6 Most Important Techniques of Inventory Control System
6 Most Important Techniques of Inventory Control System
- 1 6 Most Important Techniques of Inventory Control System
- 2 Ticks
- 3 Biology
- 4 Public health importance
- 5 Control measures
Some of the most important techniques of inventory control system are: 1. Setting up of various stock levels. 2. Preparations of inventory budgets. 3. Maintaining perpetual inventory system. 4. Establishing proper purchase procedures. 5. Inventory turnover ratios. and 6. ABC analysis.
1. Setting up of various stock levels:
To avoid over-stocking and under stocking of materials, the management has to decide about the maximum level, minimum level, re-order level, danger level and average level of materials to be kept in the store.
These terms are explained below:
(a) Re-ordering level:
It is also known as ‘ordering level’ or ‘ordering point’ or ‘ordering limit’. It is a point at which order for supply of material should be made.
This level is fixed somewhere between the maximum level and the minimum level in such a way that the quantity of materials represented by the difference between the re-ordering level and the minimum level will be sufficient to meet the demands of production till such time as the materials are replenished. Reorder level depends mainly on the maximum rate of consumption and order lead time. When this level is reached, the store keeper will initiate the purchase requisition.
Reordering level is calculated with the following formula:
Re-order level =Maximum Rate of consumption x maximum lead time
(b) Maximum Level:
Maximum level is the level above which stock should never reach. It is also known as ‘maximum limit’ or ‘maximum stock’. The function of maximum level is essential to avoid unnecessary blocking up of capital in inventories, losses on account of deterioration and obsolescence of materials, extra overheads and temptation to thefts etc. This level can be determined with the following formula. Maximum Stock level = Reordering level + Reordering quantity —(Minimum Consumption x Minimum re-ordering period)
(c) Minimum Level:
It represents the lowest quantity of a particular material below which stock should not be allowed to fall. This level must be maintained at every time so that production is not held up due to shortage of any material.
It is that level of inventories of which a fresh order must be placed to replenish the stock. This level is usually determined through the following formula:
Minimum Level = Re-ordering level — (Normal rate of consumption x Normal delivery period)
(d) Average Stock Level:
Average stock level is determined by averaging the minimum and maximum level of stock.
The formula for determination of the level is as follows:
Average level =1/2 (Minimum stock level + Maximum stock level)
This may also be expressed by minimum level + 1/2 of Re-ordering Quantity.
(e) Danger Level:
Danger level is that level below which the stock should under no circumstances be allowed to fall. Danger level is slightly below the minimum level and therefore the purchases manager should make special efforts to acquire required materials and stores.
This level can be calculated with the help of following formula:
Danger Level =Average rate of consumption x Emergency supply time.
(f) Economic Order Quantity (E.O.Q.):
One of the most important problems faced by the purchasing department is how much to order at a time. Purchasing in large quantities involve lesser purchasing cost. But cost of carrying them tends to be higher. Likewise if purchases are made in smaller quantities, holding costs are lower while purchasing costs tend to be higher.
Hence, the most economic buying quantity or the optimum quantity should be determined by the purchase department by considering the factors such as cost of ordering, holding or carrying.
This can be calculated by the following formula:
where Q stands for quantity per order ;
A stands for annual requirements of an item in terms of rupees;
S stands for cost of placement of an order in rupees; and
I stand for inventory carrying cost per unit per year in rupees.
2. Preparation of Inventory Budgets:
Organisations having huge material requirement normally prepare purchase budgets. The purchase budget should be prepared well in advance. The budget for production and consumable material and for capital and maintenance material should be separately prepared.
Sales budget generally provide the basis for preparation of production plans. Therefore, the first step in the preparation of a purchase budget is the establishment of sales budget.
As per the production plan, material schedule is prepared depending upon the amount and return contained in the plan. To determine the net quantities to be procured, necessary adjustments for the stock already held is to be made.
They are valued as standard rate or current market. In this way, material procurement budget is prepared. The budget so prepared should be communicated to all departments concerned so that the actual purchase commitments can be regulated as per budgets.
At periodical intervals actuals are compared with the budgeted figures and reported to management which provide a suitable basis for controlling the purchase of materials,
3. Maintaining Perpetual Inventory System:
This is another technique to exercise control over inventory. It is also known as automatic inventory system. The basic objective of this system is to make available details about the quantity and value of stock of each item at all times. Thus, this system provides a rigid control over stock of materials as physical stock can be regularly verified with the stock records kept in the stores and the cost office.
4. Establishing Proper Purchase Procedures:
A proper purchase procedure has to be established and adopted to ensure necessary inventory control. The following steps are involved.
(a) Purchase Requisition:
It is the requisition made by the various departmental heads or storekeeper for their various material requirements. The initiation of purchase begins with the receipts of a purchase requisition by the purchase department.
(b) Inviting Quotations:
The purchase department will invite quotations for supply of goods on the receipt of purchase requisition.
(c) Schedule of Quotations:
The schedule of quotations will be prepared by the purchase department on the basis of quotations received.
(d) Approving the supplier:
The schedule of quotations is put before the purchase committee who selects the supplier by considering factors like price, quality of materials, terms of payment, delivery schedule etc.
(e) Purchase Order:
It is the last step and the purchase order is prepared by the purchase department. It is a written authorisation to the supplier to supply a specified quality and quantity of material at the specified time and place mentioned at the stipulated terms.
5. Inventory Turnover Ratio:
These are calculated to minimise the inventory by the use of the following formula:
Inventory Turnover Ratio
= Cost of goods consumed/sold during the period/Average inventory held during the period
The ratio indicates how quickly the inventory is used for production. Higher the ratio, shorter will be the duration of inventory at the factory. It is the index of efficiency of material management.
The comparison of various inventory turnover ratios at different items with those of previous years may reveal the following four types of inventories:
(a) Slow moving Inventories:
These inventories have a very low turnover ratio. Management should take all possible steps to keep such inventories at the lowest levels.
(b) Dormant Inventories:
These inventories have no demand. The finance manager has to take a decision whether such inventories should be retained or scrapped based upon the current market price, conditions etc.
(c) Obsolete Inventories:
These inventories are no longer in demand due to their becoming out of demand. Such inventories should be immediately scrapped.
(d) Fast moving inventories:
These inventories are in hot demand. Proper and special care should be taken in respect of these inventories so that the manufacturing process does not suffer due to shortage of such inventories.
Perpetual inventory control system:
In a large b essential to have information about continuous availability of different types of materials and stores purchased, issued and their balance in hand. The perpetual inventory control system enables the manufacturer to know about the availability of these materials and stores without undergoing the cumbersome process of physical stock taking.
Under this method, proper information relating to receipt, issue and materials in hand is kept. The main objective of this system is to have accurate information about the stock level of every item at any time.
Perpetual inventory control system cannot-be successful unless and until it is accompanied by a system of continuous stock taking i.e., checking the total stock of the concern 3/4 times a year by picking 10/15 items daily (as against physical stock taking which takes place once a year).
The items are taken in rotation. In order to have more effective control, the process of continuous stock taking is usually undertaken by a person other than the storekeeper. This will check the functioning of storekeeper also. The items may be selected at random to have a surprise check. The success of the system of perpetual inventory control depends upon the proper implementation of the system of continuous stock taking.
6. ABC analysis:
In order to exercise effective control over materials, A.B.C. (Always Better Control) method is of immense use. Under this method materials are classified into three categories in accordance with their respective values. Group ‘A’ constitutes costly items which may be only 10 to 20% of the total items but account for about 50% of the total value of the stores.
A greater degree of control is exercised to preserve these items. Group ‘B’ consists of items which constitutes 20 to 30% of the store items and represent about 30% of the total value of stores.
A reasonable degree of care may be taken in order to control these items. In the last category i.e. group ‘Q’ about 70 to 80% of the items is covered costing about 20% of the total value. This can be referred to as residuary category. A routine type of care may be taken in the case of third category.
This method is also known as ‘stock control according to value method’, ‘selective value approach’ and ‘proportional parts value approach’.
If this method is applied with care, it ensures considerable reduction in the storage expenses and it is also greatly helpful in preserving costly items.
Ticks are arthropods that suck blood from animals and humans. They occur around the world and are important as vectors of a large number of diseases. Among the best-known human diseases transmitted by ticks are tick-borne relapsing fever, Rocky Mountain spotted fever, Q fever and Lyme disease. Ticks are also important as vectors of diseases of domestic animals and they can cause great economic loss. Two major families can be distinguished: the hard ticks (Ixodidae), comprising about 650 species, and the soft ticks (Argasidae), comprising about 150 species. Ticks are not insects and can easily be distinguished by the presence of four pairs of legs in the adults and the lack of clear segmentation of the body (Fig. 4.23).
Ticks have a life cycle that includes a six-legged larval stage and one or more eight-legged nymphal stages (Fig. 4.24). The immature stages resemble the adults and each of them needs a blood-meal before it can proceed to the next stage. Adult ticks live for several years, and in the absence of a blood-meal can survive several years of starvation. Both sexes feed on blood, the males less frequently than the females, and both can be vectors of disease. Disease organisms are not only passed from one host to another while blood is being taken: female ticks can also pass on certain disease agents to their offspring.
Fig. 4.24. Life cycle of the soft tick, Ornithodoros moubata (30).
The adults are flat and oval in outline and have tough, leathery, wrinkled bodies. The mouthparts are situated underneath the body and are not visible from above. The eggs are laid in the places where the adults rest, such as cracks and crevices in the walls and floors of houses and in furniture. The larva, the five nymphal stages and the adults all actively search for hosts from which to take blood-meals. After feeding, which lasts about 30 minutes, they drop to the ground. Most species can survive for more than a year between blood-meals, and some for more than 10 years.
The soft ticks live apart from their hosts and are most common in the nests and resting places of the animals on which they feed. Some species, such as the chicken tick and the pigeon tick (Argas species) may feed on humans when the preferred hosts are not available.
Species that commonly feed on humans are found around villages and inside houses (Fig. 4.25). Their habits are comparable to those of bedbugs: ticks often emerge from hiding places at night to suck the blood of humans and animals. Some species are common on travel routes, in rest houses and camping sites, and in caves and crevices.
The adult hard ticks are flat and oval in shape and between 3 and 23 mm long, depending on the species (Fig. 4.26). The mouthparts are visible at the front of the body, differentiating them from the soft ticks. In contrast to the soft ticks they have a shield-like plate or scutum behind the head on the back of the body, and there is only one nymphal stage (Fig. 4.27).
The eggs are deposited on the ground in large numbers. The larvae are very small, between 0.5 and 1.5 mm in length; they climb up vegetation, wait until a suitable host passes by, then climb on to it and attach themselves at a preferred feeding site, such as in the ears or on the eyelids.
After several days, when fully engorged, they drop to the ground, seek shelter and moult to the nymphal stage, which in turn seeks a blood-meal (Fig. 4.28), engorges, detaches itself and moults into an adult. The adult females climb up vegetation to wait for a suitable host, remaining on it for one to four weeks, then drop to the ground and seek shelter in cool places under stones and leaf litter, where they lay their eggs.
Fig. 4.25. Ornithodoros soft ticks are common in traditional-style mud-built houses with mud floors in some parts of Africa.
Fig. 4.26. Hard ticks.
(a) The bont tick, Amblyomma hebraeum, vector of spotted fever due to Rickettsia conori in southern Africa.
(b) The sheep tick, Ixodes ricinus, vector of tick-borne (Central European) encephalitis.
(c) The Rocky Mountain wood tick, Dermacentor andersoni, vector of spotted fever due to Rickettsia rickettsii in North, Central and South America (by courtesy of the Natural History Museum, London).
Fig 4.27 Life cycle of a hard tick (Ixodes) showing a female with a large mass of eggs, and a single nymphal stage (30).
Most species of hard tick feed on three different hosts: one each for the larva, nymph and adult. However, some species feed on only one or two hosts. Because they remain attached to their hosts for several days, the hard ticks may be carried over large distances. The combination of feeding on different hosts and travelling considerable distances partly explains their importance as disease vectors.
Public health importance
Ticks can cause painful bites; heavy infestations, not uncommon in animals, can cause serious loss of blood.
Tick-borne relapsing fever
This disease is caused by a microorganism of the genus Borrelia. It is transmitted by biting soft ticks of the genus Ornithodoros in many countries in the tropics and subtropics and also in Europe and North America. The ticks usually feed quickly at night in or near houses, and then leave the host (31).
The disease causes bouts of fever alternating with periods without fever. Death occurs in about 2-10% of persons who are untreated.
Treatment is possible with tetracycline or its derivatives.
Prevention requires measures to control soft ticks and to avoid their bites.
Hard ticks inject into the body with their saliva certain toxins that can cause a condition in people and animals called tick paralysis. It appears 5-7 days after a tick begins feeding, paralysing the legs and affecting speaking ability, swallowing and breathing. It occurs worldwide and is most common and severe in children aged up to two years. Treatment involves removing the tick.
Tick-borne rickettsial fevers
This group of diseases is caused by closely related Rickettsia microorganisms transmitted by tick bites or contamination of the skin with crushed tissues or faeces of the tick.
· Spotted fever due to Rickettsia rickettsii occurs in Brazil, Canada, Colombia, Mexico, Panama and the USA.
· Spotted fever due to R. sibirica occurs in Japan, the Russian Federation and the Pacific.
· Spotted fever due to R. conori is found in the Mediterranean region, Africa and southern Asia.
· Spotted fever due to R. australis occurs in Queensland, Australia.
· Q fever, caused by Coxiella burnetii, has a worldwide distribution and is commonly present in abattoirs, meat-packing and meat-rendering plants, diagnostic laboratories, stockyards and poultry farms. It is transmitted to humans mainly by the consumption of milk and meat from contaminated cattle or the inhalation of dried infected tick faeces by people working with cattle.
Symptoms in humans are sudden fever persisting for several weeks, malaise, muscle and joint pains, severe headache and chills. A rash sometimes spreads over the entire body. Death may result in about 15-20% of persons if the disease is misdiagnosed or left untreated.
Antibiotics such as tetracycline or chloramphenicol can be used.
Tick bites should be avoided and attached ticks should be removed rapidly and carefully. Several hours of attachment are needed before the Rickettsia organisms can infect humans.
Lyme disease (erythema chronicum migrans) is a severe and often debilitating condition caused by a spirochaete, Borrelia burgdorferi. Acute Lyme disease is a flu-like illness, characterized by an expanding red rash in about 50% of patients, accompanied by fever, fatigue, and muscle and joint pain. Weeks or even months after the infecting tick bite, patients may experience swelling and pain in large joints (knee, elbow), encephalitis, facial palsy, ocular lesions and carditis, irrespective of whether a rash occurred in the acute phase. Later, perhaps years after the bite, there may be cartilage erosion (arthritis) and neuromuscular dysfunction (Fig. 4.29). Lyme disease occurs principally in northern temperate regions of the world, including China, Europe, the USA and the former USSR.
Fig. 4.29. A typical symptom of Lyme disease is swelling and pain in the large joints, such as the knees, and chronic arthritis.
The disease is transmitted mostly by Ixodes ticks, commonly in the summer when the nymphs are abundant. Small rodents, especially mice, serve as reservoirs of infection while large mammals serve principally as hosts maintaining tick populations. The larvae acquire infection while feeding on mice, and nymphs or adults can transmit spirochaetes during subsequent blood-meals. In the northern temperate zone, where it occurs most intensely, Lyme disease has become more common as deer populations have increased and as this critical host has adapted to living in closer proximity to people. In many areas, Lyme disease is acquired in the suburban residential environment (32).
Further development of the disease in adults may be reduced or prevented by treatment with tetracycline or its derivatives for 2-4 weeks, and in children by treatment with penicillin.
Prevention requires avoidance of tick habitats and bites, and vector control. Personal protection may be possible by the use of repellents on the skin and clothing in tick-infested areas. The removal of attached ticks within 24 hours may prevent spirochaete transmission. Prophylactic antibiotic therapy may be desirable following the bite of an infected tick. New molecular assays are commercially available for detecting the spirochaetes in tick samples.
Tularaemia, also known as rabbit fever, deerfly fever and Ohara disease, is caused by the infectious agent Francisella tularensis. The symptoms, which vary according to how the agent enters the body, include headache, chills, fever and the swelling of lymph nodes. The disease occurs in Europe, Japan, North America and the former USSR.
Transmission takes place through the bites of ticks and deerflies (see Chapter 1) or as a result of handling infected animals such as rabbits and other game. Hunters and forest workers are at the highest risk of infection.
Antibiotics such as streptomycin can be used to treat the disease.
Tick bites and tick habitats should be avoided, impermeable gloves should be worn when skinning and dressing game animals, wild game meat should be thoroughly cooked, and untreated drinking-water should be avoided in areas where the disease occurs.
This is a group of viral diseases causing acute inflammation of the brain, spinal cord and meninges. The symptoms vary in severity with the type of disease. Many infections do not result in disease. Severe infections may cause violent headaches, high fever, nausea, coma and death.
· Far Eastern tick-borne encephalitis is found in the far east of the former USSR.
· Central European tick-borne encephalitis occurs in Europe from the Urals to France.
· Louping ill is a disease of sheep in the United Kingdom which sometimes affects people.
Transmission and prevention
These diseases are transmitted by biting ticks and by the consumption of milk from infected animals. No specific treatment is available but vaccines have been developed against some of the diseases. Prevention requires avoidance or rapid removal of ticks.
Principal hard tick vectors
Usually various tick species act as vectors for any one disease and their importance varies from region to region.
Deer tick, Ixodes dammini
Spotted fever due to:
American dog tick, Dermacentor variabilis
Asiatic wood tick, Dermacentor silvarum
Brown dog tick, Rhipicephalus sanguineus
Wattle tick, Ixodes holocyclus
Lone star tick, Amblyomma americanum
American rabbit tick, Haemaphysalis leporis-palustris
Far Eastern tick-borne encephalitis
Taiga tick, Ixodes persulcatus
Central European tick-borne encephalitis
Castor bean tick, Ixodes ricinus
Kyasanur Forest disease
A tick of birds and monkeys, Haemaphysalis spinigera
Colorado tick fever
American wood tick, Dermacentor andersoni
Crimean-Congo haemorrhagic fever
A tick of birds and mammals, Hyalomma marginatum
Kyasanur Forest disease occurs in parts of India.
Omsk haemorrhagic fever is found in south-western Siberia; it causes severe disease and death in muskrat handlers; it is mainly waterborne, although it is found in hard ticks.
Colorado tick fever is a moderately severe disease that occurs in western North America.
Crimean-Congo haemorrhagic fever is an acute, often severe and fatal disease found in parts of Africa, Asia and Europe.
Fields and forests infested with ticks should be avoided if possible. In Africa, bites by the soft tick Ornithodoros moubata, the vector of relapsing fever, can be prevented by avoiding old camp sites and by not sleeping on floors of mud houses. Beds, especially metal ones, may provide some protection because the ticks have difficulty in climbing the legs. However, they may still be able to reach hosts by climbing up the walls.
Effective repellents that prevent ticks from attaching to the body include deet, dimethyl phthalate, benzyl benzoate, dimethyl carbamate and indalone (33). These substances can be applied to the skin or clothing. On the skin, repellents often do not last more than a few hours because of absorption and removal by abrasion. On clothing they last much longer, sometimes for several days (34). For more information on repellents, see Chapter 1.
Clothing can provide some protection if, for example, trousers are tucked into boots or socks and if shirts are tucked into trousers. Clothing should be removed and examined for the presence of ticks after a tick-infested area has been visited.
People who frequently enter tick-infested areas should consider impregnating their clothing by spraying (35, 36) or soaking with a pyrethroid insecticide such as permethrin or cyfluthrin. Ticks crawling up trousers or shirts are quickly knocked down. Thus, not only is biting prevented but the ticks are also killed. Pyrethroid treatment of clothing is additionally effective against mosquitos for a month or longer (34). Information on how to treat clothing with a pyrethroid insecticide is given in Chapter 1.
Removal of attached ticks
During and after visits to tick-infested areas it is important to examine the body frequently for ticks. They should be removed as soon as possible because the risk of disease transmission increases with the duration of attachment.
A tick should be removed by pulling slowly but steadily, preferably with forceps to avoid contact between the fingers and the tick’s infective body fluids. The tick should be grasped as close as possible to where the head enters the skin, so as not to crush it, and care should be taken not to break off the embedded mouthparts, as they may cause irritation and secondary infection. Some veterinarians may have a special tool for quick removal of ticks from dogs.
The following methods may induce soft ticks to withdraw their mouthparts: touching with a hot object such as a heated needle tip; dabbing with chloroform, ether or some other anaesthetic. With hard ticks these methods only work immediately after biting because they are attached with a saliva cement that prevents them from quickly withdrawing their mouthparts. In areas where ticks are only a nuisance they can be coated with oil, paraffin, vaseline or nail varnish to prevent them from obtaining oxygen. Hard ticks then dissolve the cement so that they can withdraw their mouthparts, but this may take several hours. However, these methods are not recommended in areas where ticks are vectors of disease, as they work too slowly and may cause ticks to regurgitate into wounds, injecting disease organisms. In such circumstances it is recommended to pull the ticks out immediately, even if the head is left in the wound.
Application of insecticides to animals
Domestic animals are often hosts to ticks that can feed on humans and transmit disease to people and animals. Insecticides applied directly to the bodies of these animals in the form of dusts, sprays, dips or washes can be very effective. Pour-on formulations are applied over the animals’ backs. The insecticide (a pyrethroid) is distributed over the whole body by tail and other movements.
Insecticidal powders or dusts can be applied by means of a shaker, puff-duster or plunger-type duster. Insecticidal sprays are applied with hand-compression sprayers. The same insecticides and dosages can be used as for the control of fleas (see Table 4.2). It is particularly important to treat the back, neck, belly and the back of the head.
Plastic collars impregnated with an insecticide for the control of fleas in dogs and cats (see Table 4.2) are only partially effective against most species of tick.
Spraying insecticides in houses and resting places for animals
Ticks can be killed by insecticides sprayed on floors in houses, porches, verandas, dog kennels and other places where domestic animals sleep. Suitable residual sprays are indicated in Table 4.4 (see also p. 246).
Houses infested with soft ticks (Ornithodoros) can be sprayed with lindane (0.2g/m 2 ) or another insecticide formulation. Special care must be taken to treat the hiding and resting places of ticks in cracks and crevices in walls, floors and furniture. Residual house-spraying against malaria mosquitos has often resulted in a reduction in the numbers of ticks (see also p. 241).
Table 4.4 Insecticidal formulations used against ticks
Dipping, washing or spray-on
malathion (5%), dichlorvos (0.1%), carbaryl (1%), dioxathion (0.1%), naled (0.2%), coumaphos (1%)
Insecticidal powder (dust)
carbaryl (5%), coumaphos (0.5%), malathion(3-5%), trichlorphon (1%)
Residual spray on floors, etc.
oil solutions or emulsions of DDT (5%), lindane(0.5%), propoxur (1%), bendiocarb (0.25-0.48%), pirimiphos methyl (1%), diazinon (0.5%), malathion(2%), carbaryl (5%), chlorpyrifos (0.5%)
Ultra-low-volume fogging (area spraying)
organophosphorus insecticides, carbamate compounds and pyrethroids
Flea and tick collars for dogs and cats
dichlorvos (20%), propoxur (10%), propetamphos(10%), permethrin (11%)
Impregnated mosquito nets
Soft ticks that habitually feed indoors on sleeping persons can be controlled with impregnated bednets (5) (see also p. 240 and Chapter 1).
Large-scale control activities are sometimes carried out in recreational areas or in areas where ticks transmit tick-borne diseases. It is often economical and effective to integrate several methods into a comprehensive control strategy (37). Possible components of an integrated strategy are as follows:
· Surveillance: sampling to identify tick habitats where control is needed.
· Vegetation management: physical or chemical measures to reduce and isolate tick habitats.
· Host management: removal or exclusion of host animals.
· Targeted chemical control: pesticide applications against ticks, targeted at the tick host or habitat.
· Cultural practices: lifestyle changes to limit exposure to ticks.
· Personal protection: protective clothing; repellents; checking for and removing of ticks.
Area spraying with insecticides
Spraying ticks directly in their natural habitats in forests and fields may control outbreaks of certain tick-borne diseases (e.g. Lyme disease (38) and tick-borne encephalitides). Large areas may be treated by ultra-low-volume spraying of liquid acaricide concentrates from fixed-wing aircraft or helicopters. Small areas may be sprayed by means of motorized knapsack sprayers or mist-blowers, applying either ultra-low-volume formulations or formulations of water-based emulsions or wettable powders. Control lasts for a month or longer, depending on conditions and the size of the treated area. Suitable biodegradable insecticides are shown in Table 4.4 (39—44).
In, for example, parks and camp sites, ticks can be controlled by removal of the vegetation serving as their habitat (37, 45). This can be done by cutting, mowing or applying herbicides.
Tick populations can be reduced by removing the animals on which they usually feed. Fences can be used to exclude larger animals such as deer (37).
Insecticide-treated nesting material
Nest-building rodents serve as natural reservoirs or critical hosts for many vector-borne infections, including Lyme disease, several of the tick-borne encephalitides, and others. One host-targeted vector control strategy uses insecticide-impregnated nesting material directed at the rodent reservoirs of Lyme disease spirochaetes. In the USA, white-footed mice serve as the principal reservoirs. Larval deer ticks become infected while feeding on these mice, and nymphs derived from mouse-fed larvae become infected vectors. Mice actively harvest soft material for their nests; when they incorporate cotton nesting material treated with 7-8% permethrin, their tick infestations are virtually eliminated.
This method has been used in residential areas bordering woodlands and parklands in the northern USA to reduce the abundance of infected nymphal ticks (46, 47). The treated nesting material is protected in dispensing tubes (4 cm in diameter by 20 cm in length) and is placed about every 10 m in mouse habitats. The impregnated material is made using a patented method of soaking cotton in a permethrin emulsion and then drying it.
Clearly, mice must find and use the nesting material if this method is to work, and failures have been reported (48). However, when used properly, such a host-targeted treatment can significantly reduce the abundance of infected ticks, using up to 20 times less active ingredient and at less cost than insecticidal spray treatments. Community-wide programmes, where all properties in a neighbour-hood receive treatment, have proved most effective.