Literature Review and Discussion

According to one text-book Fowl Cholera "usually appears as a septicaemic disease associated with high morbidity and mortality, but chronic and benign conditions often occur" (Rhoades and Rimler, 1991). This incident had some of the characteristics of the acute disease (high morbidity and mortality) and of the chronic disease (location of lesions). While the lesion distribution resembled the chronic disease the intensity of these lesions (especially in the wattles of males) was very severe. For these reasons it was considered more appropriate to call the problem "Acute Localized Pasteurellosis or Fowl Cholera".

We cannot be certain when and how this flock became infected. Birds artificially infected with the bacteria develop disease within 24-48 hours, those placed in contact with infected birds may take 2 weeks or more (Rhoades and Rimler, 1991). However birds in rear are very much more resistant to the disease than adults. It is just possible that they became infected at the rearing site and the stress of the move and mixing resulted in the infection becoming evident. It is known that other diseases can interact with fowl cholera. For instance, meat turkey flocks that had antibodies to Newcastle disease virus and/or Mycoplasma meleagridis had an increased risk of having an outbreak of fowl cholera (Carpenter et alii, 1991). Results of routine serology for Mycoplasma gallisepticum M. synoviae, Newcastle Disease, Infectious Bronchitis and Avian Influenza were either negative or within the normal range for birds of this type and age. It seems likely that a small proportion of birds (probably in house 3) became infected during transport or soon after. It seems unlikely that the nest-boxes from another farm were the vector unless some moist organic material came with them (e.g. soil from around the houses). It also seems unlikely that the infection was already at this farm since no similar problem was noted at any point in the previous flock.

The serotype of the P.multocida isolated from this outbreak was not among those commonly associated with domestic poultry. In chickens, 54% of the isolates were 3 X 4 and 19% were serotype 1 (Hofacre and Glisson, 1986). 7,16 was not among the commoner serotypes in a series of 152 isolates obtained from chickens (Rhoades and Rimler,1990). Isolates of sero-types 1,3,and 4 were found to be associated with mortality in broilers in the south-east of the USA (Sander and Glisson, 1989)

Serology was not used in the primary diagnosis of this case. An Elisa test has been developed to measure serum antibody response to P.multocida in vaccinated and challenged chickens (Briggs and Skeeles, 1984).but the degree of antibody response in the Elisa test depends on the sero-type of the antigen used (Avakian et alii, 1986). Highest titres when the immunizing antigen is homologous with the measuring antigen. Antigen based on the CU strain is preferred for measuring response both to itself and commercial polyvalent vaccines.Birds with an Elisa titre of over 475 after vaccination with a live (CU) vaccine were highly resistant to challenge (Briggs et alii, 1985). Avakian and Dick (1985) developed a modification of the technique to facilitate sample collection with filter papers. Eluates of whole blood, obtained by overnight elution of two 4.8-mm discs in 200 mcll of buffered saline at 4C, were equivalent to a 1:20 dilution of serum.

Pasteurella has a very low propensity for acquiring adaptive resistance when grown in the presence of oxytetracycline (Champlin et alii,1988). In turkeys a single I/M dose of 152 mg/kg long acting tetracycline resulted in therapeutic blood concentrations over at least 72 hours (Skeeles et alii, 1985). Tissue residues were "within tolerance" by 3 weeks post-injection. The combined use of in-feed chlortetracycline for all birds with injection of long-acting tetracycline in all males and obviously affected females was highly effective in bringing this outbreak under control.\par \par The continuing mortality associated with cannibalism led the company to consult us with regard to the possibility of using a "tranquilizer" in the birds. Currently there is no such product licensed for poultry in the U.K. Reserpine (Sermix, Ciba Geigy) had been licensed for the control of aortic rupture in turkeys, but is no longer available either in the U.K. or elsewhere in Europe. It seemed a reasonable assumption that the cannibalism might be being exacerbated by reduced mobility in the birds with joint and wattle lesions. An anti-inflammatory treatment was sought. Aspirin (acetyl salicylic acid) has been recommended as an anti-inflammatory agent in poultry (Rosoff, 1975) and been used for the control of the adverse effects of high temperature (Oluyemi and Adebanjo, 1978).\par \page Aspirin has also been used but found to be ineffective in the control of perirenal haemorrhage syndrome in turkeys (Frank et alii, 1990). Aspirin is the only nonsteroidal anti-inflammatory drug approved for use in animals intended for food production in the United States (Kopcha and Ahl, 1989). The typical published dosage of aspirin is 1500 ppm in feed (Rossoff, 1975). Soluble aspirin (sodium salicylate) in drinking water was considered to be a more appropriate alternative and an equivalent dosage was estimated at 1000 ppm (1g/litre).

This flock had not been vaccinated because it was policy only to vaccinate flocks being reared or transferred to sites with a known Fowl Cholera history. The males which were added were subjected to a necessarily abbreviated programme of vaccination and antibiotic medication. These birds appear to have been protected therefore there is every possibility that vaccination would have greatly reduced or avoided the effects of this disease outbreak. Antibody responses to vaccination are higher and much more uniform as birds increase in age (at least between 1 and 6 weeks according to Dick and Avakian, 1991). In a study of different immunization programmes in dwarf broiler parent chickens antibody titres were usually higher in birds that received bacterins than in those receiving live vaccines, yet overall protection was still greater in those birds that received the live cholera vaccine twice (Avakian et alii, 1989). No live vaccines are currently available for Fowl Cholera in the U.K. Whether their advantage (improved immunogenicity, ease of application) would outweigh the risk of reversion to virulence is, as yet, debatable.