B. The principal vectors: biology and control methods

Mosquitoes
Lice
Flies
Fleas
Rodents

General introduction

Knowledge, however small, of the biology of each vector is a sine quae non of effective control measures.

It is useless to try to combat body lice by treating the hair, or to try to control Culex larvae by treating clean standing water. It is vital to know how, where and when to act.

Nevertheless, it is possible to identify some principles common to all vector control programmes:

- The aim should be to make the local environment unfavourable for the development and survival of the vector (environmental hygiene).
- Combat is generally more effective if * is focused on immature forms of the vector.
- Complete eradication is frequently unattainable; the objective should be to keep the vector population below a level at which it poses a too great risk of an epidemic.

Mosquitoes

Mosquitoes form the largest group of vectors of medical importance, with more than 3,000 species. Their life cycle is closely linked with water.

Life cycle

There are four stages of development. The first three are aquatic (egg, larva, nymph), and generally last for two weeks.

The adult (or imago) feeds on plant sap. Its lifespan varies from one to several months. The maturation of eggs in the female needs a blood meal (except for the first laying in Culex species).

Larvae may thrive in any water body except for deep water such as large river, lake or sea.

Certain genera (Aedes) use sites where the water level is variable (dependent on rainfall), such as tree trunks or leaves. Others are more specific to stagnant and heavily polluted water (Culex).

The activity of the adult varies with species: the sphere of action, hours and places of activity, type and specificity of food, etc.

Thus, in order to carry out effective control it is important to find out what species is concerned and what are its specific features.

Control methods

ENVIRONMENTAL HYGIENE

The main aim is to alter the environment so as to make it unfavourable for the reproduction of the species concerned.

-Aedes aegypti

This species is often associated with human dwellings where it breeds in any open container of water. Control measures aim at getting rid of these types of sites, or at protecting them (with a cover, or mosquito netting with mesh <0.7 mm).

Large water containers should be frequently emptied (at least once per week).

- Culex

Control measures aim to eliminate bodies of stagnant water loaded with organic matter (e.g. in latrines), or if not, then to eliminate surface vegetation in stagnant water and ensure that these water bodies are at least 1.2 m deep.

- Anopheles

Control measures are identical to those for Culex, although Anopheles have a wider distribution. Land drainage and filling also has an impact but this is rarely possible.

However, it may be necessary to couple these environmental hygiene and improvement measures with chemical control of larvae and adults.

In this case the relevant government department should be called in, as the misuse of insecticides, particularly in the aquatic environment may cause irreparable ecological disturbance and favour the development of resistance, etc.

In the case of a viral epidemic (encephalitis, yellow fever, etc.), individual protection should be done by using:

- mosquito nets (mesh diameter less than 0 - 7 mm) on openings of houses and over beds;
- repellent creams and lotions

CHEMICAL CONTROL

- Larvicides

· fuel oil: spread over the whole water surface (little used).
· paraffin: 30 l/ha, 1 glass/latrine/week.
· malathion: 224 to 692 g active product/ha.
· temephos: 56 to 112 g active product/ha.
· deltamethrin: 2.5 to 10 g active product/ha.

Chemicals formulated in granules or in emulsifiable concentrate.

- Adulticides

· Persistent treatment

- deltamethrin:0.05 g active product/m2
- malathion: 1 to 2 g active product/m2
- permethrin: 0.5 g active product/m2
- propoxur: 1 to 2 g active product/m,
- pirimiphos-methyl: 1 to 2 g active product/m2

· Aerial application

Confined to specialists.

Lice

Only the body louse is a potential vector. It is found almost exclusively between the skin and clothes. Transmission of possible pathogens through the louse's excretions (e.g. typhus) or by the louse being crushed (e.g. recurrent fever).

The louse population may grow when personal hygiene is poor (lack of water, soap or clothes), where there is overcrowding and when it is cold. A situation of a risk of epidemic may be reached, demanding emergency measures, including the use of an insecticide.

Before any action is taken, a study of the resistance of the lice to insecticides should be done by a competent laboratory or by an experienced person in the field. In the absence of resistance (which is rare), DDT or malathion may be used. If in doubt, propoxur (or permethrin) is almost always a good choice.

Powder is used, as treatment of the body. The concentration depends on the product, but 30 g of powder per person is the standard dose. Powder is applied to the fully clothed subject at the neck, half at the front and half at the back, then spread by rubbing. Pay special attention to belts and socks if these are worn. Bedding, blankets and clothes may also be treated, in a plastic bag for example.

Every bout of fever or large drop in temperature (e.g. on death) promotes the movement of lice to a new host. Patients should therefore be disinfected before being admitted to a hospital structure.

Control measures include:

- Improvement of hygiene and reduction of overcrowding.
- Preventive treatment of clothes, blankets etc distributed (by immersion, spraying or powdering).
- Active treatment of every body by powdering (treatments, 1 week apart).
- Treatment of clothes by damp heat (1 hour at 70d by dry heat (ironing).

Only this method ensures the elimination of lice, ticks, and bacteria (but it uses a lot of energy).

It must not be forgotten when treating that dust from clothing etc. is contaminating (excrete of lice).

- If possible, inform people of the danger of crushing lice between finger nails or teeth (risk of spread of borrelia).

Equipment

- Powder blower (hand or knapsack)
- Soupspoon: still the cheapest and most practical (1 soupspoon = 15 g: use one at the front and one at the back).
- Powdering tin or pierced bag.
- Bath, tub or simple knapsack sprayer for liquid application.

Informing the population and training operators are two essential parts of a control programme.

Precautions:

- Powders: dust masks and gloves for teams of operators doing treatment.
- Liquids: see Precautions for use.

When treating head lice, malathion is, the usual choice of insecticide (except if there is resistance). Prepare an aqueous solution of 4% active product, add a gentle detergent if possible (e.g. fabric detergent or shampoo, to aid the penetration into ticks) and apply about 15 ml to the head of each person. Leave to act for 24 hours before rinsing. Do not use on children of less than three years.

Deltamethrin may also be used, as a solution of 0.03% active product (2.5 ml of concentrated solution/litre or one sachet of 33 g/litre of water), or a 0.05% solution of permethrin. This treatment should be avoided if at all possible.

Flies

General points

This order is composed of a wide range of species which differ in their ecology, their behaviour and their medical importance.

The domestic fly has a life cycle of 10 to 30 days, depending on temperature.

The larvae develop in mammal and bird excrete, in waste waters, and in decomposing organic matter (medical and domestic refuse). The adult which lives for 2 to 8 weeks has a radius of activity of about three kilometres. It feeds on moist or liquid substances rich in sugars and proteins.

This species is closely linked to the human environment and frequently moves between contaminated areas (excrete, medical wastes, etc.), food and drink, thus playing a disease-carrying role.

Other species are of particular medical importance due to their specific biology: those attracted by lacrymal or nasal secretions, open wounds, domestic refuse, dead bodies, etc. They are important in the spread of conjunctivitis, plan (yaws), trachoma, dysentery, etc.

Glossinia (Tsetse fly, vector of trypanosomiasis) and glossinnia (black fly, vector of onchocerciasis) demand the implementation of a specific programme because of their medical importance and their biology; the complexity of such a programme is beyond the scope of this guide.

Control methods

Larval sites, i.e. the places where eggs are laid and where the larvae develop, are very often a product of human activity.

The basic principle of all control measures should be to reduce or remove these sites,or to prevent access by flies. Without these environmental hygiene measures, all control efforts are in vain.

In practical terms, these measures focus on:

- Animal excreta: cleaning of stockraising areas.
- Refuse: organize collection and ensure disposal by burning or burial under at least 30 cm of soil. Make covered refuse containers available (in particular in kitchens, laboratories and health centres).
- Spilled food: make smooth floors in feeding centres (smoothed cement or plastic sheeting) to aid cleaning.
- Wastewater: ensure good removal and disposal, particularly at washing areas for clothes and cooking ustensils.

Recourse to chemical products should be avoided as much as possible, because of the rapidity of appearance of resistant strains which render this option costly and ineffective.

In practice:

- In the case of massive infestation by larvae of a defecation trench or latrines, used engine oil, diesel or kerosene is used to spread an impermeable layer which asphyxiates the larvae (but take care of the risks of polluting the water table). Ashes or earth may be added to latrines pits (after each defecation, or at least every morning and night), to reduce the contact between flies and excreta.

- In the case of an epidemic, when the presence of flies creates a risk of increased spread of the pathogen, and when an operating theatre or treatment room must be protected, the use of larvicides and alduticides may be considered, but always in tandem with environmental hygiene measures.

· Destruction of larvae

Spraying of larval sites with an emulsion or a suspension until the surface is completely wetted.

· Destruction of adults

Treatment with a residual effect should be done on the surfaces where flies land at night, as it is here that there is the longest contact time. These places may differ according to the species and the climate. In general they are external surfaces of building (in hot countries), trees, fences, dustbins, animal shelters, etc.

The insecticide is applied in an emulsion or a suspension.

Malathion (5% solution, 1 to of active or deltamethrin (see technical brief) are the most suitable.

To give a permanent protection to certain places (e.g. operating theatre, dispensary, kitchen), strips of gauze or cotton impregnated with insecticide may be used (1 m length/m2 of ground surface area). Renew the impregnation every two months. Where there is a concentration of flies in a limited area (e.g. in a feeding centre), poisoned baits may be used (though there is a risk of poisoning of children and poultry). These baits should always be placed outside (on the windowsill, door, etc). They may be:

- Dried food mixed with a toxic product (1 to 2%). The bait is then spread at a rate of 6g/ 10 m2. or
- A 10% aqueous sugar solution mixed with the toxic product (0.1 to 0.2%), which is spread with a watering can or sprayer.
- Fly traps.

Fleas

Fleas are blood-feeding (haematophagic) insects with a close relationship with their host. Their developmental stages all occur on land.

The flea is usually a specific parasite but it is capable of changing its host (wild or domestic rodent, man, etc.) in certain circumstances.

Biology

The development cycle lasts about one month. The eggs are laid in dusty places in houses or in rodents nests.

The larvae are found in dark places (negative photo-trohism).

In the adult stages both sexes are blood feeders.

Contamination of the host may take place through a bite (plague) or excrete (typhus), so dust may be highly contaminating.

Control methods

Control is essentially by chemical methods, even if cleaning dwellings and burning dust gets rid of eggs and larvae.

It is essential to determine the sensitivity to any given insecticide because of the many problems of resistance.

Bedding and clothes are treated with an insecticide powder.

Disinfectant products (chlorine solution, 4% cresyl, etc.) are effective against eggs and larvae.

In the case of a risk of epidemic (plague) it is essential to destroy the population of fleas without harming the host species (e.g. rat) because of the risk of human infestation would be increased.

Control is done by putting insecticide powder on the rodent's trails or in their nests. Permethrin (0.5%), propoxur (1%) or any other effective insecticide is used. (Organochlorines are generally ineffective.). The powder is laid down in lines 50 cm long and 3 mm wide near to a non-poisonous bait, to attract the maximum number of rodents.

Rodents

General points

Rodents make up about half the mammal population and thus play an important role as reservoirs of pathogens in the transmission of diseases to other mammals.

Domestic rodents (and those of medical importance) are composed essentially of three species of the muride family: the black rat, the brown rat and the mouse.

Methods of transmission

Pathogenic agents are transmitted by:

- an ectoparasite of the rodent which thus plays only a secondary role (e.g. plague);
- rodent excrete (e.g. salmonella, leptospirosis);
- a bite.

Biology

- The black rat

The adult measures about 40 cm from head to tail and weighs 250 g.

The muzzle is pointed, the ears round and protruding from the fur, and the eyes protuberant. The animal may be dark grey or brown.

The nest is built generally on the ground, in vegetation or in trees (and exceptionally in a burrow or sewer).

In houses the nests are generally built under the roof and the territory is more "aerial" than terrestrial with a radius of about sixty meters.

Its diet is very varied (vegetable and animals). Sexual maturity is reached at two months (5 litters of 7 to 8 rats per month).

- The brown rat (or sewer rat)

The colour is generally brown. It is bigger than the black rat and may exceed 400 g. Its muzzle is rounded and the eyes and ears are smaller than those of the black rat.

This species is not well adapted to hot countries. Nests are built in burrows (entrance diameter: about 8 cm).

The diet is less varied than that of the black rat. The brown rat prefers refuse and human wastes. Its biology is identical to that of the black rat.

- The domestic mouse

This is a well known universal species. Its biology is similar to that of the rats. It can survive with the water enclosed in food (flour, etc.) whereas the rat needs "free" water (e.g. infusion liquids).

Its maximum weight is 20 g, and its length 20 cm. For two animals of the same size, the head and feet are larger in the young rat.

Nests are built in any place where there is an accumulation of material for making the small shelter which the mouse needs, making control difficult, specially as the adult's sphere of activity is never more than a few meters.

Control methods

ENVIRONMENTAL HYGIENE

As for any other vectors, this means making the environment unsuitable for rodents, working on two fronts:

- Food

· packaging of stored food
· disposal of refuse

- Reproduction: eliminatation of likely sites (refuse tips, waste packaging scrub).

PROTECTION AGAINST RODENTS

The aim is to prevent access by rodents to important or vulnerable areas (food stores, infusion liquids, etc.).

- Block or protect all openings greater than 6mm with cement or metal netting
(1 mm wire, mesh less than 6 mm dia).
- Fit discs on cables joining roofs.
- Paint a smooth band on walls at 1m from the ground to prevent passage on rough vertical surfaces.
- Fix galvanized sheet (1mm) at the bottom of doors and on the skirtings.

Stores are always places where people come in and out and where doors are often left open. It is therefore useful, in addition to the above mentioned measures, to organise stores according to the following points:

- Repair all broken packages;
- Leave a passage (1m) between walls or pillars and stacks, to allow inspection;
- Stack bags with care, leaving a sufficient space between the top of the stack and the roof;
- Clean the store daily and never let a stack remain intact for more than a month.
- Inspect the store at least once per week, looking for:

· insects,
· signs of damage on the bags (water, mould, fermentation, etc.),
· the presence of rodents,
· empty bags and refuse not thrown away.

TRAPPING

This method never achieves complete eradication by itself. It may be used to get rid of the last few survivors of a chemical control campaign or individuals of an isolated and small infestation.

Spring traps are the only efficient models. The key points to follow are:

- Many traps should be used.
- The traps should be placed perpendicular to the rodents' trails.
- Rat traps should be left unset for several days so that the rats become used to them (except for mouse traps). A well placed trap does not need bait.

POISONING (POISONED BAIT)

Two classes of product are used:

- Single dose poisons

These are only effective if the animal ingests a lethal dose at the first feed, otherwise it will not go back to the bait.

These substances are extremely toxic and in addition they need special skills and experience in rodent control if their use is to be at all effective. For information, they are:

· Zinc phosphate (1 to 5%),
· fluorocetamide (2%),
· sodium fluoroacetate (0.25%),
· certain anticoagulants.

- Multiple-dose poisons

These are anticoagulants with a cumulative effect used at low doses, which have two advantages:

· The slowness of their effect allows the animal to absorb a lethal dose before the first effects are felt.
· Their mode of action makes them less dangerous to man and other domestic animals, and there is an effective antidote.

Products in use

See the table on the following page.

The bait is bought ready to use, or made with broken cereal grains which are soaked over night to moisten them to make a thick paste. Sugar may be added (5%) to make it more attractive, as it is important to persuade the animal away from its usual food and get it to stick to this new diet.

The poison is then mixed in, and the bait is laid on the rodents' trails in piles of 25g (mice) or 200g or more (rats).

These baits should be out of reach of other animals. They can be laid in particular ways (e.g. a slightly open plastic bag, a short piece of pipe, a small wooden box, etc.).

Effects should be seen in 3 weeks for the brown rat, 4 to 5 weeks for the black rat and the mouse.

The baits are always renewed, and left in excess. The dead bodies should be disposed of quickly and well: pick them up twice per day and put them in a plastic bag (with their fleas).

Chemical control will only be effective if it is complemented by environmental hygiene and if both efforts cover the whole area concerned.

Periodic inspection should allow renewed control measures before being faced with a new massive infestation.

Figure