When considering the life of the mosquito, we can say that it has a very extraordinary adventure. It begins with a bite, a painless bite. The mosquito comes in the night, alights on an exposed patch of flesh, and assumes the hunched, head-lowered posture of a sprinter in the starting blocks. Then she plunges her stiletto mouth-parts into the skin. The mosquito has long, filament thin legs and dappled wings; she’s of the genus Culex (Cx), the only insect capable of harboring the diseases in animals and humans. And she’s definitely a she: Male mosquitoes have no interest in blood, while females depend on protein rich hemoglobin to nourish their eggs. Female mosquito that bites an infected human or animal can pick up pathogens along with the blood meal and can easily pass to other non-infected humans and animals. During pathogens pick and drop service mosquito and pathogen do not harm each other but the pathogen reproduces inside mosquito body.
Factors like ground water and their environs included: ponds filled by gutters draining, grassy organic pools, borrow pits filled by irrigation or rain water, drying irrigation water courses and temporary rain filled pools are positive breeding habitat for mosquitoes. In Pakistan, the population dynamics of most ground water breeding mosquitoes are dependent upon temperature, irrigation practices, rainfall, with peak relative abundance occurring during the pre-monsoon period. Among the major mosquitoes borne viral infections, west nile virus (WNV) is an important threat to human and animal health. The virus exists in nature primarily through a transmission cycle involving mosquitoes and birds but sometimes also isolated from tick vectors (Argas, Hyalomma, and Ornithodoros). Mosquitoes become infected with WNV when they feed on vertebrate hosts which are mainly infected birds.
In humans, infection with WNV can cause clinical or subclinical symptoms. Clinical symptoms vary from temporary fever to serious encephalitis (swelling of brain). The disease can be severe in the elderly, but is usually mild in healthy adults and children. The incubation period for WNV is 3-6 days and onset of disease symptoms is usually sudden, beginning with a sustained, elevated fever. Clinical infection can include severe headache; a rash, usually on the trunk; and swollen lymph nodes. Symptoms of WNV infection can also include eye, muscular and back pains, and gastrointestinal problems. In severe cases, there are often symptoms of encephalitis with eventual neurological involvement and sometimes death. Humans experience a low level of viremia that lasts about 6 days with 5 to 13 per cent mortality rate.
In animals, clinical signs of WNV infection have only been observed in equines (horses, donkeys, mules and zebra), even though most equines infections are asymptomatic and experienced 25 per cent mortality. As with horses, cattle, sheep, and camels can also be infected with WNV, but clinical symptoms and viremia capable of infecting arthropod vectors have not been reported from these hosts. Birds, however, do experience viremia capable of infecting arthropod vectors (mosquitoes and ticks).
WNV is widely distributed in Asia, Africa, and America. However, worldwide different ecological investigations suggested that WNV probably persists naturally in bird populations with occasional transmission with infected migratory birds to mammals of another region. In India members of the vishnui complex (pseudovishnui, tritaeniorhynchus and vishnui) and perhaps Cx quinquefasciatus are apparently important in transmission. Throughout the distribution of WNV, low anthropophagic feeding patterns by the presumed vectors have led several workers to suggest that Cx quinquefasciatus could be important in transmitting WNV to man and other mammals.
In Pakistan, WNV has been isolated from febrile patients and pools of Cx quinquefasciatus and “Cx tritaeniorhynchus” (possibly pseudovishnui + tritaeniorhynchus) mosquitoes. The residents of several villages have demonstrated a serological picture of endemic WNV exposure. Local strains of Cx tritaeniorhynchus are highly susceptible to WNV in the laboratory; however, host selection patterns in nature appear unsuitable for WNV maintenance in birds, since over 90 per cent of reacting blood smears reported to be positive for bovids.
An important question concerning WNV movement and transmission throughout Pakistan is: What can be done to monitor the movement and introduction of WNV in new cities and localities throughout Pakistan? The answer is simple: surveillance, surveillance, surveillance, and more surveillance, and surveillance mainly includes: vector surveillance, amplification host surveillance, meteorological surveillance, and virus surveillance. Moreover, mosquito and vector control programs throughout the Pakistan already have considerable experience for dengue virus infection. Programs similar to this should be considered for areas that are at high risk for arboviral transmission.
So, an accurate risk assessment of a vector borne epidemic will give all public health authorities time to institute control strategies and public awareness campaigns that will reduce the impact of an epidemic. Authorities should make their decisions about what strategies to use based on scientific information about the pathogen and vectors involved, and local or regional environmental conditions. It is generally accepted that it would be enormously costly and very difficult to vaccinate large human populations to prevent a vector borne epidemic. However, vector control and personal protection against vectors and the diseases they carry are the best way to avoid infection with vector borne pathogens.
So, it is recommended to protect against biting arthropods, particularly when they are infected with dangerous pathogens. Make sure screens are in good repair to prevent mosquitoes from entering houses. If you must enter areas where there is a threat of encountering infected mosquitoes, wear protective clothing. Finally, use a personal insect repellent that provides a reasonable complete protection time (CPT). The CPT is the total time following repellent application that the treated individual will remain bite free. Unfortunately, Pakistan has not good mosquito and arthropod control programs. However, in order to efficiently cover the area of infections related to mosquitoes borne infections try to reduce mosquitoes breeding sites; focal applications of insecticides directed against adult and immature mosquitoes at breeding habitats and their environs; public service announcements to educated residents about the vector, the disease, and disease avoidance; tips help to prevent home invasion by infected vectors; and information about the most effective means of personal protection.
The writers are associated with the Department of Entomology, University of Agriculture, Faisalabad.
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