Fowl cholera is a serious, highly contagious disease which is caused by a bacterium in a range of avian species including chicken, turkey and water fowl. The disease can range from acute septicaemia to chronic and localized infections and the morbidity and mortality may be up to 100%.
The route of infection is oral or nasal with transmission via nasal exudate, faeces, contaminated soil, equipment and people. The incubation period is usually 5-8days.
The bacterium is easily destroyed by environmental factors and disinfectants but may persist for prolonged periods in soil. Reservoirs of infection may be present in other species such as rodents, cats and possibly pigs. Predisposing factors include high density and concurrent infections such as respiratory viruses.
Fowl Cholera Etiology and Transmission
Pasteurella multocida, the casual agent, is a small, gram-negative, non-motile rod with a capsule. P multocida is considered a single species although it includes three subspecies: multocida, septic and gallicida. Subspecies multocida is the most common cause of disease, but septic and gallicida may also cause cholera-like disease.
Although P multocida may infect a wide variety of animals, strains isolated from non-avian hosts generally do not produce fowl cholera. Strains that cause fowl cholera represent a number of immunotypes (or serotypes), which complicates efforts at widespread prevention using bacterins. The organism is susceptible to ordinary disinfectants, sunlight, drying and heat.
Turkeys and waterfowl are more susceptible than chickens, older chickens are more susceptible than young ones and some breeds of chickens are more susceptible than others.
Chronically infected birds and asymptomatic carriers are considered to be major sources of infection. Wild birds may introduce the organism into poultry flock, but mammals (including rodents, pigs, dogs and cats) may also carry the infection.
However the role of these as a reservoir has not been thoroughly investigated. Dissemination of P multocida within a flock and between houses is primarily by excretions from the mouth, nose and conjunctiva of diseased birds that contaminate their environment.
In addition, P multocida survives long enough to be spread by contaminated crates, feed bags, shoes and other equipment. The infection does not seem to be egg-transmitted.
Clinical findings vary greatly depending on the cause of disease. In acute fowl cholera, finding a large number of dead birds without previous signs is usually the first indication of disease. Mortality often increases rapidly.
In more protracted cases, depression, anorexia, mucoid discharge from the mouth, ruffled feathers, diarrhoea, and increases respiratory rate are usually seen. Pneumonia is particularly common in turkeys.
In chronic fowl cholera, signs and lesions are generally related to localized infections of the sterna bursae, wattles, joints, tension sheaths, and footpads, which often are swollen because of accumulated fibrinosuppurative exudate. There may be exudative conjunctivitis and pharyngitis. Torticollis may result when the meninges, middle ear, or cranial bones are infected.
Fowl Cholera Lesions
Lesions observed in peracute and acute forms of the disease are primarily vascular disturbances. These include general passive hyperaemia and congestion throughput the carcass, accompanied by enlargement of the liver and spleen.
Petechial and ecchymotichemorrhages are common, particularly in subepicardial and subserosol locations. Increased amounts of peritoneal and pericardial fluids are frequently seen.
In addition, acute oophoritis with hyperaemic follicles may be observed. In sub-acute cases, multiple, small, necrotic foci may be disseminated throughout the liver and spleen.
In chronic forms of fowl cholera, suppurative lesions may be widely distributed, often involving the respiratory tract, the conjunctiva, and adjacent tissues of the head. Caseous arthritis and productive inflammation of the peritoneal cavity and the oviduct are common in chronic infections.
A fibrino-necrotic dermatitis that includes caudal parts of the dorsum, abdomen, and breast and involves the cutis, subcutis, and underlying muscle has been observed in turkeys and broilers. Hidden necrotic lung lesions I n poultry should always raise suspicion of cholera.
Fowl Cholera Diagnosis
Although the history, signs and lesions may aid diagnosis, P Multocida should be isolated, characterized and identified for confirmation. Primary isolation can be accomplished using media such as blood agar, dextrose starch agar, or trypticase soy agar.
Isolation may be improved by the addition of 5% heat-inactivated serum. P multocida can be readily isolated from viscera of birds dying from peracute/acute fowl cholera, whereas isolation from suppurative lesions of chronic cholera may be more difficult.
At necropsy, staining of smears obtained from the liver will be useful in the case of acute cholera. In addition, immunofluorescent microscopy and in situ hybridization have been used to identify P multocida in infected tissues and exudates.
Polymerase Chain Reaction (PCR) has been used for the detection of P. multocida in pure and mixed cultures and clinical samples. This method may help identify carrier animals within flocks. However, the specificity and sensitivity of the PCR must be improved.
Serologic testing can be done by rapid whole blood agglutination, serum plate agglutination, agar diffusion tests, and ELISA. Serology may be used to evaluate vaccine responses but has very limited value for diagnostic purposes.
Several bacterial infections may be confused with fowl cholera based solely on the grass lesions. Escherichia coli, Salmonella enteric, Ornithobacterium rhinotracheale, gram-positve cocci, and Erysipelothrix rhusiopathiae (erysipelas) may all produce lesions indistinguishable from those cased by P. multocida.
Fowl Cholera Prevention
Good management practices, including a high level of biosecurity, are essential to prevention. Rodents, wild birds, pets, and other animals that may be carriers of P multocida must be excluded from poultry houses.
Adjuvant bacterins are widely used and generally effective, autogenousbacterins are recommended when polyvalent bacterins are found to be ineffective. Thus, it is important to know the most prevalent serotypes within an area to choose the right bacterins.
Attenuated live vaccines are available for administration in drinking water to turkeys and by wing-web inoculation to chickens. These live vaccines can effectively include immunity against different serotypes of P multocida. They are recommended for use in healthy flocks only.
Fowl Cholera Treatment
A number of drugs will lower mortality from fowl cholera, however, deaths may resume when treatment is discontinued, showing that treatment does not eliminate P multocida from a flock. Eradication of infection requires depopulation and cleaning and disinfection of buildings and equipment. The premises should then be kept free of poultry for a few weeks.
Sulfonamides and antibiotics are commonly used, early treatment and adequate dosages are important. Sensitivity testing often aids in drug selection and is important because of the emergence of multi-resistant strains. Sulfaquinoxaline sodium in feed or water usually controls mortality, as do sulfamethazine and sulfadimethoxine.
Sulfas should be used with caution in breeders because of potential toxicity. High levels of tetracycline antibiotics in the feed (0.04%), drinking water, or administered parenterally may be useful. Norfloxacin administered via drinking water is also effective against fowl cholera.
However, many countries do not allow the use of quinolones in food-producing animals, including poultry, because of the risk of the development of drug resistance.
Penicillin is often effective for sulfa-resistant infections. In ducks, a combined injection of streptomycin and dihydrostreptomycin can be effective.