6 Major Diseases Caused by Mosquitoes

The following points highlight the six major diseases caused by mosquitoes. The diseases are: 1. Malaria 2. Filariasis or Elephantiasis 3. Encephalities 4. Dermatobia 5. Dengue 6. Dengue Haemorrhagic Fever.

Disease # 1. Malaria:

Malarial fever is a serious disease of man which is created by protozoan endoparasite known as Plasmodium. The malarial disease in man is caused by the four different species of Plasmodium and each species manifests a bit different from the other species.

Plasmodium lives within the RBC of man to whom it is transmitted by the bite of an infected female mosquito (Anopheles sp.). Thus the parasite has two hosts—man and Anopheles mosquito—and there is an alternation of the hosts in its life cycle.

There are four species of malarial parasite— P. vivax, P. falciparum, P. malariae and P. ovale.

Here the four species create four different kinds of malaria and these are:

1. Benign tertian malaria:

P. vivax is responsible for benign tertian malaria. In this type the fever occurs at every 48 hours interval i.e. on every 3rd day. There is a low death rate.

2. Malignant tertian malaria:

P. falciparum pro­duces a febrile disease i.e. malignant malaria. Though the fever takes place at every 48 hours interval and also the fever is irregular. The disease proves fatal unless the victim is promptly treated by anti-malarial drugs. There is a high death rate.

3. Quartan malaria:

P. malariae causes quartan malaria. The outbreak of fever sets in every 4th day.

4. Mild tertian malaria:

It is caused by P. ovale. In this type fever occurs at every 48 hrs. interval.

The transmission of malarial parasite (Plasmo­dium sp.) by the bile of infected female Anopheles mosquito to human being is called vector trans­mission. The female Anopheles mosquito may infect several persons.

The infective stage of malarial parasite is Sporozoite which is deve­loped in the gut of female Anopheles mosquito and later reaches the salivary glands of this insect vector. During the mosquito bite sporozoites are introduced into the blood stream of man along with saliva.

Disease # 2. Filariasis or Elephantiasis:

Filaria is an infec­tive disease which is caused by endoparasitic nematode, Wuchereria bancrofti living in the lymphatic tissue of man where the ovoviviparous female worms produce thousands of juveniles (microfilariae). Culex fatigans and C. papiens are the two species which act as intermediate hosts and vectors. Unlike the malarial parasite, filarial worm shows little specificity in regard to mosquito hosts.

Most microfilariae then sweep into peripheral blood stream (i.e. blood capillaries of the skin) at midnight between 12 to 2 a.m. The female Culex mosquitoes ingest microfilariae along with their blood meal when biting human. Ingested microfilariae enter into the stomach of mosquito along with the blood.

After that each juvenile (microfilaria) pene­trates the stomach and migrates throughout the haemocoel to reach the throcic muscles where they metamorphose into fully formed larvae. The fully formed larvae eventually reach the proboscis of mosquito.

The female Culex mosquito containing the matured lar­vae when bites a person for suction of blood meal, these infected larvae pass into the blood stream of man and slowly they come to the lymphatic channels where they. sexually mature, unite again and repeat the life cycle. In this way the disease, “filariasis” spread from person to person (Fig. 15.6).

Due to attack of microfilariae the lymph glands swell profusely and as a result scrotum and limbs swell unequally. This abnormal swelling is known as elephantiasis.

Disease # 3. Encephalities:

This disease is caused by a virus. Vector of this disease is either Aedes or Culex mosquitoes. So both the mosquitoes spread this disease. Sometimes man is also attacked by this virus and causes inflamma­tion of the brain known as encephalities. High fever, headache, drowsiness and inflam­mation of the brain are the symptoms of encephalities.

Disease # 4. Dermatobia:

It is one kind of skin disease which is observed in cattle, cats, dogs, sheep, rabbits and other animals including man. When the adult fly of Dermatobia hominis is ready to lay eggs, she captures a mosquito (Culex sp.) or other blood sucking fly and glues a batch of eggs to the abdomen of mosquito.

Now the mosquito sits on the body of a human being for biting, the larvae of D. hominis hatch from the eggs and penetrate the skin of the host (man) and reside there. As the larva grows it produces a swelling which has a central opening through which it breathes. These swellings are usually very painful. Development in the host requires 5-10 weeks, after which the larva escapes and pupates in the ground for an equally long period before the fly emerges.

The presence of a superficially situated swelling with a central opening, especially more than one is noticed, would lead to the suspicion of myiasis. In man the only satisfactory treatment can be made by surgical removal of the parasite.

Disease # 5. Dengue: 

Dengue or break-bone is another viral disease transmitted principally by mosquitoes (Aedes and Culex). The virus is believed to be related to that of yellow fever but differs strikingly is not attacking the liver and in producing immunity of relatively short duration. The disease commonly breaks out in explosive epidemics that spreads with amazing rapidity.

Aedes aegypti, A. albopictus, A. polynesiensis are the main vector mosquitoes species of dengue. Mosquitoes fed on dengue patients can transmit dengue for 3 days after the onset of the fever. According to Chandler and Rice, A. aegypti becomes infected after feeding on patients in the first to fifth days of the disease and can transmit the infection as early as 24 hours after an infective feed, but Siler et al obtained different results.

They found the patient to be infective from the mosquito for only 2 to 3 days and for 5 to 18 hours prior to the onset and they failed to transmit the disease in less than 11 days after a mosquito had obtained an infective feed.

The reservoir of dengue infection is both human beings and mosquito. The mosquito becomes infective by feeding on a patient from the day before onset to the 5th day of illness. The mosquito becomes infective and is able to transmit the infection after an extrinsic incubation period of 8 to 14 days. Both sexes are susceptible to dengue fever.

Sudden rise of temperature, pimples on face, intense headache, severe pains in all joints of the body are the symptoms. The illness is charac­terised by an incubation period of commonly 5-6 days. The other common symptoms include extreme weakness, anorexia, dragging pain in inguinal region, sore throat etc. The fever lasts for about 5 days and rarely more than 7 days. The death rate in this disease is low.

Disease # 6. Dengue Haemorrhagic Fever (DHF):

It is a severe form of dengue fever which is caused by infection with more than one dengue virus. This is transmitted by Aedes aegypti. The ill­ness is characterised by an incubation period of 4-6 days and it begins abruptly with high fever accompanied by facial flushing and headache, anorexia, vomiting, tenderness at the right coastal margin etc.

In DHF the severe illness is thought to be due to double infection with dengue viruses—the first infection sensi­tizes the patient while the second infection appears to form an immunological catastrophy. The fatality case in DHF is greater.