FOWL CHOLERA
DEFINITION:
Fowl cholera is a contagious, bacterial disease that affects domestic and wild birds worldwide. It usually occurs as a septicemia of sudden onset with high morbidity and mortality, but chronic and asymptomatic infections also occur.
Fowl cholera is a contagious, bacterial disease that affects domestic and wild birds worldwide. It usually occurs as a septicemia of sudden onset with high morbidity and mortality, but chronic and asymptomatic infections also occur.
Etiology and Transmission:
Pasteurella multocida,
Chronically infected birds and asymptomatic carriers are
considered to be major sources of infection. Wild birds may introduce
the organism into a 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:
Clinical findings vary greatly depending on the course 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, diarrhea,
and increased 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 sternal bursae, wattles, joints, tendon
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.
Lesions:
Lesions observed in peracute and acute forms of the disease are
primarily vascular disturbances. These include general passive hyperemia
and congestion throughout the carcass, accompanied by enlargement of
the liver and spleen. Petechial and ecchymotic hemorrhages are common,
particularly in subepicardial and subserosal locations. Increased
amounts of peritoneal and pericardial fluids are frequently seen. In
addition, acute oophoritis with hyperemic follicles may be observed. In
subacute 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 fibrinonecrotic 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. Sequestered necrotic lung lesions in poultry should always
raise suspicion of 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, bipolar microorganisms may be
demonstrated by the use of Wright’s or Giemsa stain of impression smears
obtained from the liver 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.
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. P multocida can be subgrouped by
capsule serogroup antigens into five capsular types (A, B, D, E, and F)
and into 16 somatic serotypes. Somatic serotyping is important to choose
the right bacterins for prevention. However, conventional serotyping
suffers from problems with reproducibility and reliability, and the
methods are quite laborious. Recently, a multiplex PCR has been
developed that can differentiate between different somatic serotypes.
This will be of great value as a diagnostic tool and enable more
efficient vaccine development in the future.
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 gross lesions. Escherichia coli, Salmonella enterica, Ornithobacterium rhinotracheale, gram-positive cocci, and Erysipelothrix rhusiopathiae (erysipelas) may all produce lesions indistinguishable from those caused by P multocida.
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; autogenous bacterins 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 induce immunity against different serotypes of P multocida. They are recommended for use in healthy flocks only.
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 premise 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
multiresistant 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
development of drug resistance. Penicillin is often effective for
sulfa-resistant infections. In ducks, a combined injection of streptomycin and dihydrostreptomycin can be effective.
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