House Dust Mites Get The Travel Bug

Eric Hopton for – Your Universe Online
Even microbugs need a holiday from time to time – a well-earned break from all that munching and sucking. And a bunch of them might be coming to an airplane seat near you this Christmas.
A study by a team of University of Michigan researchers has found that we share our cabin space with countless hitchhiking dust mites and a host of other bugs. The results of the research have been published in the journal PLOS ONE.
“What people might not realize when they board a plane is that they can share the flight with a myriad of microscopic passengers, including house dust mites, that take advantage of humanity’s technological progress for their own benefit,” said Pavel Klimov, who is a biologist and assistant research scientist in the U of M Department of Ecology and Evolutionary Biology.
“House dust mites can easily travel on an airline passenger’s clothes, skin, food and baggage,” added Klimov. “Like humans, they use air travel to visit new places, where they establish new populations, expand their ranges and interact with other organisms through various means.”
The genetic study was conducted by Klimov and U of M visiting scholar Rubaba Hamid. The aim was to look for connections between the different house dust mite populations in the United States and South Asia.
The results confirmed that genetic mutations were shared by mites in both the U.S. and Pakistan. This indicates that the mites are indeed travelling between continents.
“What we found suggests that mite populations are indeed linked through migration across continents, though geographic differences still can be detected,” Hamid said. “Every time a mite successfully migrates to a new place, it brings its own genetic signature that can be detected in the resident population a long time after the migration event.”
The work concentrated on two “medically important” species of mite, the American and European house dust mites, both of which are found worldwide. Scientists believe that the species probably split around 81 million years ago at a time when they lived mainly in bird nests. Today, however, house dust mites have adapted to a new host – human beings and they live happily in our homes. Beds, sofas, and carpets are full of them and cleanliness is no protection. House dust mites are around a quarter of a millimeter long and eat human skin scales.
The problem is that, though we cannot see them, house dust mites can cause serious allergic reactions, including asthma, eczema and allergic rhinitis. Over 65 million people around the world are affected each year. Often, it is not the mite itself that causes allergic reaction, but the proteins in their droppings. Mites can leave about 20 droppings every day, which can remain allergenic long after the mite itself has died.
The team studied genetic variations in the group 1 allergen gene (which encodes the most important allergy-causing protein) taken from the two species’ separate populations in the U.S. and Pakistan. This protein is used around the world in standard skin-prick tests for allergies.
In order to be fully effective, the test would need to include local genetic variants of the protein but, until now, there has been little research into these geographical variations in U.S. mites.
“We need to have a better idea about the diversity of allergenic proteins around the world, and particularly in the United States,” said Klimov.
The study discovered mutations at 14 positions along the length of the group 1 allergen gene in genetic sequences taken from American house dust mites (Dermatophagoides farinae). Of the 14 mutations, all but one were “silent,” meaning that they occur at the DNA level, but do not change the amino acid structure of the protein. It is these mutations at the protein level that are medically important, as they can alter the allergenic properties.
The research discovered that a previously unknown mutation occurred at the active site of the protein at position 197, according to Klimov. “This was a rare mutation, found in only a single population of house dust mite in South Asia,” he said. Klimov’s analysis indicates that this mutation may change the enzyme activity of the protein.
These studies are a first step towards explaining how mites have traveled from different parts of the world, but more research is needed if we are to understand the way allergenic properties, immune response and cross-reactivity of the protein work.
“Follow-up experiments to elucidate these issues are underway in our lab,” said Klimov.
Hamid is in the Department of Zoology at the Pir Mehr Ali Shah Arid Agriculture University in Rawalpindi, Pakistan. The other authors of the PLOS ONE paper are Muhammad Inam of the University of the Punjab in Lahore, Pakistan; Farhana Riaz Chaudhary of the Pir Mehr Ali Shah Arid Agriculture University; and Barry OConnor of the U-M Department of Ecology and Evolutionary Biology.
The research was supported by the U.S. National Science Foundation, the Higher Education Commission and the International Research Support Initiative Program in Pakistan, the Ministry of Education and Science of the Russian Federation, and the U.S. National Pediculosis Association.