General Information
The Orange County Vector Control District routinely conducts field surveys to determine the presence of vector-borne disease. This operation is overseen by the District Technical Director who is assisted by a dedicated staff of vector ecologists and laboratory/field biologists. The diseases of prime concern are those carried by mosquitoes, fleas, ticks, and rodents. Surveillance and detection programs are designed around each of these vectors. When a vector-borne disease is detected by routine surveillance activities, the risk options are evaluated by management. If risk is determined, then local and state health agencies are informed, including the public.
Mosquito-Borne Diseases
The primary concern with mosquito-borne disease is the transmission of encephalitis virus to humans. Since encephalitis viruses are known to be carried by certain wild birds, the District samples blood (obtained humanely) from birds from key locations throughout the County. The blood samples are tested for antibodies associated with a recent infection (via mosquito bite) of either West Nile Virus (WNV), St. Louis Equine Encephalitis (SLE) or Western Equine Encephalomyelitis (WEE) virus. Human WNV and SLE infections normally affect elderly persons, while WEE infections impact infants and adolescents. Furthermore, human WNV and SLE cases are more abundant from late summer to early fall and WEE cases usually occur in late spring and early summer.
Along with obtaining virus antibody samples from wild birds, the District also closely monitors local mosquito population numbers by simultaneously operating carbon dioxide-baited traps that selectively trap female mosquitoes searching for blood meals from a vertebrate host. Mosquito numbers and species, plus antibody information obtained from wild birds are analyzed in conjunction with prevailing weather conditions. Collectively, these factors provide an overview of public risk to exposure of being bitten by a mosquito carrying encephalitis virus.
Encephalitis Virus Disease Cycle
Plague
Plague is a natural occurring bacterial disease associated with wild rodents and fleas. In Orange County, plague has demonstrated some sporadic historical occurrence in the uplands along the Santa Ana River adjoining San Bernardino and Riverside Counties. Plague in southern California is typically associated with ground squirrels (Spermophilus sp.) and pack rats (Neotoma spp.), and very rarely with commensal rodents like the roof rat (Rattus rattus). The District routinely traps ground squirrels at parks and historic plague sites. A sample of blood is taken along with the fleas brushed from their bodies. The blood sample is tested for plague antibodies and fleas are tested for the presence of infectious plague bacteria.
Hantavirus
Hantavirus is a viral pathogen found in rodents that affects humans by attacking the lungs and producing an often (50%) fatal pneumonia. The virus is inhaled as an aerosol originating from contaminated fecal pellets (droppings) and urine. The particular strain of Hantavirus encountered locally is the Sin Nombre ("without a name") Virus (or SNV) associated naturally with deer mice (Peromyscus maniculatus) and rarely packrats (Neotoma spp.). The District traps and takes blood samples from both deer mice and packrats throughout the County. A note of caution: Wherever deer mice occur, there are individuals that invariably test positive for SNV!
Lyme Disease
Lyme Disease, carried by ticks, is caused by a single-celled bacterial parasite called a spirochete. When a tick carrying Lyme Disease spirochetes attaches and begins ingesting blood, transmission of the spirochetes does not occur immediately, but approximately six to eight hours thereafter. The disease can become very debilitating if not treated shortly after infection. Common symptoms occurring after infection, include a rash (Erythema Migrans - EM) followed by aches and pains, stiffness in joints, muscular abnormalities, and loss of equilibrium/coordination.
Lyme Disease in California is more prevalent along the north coast where the cool and wet climate favors optimal survival of the Pacific Black-legged Tick (Ixodes pacificus) vector. This tick is relatively common in Orange County, but the factors affecting the Black-legged tick that affect its spirochete infection capacity are unknown. Because the risk of Lyme spirochete transmission is probably cyclic, the District continues to regularly collect and test Pacific Black-legged Ticks and other tick species.
