San Mateo County: Tularemia Survey

Entomology Report October 2008, San Mateo County Mosquito and Vector Control District

District laboratory and operational staff trapped ground squirrels at a rural site in Portola Valley this month to survey for tularemia. Tularemia is a disease carried by wild mammals including rabbits, squirrels and mice. It can be transmitted to humans through the bite of ticks, fleas, horse flies and other insects, as well as by direct contact with infected animals. Cases of tularemia have occurred in primates housed at a site north of Alpine Road for the past several years. District staff have been working with staff from Stanford University to identify the source of infection in these monkeys. Past investigations have included trapping for mosquitoes and biting flies, flagging vegetation for ticks, and trapping small mammals. Previous attempts to collect small mammals and arthropods yielded very little. Although numerous ground squirrel burrows were observed at the site, no rodents were present.

This year, ground squirrels have re-colonized the area, prompting renewed trapping efforts. Trapping on October 22 and 23 yielded 8 squirrels. Blood samples and fleas were collected from each animal. Blood samples from the squirrels will be tested by the Centers for Disease Control (CDC). Fleas were “cleared” and identified using a taxonomic key. Clearing is a chemical process that makes the body more transparent, so that internal structures are visible. Some features that are important for determining flea species are the position and presence of hairs on the body surface, the shape of reproductive structures, and the size of body segments. Two species of fleas were found on the squirrels, Hoplopsyllus anomalus and Oropsylla montanus. Both flea species are potential vectors of tularemia and plague.

Aside from spreading disease, fleas are insects worthy of our admiration. Relative to their size, they are spectacular jumpers, second in the animal kingdom only to another insect, the froghopper. Fossils of fleas suggest that fleas have been around
for about 100 million years. It is estimated that around 2,000 different flea species exist today. During the two or three month lifespan of most fleas, a female can lay 2,000 eggs, allowing fleas to persist as a problem for a variety of animal hosts.