Matthew Balfour BVM&S MRCVS
Dermanyssus gallinae, commonly known as red mite, are recognised as the most damaging parasite of laying hens worldwide. The UK, where the majority of flocks have a red mite infestation, is no exception. An infestation will have a cascade of negative effects on bird health and welfare, as well as the economic performance of the flock. For example, even a relatively low number of red mite will cause physical irritation, raised stress levels and increased vent/feather pecking. As numbers increase, egg production will be affected, birdsmay become anaemic and mortality will rise. In addition, red mite have been implicated in the spread of pathogens such as Pasteurella, Erysipelas, E.coli and Salmonella.
Introduction to a shed
Even new layer houses can succumb to red mite infestations. A broad range of avian species are natural hosts for red mites and therefore wild birds can easily introduce them to a flock which was previously mite- free. Red mites are able to survive for 9 months without a host and therefore may be present in the range prior to stocking of the shed. Another possibility is a biosecurity breach and introduction from a poultry flock with an existing infestation; of particular concern are any equipment/tools which have been used in another poultry house and may harbour mite.
Health and welfare implications and economic losses
Even a low mite burden in a shed will have negative effects on a flock. However, without treatment numbers will grow and clinical signs will become more obvious. These may include:
Red mites will irritate and stress the birds as they feed, and studies have shown a 1.5x increase in corticosterone and 2x increase in adrenaline (two stress hormones) within affected birds. Stressed birds are more likely to feather and vent peck.
In addition, stress will cause immunosuppression (and therefore increased susceptibility to other diseases) and reduced egg production.
Typical mite densities in an infested shed range from 25,000 – 500,000 red mite per bird. With these numbers feeding every night the bird may lose up to 3% of its blood volume, causing anaemia over time. Mildly anaemic birds will have pale combs and wattles, may be lethargic and will become immunosuppressed. In severe cases birds will die.
Blood spots on eggs
Red mite will commonly feed around the cloaca of a laying hen, causing damage to the delicate skin. This can mean that eggs will sometimes get small streaks of blood deposited on the shell as they are laid, resulting in downgrading of the eggs.
Transmission of disease
As they move from bird to bird whilst feeding, mites are able to directly inoculate pathogens from one bird into the bloodstream of another. Studies have shown that Pasteurella, Erysipelas, E.coli and Salmonella can all be transmitted by red mite.
It is not surprising that the disease manifestations discussed above will translate into significant economic losses over the lifetime of a flock. This is predominantly a result of decreased egg production, increased egg downgrades, increased mortality and reduced feed conversion rate (FCR) (particularly important with current high feed prices). One 2017 study from the Netherlands estimated a €0.60 loss per hen across the country, and this figure can only have increased over time. It should be noted that this is an average figure and severely affected flocks may experience substantially higher losses. For example, average egg production which is 5% below breed standard in a flock, due to a high mite burden, will translate into £1400 per 1000 birds lost in egg sales (assuming depletion at 80wks and 91p/doz). This is before mortality, increased seconds, reduced FCR and overall poorer flock health status are taken into account.
Red mites will feed on humans when no avian hosts are available. When they feed on humans, bite reactions may or may not occur, but may show as pruritic erythematous papules, vesicles or dermatitis.
The best control strategy for red mite is to keep it out in the first place through strict biosecurity and wild-bird deterrents on the range. However, after several years in operation most laying sheds will find themselves with a red mite infestation. Unfortunately, when this happens, complete elimination is seldom possible, and strategies must focus instead on controlling red mite populations at a low level which will cause minimum discomfort to the birds. The options described below are usually best employed in combination and in consultation with your poultry vet.
Terminal cleaning and disinfection (C&D)
A good terminal C&D programme is essential for reducing the red mite population on-site to a minimum prior to new pullets arriving. This should consist of
a thorough wash-down, application of a detergent, another wash-down, drying and then application of a disinfectant active against red mite eggs (e.g. Interkokask). Care must be taken to ensure that any disinfectant is applied to a dry surface and at the dilution rate specified to kill mite eggs.
This is a unique prescription-only in-water medication which is very effective at killing red mite, even if they are at high levels in a shed. It must be given as two 12hr treatments separated by 7 days – the aim of this is to kill adult mite in the first treatment, then the next generation of mite as they emerge 7 days later. If used properly this product will kill >99% of mite in a shed, however over time mite will gradually return and more than one treatment over the lifetime of a flock maybe required. Each treatment is relatively expensive, however when the full economic impact of red mite infestation is taken into account it is usually a cost- effective option.
Insecticide spray-on products
In the past these products were the mainstay of red mite control, however in recent years resistance has developed and many formulations have been removed from the market. However, they are still useful at controlling low to moderate infestations, particularly if a product with residual action is chosen and the shed system allows for easy spraying of hot-spot areas. We always advise rotation of products used to reduce risk of resistance build-up. Care should also be taken to ensure any products used are approved for use in the presence of laying hens.