Blackhead Disease in Poults
By Alberta Agriculture
Blackhead disease, or more correctly, histomoniasis, is primarily a disease of young turkeys. Chickens are more resistant to the effects of the infection but may act as carriers of the disease-causing organism. Histomoniasis is caused by a microscopic protozoan called Histomonas Meleagridis. The name blackhead is a poor descriptive term because the heads of the birds infected with this parasite are not dark. The protozoan causes considerable damage to the liver and ceca of infected turkeys, and the untreated birds usually die.
Histomoniasis can cause considerable losses in farm turkey flocks. It is a disease commonly seen in these birds at veterinary diagnostic labs in Alberta. It is less common in commercial turkeys because their rations contain low levels of histomoniasis preventing agents.
Histomonas meleagridis is most often transmitted to turkeys in the eggs of a second parasite, the cecal worm, commonly found in both chickens and turkeys. The eggs of the cecal worm may remain infective in the soil for three years of longer and could transmit the blackhead-causing protozoan during this period.
H. meleagridis may also be transmitted by earthworms that accidentally eat the cecal worm eggs. The cecal worm larva released from the egg and the blackhead parasite within that larva may remain in the earthworm for a year of more. When chickens or turkeys eat infected earthworms, the cecal worm larvae containing the blackhead parasites are released and a blackhead infection may result.
Turkeys may acquire the blackhead organism directly from the droppings of infected birds. However, meleagridis found free in the droppings and not protected by a namatode egg die quickly, particularly during warm dry weather.
A decrease in feed consumption and loss of weight may be the first signs observed. Sick birds appear dull and depressed, and often stand by themselves with dropping trails, ruffled feathers, and a sleepy appearance. Sulphur colored yellow droppings may be observed. If birds are not treated, or if treatment is delayed, mortality may be very high. Birds dying of histomoniasis have characteristic enlarged livers with circular depressed areas and enlarged ceca containing a rather dry cheesy material (Figures 1 and 2). Recovered birds may show swollen hard and scarred livers at the time of slaughter.
The drug Dimetridazole can be used in the drinking water or feed to control outbreaks oh histomoniasis. Other drugs occasionally are used for treatment but are better used as preventatives. Sick birds should be isolated from the main flock and treated separately. Dead birds should be burned or buried deeply. All equipment used by an infected flock should be thoroughly cleaned and disinfected.
The protozoa-causing blackhead may remain infective within the eggs of the cecal worms in the soil for nearly three years; therefore each flock of new turkeys should be raised on new uncontaminated ground. Young turkeys should never be reared near older turkeys or with chickens that may carry the infection. In addition to domestic chickens, various wild birds such as pheasant and grouse may serve as reservoirs of infection for domestic turkeys.
The periodic moving of feeders, waterers, and roosts will help prevent the local buildup of infective organisms. Good sanitation and litter management will help prevent transmission of the cecal worm as well as the blackhead organism. Many histiostats or preventative drugs are available and they are commonly included in commercial turkey rations. Because of the very serious nature of blackhead in turkeys, it is advisable to develop a regular program of preventative drug treatment.
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