MARYLAND WEST NILE VIRUS SUMMARY, 2001

Introduction:

Each year the state of Maryland conducts surveillance for West Nile virus (WNV) and other arboviruses in order to determine the level of arboviral activity in the state. As one of a family of viruses found in temperate regions of Europe, Africa, and (most recently) North America, West Nile virus is transmitted by mosquitoes and has been shown to infect horses and both domestic and wild birds. It can also pose a threat to human health. In order to minimize the risk of infection to humans, public health officials regularly monitor WNV and other arboviruses through a systematic process involving a partnership between select state agencies.

The Maryland Department of Health and Mental Hygiene (DHMH), in collaboration with the Maryland Department of Agriculture (MDA) and the Maryland Department of Natural Resources (DNR), has implemented a disease surveillance system to detect West Nile virus and other arboviral activity in Maryland. This system includes four major components: mosquito, avian (bird), veterinary, and human. This report summarizes WNV surveillance testing results in 2001.

Major Components:

Mosquito Surveillance:

The Maryland Department of Agriculture (MDA) sampled all 24 jurisdictions in the state. This sampling process involved trapping, collecting, and sorting mosquitoes into groups ("pools") by genus and species. All mosquito pools were tested for the presence of three viruses: West Nile virus (WNV), Eastern equine encephalitis virus (EEE), and St. Louis encephalitis virus (SLE). Of the total of 12,807 mosquito pools (192,682 mosquitoes) that were submitted for viral testing, 18 pools (0.14%) tested positive for West Nile virus (most of them in Baltimore city) and four tested positive for EEE. In addition, one pool tested positive for Jamestown Canyon virus (this result was confirmed by the Centers for Disease Control & Prevention), a virus that has had wide geographic distribution throughout North America but had not previously been documented in Maryland.

Avian Surveillance:

Live Bird Surveillance:

The Department of Natural Resources (DNR) Fish and Wildlife Health Program conducted live bird surveillance in the form of an ongoing wild and captive bird serosurvey. In 2001, 69 wild bird blood samples were submitted to the National Wildlife Health Center in Wisconsin. All wild bird samples were negative for WNV antibody.

Dead Bird Surveillance:

A West Nile Virus Surveillance Call Center was operational between May and November 2001. The toll-free information line, which operated 24 hours/day, 7 days/week, featured pre-recorded informational messages regarding WNV and related issues. Trained operators were also available during business hours and on weekends to accept reports of dead birds. The operators relayed dead bird reports to local WNV coordinators in each jurisdiction, who in turn coordinated dead bird pick-up and sent selected bird specimens to the DHMH Laboratories for arboviral testing. Between May 1 and November 1, 2001 the Call Center received a total of 30,327 calls, of which 58% were to report dead/dying birds. A total of 1503 birds were tested for WNV, EEE, and SLE.

Four hundred and fifty-four (454) birds from 11 jurisdictions tested positive for WNV (436 crows, 16 blue jays, 1 hawk, and one 1 non-crow). In addition, one blue jay from Worcester County tested positive for EEE.

Veterinary Surveillance:

All Maryland veterinarians registered with the Maryland State Board of Veterinary Medical Examiners were contacted by the Maryland State Public Health Veterinarian in a mass mailing alerting them to the clinical signs and symptoms of West Nile and other arboviral infections in horses and small animals (cats and dogs).

Thirty-four (34) horses were tested for the presence of WNV antibody as well as for EEE and SLE. In addition, three (3) horse brain tissue specimens were submitted for reverse transcriptase polymerase chain reaction (RT-PCR) testing; all three were negative for WNV. Seven confirmed cases and one probable case of WNV were identified. Of the seven horses that had a confirmed WNV infection, one horse in Baltimore County was euthanized.

The United States Department of Agriculture (USDA) conditionally approved an equine WNV vaccine on August 1, 2001, and information about its availability was distributed to veterinarians throughout the state. A total of 35,010 doses were distributed between August and December 2001. In addition to the horse submissions, several other animal specimens were submitted to DHMH laboratories for arboviral testing in 2001, including 151 bats, 1 goat, 1 dog, and 1 cat. All of these specimens tested negative for WNV.

Human Surveillance:

Guidelines were issued to all local health departments (LHDs) concerning enhanced passive surveillance of human encephalitis (inflammation of the brain) and aseptic meningitis (inflammation of the membranes that envelop the brain and spinal cord) cases in order to confirm or rule out WNV as a cause. The Acting State Epidemiologist notified infection control professionals (ICP), emergency department directors (EDD), and physicians in Maryland hospitals of the existence of the human arbovirus surveillance project and requested that they report any suspected cases of encephalitis or meningitis to the appropriate LHDs. Laboratory technicians at DHMH Laboratories tested samples of human blood and cerebrospinal fluid (CSF) and also performed viral culturing of select specimens. In addition to WNV, human blood samples were also tested for LaCrosse, Eastern and Western equine, and St. Louis encephalitis viruses. In total, 440 humans were tested for arboviral infection in 2001. Six humans tested positive for antibody to WNV in Maryland, and two of them died. One human tested positive for LAC antibody.

Other Components

Mosquito Control:

MDA conducted a mosquito control program that involved ground spraying of the adulticide Permethrin in designated regions. Total adult mosquito control in Maryland for 2001 covered 4230 acres.

Pesticide Toxicity Surveillance:

The Office of Environmental Health Coordination (OEH) established three systems for obtaining pesticide toxicity surveillance information: (1) an online physician reporting system, (2) the Environmental Protection Agency (EPA) National Pesticide Information Center, and (3) five sentinel hospitals in areas where spraying occurred. Physicians and emergency room triage nurses were instructed to document and report any pesticide-related illnesses of patients seen in their facilities. The Maryland Poison Control Center (Baltimore) and the National Capital Area Poison Control Center (DC) also responded to calls from citizens regarding pesticide-related symptoms. No cases of human pesticide toxicity-related illnesses were reported from any of the three surveillance systems.

Summary and Conclusion:

The expansion of mosquito, bird, and human surveillance efforts enabled the Department of Health and Mental Hygiene to gather vital information about the presence of West Nile virus and other arboviruses (including EEE and LAC) in Maryland. The positive dead bird and mosquito testing results collectively represent a viable indicator of WNV activity in Maryland, and emphasize the need for continuing mosquito and avian surveillance. Likewise, the human incidence of WNV and LAC suggest that these arboviruses may pose an ongoing threat to Maryland residents. These viruses will be monitored further in 2002.

Dissemination of information about WNV and other arboviruses also promoted the success of the 2001 surveillance program. Timely communication between DHMH and veterinarians, physicians and other health professionals, and agencies proved valuable for gaining the cooperation of those key personnel. Similarly, public education in the form of literature distribution and media campaigns strengthened the effectiveness of dead bird reporting and will remain an integral part of the 2002 surveillance plan. The results described above provide preliminary evidence of arboviral activity in the state and will help to shape surveillance efforts for 2002.

Return to Fact Sheet Index

Maryland Department of Health & Mental Hygiene — Epidemiology & Disease Control Program