In this article we will discuss about the diseases caused by mosquitoes. The diseases are: 1. Yellow Fever 2. Dengue Fever 3. Chikungunya.
Disease # 1. Yellow Fever:
It is transmitted by Aedes and other mosquitoes and is an acute specific viral fever of short duration, varying in severity. The severe cases are characterised by toxic jaundice, albuminuria and haemorrhages (apistaxis, haematemesis, melaena), etc. It occurs endemically or epidemically in certain geographical areas of Africa and South America.
The limited outbreaks occur in forest areas. The yellow fever has not been recorded in Asia, although the potential vectors like A. aegypti abound there. The strict control of immigrants, disinfection/disinfestation of aircrafts and ships has so far prevented the entry of this virus in the Asian continent. The virus can be eradicated in man as well as in the reservoir hosts.
Vector mosquitoes can be destroyed and the human beings can be vaccinated against yellow fever. The persons going from or coming to infected areas require international certificate of yellow fever vaccination. The effect of vaccination is valid 10 days after the date of vaccination and extends up to 10 years.
Disease # 2. Dengue Fever:
It was Rush, who in 1780, gave the first clear cut account of dengue fever from Philadelphia and called it ‘break-bone’ fever. Graham in 1905 was the first to prove the mosquito transmission of dengue. A. aegypti as the carrier of dengue was proved in 1906 by Bancrofti in Australia.
The disease (domestic dengue) is caused by an arbovirus (B group) existing in four forms, viz. Dengue 1, Dengue 2, Dengue 3 and Dengue 4, all of which are transmitted chiefly by A. aegypti. Other species of the genus Aedes (A. albopictus, A. squtellaris, A. albimanus and A. hebrideus) and Armigeres obtarbans also transmit this viral infection. The domestic mosquito (A. aegypti) maintains the disease cycle in man, while A. albopictus and others living in the bush or forests help in the maintenance of infection, among monkeys (jungle dengue).
The disease is widely distributed in the tropical and sub-tropical areas. It occurs in the Eastern Mediterranean countries, Africa, India and South-east Asian countries, in the Far East and in the Hawaiian and Caribbean Islands. It also occurs in southern United States and Australia.
The disease may be benign or may carry a serious prognosis. The former is a classical or normal dengue fever which occurs in infants and young children with mild febrile illness and maculopapular rash. Older children or adults may suffer from classical symptoms of high fever, headache, myalgia and rashes.
The dengue haemorrhagic fever (DHF) and its more severe form, the dengue shock syndrome (DSS) are more serious diseases. In epidemic form, Dengue 3 and Dengue 4 may cause high morbidity and even high mortality in children, the aged and the infirm. Calcutta city has already experienced outbreaks of haemorrhagic fever of the type Dengue 3 and Dengue 4 during 1963.
In 1996, there was an epidemic of dengue fever in Delhi with more than 7000 DHF cases and about 300 mortalities. The dengue appeared in epidemic form in 2006 with more than 13000 cases and 200 deaths. Recently, in 2009-12, dengue incidence and mortality rose by more than 100 per cent in India. Between 2009 and 2012, cases grew from 15,000 to 37,070 and deaths from 110 to 227.
The dengue fever is an acute febrile illness clinically characterized by haemorrhagic phenomenon and a tendency to develop a shock syndrome which may be fatal. The diagnostic symptoms are acute onset, high continuous fever lasting to 2-7 days, with various haemorrhagic manifestations like petechiae, purpura, achymosis epistaxis, gum-bleeding, hematemesis and/or melena, the enlargement of liver and shock manifested by rapid and weak pulse, narrow pulse pressure or hypotension, etc.
The patient is restless with cold clammy skin. Incubation period in man is 4-10 days. The mosquito becomes infected only during the first 3 days of patient’s illness. Incubation period in mosquito varies from 8 to 11 days. Once infected, the mosquito remains so for its life and when the mosquito introduces saliva into the man’s skin during feeding it transmits infection.
There is no transovarial transmission with respect to the mosquitoes. Although in India, it is becoming endemic in some parts, it still exists in epidemic form in certain parts of South-east Asia like Thailand, Myanmar and Malaysia. The preventive measures include the vector control and the screening of all early cases so as to avoid the mosquitoes becoming infective.
Disease # 3. Chikungunya:
It is a viral (arborvirus-A) disease characterized by high fever and severe pains in the trunks and joints. This disease occurs in Africa and Asia. In India, it was detected in Calcutta during 1963 during an outbreak of haemorrhagic fever. Subsequently, this disease was also reported to occur in Chennai and Vellore in South India.
The recent outbreak of this disease occurred in 2006 and the affected states were Andhra Pradesh, Delhi, Gujarat, Karnataka, Kerala, Madhya Pradesh, Maharashtra, Pondicherry, Rajasthan and Tamil Nadu. Many mosquitoes act as vectors for the transmission of this disease.
A. aegypti, A. africanus (Theobald), C. quinque-fasciatus Manosnia. spp. are active in Africa, while A. aegypti, C. quinquefasciatus, C. tritaeniorhynchus, Giles and C. gelidus Theobald are the vector species incriminated in Thailand. In India, A. aegypti remains the sole vector of this disease. The incubation period in mosquitoes is 14 days. Besides man, monkeys and apes play some part as reservoirs of this virus.