by George Looby
In their never ending quest to find answers to all of the questions regarding Eastern Equine Encephalitis researchers continue to study all of the contributing factors in the ever-changing picture. EEE has long been recognized as one of the most deadly insect borne diseases affecting man, horses and game birds in the eastern U.S. In much of the region it appears in mid-summer extending on into late fall coming to an end with the first killing frost. Conscientious horse owners for many years have incorporated yearly vaccinations into their horse health programs with good to excellent results. The symptoms in infected animals can best be described as that of a mental stupor progressing on to death in a large percentage of cases. Those that may survive are often left with significant neurological deficits. In the horse there is no specific treatment except that of a supportive nature. Since 2003 the CDC reports there have been 55 human cases of this disease resulting in 23 deaths.
It has long been known that game birds, particularly pheasants and quail, play a significant role in the human, horse and bird cycle but the role of birds in the wild contained many unanswered questions. It was Dr. Goudarz Molaei, Research Scientist at the Connecticut Agricultural Experiment Station (CAES) in New Haven, CT and co-workers who developed a research project to look at the incidence of the EEE virus in the wild bird population in the northeastern states. Dr. Theodore G. Andreadis, Director of the Station served as one of the co-investigators in this trial together with M .C. Thomas, T. Muller, J. Medlock, J. Shepard and P.M. Armstrong. One of the factors that stimulated their interest was that the distribution pattern of the disease has changed over the past decade moving further into northern New England and southern Canada where it had not been seen before.
It was decided to examine the blood feeding habits of the mosquito on wild birds in an attempt to determine which one(s) were most involved in the transmission of EEE. Entomologists have named this insect Culisetea melanura. This mosquito prefers to live in freshwater woodland swamps so four areas that fit this preference were selected to conduct this study. Chester, Madison and Killingworth, CT were towns selected along with North Stonington located on the Rhode Island border in the southeastern corner of the state. Each of these towns had swamps that provided near ideal habitat for the mosquito to be studied.
This study had at least three objectives: 1) Quantify which species of birds the mosquito had a preference for. 2) Which birds were most often bitten and infected by the mosquito and 3) Determine the extent to which preferences shape virus transmission in the four areas being studied. One of the most interesting findings was that the mosquito had a definite preference for certain birds over others and that this apparently was not that the population of a given species outnumbered another. The scientists developed a sophisticated mathematical model that took into account that factor.
A highly knowledgeable bird watcher became a member of the team and it was this individual’s responsibility to monitor and record all of the numerous species found within each of the four swamps. The number of species observed was quite remarkable, a bird watchers paradise by any standard. The season for observations ran from May to October for two years (2010 – 2011). The mosquito under study fed on over 65 different species of birds. It became apparent that the Wood Thrush was at the top of list when it came to the most favorite species list for a blood meal by the resident mosquitoes. Other birds that were considered top targets included the American Robin, Tufted Titmouse, Common Grackle, Chipping Sparrow, Black Capped Chickadee, Northern Cardinal and Warbling Vireo.
At each of the swamps collection boxes were set up to trap the mosquitoes. Once a day a hand held aspirator was used to pull the blood filled mosquitoes out of the traps. They were then placed in coolers and transported to the lab. At the lab, technicians sorted out the mosquitoes by species using dissecting microscopes and identification keys. Those with viable blood meals were transferred to tiny (1.5 ml.) micro tubes and were placed in an ultra low temperature freezer. The next step was extracting the blood from the specimens collected and analyzing it for its various components which allowed the investigators to determine which birds had been fed upon by the mosquitoes under study. Each creature has a unique genetic make up based on DNA sequences that allow investigators to determine with great accuracy the origin of that sample
A total of 6234 female Cs. Melanura were collected over the two years of the study. The preferred host in the majority of cases was the Wood Thrush. The number of Wood Thrushes in the total population of birds was small but the number of mosquitoes captured that had fed on that species was high, leading the researchers to conclude that the Wood Thrush was the most important source of new infections in the entire population. The reason for this preference remains unknown at this point. Of the samples collected from mosquitoes captured in Killingworth late in the season, 80.65 percent were obtained from Wood Thrushes. Similar numbers were obtained from the other sites.
These findings will help researchers in further studies relating to the spread and possible control of this most serious mosquito borne disease. As this new knowledge is studied we can be sure that it will be put to good use in the ongoing battle against EEE.
The Wood Thrush’s role in Eastern Equine Encephalitis (EEE)
by George Looby