438 PARASITISM AND DISE4SE l.\ RUFFED GROUSE 



of these outbreaks is usually unknown but observations on chronic infections of ulcerative 

 enteritis suggest that all cases do not terminate fatally and that carriers exist. 



Blackhead or Enterohepatitis (caused by Histomonas meleagridis) 



Cause. Tyzzer™ demonstrated that the causative agent of blackhead or enterohepatitis was 

 not an amoeba but a flagellate protozoan which he named Histomonas meleaf^ridis. Cultures 

 of this organism, when fed to turkey poults or when injected into the cloaca, produced typi- 

 cal lesions of the disease. 



Nature of the Disease. Blackhead is a disease affecting the ceca and liver of turkeys, 

 chickens, grouse, quail, and other gallinaceous birds. Grouse are among the more susceptible 

 birds. The primary sites of infection are the ceca. The walls of either one or both of these 

 become tremendously thickened due to invasion by the parasites of the mucosal and muscular 

 layers. The surface epithelium becomes necrotic and eroded. A typical cheesy core adhering 

 firmly to the cecal wall is generally found. It is believed that spread of the parasites to the 

 liver takes place by way of the blood stream. Here the lesions take the form of depressed 

 shallow ulcers similar to those found in turkeys. In cases where the disease has run an acute 

 course, the liver may be covered with white necrotic foci several millimeters in diameter. 



Dissemination. The researches of Tyzzer and others have demonstrated that the causative 

 organism, when eliminated with the feces, is highlv infectious. Furthermore, it has been proven 

 that chickens may act as carriers of Histomonas meleagridis without being affected thereby. 

 The organisms are also shed by birds that are suffering from the initial stages of the dis- 

 ease. An important contribution to our knowledge of the transmission of enterohepatitis was 

 made by Graybill and Smith"" and Tyzzer. who found that feeding embryonated cecal worm 

 eggs to turkeys resulted in blackhead infection. Although the evidence supporting this manner 

 of transmission is sound, no one has as yet demonstrated the presence of the protozoan with- 

 in the cecal worm egg. Furthermore, it is recognized that blackhead can also be found in 

 turkeys not parasitized with cecal worms. 



The disease occurs in both young and old grouse and at times may affect entire i)ens with 

 the rapidity of an acute contagious infection. Outbreaks of enterohepatitis have occurred in 

 pens of artificiallv reared young grouse that had iiccn on wire screen floors from the dav they 

 were hatched. Some of these birds succunilu'd to tlir infci tion in spite of all precautions that 

 were taken to prevent access to dro])pings and to prevent fecal contaniinalion of the feed 

 and water. It goes without saying that these birds were free from all worm parasites. 



A number of investigators have found that infection with blackhead can result from exposure 

 to ground that had not been occupied by gallinaceous birds for over a year. However, a re- 

 sistant form of the causative organism has not been found as yet. It is difficult to reconcile 

 these apparently contradictory facts. Much more needs to be learned about the transmission 

 of this disease. 



Control. The primarv requisite in both |)revention and control of blackiiead is the main- 

 tenance of the birds on wire. This j)ractice. together with tli<' elimination of poultry from 

 farms on wliirli snsccplililc birds arc raised, results in a reduction of losses from this disease 

 to a very low level. 



There niav. liowcncr. be oiitiircaks at times. e\cn under these conditions. Strict sanitation 

 and removal of suspected birds may bring the disease under control before it affects any 

 numbers. In certain instances, the administration of a milk lliisli* has seemed to stem the 



* 30 In 10 III*, of ctflr.l fikitii niMk mixi-.l willi rmli ino lh» <il moll. 



