Blackhead disease in poultry
In one of the most unique forms of disease transmissions known to biology, the cecal worm (Heterakis gallinarum) and the protozoan (Histomonas meleagridis) have been interacting with birds (mainly turkeys and broiler breeders) to perpetuate a serious disease called Blackhead (histomoniasis) in poultry.
Blackhead disease of turkey was first documented in the United States about 125 years ago in Rhode Island (Cushman, 1893). It has since become a serious limiting factor of poultry production in the U.S.; potential mortalities in infected flocks can approach 100 percent in turkeys and 20 percent in chickens (McDougald, 2005).
The biology of histomoniasis is quite complex as several species of organisms can be involved in the transmission, pathogenicity, and maintenance of the disease in the environment—the protozoan parasite Histomonas meleagridis (Smith, 1895); the potential hosts (turkey, chicken, pheasant, ruffed grouse, bobwhite quail, Japanese quail, guinea fowl, chukar partridge, and peafowl) (Lund and Chute, 1974); the cecal worm (Heterakis gallinarum) vector or carrier of the protozoan parasite (Gibbs, 1962; Park and Shin, 2010); the potential vectors or carriers of the worm vector (earthworms, house flies, flesh flies, grasshoppers, and sow-bugs) (Lund et al., 1966; Spindler, 1967); and the bacteria and cecal coccidia that can cause secondary infections to the hosts (McDougald and Hu, 2001).
1. Through some biological mechanisms, the protozoan Histomonas meleagridis enters the body and eggs of the cecal worm in the cecal environment of an infected bird host. Cecal worms (Heterakis gallinarum) are parasites of turkeys, chickens and other birds; Histomonas meleagridis probably just started as a parasite of cecal worms before it evolved into a parasite of turkey and other birds.
2. The eggs of the cecal worms (containing the histomonad protozoan) are excreted by the infected bird into the poultry barn litter and other environment outside the host; these infective cecal worm eggs are picked up by ground-dwelling organisms such as earthworms, sow-bugs, grasshoppers, and house flies. Infective cecal worm eggs can persist in the environment for up to three years (Farr, 1959).
3. Healthy turkeys [Fig. 2] and game birds ingest infective histomonad protozoans (contained in the eggs of cecal worms) directly or by ingesting ground-dwelling earthworms and invertebrates carrying the cecal worm eggs (containing the histomonad protozoan parasites); infection in the birds starts by the release of the histomonad protozoan when the cecal worm eggs hatch into larval cecal worms in the ceca of the birds.
4. The histomonad protozoans, as well as the cecal worms, proliferate and infect the ceca of the digestive system of the host birds; the histomonad protozoans eventually spread and cause injuries to the liver of the host.
5. Pathogenic bacteria and coccidia already present in the digestive system of the host bird interact with the histomonad parasites and cecal worms to cause full pathogenicity of the histomoniasis (blackhead or enterohepatitis) disease.
6. The cycle starts anew in the ceca of infected birds when free histomonad protozoans enter newly-formed cecal worm eggs in the body of adult female cecal worms.
7. Once started in the flock, lateral transmission of the disease can also occur in turkeys by the direct entry of free histomonad protozoans (in the absence of cecal worms) through the cloacal opening via bird-to-bird contact or from the infective excreta on the barn litter. Lateral transmission has not been shown in chickens; infections in chickens appear to be dependent on the presence of cecal worms as carriers of the histomonad parasite (McDougald, 2005).
8. Symptoms of histomoniasis in turkeys are ruffled feathers, drooping wings, drowsiness, huddled birds, reduced food intake, reduced weight gain, and bright-yellow excreta; clinical symptoms develop from 12–15 days after infection and mortality may follow at 15–21 days after infection (Hu, 2002).
Keep turkeys apart from chickens and game birds
Turkeys are much more susceptible to histomoniasis than chickens and game birds such as pheasants and quails; cecal worm eggs potentially carrying histomonad protozoan parasites excreted by chickens and pheasants will also infect healthy turkeys. To prevent cross-infections, turkeys must be kept separate and far from where chickens and other bird species are being raised. Because infective cecal worm eggs can persist in the environment for up to three years, potentially contaminated litter and bedding materials must be kept away from turkeys as much as possible, or not used or recycled at all in turkey barns. The sand grits used in turkey feeds must be inspected under the microscope for the potential presence of cecal (or other helminths) eggs before being added to the feed.
Use anthelmintic (dewormer) to control cecal worms
Because the histomoniasis disease cycle can only be initiated when healthy birds ingest cecal worm eggs containing the infective histomonad protozoan parasite, and because cecal worms on their own (without the histomonad parasite) are recognized parasites of turkeys, chickens and other birds, it is recommended that cecal worms be controlled using available effective anthelmintics. According to McDougald (2005), the lack of suitable treatment drugs or vaccines for controlling histomoniasis emphasizes the importance of prevention by worm control and management.
Control house flies and limit bird access to earthworms
House flies had been conclusively shown to be vectors or carriers of cecal worm eggs through research that involved feeding healthy birds with house flies and other invertebrates collected from areas where birds with histomoniasis were kept (Spindler,1967). Experiments conducted by Lund et al. (1966) similarly confirmed that earthworms can act as vectors of both the cecal worm and histomonad parasites. Thus, long-term control of histomoniasis must include limiting the access of healthy turkeys and chickens to earthworms as well as adopting an effective house fly control program.