SENTINEL FLOCK SURVEILLANCE PROGRAM - TEST RESULTS

Interpreting Test Results

Periodic monitoring of senitnel flocks enables the state to monitor for the presence of viral agents that are transmitted by mosquitoes.  Throughout the arboviral season, blood samples are drawn from each hen every other week using proper animal care procedures by trained personnel.  The North Carolina State Laboratory of Public  Health (SLPH) tests each serum sample for antibodies for Eastern Equine Encephalitis (EEE), Highlands J (HJ), Saint Louis Encephalitis (SLE), and California Group (CAL) viruses. Because of serological cross-reactions amoung some of the viruses mentioned, the LAC reagent will detect antibody to West Nile virus (WNV).  Results are promplty reported to Public Health Pest Management as well as mosquito control programs and local health departments. Each blood sample is processed to separate sera which is tested for the presence of antibodies to the above mentioned viral agents.  If antibodies are present, the cells on the slide will react by fluorescing a bright green color. The percentage of cells that demonstrate fluorscence gives an indication of the amount of antibodies that are present in the sample.  A non-reactive sample and a highly reactive sample are shown below for comparison.

Negative

The few fluorescent cells are an artifact of the testing process and not an indication of positive results.

Reactive

A large quantity of fluorescent cells are dispersed evenly throughout the sample in sufficient brilliance to indicate a non-specific reading.

The Indirect Fluorescent Antibody (IFA) test used by the NC SLPH is very sensitive and some non-specific fluorescence is occasionally seen, even with negative sera. Serological samples are evaluated on a scale from negative to 3+ fluorescence. A reaction classified as 2+ or 3+ is considered highly reactive and indicates the presence of anitbody to the virus in question, or in some cases, a closely related virus (for example SLE and WNV cross-react with each other).  Specimens that demostrate weak fluorescence maybe classified as +/- or occasionally 1+.  Subsequent specimens from these hens will determine if the weakly reactive test in non-specific or represents the initial production of virus-specific antibody. Hens that have been laboratory confirmed to have been infected with one of the viruses in question will be removed from the monitoring flock and replaced with a new chicken.

Positive

Here is a close up view of a reactive slide. This slide shows high reactivity, and is an example of a 2-3+ category.  Note the bright, even green fluorescence on a high proportion of the cells.

It’s important to remember that any laboratory results testing positive are promptly reported to the Public Health Pest Management Section (PHPM), the local mosquito control progam and the local health departments.  If deemed appropriate mosquito control measures are implemented to limit human exposure to the virus.