Control of Dermanyssus gallinae, the poultry red mite using Integrated Pest Management

27 February 2023

Saba Amir, IBERS, Aberystwyth University.

• Dermanyssus gallinae is a serious threat to laying hens and egg production in many parts of the world
• Acaricide resistance and changes in pesticide legislations have made the management of D. gallinae challenging
• Integrated Pest Management (IPM) offers a sustainable approach for control and management of D.gallinae which is good for the birds, humans and the environment

Introduction

The poultry red mite (PRM), Dermanyssus gallinae, is a major threat to egg-laying hens in many parts of the world. It causes serious health and welfare issues and huge economic losses. It is estimated that in Europe alone, losses due to PRM are worth €130 million. Between 60% and 85% of commercial egg-laying facilities in the UK may have PRM infestations. It has been implicated as a transmission vector for many pathogens between hens like Borrelia anserine (causative agent of borelliosis), and fowl pox virus and some zoonotic pathogens like Salmonella. Control of PRM has never been straightforward but as the poultry industry moves away from the conventional cage systems PRM is likely to become more abundant and challenging to control. Traditionally it has been treated with synthetic acaricides but the particular lifestyle of the mite, hiding away in cracks and crevices makes it difficult to reach. Thereby only a small population of the mite is exposed to the acaricide sprays. Moreover, the resistance to acaricides is increasing and treatment failure has been reported worldwide. In Europe many acaricides have been unlicensed for use due to consumer and safety regulations. A promising course of action is the Integrated Pest Management System (IPM) which has been developed by the Mite Control Project funded by the Interreg North- West Europe Programme (the European Regional Development Fund). This technical article briefly covers the biology of the insect and the health and welfare issues of hens and focuses on the control of PRM using the 8 steps of IPM.

Biology of D.gallinae

D.gallinae is a small blood sucking ectoparasite, living off the body of the host hiding in cracks and crevices near the hen’s resting place. It measures about 1.5 mm in length and varies in colour from grey to brown/red depending on feeding status. It spends much of its life cycle off the body of the host and visits its host to feed mainly between sunset and sunrise. Its most active hours are 5 to 11 hours after darkness. During daytime the mites live concealed in all possible crevices, for example in walls or floors, hosts’ nests, under the cribs and roosts, on dried litter, egg conveyor belts, cardboard boxes and transportation cages.

Fig 1: D.gallinae as seen under a microscope

There are 5 stages in its life cycle; egg, larva, protonymph, deutonymph, and adult as shown in the figure:

Conditions in poultry houses including temperatures between 20 – 25^oC and relative humidity of 70%, favour reproduction of PRM. The life cycle can be as short as 5.5 to 7 days at 25–37^oC and as long as 17 days at 20^oC. PRM may be present year-round, but highest densities occur during hot and humid seasons. As the turnover of laying hens exceeds more than a year, such conditions can be encountered during any production cycle giving PRM ample opportunity to proliferate. PRM may survive long enough to infest any new flock, in extreme cases, it can survive for up to 8 months without a meal. Therefore, even when the houses are empty between production cycles, PRM cannot be wiped out.

Fig 2: Life cycle of poultry red mite, D.gallinae

Pathology of parasitized birds

At a sublethal level PRM causes

1. Significant stress to hens due to lack of sleep and self-pecking
2. Increased feed intake and decreased feed efficiency
3. Reduction in egg numbers and egg quality due to shell thinning and blood spotting
4. Decline in bird condition

Severe infestations can lead to more serious effects like anaemia, aggressive feather-pecking and cannibalism and finally death.   

Fig 3: Consequences of PRM infestation in hens

Conventional control of PRM

Conventional control of PRM has involved use of synthetic acaricides but their tendency to seek refuge in difficult to reach places and live for long periods without a blood meal has hampered their control. In the UK phoxim-, abermectin- and pythreroid- based acaricides have been approved for use but resistance to these chemicals is widely reported and observed. A 2004 survey of British farms revealed that more than 60% had experienced acaricide resistant infestation and inappropriate use of products has worsened the problem since then. Stricter legislation regarding active ingredients has led to reduced product availability. For example, the once popular organophosphate fenitrothion is no longer licensed in the UK.  Additional limitations on the use of conventional acaricides include extended product withdrawal times following application and prohibitions against treatment while birds are laying eggs. These regulations are put in place to reduce the risk of product residues, which are reportedly a global problem. A transition away from synthetics is being driven by consumer awareness and the demand for pesticide-free goods.

Integrated Pest Management Strategy (IPM)

Integrated Pest Management (IPM), developed by the Mite Control Project is a strategy to control pest species, which is sustainable for animals, humans, and the environment. It consists of eight steps, in which prevention of introduction of mites in poultry houses, and monitoring of the pest are essential for sustainable control. Primarily, strategies and tactics that are safe for the environment and don’t include chemicals are employed to prevent and control the pest species. Chemical treatments are only used as a last resort once non-chemical ones have failed and an action threshold has been reached. The sections below provide an overview of each step of the IPM strategy for the control of PRM on layer farms.

Step 1. Prevention and Population Suppression

The very first step of an IPM approach is preventing new populations of PRM from entering and spreading in layer houses. PRM infestations occur not only on layer farms but also in breeder flocks and rearing farms, and they can persist and spread through the transportation of pullets, eggs, and manure but good biosecurity measures can reduce their dispersal throughout facilities. Preventive measures must be considered both during production and in between egg-laying rounds, as well as when constructing or installing new facilities.

During production

1. The first measure is to keep the number of external visitors limited and apply strict biosecurity regulations such as wearing company provided PPE, hygiene barriers and wearing separate boots in separate houses for staff and visitors.
2. Pullets and their crates when delivered must be mite free. Therefore, it is essential that pullet breeders monitor infestation in their premises and share their monitoring data with the egg producer.
3. Egg trays and egg containers which are regularly brought in and out of the facilities must be kept mite free.
4. A good pest control programme is essential as PRM is known to spread via vermin.
5. Dead birds must be removed as fast as possible as PRM can survive on them. A cadaver storage room close to the layer houses or up against the outer wall of the laying hen facility must be avoided, and such storage facilities should be frequently cleaned and disinfected after the cadavers have been removed.
6. Manure must be stored separate from the layer house as it can be a source of reinfection.
7. All crack and crevices must be sealed.
8. Manure must be removed frequently at least times 6 times per week. Cleaning of the manure belts could also help if the same belt goes through different houses.

Between production

Empty houses offer opportunities to reach places that cannot be cleaned in the presence of hens, for example:

1. Cleaning with hot water and soap is strongly advised over dry cleaning (table below illustrates all the steps)
2. Where finances and infrastructure allow, heating the poultry house gradually to over 45 ^oC for at least 2 days. This lures mites out of their hiding places.
3. Acaricidal chemicals can also be sprayed in empty houses. But in the context of IPM, chemical/synthetic treatments should not be employed preventively; rather, they should only be utilised when non-chemical preventative and curative treatments are not sufficient.

Table 1: Cleaning actions to be executed during the empty period for optimal control of PRM infestations in a layer house according to Mul et al. 2020

Step 2. Mite monitoring system

Without proper monitoring tools infestations are only noted when aggregates of mites are visible, or blood spots are seen on eggs or workers experience bites. But by this point it is usually too late for successful control to be applied. Tools include:

1. Mite Monitoring Score (MMS) – This involves a visual inspection of 1 m² of area at different points and levels in the house and giving a score of (0-4) to estimate the infestation rate. It is a visual perception and is therefore not sensitive enough.
2. Manual Traps - Various manual traps are commercially available such as corrugated cardboard traps, water trap, adhesive tape, AviVet trap, Velcro trap. They are placed at different locations in the house. Mites are counted or weighed manually as per instructions provided by the trap either by the farmer or by sending it back to the supplier to be assessed in a laboratory. These are reliable but can be labour intensive.
3. Automated Mite Counter – This trap counts the mite as they enter it. Once counted, the mites are removed by air suction into a filter. The filter must be cleaned weekly to prevent blockage.

Fig 4: Overview for the interpretation of monitoring results and thresholds defined by AviVet.

Step 3. Treatment decisions based on monitoring and thresholds

When PRM populations increase and reach the threshold set in the monitoring plan, chemical treatment is applied. By adopting this "action threshold," treatment/activity is not carried out too quickly or excessively, preventing adverse environmental consequences, unnecessary expenses, and the emergence of resistance. On the other hand, the treatment is not started too late, leaving room for effective control. Currently there is no prescribed threshold for PRM and the Mite Control project is working to determine a suitable threshold.

Step 4. Non Chemical Treatment Methods

In IPM, synthetic acaricides are avoided; the non chemical treatments listed below can be used preventatively under step 1 or curatively when the mite population has exceeded the threshold.

1. Plant based products – They have acaricidal and toxic activity but also work as a repellant or an attractant. They have low to none toxic effects on mammmals and the environment and can reach mite hiding places in vapour form. Several studies have demonstrated the efficacy of plant based products like eugenol, the bioactive compound of clove and neem seed extract against PRM, but these can be short lived compounds.
2. Biological control – Use of natural enemies against  pests is widely used in horticulture. The environmental impact is minimised and the risk of emergence of resistance is reduced by adopting biological control methods. Predatory mites are typically released as a preventative measure.  The strategy for numbers, frequency, and release places depends on the number of hens and the housing arrangement.

Fig 5: Eugenol from cloves (left) and neem seed extract (right) are promising plant based products for control of PRM

iii. Physical methods – Inert dusts like synthetic silica dust or natural diatomaceous earth can be used to treat PRM. Only natural diatomaceous earth is allowed on organic farms. They work by drying out the mites' outer skeleton, making resistance unlikely, but can pose a respiratory hazard to humans due to fine particle size. Liquid silica products are also available, but their efficacy may decline due to dust and debris buildup. Cleaning surfaces before applying silica can increase efficacy. Heat treatment (discussed in step 1) is also effective in controlling mites.

Fig 6: Inert dusts, diatomaceous earth (left) and silica sand (right) used as phyical aides to control PRM

1. Combination of above methods – Many studies have demonstrated that a combination of the above methods is more efficacious than using one of these.

Step 5. Use of specific chemical acaricides

IPM allows the use of chemical acaricides but only as a last resort. Careful choice and proper dosage of acaricides is necessary to prevent resistance and increase success. Selective products are preferred to minimize impact on non-target species, but none are fully selective for PRM. Regulations vary by EU country. A fluralaner-based drug, administered through drinking water, has been recently marketed. It is prescription only and has a quick killing effect after a blood meal, with 0 day withdrawal for eggs and 14 day withdrawal for meat and offal. It is allowed in organic farming with extended withdrawal periods.

Step 6. Reduction in use of pesticide

Reduction in the use of chemical pesticides can be achieved by using targeted pesticide delivery such as spraying on hotspots or using traps impregnated with pesticide instead of spraying the whole house. By treating locally, the total amount of pesticides used is reduced and negative impacts on natural enemies are reduced. The fluralaner based product adminstered in drinking water is another option of targeted delivery.

Step 7. Antiresistance strategies

Resistance of PRM to chemical acaricides like carbamates, pyrethroids, and the organophosphate phoxim has been widely reported. The prospect of resistance emergence is increased when products are applied in incorrect dosages, or too often, which is reinforced by the limited number of allowed chemicals. To warrant the success of IPM for controlling PRM, actions to prevent resistance emergence against natural as well as chemical treatments should be taken into consideration. To decrease the emergence of resistance the following steps can be undertaken

1. not under-dosing or exceeding the recommended frequency of application of a product.
2. using targeted delivery as discussed in step 6.
3. combining and/or rotating products with different modes of action.
4. Applying preventative management actions properly can reduce the need for chemical acaricides and hence the emergence of resistance.

Step 8. Evaluation

Evaluation is necessary to determine effectiveness and to make adjustments to IPM strategies. Monitoring PRM populations regularly to assess impact of treatments and IPM strategy is crucial. A universal IPM strategy is impossible due to variables such as temperature, humidity, husbandry practices, hen breed, and economic factors. The goal is to create dynamic, adaptable strategies that continuously assess a method used at a particular farm.

Summary

The poultry red mite is the most important blood-sucking ectoparasite in egg-laying hen facilities. This mite poses serious animal health and welfare concerns, adversely affecting productivity of hens leading to huge economic loses. Bites on poultry workers are not uncommon. Conventional control using chemical acaricides is leading to emergence of acaricide resistant populations and the scarcity of chemicals licensed to use, due to safety regulations, implies that urgent alternative treatments are needed. The Mite Project has developed IPM strategies based on the principals of established IPM protocols in horticulture. It combines eight different steps, in which prevention of introduction and monitoring of the pest are key. Further, it focusses on non-chemical treatments, with chemicals only being used as a last resort. A major knowledge gap lies in the determination of the action and economic threshold to decide at which point application of chemical acaricides is required. The lack of a vaccine against PRM also poses some hinderance to the success of IPM. The Mite Control Project is working to address these issues.

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