Blackhead diseaseFrom Wikipedia, the free encyclopedia

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Large, pale areas in the liver of a bird infected with Histomonas sp

Blackhead disease (also known simply as blackhead) is a commercially important avian disease that affects chickens, turkeys and other poultry. The disease carries a high mortality rate and affects the liver and cecum. It is a form of histomoniasis caused by the protozoan parasite Histomonas meleagridis. It is (only?) transmitted by another parasite; the nematode Heterakis gallinarum [1] when it is ingested along with soil containing (remains of) feces. Earthworms can act as a paratenic host [2].

Poultry (especially free-ranging) and wild birds, commonly harbor a number of parasitic worms with only mild health problems for them. Turkeys are much more susceptible to getting blackhead than are chicken. Thus chicken can be infected carriers for a long time because they aren't removed or medicated by their owner, and they don't die or stop eating/defecating. Heterakis gallinarum eggs can remain infective in soil for four years [3] thus there is a high risk of transmitting blackhead to turkeys if they graze areas with chicken feces [4] in this time frame. Thermophilic composting is known to sanitize soil from ascarid (another nematode) eggs.

The most common symptom of Blackhead disease is yellow watery bird droppings. To reduce the spreading of Blackhead disease, the sick birds must be removed and their litter changed.

Histomonas meleagridis

From Wikipedia, the free encyclopedia

Jump to: navigation, searchHistomonas meleagridis

Scientific classification

Domain: Eukaryota

(unranked): Excavata

Phylum: Metamonada

Class: Parabasalia

Order: Trichomonadida

Family: Monocercomonadidae

Genus: Histomonas

Species: H. meleagridis

Binomial name

Histomonas meleagridis(Smith, 1895)

Histomonas meleagridis is species of parasitic protozoan that infects chickens, turkeys, peafowl, quail and pheasants, causing blackhead disease, infectious enterohepatitis, or histomoniasis. H. meleagridis can infect many birds but it is most deadly in turkeys.

[edit] Etiology

H. meleagridis between flocks is transmitted in eggs of Heterakis gallinarum, a cecal nematode which in turn can be transmitted by earthworms that ingested the nematode eggs. But within turkey flocks H. meleagridis is also known to be directly transmitted from bird to bird. Histomonads, either released from the Heterakis nematode larvae in the ceca or after direct infection via the cloaca, begin to replicate rapidly in the cecal tissues. They migrate to the submucosa and muscularis mucosae and cause severe necrosis. The parasites then move to the liver through the vascular system. There they cause typical crater-like necrotic lesions. Mortality in turkey flocks can be very high (over 80%).

Blackhead Disease in Poults

About Blackhead Disease [Histomoniasis]

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.

Life History of the Disease

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.

Signs

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.

Treatment

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.

Prevention and Control

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.

Control of Blackhead Disease

A thorough review of this hard-to-control disease of turkeys, chickens and other farmed birds was presented by Dr Larry R. McDougald of the University of Georgia at the 2008 North Carolina Turkey Days.

Blackhead disease continues to cause sporadic but severe disease losses in commercial turkey flocks. Losses are more common but less severe in chickens, particularly broiler breeder pullets, and in game bird flocks reared in confinement and on open range. The wild turkey, a common game bird in North America, as well as peafowl and certain other gallinaceous birds often fall victim to this disease.

While death losses often reach 80- 100 per cent in domestic turkeys, a different face of the disease is seen in chickens. Broiler breeder pullets may suffer 10% mortality, extensive culling losses, and poor uniformity at time of lay. In the absence of any highly effective treatment drugs, emphasis on control is placed on prevention and containment by management and quarantine. Fortunately, research has been initiated in several laboratories in the USA and Europe, which are yielding important new findings which will be of value in planning future control programs. The older literature and some new findings were reviewed by McDougald (2005).

Additions to the life cycle of Histomonas meleagridis

At one time, researchers and diagnosticians believed that infections in turkey flocks arose from the ingestion of embryonated eggs of the caecal worm Heterakis gallinarum, or ingestion of earthworms that were carrying larvae of the caecal worm. This mechanism did not explain the phenomenon of rapid spread of blackhead through a flock of turkeys, and led some to question whether other intermediate hosts might be involved.

This question was addressed more recently in experiments where uninfected turkey poults were placed in pens alongside other directly inoculated poults, in the absence of any other possible carrier or host (McDougald and Hu, 2003). The results were clear-cut and dramatic. The uninoculated birds readily contracted the infection, became sick and died. By the end of the study, all of the birds had died or were sick with blackhead (Figure 1).

Figure 1. Mortality from blackhead by indirect exposure

Direct transmission of blackhead disease from directly inoculated turkey poults to uninoculated poults in the absence of intermediate hosts or other carriers.

Depending on the exposure level, all of the uninoculated birds died or became ill from histomoniasis.

Further work on this means of acquisition of infection showed that the oral route was not involved, and that birds probably became infected by intake of liquid faeces in a process known as cloacal drinking (16).

The implication of this finding was that if healthy birds could be separated from sick birds, it would be impossible for the infection to spread through the flock. This could be accomplished by means of migration barriers, to divide the house into smaller sections, thereby limiting risk. This concept has been used successfully in some instances.

Where do turkeys get infections?

It is common that veterinarians and other investigators are unable to find Heterakis worms associated with outbreaks in turkeys. If the above discussion is considered, then it would be logical that the infection comes from outside the flock, probably tracked inside by a worker on the shoes. The source of such contamination is most likely chickens, which are often found not too distant from the turkey flock.

A review of the literature shows that chickens, among the domestic gallinaceous birds, are the best hosts for Heterakis worms, and that the eggs produced by these worms in chickens are the best for causing disease when inoculated into turkeys. Lund and Chute (1969) found that young chickens were 16 times as effective as mature chickens in hosting caecal worms, and that young turkeys were almost negligible in this respect. Lund and Chute (1973) tested eight species of gallinaceous birds and found that the Chinese ringneck pheasant was the best host for caecal worms, followed by chickens and guinea fowl.

In modern poultry production, it is not unusual for farms used for one type of poultry to be converted to rearing of another type. Probably the most disastrous example is the conversion of broiler breeder farms to the rearing of turkeys. It is commonly agreed that all broiler breeder farms are heavily contaminated with caecal worm (Heterakis gallinarum) eggs, which are the only known biological vector of the blackhead organism. (Earthworms can harbour caecal worms until they are eaten by chickens or turkeys, but this is only an 'extra' reservoir of infection and not a necessary part of the life cycle). Such farms reportedly remain infective to turkeys for many years.

Where did blackhead disease come from?

The first cases of blackhead in turkeys were reported from Rhode Island in 1892. It is interesting that the Chinese ringneck pheasant was introduced into the USA in 1881, and was soon propagated widely for release as a game bird. The chukar partridge has also been imported and propagated for release in many areas.

Blackhead outbreaks decimated the turkey industry in the New England area, and followed the farmers to the Midwest, Canada and to the far West. Up to World War II, blackhead was the leading cause of mortality in turkeys. Game birds are usually overlooked as a source of infection, but obviously could be another reservoir of infection as they may be present in areas where turkeys are raised. As shown by Lund, the pheasant and chukar are far better hosts for Heterakis than even chickens, and suffer little from the effects of histomoniasis, making them ideal as reservoirs of the disease.

How is blackhead controlled by management?

Prevention of blackhead in turkeys by management is two-fold:

1. prevention of exposure by quarantine or isolation, especially avoiding any contact with chickens or game birds, and

2. use of migration barriers to prevent commingling of infected birds with uninfected birds.

It is interesting that the contagious spread of blackhead by direct contact of birds is not as important in chickens as in turkeys (Hu et al. 2006). Thus, it is likely that infections in chickens result only from ingestion of Heterakis ova. Also, farm owners should be aware of the hobbies of their workers and discourage the keeping of backyard chickens, pheasants, chukars or fighting cocks.

Could a vaccination against histomoniasis be developed?

Previous investigators considered immunization an impractical approach to control of blackhead disease. Repeated infection and treatment with

dimetridazole produced turkeys that were resistant to re-infection after three infection/treatment cycles.

Recent experiments in our laboratory were successful in demonstrating some protection when turkey poults were given two or more inoculations with an antigen consisting of freeze/thawed cultured H. meleagridis. A single inoculation failed to offer protection when birds were challenged within a few weeks.

More work is needed to identify the best vaccination regime, the amount of antigen needed, and the possible contribution of bacteria to the host immune response.

Does Histomonas respond to anticoccidials or antibiotics?

The answer to this is essentially no. Like some of its common relatives Trichomonas and Giardia, it is anaerobic and lacks mitochondria. These organisms make energy by an anaerobic process involving special organelles called hydrogenomes, This explains why histomonads do not respond to these other types of chemotherapeutic agents; they simply lack the metabolic machinery to be interfered with.

Sometimes birds infected with blackhead, particularly chickens, will seem to respond to antibiotic treatment. However, this is probably because of secondary infections with bacteria that could be affected by the drug.

Antibiotics normally have little beneficial effects on turkeys during a blackhead outbreak. Early and frequent preventive use of wormers (benzimidazole) can be of benefit in chickens because worms are the primary source of infection (6). In turkeys, where the infection spreads easily from bird to bird, this is probably not of value after outbreaks start.

Discussion and Conclusions

Even though losses from blackhead disease continue to be highly significant, some progress has been made in recommendations for control by management. Chickens and game birds are likely the most important source of infection for turkeys and other birds, as they are prolific in generation of infective cecal worm ova.

The life cycle of Histomonas has been reconsidered, after the discovery that turkeys could become infected from direct contact with other birds or contaminated faeces.

The unique structure and metabolism of Histomonas makes these organisms immune to treatment with anticoccidials and antibiotics, but antibiotics are usually considered beneficial to treat secondary bacterial infections

After its discovery in 1893 in Rhode Island, blackhead disease was reported across the continent

and soon in many other countries. It decimated the turkey industry in New England and followed

production like a faithful shadow. Blackhead disease causes high mortality in turkeys, sometimes

approaching 100% of a flock. In chickens, the mortality may be 10%–20% with high morbidity,

although many outbreaks pass unnoticed. Early workers identified Histomonas meleagridis, a

protozoan related to Entamoeba histolytica, Giardia lamblia, and Trichomonas, as the causative

agent. Like many other parasites, its life cycle is complex, involving as an intermediate host, the

common cecal worm Heterakis gallinarum. The necessity for bacteria for Histomonas to become

virulent in the turkey and chicken, notably Escherichia coli, Bacillus subtilis, and Clostridium

spp., was discovered by research in gnotobiotic birds. Changes in management brought the

disease under control, although it remained the first cause of mortality in turkeys until modern

antihistomonal products were developed after WWII. The ban of nitroimidazole products in the

United States and Europe was followed by an upsurge in reported cases in turkeys and chickens.

Immunization is not an option for prevention, as birds do not reliably become resistant to

reinfection after suffering a primary exposure. Recent research demonstrated that histomoniasis

could spread rapidly through a flock of turkeys by direct contact, probably involving the

phenomenon of cloacal drinking. Direct transmission was not demonstrated for chickens,

stressing dependence on H. gallinarum as the source of infection. The lack of suitable treatment

drugs or vaccines emphasizes the importance of prevention by worm control and management.