly necessary as the enormous variety of chigger species present around the globe creates a unique risk.
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