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How to identify it
- A dark brown flea, medium-sized compared with most fleas, with distinctively long palps. Males and females differ in the shape of the genitalia: males have a rounded clasper with two long strong bristles and four weak ones near the finger, and the female's sperm-storage organ has a bulging body and a crooked tail.[1]
- Where it is found is a better clue than appearance. It lives in the nests of ground squirrels in rural settings, rarely feeding on other rodents, and nest material may include twigs and leaves or even cotton and upholstery stuffing. Eggs are about half a millimeter, larvae are white, legless and eyeless, and the pupa spins a silk cocoon.[1]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Oriental rat flea (Xenopsylla cheopis) | The flea of urban rat-borne plague; the ground squirrel flea is the rural, western counterpart that lives with ground squirrels.[1] |
| Sticktight flea (Echidnophaga gallinacea) | Was collected from the same California ground squirrel burrow systems as Oropsylla montana, but it attaches tightly and stays on its host rather than living mostly in the nest.[3] |
| Cat flea (Ctenocephalides felis) | Also turned up in squirrel burrow surveys, in much smaller numbers; it is the flea people normally find on pets indoors.[3] |
Where it lives
North America west of the Rocky Mountains, in the United States and Mexico: the Pacific coastal and Rocky Mountain regions plus Colorado, Utah, Arizona and New Mexico, extending south into Michoacan and the State of Mexico. It is native to the Nearctic and lives in temperate grassland, chaparral, forest, scrub and mountain habitats.[1]
Biology and life cycle
The flea is nidicolous: it lives in the host's nest rather than on the host, hopping on to feed and getting off again quickly. Counts on ground squirrels are highest at cooler temperatures, below about 18 degrees C, and squirrels can pick up plague after hibernation when they return to their burrows.[1]
Recorded hosts are ground squirrels, including the California ground squirrel and Spermophilus variegatus. In a California survey of 52 ground squirrel burrow systems, 560 fleas were collected, of which 158 were Oropsylla montana; the largest monthly catch came in October, and a few fleas were taken free in the burrows rather than on a host.[1][3]
Damage and signs
- This flea damages nothing structural or agricultural. Its significance is as a disease vector around squirrel colonies, which is why flea control is treated as a priority at facilities where ground squirrels live close to people or animals.[3]
Health risk
Oropsylla montana is one of the North American vectors of Yersinia pestis, the plague bacterium, and is described as a major plague vector in the western United States; Animal Diversity Web calls it the vector of plague in the countryside, as distinct from urban rat-borne plague, and notes that it is only a moderately effective vector that is not very likely to transmit the bacteria even if it bites a person. A few plague cases occur in the United States each year and are treatable with antibiotics. Fleas from squirrel burrows, including this species, are opportunistically willing to bite people, so burrows with unusual rodent die-offs are a real if small risk.[1][2][3]
Control
Prevention
If you find unusual numbers of ground squirrels or other rodents dead for no obvious reason, notify public health officials and do not handle the carcasses: squirrels are highly susceptible to plague, which has wiped out entire colonies, and the fleas leave the dead host looking for a new one.[4]
Keeping ground squirrel numbers down around homes, outbuildings and animal facilities is the practical control, since the flea lives in squirrel burrow nests. Where squirrels and people or livestock share ground, flea control is recommended as a high priority.[4][3]
How the flea transmits plague
Flea-borne plague transmission happens in two phases. In the first few days after an infectious blood meal, a flea can pass bacteria on the very next time it feeds, which researchers call early-phase transmission. Later, Yersinia pestis builds a biofilm in the proventriculus, the valve between the flea's esophagus and midgut. As the biofilm grows the valve no longer closes, so contaminated blood backflows into the bite; when the valve is completely blocked, incoming blood is stopped and then recoiled back into the wound along with bacteria.[2]
In laboratory work with this species, partially or completely blocked fleas appeared as early as two to three days after infection, with the highest incidence one to three weeks afterward; blockage developed in 21 percent of fleas infected on rat blood and 11 percent of those infected on mouse blood. A blocked flea cannot feed and eventually starves.[2][1]
Modeling in the same study concluded that in most ecological settings early-phase transmission is too inefficient to drive an epizootic; instead it thins out the susceptible animals in a population, which favors the quiet, enzootic state in which plague persists between outbreaks.[2]
Common questions
- Can this flea give me plague?
- It is one of the vectors, and it will bite people opportunistically, but it is described as only moderately effective: a bite is not very likely to transmit the bacteria. Plague cases in the United States number a few a year and are treated with antibiotics.[1]
- Where would I run into it?
- Around ground squirrel burrows in the rural West, mostly in the nest material rather than on the animal. It is not a flea of houses or pets.[1]
- What should I do about a burrow full of dead squirrels?
- Leave the carcasses alone and tell public health officials. A die-off is how a plague outbreak in a squirrel colony looks, and the fleas that fed on those animals are then looking for another host.[4]
Sources
- Animal Diversity Web, University of Michigan Museum of Zoology, “Oropsylla montana (L. Hemeyer, author; S. David, editor), as Diamanus montanus.” animaldiversity.org/accounts/Oropsylla_montana/ (accessed 2026-11-10)
- PLoS Pathogens, open access via PubMed Central, “Bland, Eisen and Hinnebusch (NIAID Laboratory of Bacteriology, Rocky Mountain Laboratories). A role for early-phase transmission in the enzootic maintenance of plague. PLoS Pathogens 18 (2022) e1011000, PMC9754260.” pmc.ncbi.nlm.nih.gov/articles/PMC9754260/ (accessed 2026-11-10)
- Journal of Medical Entomology (Entomological Society of America), “Lee, Yee, Dhuey and colleagues (University of California Davis and the California National Primate Research Center). Prevalence and Seasonality of Fleas Associated With California Ground Squirrels and the Potential Risk of Zoonotic Disease Transmission. Journal of Medical Entomology 55 (2018) 452-458, PMID 29202201.” pubmed.ncbi.nlm.nih.gov/29202201/ (accessed 2026-11-10)
- University of California Statewide Integrated Pest Management Program, “Pest Notes: Ground Squirrel.” ipm.ucanr.edu/home-and-landscape/ground-squirrel/ (accessed 2026-11-10)
- iNaturalist, “Oropsylla montana taxon page (order and family).” www.inaturalist.org/taxa/308127 (accessed 2026-11-10)