Of Mites and Hen
Northern fowl mites (Ornithonyssus sylviarum) are the dominant species of ectoparasitic mites of poultry in the United States. They feed on chicken blood, can cause severe chicken anemia, and reduce egg production by up to 15% both in egg layers (where eggs are used for direct human consumption) and broiler breeders (where eggs are hatched into pullets for broiler or poultry meat production) (Axtell, 1985). A 15% reduction in egg production in layers might mean a lost income of $22,800 per day at peak egg production (30-week-old laying hens) in a farm with 2,000,000 laying hens, 95% of the hens laying one egg per day, and an egg market value of $0.08 per table egg.
The loss in potential income might be greater in broiler breeder hen farms because of the added value when the eggs are hatched into pullets then raised for broiler chicken meat production. Northern fowl mites can also cause dermatitis and allergies among poultry farm workers; several disease-causing microorganisms had been isolated from northern fowl mites but it is currently not known if these microorganisms are vectored by northern fowl mites to poultry (fowlpox virus, Newcastle disease virus) or humans and horses (St. Louis encephalitis virus, western equine encephalitis virus) under natural settings (Axtell, 1985).
Poultry red mites (Dermanyssus gallinae), although considered the dominant poultry mite species in Europe, are usually not considered a significant problem of poultry production in the United States at this time. Poultry red mites may be found in broiler breeders in the southern United States (Axtell, 1985), but they are usually not found in significant numbers in layer and broiler production farms in this country. The exact reason why poultry red mites are dominant in Europe but not in the United States, while the opposite is true for the northern fowl mite, may be related to the so-called ecological niche concept (the way an organism makes its living in the ecosystem) (Keeton and Gould, 1986). Changes in poultry production practices (e.g., poultry housing conversion from caged to cage-free laying hens) may alter the niche and favor the proliferation of poultry red mites (Flochlay et al., 2017) over other species of mites and insects associated with poultry production. This current article will focus mainly on the northern fowl mite; readers who are interested in the importance, biology, and control of poultry red mites can refer to Flochlay et al. (2017).
Fully-grown northern fowl mites are only about one millimeter in length but they are still visible with the naked eye. The best way to visualize northern fowl mites is to pluck several feathers from the vent or cloacal area of an infested hen, place these feathers in a sealable plastic bag, and observe the mites moving away from the feathers. A microscope or magnifying lens might be needed to better see the mite eggs and small immature stages on the infested feathers. Northern fowl mites can complete their five developmental life stages (eggs, larvae, protonymphs, deutonymphs, adults) within five days (Axtell, 1985). Development is usually completed on the host chickens.
Peak northern fowl mite infestation level usually coincides with peak egg production when the laying hens are 20-30 weeks old. Blood (from chickens, wild birds, and even rodents) is the main food of northern fowl mites. Infested chickens become anemic, restless and agitated when thousands of mites try to bite and withdraw blood resulting in significantly lowered egg production, feed conversion and weight gain.
An average (from 20 or more randomly selected chickens) mite infestation level of 10-15 mites in the vent area per chicken may warrant control intervention in 20- to 30-week-old laying hens (Axtell, 1985). Northern fowl mites can multiply very quickly from a few individuals to over 20,000 mites per chicken within 10 weeks; infestation levels of over 50,000 mites per chicken can result in the loss of 6% of the total chicken blood volume per day (DeLoach and DeVaney, 1981). Uncontrolled heavy infestations with over 50,000 mites per chicken can cause mortalities due to blood loss. Adult mites can survive for up to a month away from the host (Axtell, 1985); during heavy infestations they can be seen crawling around on eggs in the conveyor belts that connect the poultry houses to the egg processing areas.
Northern fowl mites can be very hard to eliminate once established on the farm; it is best to avoid infestations in the first place. Infestations can be prevented by only using pullets from reputable farm sources that are guaranteed free of existing northern fowl mite infestations. Because northern fowl mites can also use rodents and wild birds as alternate hosts, every effort must be made to control and exclude these potential infestation sources from the chickens being raised on the farm.
Controlling northern fowl mites on infested chickens can be challenging because of the location of the infestation (at the base of feathers close to the skin) and position (in the vent area) in the chicken body. Thus, although there are currently many insecticides to choose from, these products will still need to reach the mites by penetrating the feathers in the vent area of the chicken body to be effective.
Chicken feathers are biologically designed to repel water and dust; but feathers appear to readily absorb oil and oil-based products. Based on my laboratory research in our MWI Animal Health Entomology Laboratory in 2014, oil-based products appear to penetrate feathers better than water-based or dust-formulated products. Thus, miticidal products formulated as dusts or water-based mixtures may need to be applied with more effort than oil-based products to penetrate the feathers and reach the mites located close to the skin in the vent area of the infested chickens.