The northern fowl mite, Ornithonyssus sylviarum, infests a wide variety of domestic fowl and wild birds and is the most important and common external parasite of poultry. Mites feed on blood, and heavy mite infestations can irritate and stress the birds, reducing egg production by 10 to 15 percent. A production loss of this size can add up to significant losses over the total life of a caged laying hen. Heavy populations also can reduce weight gains and, in male birds, reduce seminal fluid volume. Mites also can annoy egg handlers and other personnel.
Mites congregate first on the vent, then on the tail, back, and legs of female birds; they are more scattered on male birds. As the mite population increases, feathers become soiled from mite eggs, cast skins, dried blood, and excrement. The soiling produces the characteristic blackened feathers in the vent area. Scabs also may form in the vent area. Although death due to actual anemia is rare, birds with heavy infestations (50,000 mites/bird) can lose 6 percent of their blood daily.
Mite populations can rise rapidly after a bird first has been infested, especially during the cooler months and on young birds 20 to 30 weeks of age. Newly infested birds may support mite populations in excess of 20,000 per bird in 9 to 10 weeks. Mites do not become established on birds in large numbers until birds reach sexual maturity. Birds older than 40 weeks usually do not support many mites.
The northern fowl mite completes its entire life cycle on the bird, although it can survive off the host for 2 or 3 weeks under suitable conditions. Life cycle stages consist of egg, larva, two nymphal stages, and adult. The eight-legged adult is only 1/26 of an inch long and is usually dark red to black. Females lay two to five eggs in the fluff of feathers after each blood meal. Eggs hatch into six-legged larvae within 2 days. All other mite stages possess eight legs. Nonfeeding larvae develop in approximately 9 hours and molt into blood-feeding nymphs that develop in 1 or 2 days. Second-stage nymphs, like the larvae, do not feed and molt to adults in less than a day. The entire life cycle can be completed within a week under favorable conditions.
The detection of an initial low mite population that can be controlled effectively and economically is important in a mite-monitoring program. With early detection, only part of a caged-layer house may need to be treated. At least 10 randomly selected birds from each cage row in the entire house should be monitored weekly. The vent area should be examined under a bright light, and the feathers parted to reveal the mites. Single caged birds often have more mites than those caged in groups and, because of variation in susceptibility among birds, one bird may have mites while its cage mates are mite-free.
The following index is effective for estimating infestation levels:
0 =no mites observed
1 =1 to 2 mites
2 =3 to 9 mites
3 =10 to 31 mites
4 =32 to 99 mites
5 =100 to 300 mites
6 =301 to 999 mites
7 =1,000 to 3,000 mites
8 =3,001 to 9,999 mites
9 =10,000 to 32,000 mites
10 =more than 32,000 mites
An average index of 5 or greater for all examined birds generally indicates the need for chemical treatment.
The actual decision to treat is influenced by flock age, time of year, and distribution of the infestation in the house. It is usually not economical to treat older birds, because their mite populations are unlikely to increase. A population buildup is more likely in a young flock. Mite populations can be expected to increase in cooler months and decrease in warmer months. An infestation restricted to one part of the house may not spread, but the infested area should be closely monitored. Detection of mites in broiler-breeder operations generally means the entire flock must be treated.
Chemical control of northern fowl mites in caged-layer operations requires direct pesticide application to the vent region with sufficient pressure (minimum 100 to 125 psi) to penetrate the feathers. The spray will have to be directed upward from beneath the cages to reach the vent. A split treatment of a recommended active ingredient may increase effectiveness, since water is held better when applied to wet feathers. Mix half the insecticide in the standard amount of water for the first application, spray, and then mix the other half in another standard amount of water for the second application. Dust formulations can be purchased ready to use and may be applied to caged layers with a power blower. Treatment is difficult in broiler-breeder operations where birds are not confined to cages.