Surveillance Studies Of Aedes Stegomyia Mosquitoes In Three Ecological Locations Of Enugu, South-Eastern Nigeria.
A ONYIDO, N OZUMBA, V EZIKE, E NWOSU, O CHUKWUEKEZIE, E AMADI
mosquitoes, public health and control., surveillance, vectors
A ONYIDO, N OZUMBA, V EZIKE, E NWOSU, O CHUKWUEKEZIE, E AMADI. Surveillance Studies Of Aedes Stegomyia Mosquitoes In Three Ecological Locations Of Enugu, South-Eastern Nigeria.. The Internet Journal of Infectious Diseases. 2009 Volume 8 Number 1.
A surveillance study of
The increasing importance of
Materials and Methods
The Study Area
Enugu, the study area, is geographically located between latitudes 60 and 70 North of Equator and between longitudes 70 and 80 East of the Greenwich Meridian. It is a cosmopolitan city and the political capital of Enugu State in Southeastern Nigeria. It lies in the rain forest belt of Nigeria although it has derived savannah vegetation resulting from urbanization and other human activities. It has eight months of rainfall (April to November) and four months of dry season (November to March). The mean annual rainfall is 2000mm while the mean daily temperature varies from 22 - 280C in the rainy season to 28 - 320C in the dry season. Enugu is surrounded by Udi hills. It has an undulating topography with good natural drainage systems.
Enugu metropolis also serves as the commercial nerve centre of Enugu State and the administrative headquarters of many National and State Institutions. In addition, it has many secondary and tertiary institutions of higher learning. Also found within the metropolis are many privately owned clinics, primary health centers, general hospital and tertiary health institutions including the University of Nigeria Teaching Hospital, National Orthopedic Hospital and Parklane General Hospital. The city has a teeming population of about 3 million people.
The Study Sites
The study sites were No.33 Park Avenue compound at GRA, which also served as the office compound of the National Arbovirus and Vectors Research Centre Enugu, a subsidiary of the Federal Ministry of Health. It is situated in a low-density area of the metropolis and it was intended to represent a typical residential home in Enugu. The second ecological site was the Gmelina Forest Reserve of the Coal Corporation of Nigeria. The Gulf Course separates it from the GRA. It was used by the coal corporation to produce logs for the support of coal mines. It was intended to represent a forest condition throughout the year. The third ecological site was the Ekulu Riverbanks between the GRA and Trans-Ekulu Area of the metropolis. It was assumed to provide water for the breeding of the mosquitoes throughout the year.
Collection of the Mosquitoes
Table 1 shows the monthly collection of mosquitoes from the selected 3 ecological locations. A total of 5,251 mosquitoes made up of four species namely,
Out of 588 traps set at each location, 303 ovitrap paddles (51.53%) were positive with
Mosquito yield from the paddles were as follows: the 303 egg-positive paddles from Park Avenue yielded 3,186 mosquitoes at a rate of 10.51 eggs per positive paddle. The 152 egg-positive paddles from the Gmelina forest canopy yielded 927 mosquitoes at a rate of 6.09 eggs per egg positive paddle. The 188 egg-positive paddles from the Ekulu River banks yielded 1,138 mosquitoes at the rate of 6.05 mosquitoes per egg-positive paddle. From these results, each paddle set at Park Avenue compound yielded almost twice as much mosquitoes hatched out from each egg-positive paddles from the Gmelina Forest and Ekulu River banks respectively.
Out of the four mosquito species collected,
Table 2 shows the variations in mosquito populations, egg-positive paddles and the mosquito yields per paddle at the various periods of the year. At No.33 Park Avenue GRA, the mean egg-positive paddles per month varied from 22 egg-positive paddles between January and March (hot dry period of the year), to 29 egg-positive paddles between April and September (warm humid period /wet season), and 21 egg-positive paddles between October and December (cold dry period). The mean monthly mosquito yields from the paddles varied from 148 mosquitoes in the hot dry period to 324 mosquitoes per month, during wet season and 240 mosquitoes during cold-dry periods. The mean number of mosquitoes hatching out per egg-positive paddle ranged from 6.7 mosquitoes per paddle in hot dry period, to 11.2 mosquitoes per paddle in the wet period and 11.4 mosquitoes per paddle in cold dry period.
At the Gmelina forest, the monthly egg-positive paddles ranged from 8 egg-positive paddles per month in the hot dry period to 17 egg-positive paddles in the wet season and 8 egg-positive paddles in the cold dry period. The mean numbers of mosquitoes per month collected were from 37 mosquitoes per month in hot dry period, to 87 mosquitoes per month in wet season and 70 mosquitoes in cold dry period. The mosquito yield per egg-positive paddle was from 4.6 mosquitoes per paddle in hot dry periods to 5.2 mosquitoes per paddle in wet months and 8.6 mosquitoes per paddle in cold dry months.
At the Ekulu River banks, the monthly egg-positive paddles varied from 9 egg-positive paddles per month in hot dry period to 19 egg-positive paddles in the wet months and 10 egg-positive paddles in cold dry months. The mean numbers of mosquitoes collected per month varied from 27 mosquitoes per month in the hot dry period, to 125 mosquitoes per month in the wet months and 68 mosquitoes per month in the dry cold months. The mosquito yields per egg-positive paddle varied from 3 mosquitoes per paddle in hot dry months to 6.6 mosquitoes per paddle in the wet months and 6.8 mosquitoes per paddle in cold dry months.
In all the locations, the mean number of egg-positive paddles, the mean number of mosquitoes collected and the mean number of mosquitoes hatching out per egg-positive paddle were least in the hot dry periods (January – March), followed by those of the cold dry period (October to December). The numbers were significantly higher during the wet months (April to September).
Figure 1 shows the monthly variation in the numbers of mosquitoes collected during the study period. At Ekulu River banks, a population of about 450 mosquitoes was obtained in the month of April, which sharply declined but showed two minor peaks in September and November months. At the Gmelina forest, a peak population was obtained in May, which again declined but showed two minor peaks in September and November months respectively.
Egg-positive paddles were collected from the three ecological locations in all the months of the year. This suggests that
The mosquito collections from Park Avenue compound were about twice the collections from the other ecological locations. The Park Avenue compound was intended to represent a normal compound in the city operating under water scarcity and abundance depending on water supply from the water cooperation. However the preponderance of
The collection of more
The mean number of egg positive-paddles, the mean number, of mosquitoes collected and the number of eggs hatching out per paddle were found to be low during the hot-dry and cold-dry periods but significantly high during warm humid periods. The cold dry period (October to December) comes at the end of rains and usually marks the beginning of dry season. This period varies from place to place and year to year and sometimes may not be pronounced and therefore passes unnoticed. It is usually characterized by cold spells and dry dusty winds that may be deleterious to the survival of many organisms. The hot dry period (January to March) is usually marked with hot sunny days, high temperatures and low relative humidity with little or no rainfall during this period. This also makes the survival of many organisms difficult and therefore accounts for low figures within the period. The warm humid period or wet season (April to October) is usually characterized by rains, high relative humidity and moderate temperatures favorable for the survival of organisms during this period.
From the graphs, a high peak of mosquito population was obtained between April and June in all the three ecological locations. A sharp decline in mosquito populations between June and August, and a small peak followed this in September to November. The major mosquito population peaks obtained between April and June correspond with the beginning of the warm humid weather favorable to the growth of organism. Also the rains came at intermittent intervals giving sufficient time for the mosquitoes to hatch out and develop within the small pools of water. The sharp decline from June depicts heavy rains, which flushes and destroys developing mosquito eggs and larvae leading to fall in population. The small peak in September may be as a result of short intervals in August break while the November peak corresponded with the end of rains in which rains were again at intervals giving sufficient time for immature stages to develop, while the final decline were due to the drying up of the little pools of water left by the rain, thereby limiting reproduction and development.
This study thus reveals that adult female