Coronaviral Enteritis of Turkeys
(Bluecomb, Mud Fever, Transmissible Enteritis)
Coronaviral enteritis has been identified in turkeys in the USA, Canada, Brazil, Italy, UK, and Australia. Turkey coronavirus (TCV) affects turkeys of all ages; however, clinical disease most commonly is seen in young turkeys during the first few weeks of life. Turkeys likely are the only natural host for TCV.
TCV is shed in droppings of infected birds and spread horizontally through ingestion of droppings and dropping-contaminated fomites. Infected turkeys shed the virus for several weeks after recovery from clinical disease. The virus generally spreads rapidly through a flock and from flock to flock on the same or neighboring farms. Mechanical movement of the virus may occur by people, equipment, vehicles, and insects. Darkling beetle larvae and domestic house flies are potential mechanical vectors. Wild birds, rodents, and dogs also may serve as mechanical vectors. There is no evidence that TCV is egg transmitted; however, poults may become infected in the hatchery via contaminated personnel and fomites such as egg boxes from infected farms.
Clinical signs of coronaviral enteritis occur suddenly. Morbidity generally approaches 100%, but mortality is variable; high mortality may occur depending on the age of the birds, concurrent infection, and management practices.
Turkeys affected by coronaviral enteritis may exhibit:
Breeder hens exhibit a rapid drop in egg production. Egg quality also is affected; hens produce white, chalky eggs lacking normal pigmentation.
Gross lesions of coronaviral enteritis are seen primarily in the intestines. The duodenum and jejunum generally are pale, thin-walled, and flaccid; ceca are distended with gas and watery contents. Atrophy of the bursa of Fabricius also may be seen.
Microscopic lesions consist of a decrease in villous length and increase in crypt depth; the columnar epithelium changes to a cuboidal epithelium, and these cells exhibit a loss of microvilli. There is a decrease in the number of goblet cells, separation of enterocytes from the lamina propria, and infiltration of the lamina propria with heterophils and lymphocytes.
In the bursa of Fabricius, the normal pseudostratified columnar epithelium is replaced by a stratified squamous epithelium, and intense heterophilic inflammation is seen within and underneath the epithelium.
Diagnosis of coronaviral enteritis in turkeys generally requires laboratory assistance, because other enteric pathogens of turkeys may cause similar clinical signs and lesions. Laboratory diagnosis is based on virus isolation, serology, or detection of viral antigens by immunohistochemistry or viral RNA by reverse transcriptase PCR.
Preferred clinical samples for diagnostic analyses include serum, intestinal contents, and fresh tissues (intestines and bursa of Fabricius); these samples should be kept cold (on ice at 4°C) at all times.
Coronaviral enteritis must be distinguished from other enteric viral, bacterial, and parasitic infections, including those caused by astrovirus, rotavirus, reovirus, Salmonella spp, and crytosporidia.
Prevention of coronaviral enteritis in turkeys through sound biosecurity practices is the preferred method of control. No commercial vaccine is available. Infected turkeys shed virus in droppings for prolonged periods after recovery; these turkeys, their feces, and the materials their feces contact are potential sources of infection for other susceptible turkeys.
Feces from infected turkeys can be carried on a variety of fomites, including clothing, boots, equipment, feathers, and vehicles. Other potential vectors include wild birds, rodents, dogs, and flies.
Elimination of TCV from contaminated premises is accomplished by depopulation followed by thorough cleaning and disinfection of houses and equipment. After cleaning and disinfection, premises should remain free of birds for a minimum of 3–4 weeks.
There is no specific treatment for TCV enteritis. Antibiotic treatment reduces mortality but not growth depression, most likely by controlling secondary bacterial infections. Effective management procedures to reduce mortality include raising brooder house temperatures and avoiding crowded conditions.
Coronaviral enteritis affects turkeys of all ages, but clinical disease occurs most commonly in young turkeys during the first few weeks of life. Breeder hens also may experience a drop in egg production along with decreased egg quality.
Coronaviral enteritis should be suspected in cases of acute enteritis with high mortality in young turkeys and decreased egg production in breeder hens.
Diagnosis of coronaviral enteritis is dependent upon laboratory diagnostics (serology, virus isolation, immunohistochemistry, or PCR). Preferred clinical samples include serum, intestinal contents, and fresh tissues (intestines, bursa of Fabricius).
Control of coronaviral enteritis is accomplished by biosecurity measures that prevent exposure of susceptible turkey to infected turkeys, contaminated fomites, or potential vectors.