San Mateo County: Tularemia Investigations

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

In response to a tularemia outbreak in primates housed in a facility in Portola Valley, the entomology staff have been investigating the role of possible vectors responsible for the disease transmission. Tularemia, also known as rabbit fever, is a potentially serious and highly infectious disease caused by a bacterium found in wild mammals, mainly rodents and rabbits. It can be transmitted to humans by a variety of means. Potential ways people can become infected include: ticks, biting flies, exposure to infected animal carcasses, eating or drinking contaminated food or water, or breathing in bacteria suspended in air. Symptoms of tularemia include: sudden fever, chills, headache, diarrhea, muscle aches, joint pain, dry cough, and weakness. This disease is treatable with antibiotics.

District staff have been examining the wildlife in the area to determine the cause of the outbreak. Burrowing ground squirrels are abundant near the facility and serve as a reservoir of infection. One group of vectors under investigation are horse flies in the family Tabanidae. These are large flies with a vicious and painful bite. They mainly feed on the blood of cattle and horses but will also attack humans and presumably other primates.

Two types of traps, a box trap and a malaise trap, were set out in the field to assess fly populations. Ground squirrels were trapped and blood samples were obtained to test for tularemia.

San Mateo County: Lyme Disease

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

The district continues to survey for Lyme disease in San Mateo County recreational parks. Disease-causing spirochetes have been detected for the first time in ticks at two additional parks this year: Water Dog Lake in Belmont and Huddart Park in Woodside (refer to table). The average prevalence of infection for San Mateo County is between 1 and 3% for adults and nymphs.

Park/Trail     Tick Stage  # Collected  Positive  Prevalence
Huddart Park
Bay Tree       Nymph            91          0         0
Richards Rd    Nymph            21          0         0
Chickadee      Nymph            84          3         3.6%*
Waterdog Lake
Lake           Adults          114          1         0.8%

*Prevalence at this site is preliminary due to small sample size.

San Mateo County: Physician Lyme Disease Survey Follow-Up

Entomology Report March 2008, San Mateo County Mosquito Abatement District

Follow-up on the physician survey on Lyme disease continued in March. Based on survey results, the district mailed educational materials to over 200 primary care physicians in San Mateo County. The mailing consisted of a letter about infection in local ticks, instructions on disease reporting, brochures, and tick identification cards.

Results of the survey were presented to the California Lyme Disease Advisory committee in Sacramento on March 12. On March 20, the district discussed the survey in a teleconference of the Communicable Disease Exchange, a forum of health departments from counties throughout California. Two counties in the coastal region have expressed interest in conducting similar surveys or using the results of the present survey to guide physician education.

San Mateo County: Surveying for Tularemia on the Coast

Entomology Report Frebruary 2008, San Mateo County Mosquito Abatement District

On February 27, the district trapped rodents south of Pescadero to test for tularemia. Tularemia is a bacterial disease of wildlife (primarily rodents and rabbits) that can sometimes be transmitted to people. Symptoms include abrupt onset of fever, chills, muscle aches and progressive weakness. When a person is infected by a tick bite, there is often a skin ulcer at the site of the bite and the local lymph nodes are swollen. The disease can be acquired from ticks or exposure to the tissues of infected animals, such as rabbits. Tularemia is not passed from one person to another and is treatable with antibiotics.

There have been two cases of tularemia in children in San Mateo County in the past 3 years. Both cases appear to have been caused by exposure to infected ticks at a location south of Pescadero along the coast highway. Tularemia has been detected in ticks collected at this site. In 2007, the bacteria was detected in 3 out of 102 adult American dog ticks (Dermacentor variabilis) and in 5 pools out of 128 adult Pacific coast ticks (D. occidentalis). The prevalence of infection in these ticks was 3 and 4%, respectively.

The survey conducted this month was aimed at assessing the level of infection in reservoir hosts. A total of 150 live traps were set and 12 animals were collected. Collections included 5 deer mice (Peromyscus maniculatus), 4 harvest mice (Reithrodonomys megalotis) and 3 California meadow voles (Microtus californicus). Blood samples obtained from these animals will be tested for serological evidence of exposure to tularemia by scientists at the Centers for Disease Control (CDC). Ultimately, the district plans to conduct monthly trapping and collect samples from at least 50 mice.

This project is a collaboration between the district, the San Mateo County Health Department, the California State Department of Public Health, and the CDC.

San Mateo County: Mosquito and Vector Control Association of California Annual Conference (Physician Survey on Lyme Disease)

Entomology Report January 2008, San Mateo County Mosquito Abatement District

Two laboratory staff presented research in the tick-borne disease symposium at the 2007 annual meeting of the Mosquito and Vector Control Association of California. Chindi Peavey, Vector Ecologist, spoke about tularemia, a disease of rabbits and rodents that can sometimes be transmitted to people. There have been 2 cases of tularemia in children in the past 3 years that have been traced to tick exposure on the coast near Pescadero. The District conducted follow-up investigations of these cases with the California department of Public Health in 2004 and 2006.

Assistant Vector Ecologist Lauren Marcus presented the findings of a physician survey performed in San Mateo County, in conjunction with the San Mateo County Health Department. Physicians were asked about their beliefs about Lyme Disease (LD). 86% of respondents believe that LD can be acquired locally. 73% of physicians note that 1 to 5 patients seek medical attention due to a tick bite annually and diagnose an average of 1.4 LD cases in 5 years. Of those who have diagnosed a patient with LD, 40% observed an erythma migrans (EM) rash and 52% stated that the patient had recalled a tick bite within three months of symptom onset. Physicians were asked what types of public education materials would be most useful. A resounding 80% wanted data on tick infection prevalence in the County, followed by 78% who wanted fact sheets on tick ecology, risk exposure, and prevention information. Finally, 73% wanted tick ID cards. Overall, the physician survey was successful in determining beliefs towards LD and what forms of educational materials were desired.

San Mateo County: Photomicroscopy – Borrelia burgdorferi (the Lyme disease spirochete)

Entomology Report January 2008, San Mateo County Mosquito Abatement District

District staff collect and test Ixodes pacificus ticks annually to detect the presence of the Lyme disease bacterium, Borrelia burgdorferi (Bb). Ticks are dissected and their guts are split in half to perform two tests on each tick. Each tick is tested by direct fluorescent antibody (DFA) technique which involves staining tick guts with a fluorescent antibody specific to Borrelia burgdorferi. If Bb is present, the stained bacteria will appear green when viewed through a fluorescent microscope. The other half of the tick gut is placed in a culture medium that supports growth of Bb. These cultures are viewed weekly to look for live bacteria. Under dark field microscopy the otherwise clear bacteria appear white against a black background.

San Mateo County: Tick Surveillance

Entomology Report November/December 2007, San Mateo County Mosquito Abatement District

This year, the lab will be measuring relative density and seasonal distribution of western black-legged ticks at five parks: Costanoa Lodge, Huddart Park, Skyline Ridge Open Space, Pulgas Ridge Open Space, and San Pedro Valley County Park. These parks represent the varied climates of the north, south, mid-, and coastal areas of the county. The study is designed to look at differences in the timing of tick activity in different parts of the county.

Additional testing for Lyme disease spirochetes will be conducted on ticks from Water Dog Lake and Camp Jones Gulch.