CSEC Biology: Vectors and Pathogenic Disease

Vectors are organisms that carry pathogens on or in their bodies without the pathogen causing any harm to them. These vectors transmit pathogens between other organisms. The relationship between a vector and the pathogen it carries could be considered an example of commensalism, since the pathogen gains transportation while the vector is (usually) neither benefited nor harmed.

Examples of vectors are rats which transport leptospirosis, fleas which transport bubonic plague, and mosquitoes which transport many varieties of pathogenic diseases.

CSEC has mandated that you study the life cycle of everyone's favourite blood-sucking pest- the mosquito. Great. You may already be familiar with some of this from your country's respective public education initiatives surrounding mosquitoes and controlling mosquito-borne diseases.

Mosquitoes pass through four stages from youth to maturity: egg, larva, pupa and adult/imago.

Egg- eggs are laid by adult females in protected areas of stagnant water, and float on the surface of the water. They remain there until hatching.

Some species lay their eggs individually while others lay theirs in attached groups called rafts.

Larva- once the egg hatches, the larval stage commences. The larvae of most species require air to survive, so they tend to hang suspended from the surface of the water. A breathing tube known as a siphon attaches the rear end of the larva to the surface of the water and allows air to reach it (sort of like a snorkel, a very disgusting snorkel).

Larvae feed on microorganisms near the surface of the water to support their growth. Throughout feeding, they outgrow their exoskeletons, cast them off, and develop new ones.

Pupa- feeding ends, but the organism still remains attached to the surface of the water through the breathing tube. Larval tissue from the larval stage reorganizes into adult/imago tissues as the pupa's skin slowly splits down the back, allowing the newly formed adult to slowly emerge.

Adult/Imago- The newly formed adult emerges from the pupa (in all its annoying, parasitic glory). Adult males remain near the breeding site where they mate with the females quickly after emergence. The male mosquitoes only live for 6-7 days, and feed on plant nectar rather than blood.

The female mosquitoes, however, are not as peaceful or short-lived. They can live for up to 5 months (with adequate food supply). They must take in blood meals (from us and other animals) along with nectar in order to nourish their eggs. After every blood meal, they oviposit (lay) their eggs, sometimes several times for their lifetimes (depending on the species).

What is the role of mosquitoes as vectors?

The pathogens transmitted by mosquitoes have two hosts, the humans and the mosquitoes. The humans are the primary host, and develop diseases because of the pathogen.

The mosquitoes are the secondary/intermediate host, because they simply carry the pathogen. The pathogen will reproduce in the secondary host, but will not cause diseases in it.

When a female mosquito takes in blood, if it contains pathogens, those pathogens can pass through the walls of her intestines and enter her salivary glands, where they can divide and proliferate.

The next time that mosquito takes in blood from a human, it will inject the pathogens along with the saliva in its salivary glands (note that it injects saliva so as to prevent the blood from clotting while it is sucking).

The mosquitoes are basically reservoirs, or 'pathogen nurseries' where the pathogens can multiply continuously and be spread through the labour of these mosquito mothers. When you think about the fact that mosquitoes are literally ensuring that pathogens continue to survive and spread, it only makes you want to eliminate them even more.

So, how do we control mosquitoes?

You've definitely heard about methods of controlling mosquito populations in the past through some catchy song courtesy of your local Ministry of Health Public Education Programme or a poster somewhere.

Since three stages of a mosquito's life are aquatic (egg, larval and pupal), cutting off their supply of water or their ability to function in water will stop them at those stages in the life cycle:

• Drain all areas of stagnant water

• Add insecticides to breeding areas

• Introduce fish (eg Tilapia) into breeding areas to feed on the larvae and pupae (known as biological control)

• Spray oils or lecithins onto still-water breeding areas to prevent the larvae and pupae from getting access to air

Our options for controlling adult mosquitoes are limited by comparison:

• Remove dense vegetation to reduce protection for adults during daylight hours

• Spray insecticides to kill adults