JPC SYSTEMIC PATHOLOGY
Signalment (JPC #3166610): Young male turkey (Meleagris gallopavo).
HISTORY: Found dead.
HISTOPATHOLOGIC DESCRIPTION: Liver: Approximately 70% of the section is effaced by a single, large focus of lytic necrosis, characterized by abundant eosinophilic cellular and karyorrhectic debris, hemorrhage, and fibrin admixed with numerous macrophages that are often hemosiderin-laden, lymphocytes, scattered foreign body type multinucleated giant macrophages, and heterophils. Multifocally within these areas of necrosis there are numerous extracellular and intrahistiocytic, 10-20 um diameter, irregularly round, lightly eosinophilic, protozoal trophozoites (histomonads) that occasionally contain single, central, 3-5 um diameter basophilic nuclei. Trophozoites are often surrounded by a 3-5um clear zone. Bile ducts are multifocally ectatic and contain luminal debris and low numbers of heterophils and macrophages, and often exhibit piling up of epithelial cells (bile duct hyperplasia). Hyperplastic bile duct epithelia occasionally contain previously described trophozoites. Adjacent less affected hepatic parenchyma is characterized by swollen hepatocytes with microvacuolated cytoplasm (degeneration), and there are increased numbers of small bile duct profiles (bile ductular reaction) and increased numbers of macrophages. There are low numbers of periportal lymphocytes, plasma cells, and macrophages.
Cecum: Affecting 95% of the section, there is diffuse, transmural, lytic necrosis and granulomatous inflammation characterized by loss of normal, well-defined intestinal layers and replacement by large numbers of macrophages, lymphocytes, fewer plasma cells and heterophils, admixed with eosinophilic and karyorrhectic debris, fibrin, edema and mild hemorrhage. Transmurally there are numerous previously described protozoal trophozoites, often found within vacuoles or within macrophages. The overlying mucosa is diffusely effaced to the level of the muscularis mucosa aside from one small focus of remaining crypts, which are are either hyperplastic, characterized by cells piling up to 2-3 layers deep with increased mitoses, or dilated and filled with necrotic cellular debris, fibrin, degenerate protozoal trophozoites, macrophages, and heterophils (crypt abscess). The inflammatory infiltrate surrounds and separates often necrotic or degenerate myofibers of the tunica muscularis.
- Liver: Hepatitis, necrotizing and granulomatous, random, multifocal to coalescing, marked, with numerous intrahistiocytic and extracellular protozoal trophozoites, turkey, avian.
- Cecum: Typhlitis, fibrinonecrotic, lymphohistiocytic, and heterophilic, diffuse, transmural, marked, with numerous intrahistiocytic and extracellular protozoal trophozoites.
ETIOLOGIC DIAGNOSIS: Hepatic histomoniasis
CAUSE: Histomonas meleagridis
SYNONYMS: Blackhead, infectious enterohepatitis, typhlohepatitis
- Well-known, economically important protozoan that causes severe necrotizing typhlitis and hepatitis in a wide range of birds including turkeys, chickens, peafowl, grouse, quail, other gallinaceous birds, and ducks
- Turkey poults are most susceptible (as well as pheasants and peafowl), but disease is common but mild and rarely recognized in chickens therefore they may serve as carriers
- This disease once limited expansion of the turkey industry, but is now uncommon in turkeys, occurring typically when turkeys are raised where chickens were previously located; there have been recent outbreaks in commercial pullet flocks
- The protozoan is anaerobic and exists in either flagellated or amoeboid forms (amoeboid in tissue)
- The cecal worm, Heterakis gallinarium, is the intermediate host; earthworms can be paratenic hosts for gallinarum larvae
- Heterakis gallinarum itself is not considered a major threat, but is a major carrier/vector for Histomonas mileagridis; the lifecycle of gallinarum is similar to other ascarids (direct, eggs passed in feces become infective within 2-3 weeks)
- Ingestion of: 1) cecal worm (Heterakis gallinarum) infected with meleagridis (embryonated eggs or larvae), 2) earthworms that harbor H. meleagridis-infected cecal worm larvae within their tissues, or possibly 3) infected fresh feces (less important) à H. gallinarum nematode larvae localize in cecum, molt à H. meleagridis trophozoites released into the cecal lumen à trophozoites invade cecal wall à lose flagella (appear amoeboid in tissue) à vascular invasion à dissemination to the liver à necrotizing hepatitis à +/- spread to other organs
- Disease appears to require secondary bacteria (cohabitating cecal bacteria such as coli, Clostridium perfringens, Bascillus subtilis in turkeys, and protozoa in chickens such as Eimeria tenella); experimentally, H. meleagridis was not pathogenic in the absence of bacteria
- meleagridis trophozoites can survive in the external environment for a few hours, but remain viable for up to 2 years within H. gallinarum eggs
TYPICAL CLINICAL FINDINGS:
- Morbidity and mortality is high in young turkeys (up to 100%), older birds tend to be more resistant
- In turkeys, incubation period between exposure and disease is 7-12 days
- Initial signs include listlessness, moderate anorexia, drooping wings, and yellow feces (“sulfur colored”); chicken feces may contain some blood; head parts may be cyanotic (“blackhead”) but often are not
- Later signs include depression, eyes closed, head drawn close to the body, emaciation
TYPICAL GROSS FINDINGS:
- Well-developed gross lesions are pathognomonic: typhlitis and characteristic hepatic lesions
- Ceca: Bilateral distension/enlargement of ceca with thickening of the walls, typically ulcerated mucosa, and often contain caseous cecal cores (yellow, gray, or green, may be laminated) that may have been expelled in chronic cases
- Liver: Irregularly round, depressed, target-like (bulls-eye) areas of necrosis surrounded by a raised ring of hemorrhage; often yellow to gray but may be green to red, are often 1-2cm in diameter but may coalesce
- Other lesions: Kidney and spleen may have small white nodules on surface, peritonitis if the cecal wall becomes perforated
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Histomonads: extracellular and intrahistiocytic, round, 10-20um, PAS-positive trophozoites with central dark nuclei; may be difficult to distinguish from macrophages on H&E
- Cecum: Bilateral, mucosal to transmural, fibrinonecrotic typhlitis with fibrinonecrotic to hemorrhagic cecal cores; histomonads in necrotic foci; granulomatous inflammation in more chronic lesions
- Liver: Portal to multifocal-to-coalescing heterophilic granulomas and hepatocellular necrosis, histomonads in necrotic foci
- Kidney, spleen, bursa of Fabricius, air sacs, lungs, pancreas, proventriculus: May also have foci of necrosis with associated histomonads, granulomatous inflammation, few multinucleated giant cells
ADDITIONAL DIAGNOSTICS TESTS:
- Special stains: PAS-positive
- Cytology: histomonads may be identified on smears of cecal contents or scrapings of cecal or liver lesions
Necrotic cecal cores:
- Eimeria tenella
- Salmonella (Pullorum disease, Paratyphoid)
Other histomonads of the cecum:
- Histomonas wenrichi is larger, has 4 flagella, and is nonpathogenic
- Chickens and ring-necked pheasants may carry the disease and show minimal/no clinical signs; turkeys, pheasants (other than ring-necked), and peafowl are highly susceptible; ostrich, rhea, guinea fowl, chukar, grouse, quail (tend to not develop cecal lesions despite high mortality), and partridges can be infected
- A recent study (Clarke 2017) identified coinfection with meleagridis and Tetratrichomonas gallinarum in 1 of 5 H. meleagridis-infected peafowl (undetermined significance), and there was no evidence of Heterakis gallinarum infection in any of the 5 H. meleagridis-infected peafowl
- Clarke LL, Beckstead RB, Hayes JR, Rissi DR. Pathologic and molecular characterization of histomoniasis in peafowl (Pavo cristatus). Vet. Diagn. Invest. 2017;29(2):237-241.
- Crespo R, Franca MS, Fenton H, Shivaprasad HL. Galliformes and columbiformes. In: Terio KA, McAloose D, St. Leger J. Pathology of Wildlife and Zoo Animals. London, England: Elsevier; 2018:759-761.
- Fitz-Coy SH. Parasitic diseases. In: Boulianne M., ed. Avian Disease Manual. 7th Jacksonville, FL: American Association of Avian Pathologists; 2013:158, 161, 168-169, 175, 178.
- Smith DA. Palaeognathae: apterygiformes, casuariiformes, rheiformes, struthioniformes; tinamiformes. In: Terio KA, McAloose D, St. Leger J. Pathology of Wildlife and Zoo Animals. London, England: Elsevier; 2018:645.