The Eco-Epidemiology of Pacific Coast Tick Fever in California

Rickettsia philipii (type strain “Rickettsia 364D”), the etiologic agent of Pacific Coast tick fever (PCTF), is transmitted to people by the Pacific Coast tick, Dermacentor occidentalis. Following the first confirmed human case of PCTF in 2008, 13 additional human cases have been reported in California, more than half of which were pediatric cases. The most common features of PCTF are the presence of at least one necrotic lesion known as an eschar (100%), fever (85%), and headache (79%); four case-patients required hospitalization and four had multiple eschars. Findings presented here implicate the nymphal or larval stages of D. occidentalis as the primary vectors of R. philipii to people. Peak transmission risk from ticks to people occurs in late summer. Rickettsia philipii DNA was detected in D. occidentalis ticks from 15 of 37 California counties. Similarly, non-pathogenic Rickettsia rhipicephali DNA was detected in D. occidentalis in 29 of 38 counties with an average prevalence of 12.0% in adult ticks. In total, 5,601 ticks tested from 2009 through 2015 yielded an overall R. philipii infection prevalence of 2.1% in adults, 0.9% in nymphs and a minimum infection prevalence of 0.4% in larval pools. Although most human cases of PCTF have been reported from northern California, acarological surveillance suggests that R. philipii may occur throughout the distribution range of D. occidentalis

Detection of West Nile Virus Infection in Birds in the United States by Blocking ELISA and Immunohistochemistry

A blocking ELISA targeting an immunodominant West Nile epitope on the West Nile Virus NSI protein was assessed for the detection of West Nile-specific antibodies in blood samples collected from 584 sentinel chickens and 238 wild birds collected in New Jersey from May-December 2000. Ten mallard ducks (Anus platyrirynchos) experimentally infected with West Nile virus and six uninfected controls were also tested. The ELISA proved specific in detecting WNV antibodies in 9/10 chickens and 414 wild birds previously confirmed as positive by Plaque Reduction Neutralization test (PRNT) at the Center for Disease Control, Division of Vector Borne Diseases, Fort Collins, CO, USA (CDC). Nine out of the ten experimentally infected mallard ducks also tested positive for WN antibodies in the blocking ELISA, while 616 uninfected controls did not. Additionally, 1705 wild birds, collected in New Jersey from December 2000-November 2001 and Long Island, New York between November 1999 and August 2001 were also tested for WN antibodies by the blocking ELISA. These tests identified 30 positive specimens, 12 of which had formalin-fixed tissues available to allow detection of WN specific viral antigen in various tissues by WNV-specific immunohistochemistry. Our results indicate that rapid and specific detection of antibodies to WN virus in sera from a range of avian species by blocking ELISA is an effective strategy for WN Virus surveillance in avian hosts. In combination with detection of WN-specific antigens in tissues by immunohistochemistry (IHC) the blocking ELISA will also be useful for confirming WN infection in diseased birds. Key Words: West Nile Virus-Blocking ELISA-Immunocytochemistry-Wild birds-Arbovirus-Serum-blood spot. Vector-Borne Zoonotic Dis. 3, 99-110

The Eco-epidemiology of Pacific Coast Tick Fever in California.

Rickettsia philipii (type strain "Rickettsia 364D"), the etiologic agent of Pacific Coast tick fever (PCTF), is transmitted to people by the Pacific Coast tick, Dermacentor occidentalis. Following the first confirmed human case of PCTF in 2008, 13 additional human cases have been reported in California, more than half of which were pediatric cases. The most common features of PCTF are the presence of at least one necrotic lesion known as an eschar (100%), fever (85%), and headache (79%); four case-patients required hospitalization and four had multiple eschars. Findings presented here implicate the nymphal or larval stages of D. occidentalis as the primary vectors of R. philipii to people. Peak transmission risk from ticks to people occurs in late summer. Rickettsia philipii DNA was detected in D. occidentalis ticks from 15 of 37 California counties. Similarly, non-pathogenic Rickettsia rhipicephali DNA was detected in D. occidentalis in 29 of 38 counties with an average prevalence of 12.0% in adult ticks. In total, 5,601 ticks tested from 2009 through 2015 yielded an overall R. philipii infection prevalence of 2.1% in adults, 0.9% in nymphs and a minimum infection prevalence of 0.4% in larval pools. Although most human cases of PCTF have been reported from northern California, acarological surveillance suggests that R. philipii may occur throughout the distribution range of D. occidentalis

<i>Dermacentor occidentalis</i> collection records in California by month; adults were collected from 1948–2014, nymphs 1949–2014, and larvae 1969–2014.

<p>Red Arrows indicate illness onset month for human cases of Pacific Coast tick fever (numbers inside arrows represent onset of symptoms for human cases per month).</p

Prevalence of <i>Rickettsia philipii</i> and <i>Rickettsia rhipicephali</i> detected in <i>Dermacentor occidentalis</i>, by county, California, 2009–2015.

<p>Prevalence of <i>Rickettsia philipii</i> and <i>Rickettsia rhipicephali</i> detected in <i>Dermacentor occidentalis</i>, by county, California, 2009–2015.</p

Number of nymphal <i>D</i>. <i>occidentalis</i> collected biweekly at Jack London State Park, Sonoma County, California, May-October, 2009–2010.

<p>Number of nymphal <i>D</i>. <i>occidentalis</i> collected biweekly at Jack London State Park, Sonoma County, California, May-October, 2009–2010.</p

Counties where <i>Dermacentor occidentalis</i> ticks have tested positive (green) and negative (blue) for <i>Rickettsia philipii</i> (Lane et al., 1981 and Philip et al., 1981; present study).

<p>Counties with no ticks tested (white). Counties where Pacific Coast tick fever confirmed cases were acquired (red dots, number indicates number of cases), California, 2005–2014.</p

Serological results from all human Pacific Coast tick fever case patients, California, 2008–2014.

<p>Serological results from all human Pacific Coast tick fever case patients, California, 2008–2014.</p