Red mite is a pervasive pest in both the laying hen and broiler breeder sectors. It can cause significant production problems, act as a vector for disease and even increase mortality. Jake Davies reports from Scotland where research is underway to develop a vaccine.
The poultry red mite constitutes a substantial cost to laying hen production and there are few ways to control it. The pest can feed on up to 5% of a bird’s blood overnight, causing stress and anaemia, as well as lowering immunity and increasing mortality. The cost of red mite across Europe has been put at € 230 million in lost production every year. That stress can induce feather pecking, cannibalism and increased feed consumption. Besides this, red mite is known to carry diseases such as salmonella, E. coli and mycoplasmas. And it is pretty pervasive.
Red mite live alongside many bird species but concentrated in the setting of a commercial poultry shed they can become a problem if populations are able to build up. New farms will often have a few months’ grace but the pest can be introduced by a wide range of things, such as people, pullets, egg trays or even carried in on wildlife. A mature adult will lay a clutch of eggs which takes about 2-3 days to hatch into larvae. From there, it is 1-2 days before they develop into protonymphs when they will take their first feed. Another 5 days and the mite is a fully grown adult, ready to lay eggs – and each female will lay up to 300 over her lifetime, demonstrating just how quickly populations can escalate. A normal infestation level could be up to 50,000 mites per hen and, in the case of a severe infestation, more than half a million mites per bird may be found in sheds.
Research suggests that about 90% of farms in most Western European countries are infested, with free-range and barn units thought to be more susceptible than colony farms. But control measures are limited, at least in Europe. Most farmers will re-purpose insecticides as acaricides, says Dr Alasdair Nisbet, head of the Vaccines Pillar at the Moredun Research Institute. “Some of the older classes of these, like organophosphates and spynosyns, were effective, but we’re starting to see resistance.” There is a single specific red mite deterrent active on the market that is proving to be effective, Dr Nisbet added, but it is not wise to rely too heavily on one product. This is what led to investigations of whether a vaccine might be an effective tool in controlling red mite. “Initially this was based around taking extracts of the mite and testing them to see if they were effective, that has since evolved into the development of a synthetic commercialisable vaccine for poultry red mite.”
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One of the first approaches was to take a simple extract of the parasite and inject that into hens. This generated antibodies in the hens which subsequently attacked mites when they fed on hens’ blood. In an experiment with the novel vaccine, 800 birds were split into 2 groups with half vaccinated and the other half free of red mite control. Red mite were introduced to the birds and after 4 months the vaccinated hens had 75% fewer red mites than those in the control group. “This demonstrated the proof of principle that we could control this parasite with a vaccine. It also opened the door to autogenous vaccines,” said Dr Nisbet.
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For commercial operations, however, it would be necessary to identify the components that control red mite and produce them as synthetic proteins. “Then you would have a commercialisable vaccine.” The next stage took some 3 years. Working out which components produced the antibody reaction in hens and synthetically reproducing them. “Unfortunately, the vaccine didn’t work. That’s often the case when you’re trying to develop a synthetic vaccine made from a native extract.”
This is really a mini tick that we’re dealing with. That gave us cause for optimism…” – Dr Nisbet.
The research team “took a step back” to work out what was needed to create a vaccine. A well-annotated genome was one element necessary to help understand more about how the red mite works. “When we first sequenced the genome we were really surprised by how big it is.” Another surprise was how closely related it was to ticks. “This is really a mini tick that we’re dealing with. That gave us cause for optimism because the only other commercialised vaccine against an ectoparasite is against the tick.”
Another tool was aimed at creating a way to reliably test vaccines on a small scale without going into the field for more expensive large-scale trials. The result was a small ‘teabag’ that opens at one end so that mites can be added. The mesh is just large enough for them to put their heads through to bite birds but not big enough for them to escape. Once attached to birds’ legs it gives an easy way to assess how effective prototype vaccines are. “That’s been a real refinement of the process,” Dr Nisbet added.
The genome was used to understand which genes are switched on in mites after they feed. One protein breaks down haemoglobin, interrupting the mite’s ability to digest food. That was mixed with an adjuvant and tested using the teabag tester. Results from that protein found a high antibody response. Mites that fed on inoculated birds’ blood produced 50% fewer eggs. “So, we’ve now gone through a rational process for identifying a vaccine, we now need to know if that is enough to suppress populations in the field. Do we have to add other antigens to make it better and actually kill mites, rather than just suppress populations?” While promising, Dr Nisbet says that the idea is not to replace other treatments against red mite but to complement them as part of an integrated pest management system. “There’s been a huge amount of work done over a great many years and we’re now at a really exciting point in the project,” Dr Nisbet concluded.
*This article was based on a poultry and pig health and welfare session hosted by the Moredun Research Institute.