Mice and mites (and more)

Small mammals (including mice, voles, and shrews) are found all over the world, and many species are happy to live next to or even in the same house as humans. They can play an important role in vector-borne diseases by harboring and supporting a wide range of fleas, ticks and mites. A recent study in Germany examined the diversity and drivers of these parasite communities found on small mammals in different habitats that humans frequent.

Why are small mammals important?

Small mammals refer to more than 2,800 species of mammals in the three commands encompassing: rodents (e.g. mice, voles, rats), tree shrews and order Eulipotyphla (eg shrews, hedgehogs, moles). This last command was previously called Insectivora and included more species – but since found inaccurate and has been amended. These “ small ” mammals (the largest rodent, Capybaras, weighs up to 79 kg – see also a recent article on Bugbitten) provide important ecosystem services, are incredibly diverse, and many species thrive in human-modified environments.

Small mammals can provide important ecosystem services such as pest control (but can also cause damage to the crops themselves), pollination and seed dispersal. However, they are also of concern for human health. They can wear highly virulent pathogens which affect humans and cause severe morbidity (eg. Lassa fever, Hantavirus pulmonary syndrome). In addition, they are important for maintaining ectoparasite populations and, as such, can be very important for vector-borne diseases (eg. Lyme disease). However, in some areas we know very little about small mammal communities and the series of ectoparasites they carry. This severely limits our ability to understand the risk of vector-borne disease in these populations.

What did this study do?

To help understand the diversity and abundance of ectoparasites found on small mammals, scientists recently investigated communities of small mammals found in different habitats in Germany. In total, they captured and examined 775 small mammals from three different habitats: one small urban park, a tall mature forest, and one recreational zone (“rebuilding” nature reserve).

After capturing animals, the scientists measured the characteristics of the individual – including species, approximate age (juvenile, adult) and weight. Each individual was also carefully screened for ectoparasites and all lice or fleas, ticks and mites were collected and placed in separate containers. Back in the lab, ectoparasites were carefully identified to species – fleas and ticks were identified with molecular tools and mites were identified to species using morphological keys. After identification, scientists built statistical models to help understand how habitat and host species affected the communities and abundance of the ectoparasites found.

Some of the parasites found on small mammals in this study: (A) a Ctenophthalmus agyrtes chip, (B) a Ixodes ricinus tick, and (C) a sketch of a Laelaps agilis moth.

What did they find?

In total, scientists found 5691 different ectoparasites belonging to 27 different species! The urban site had the fewest number of species (another 12 different species!) While the recreational site had 20. Most individuals (88.9%) had at least one ectoparasite. The average infestation rate was 7.34 ectoparasites per host, often from one or two different species. But one individual was found with six species of ectoparasites.

Only two species of small mammals have been found – the bank vole (Myodes glareolus) and the yellow-necked mouse (Apodemus flavicollis). The prevalence and diversity of each group of ectoparasites and their relationship to habitats and host species are detailed below:

Lice or fleas

  • 58% to 66% of hosts had fleas in each habitat
  • 11 species of fleas were found
  • Fleas were more abundant in forest sites, but this was not significant.
  • There was no difference between the two host species

Ticks

  • 61% to 83% of hosts had ticks in each habitat
  • 3 tick species were found
  • Includes important Lyme host: Ixodes ricinus
Model estimates for the presence of ticks, modified from Figure 1. The further to the right a point is, the more likely it is to have ticks compared to the reference (first host / habitat / season in each category).
  • Ticks were more common on yellow-necked mice and wooded sites
  • Ticks were more common in spring and summer, compared to fall / winter and more abundant in spring

Mites

  • 20% to 47% of hosts had ticks in each habitat
  • 10 species of mites were found
  • Mites have been found more often and in greater numbers on the yellow-necked mouse

Yellow-necked mice are larger than voles and may account for the greater abundance of ticks and fleas seen on this species. Scientists have also found that the warmer seasons support more ectoparasites, which is in line with previous work. The diversity the authors found is higher than expected – pointing out that we have a lot to learn about small mammals and the ectoparasites they carry!

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