Fowl cholera - Home: OIE · 2009-02-06 · Although fowl cholera probably occurs world-wide (81) and has been studied extensively for many years, the epidemiology of the disease remains

Rev. sci. tech. Off. int. Epiz., 2000,19 (2), 626-637

Fowl cholera J.P. Christensen & M. Bisgaard

Department of Veterinary Microbiology, The Royal Veterinary and Agricultural University, Stigbojlen 4, 1870 Frederiksberg C, Denmark

Summary Pasteurella multocida subspec ies multocida is the most common cause of fow l cholera, a l though P. multocida subspec ies septica and gallicida may also cause fow l cholera- l ike disease to some extent. However, the v i ru lence proper t ies of the di f ferent subspec ies for var ious hosts have not been e luc idated. The seventy and inc idence of P. multocida in fect ions may vary considerably depending on several fac to rs associated w i t h the host ( inc luding spec ies and age of infected birds), the env i ronment and the bacter ia l s t ra in. No single v i ru lence fac tor has been assoc ia ted w i t h the observed var ia t ion in v i ru lence among strains. Possible v i ru lence fac to rs inc lude the fo l low ing : the capsu le , endotox in, outer membrane proteins, i ron binding systems, heat shock proteins, neuraminidase product ion and ant ibody c leaving enzymes. No RTX toxins (repeats in toxin) appear to be produced by P. multocida, but P. multocida exotoxin (PMT) could contr ibute to v i ru lence in some avian infect ions. The epidemiology of f o w l cholera appears complex. Tradit ional serotyping systems are only of l imited use in epidemio logica l studies. In recen t years, molecular typ ing methods have been appl ied to avian strains of P. multocida of di f ferent or ig in. The results obtained using these newer methods ind icate that w i ld birds may be a source of infect ion to commerc ia l poultry. Documentat ion suggest ing tha t mammals play a similar role is not as comprehens ive , but the possibi l i ty cannot be exc luded. Carrier birds seem to play a major role in the t ransmiss ion of cholera. Surviving birds f rom diseased f locks appear to represent a risk, but more recent invest igat ions indicate tha t carr iers of P. multocida may exist w i th in poultry f locks w i t h no history of previous outbreaks of f o w l cholera. The s ign i f icance of this awai ts fur ther invest igat ion.

The site of infect ion for P. multocida is general ly bel ieved to be the respi ratory t rac t . The outcome of in fect ions may range f rom peracute /acu te in fect ions to chronic infect ions. In the fo rmer type of in fect ions, f e w c l in ical signs are observed before death and the lesions w i l l be dominated by general sept icaemic lesions. In chronic forms of P. multocida in fect ions, suppurat ive lesions may be w ide ly d ist r ibuted, often involving the respi ratory t rac t , the conjunct iva and adjacent t issues of the head. Diagnosis is a lways dependent upon isolat ion of the organism. For the detect ion of subc l in ica l in fect ions, mouse passage of re levant samples is recommended , but polymerase chain react ion and isolat ion at tempts on select ive media may represent a l ternat ives. Conf inement is probably the most ef fect ive w a y to prevent in t roduct ion of P. multocida. However, extensive management systems dominate in many parts of the w o r l d , and under such c i rcumstances vacc ina t ion is recommended as a prevent ive measure. Unfortunately, the deve lopment of safe and ef f ic ient live vacc ines stil l poses problems. As a result , cont ro l remains dependent on bacter ins w h i c h exhibit s igni f icant d isadvantages compared to live vacc ines .

Keywords Avian diseases - Epidemiology - Fowl cholera - Identification - Pasteurella multocida -Pathology-Prevention.

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Introduction For decades, the term 'avian pasteurellosis' has been used to refer to a group of diseases caused by pasteurellae and pasteurellae-like organisms. Important poultry pathogens such as Yersinia pseudotuberculosis (86) , Riemerella anatipestifer (84) and Ornithobacterium rhinotracheale (94) have all been excluded from the family Pasteurellaceae sensu stricto (15) and will consequently not be described. The genus Pasteurella sensu stricto presently includes at least eleven species, but only seven species (P. multocida with its three subspecies, P. gallinarum, P. avium, P. volantium, P. anatis, P. langaa and P. sp. A) have been associated with avian hosts (6). Among these, P. multocida is considered the causative agent of fowl cholera (81) . Apart from P. multocida, none of the above species appear to be involved as aetiological agents in acute cholera-like disease or to be of any major economic importance in birds (6), and will therefore not be covered by this review.

The taxonomy of organisms previously reported as P. haemolytica remained unsolved for many years. Recent investigations have reclassified mammalian isolates as Mannheimia, a new genus containing at least five species (2), while avian isolates belong to another new genus within the family Pasteurellaceae which has not yet been named (14) . These investigations have confirmed that host specificity seems to exist for many species of the family Pasteurellaceae. Similar taxonomic investigations are in progress for the P. multocida complex, but results are not yet available. Consequently, the following text will be based on published data only.

Description of the aetiological agent Three subspecies of P. multocida (P. multocida subspecies multocida, septica and gallicida) are recognised (65). Pasteurella multocida subspecies multocida is the most common cause of disease, but subspecies septica and gallicida may also cause fowl cholera-like disease to some extent (43). Pasteurella multocida subspecies gallicida is mainly associated with web-footed birds (35; J.P. Christensen and M. Bisgaard, unpublished observations), but has also been reported in pigs (10). The relationship between subspecies and serovars of P. multocida obtained by published serotyping systems has not been elucidated. For many years, passive haemagglutination tests have formed the basis for a serogrouping system based on specific capsule antigens (13, 67) , whereas tube agglutination and gel diffusion precipitin tests have been used to detect somatic antigens (40, 66, 67) . Five capsular (A, B, D, E and F) and sixteen somatic (1-16) serovars of P. multocida are currently recognised (80). All but serotypes 8 and 13 have been isolated from avian hosts (67), as have capsular types A, B, D and F (80, 81) . However,

subspecies multocida and serovar A appear to be the most frequently isolated subspecies and serogroup from cases of the most severe form of fowl cholera (78, 80) . Several of the sixteen somatic serovars have been demonstrated among serovar A isolates, just as somatic serotype variation has been shown to occur within serovars B, D and F (78) . Isolates that have multiple somatic antigens are often encountered and are considered distinct serotypes (97) . Although the somatic serovars 1,3 and 3,4 within serovar A apparently dominate among strains isolated from fowl cholera in England and the United States of America (21 , 79), no particular serovar appears to be more or less virulent than others. Lee et al. demonstrated that different isolates of the common serovar A:3,4 vary greatly in virulence (55) . Virulence properties of the different subspecies for different avian hosts are unclear.

No single factor has been associated with the strain variation in virulence observed (56). The capsule is regarded as a major virulence factor of avian P. multocida (37, 38 , 87, 93) , but other factors probably influence the outcome of infections. Other virulence factors suggested include the following:

- endotoxin (20, 25, .26, 57, 76) - outer membrane proteins (92) - iron binding systems (34, 49 , 6 9 , 1 0 0 ) - heat shock proteins (59) - neuraminidase production (51 , 58) - antibody cleaving enzymes (72).

Toxins other than endotoxin may also play a role in the pathogenesis of fowl cholera. Pasteurella multocida toxin, which is at least partly responsible for the lesions observed in atrophic rhinitis in pigs (27), cannot be excluded as a possible virulence factor in some of the lesions observed in avian infections with P. multocida (15). Production of RTX toxins (repeats in toxin), which is of major importance in the pathogenesis of some members of the family Pasteurellaceae (24), has not been observed in P. multocida. One of the few factors which may be of practical value as a virulence marker is the ability of P. multocida to resist killing by serum components (56). Very little is known about the molecular basis of diseases caused by P. multocida in avian species (15), and genetic evidence for the role of virulence factors is lacking, even for some of the factors currently considered to have the most influence over virulence (e.g. the capsule) (1).

Factors other than those associated with the bacteria may influence the outcome of P. multocida infections. Although most species of birds are considered susceptible to infection with P. multocida (P. multocida has been isolated from more than 100 different species of birds [9]), different species of birds differ significantly in susceptibility to infection. Among domestic fowl, turkeys are probably the most susceptible species. Web-footed birds also seem highly sensitive to infection, since outbreaks regularly cause massive losses among waterfowl (9), whereas chickens are considered relatively resistant (81). This was clearly demonstrated by

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experimental infections of several species of birds using an outbreak clone of P. multocida ssp. multocida originally isolated from eiders (16) . Intratracheal challenge of seventeen-week-old chickens with 1 0 4 colony-forming units did not result in mortality. The organism could not be detected in either the liver or the spleen 48 h after infection, although typical lesions of the lungs were observed in the majority of the birds. In contrast, partridges of the same age all died within 2 4 h of infection. Infection of three-week-old turkeys also resulted in 100% mortality within 2 4 h. Pheasants appeared to be of intermediate susceptibility to infection, with approximately 5 0 % mortality observed after 24 h (71). Other factors which have been reported to affect the severity and incidence of the disease include environmental factors (e.g. crowding), climate (85), concurrent disease (19), nutritional stress (23) and age of the host (50). Age markedly influences the outcome of infection, at least in chickens, where birds less than sixteen weeks old are relatively resistant. Under natural conditions, mortality may range from only a few percent to close to 100%, depending on the factors mentioned above (81).

Epidemiology Although fowl cholera probably occurs world-wide (81) and has been studied extensively for many years, the epidemiology of the disease remains controversial, and many aspects are not yet fully understood. Basic knowledge, such as the route of introduction of fowl cholera into a flock, is still lacking. Due to genotypic variation within serotypes, serotyping in many cases does not provide sufficient detailed information to determine the epidemiology of infections (12, 16, 53 , 89 , 97) . Within the last ten years, DNA (deoxyribonucleic acid) fingerprinting in the form of restriction endonuclease analysis and ribotyping has been applied to avian strains of P. multocida of different origins, including strains obtained from wild birds (16, 17, 63, 98 , 99) , and has also been shown to be of value in studying the epidemiology of fowl cholera in turkeys (12, 18, 90) . The restriction endonucleases Hpall and Hhal are reported to be among the most suitable for epidemiological studies (16, 98 , 99) .

Pasteurella multocida is a fairly delicate organism which is easily inactivated by common disinfectants, sunlight, drying or heat, and experiments suggest that P. multocida will survive for a maximum of thirty days in the environment (e.g. water or soil) (4, 8 1 , 82) . Consequently, contaminated environments are not thought to serve as reservoirs for periods of more than thirty days, although as yet unknown factors could have a protective role.

As the habitat of P. multocida is broad, including mucosal surfaces of mammals, birds and humans, many sources could act as a potential reservoir (6). An exchange of P. multocida ssp. multocida between wild birds and domestic poultry is

reported to be possible, and wild birds are capable of spreading the disease to new areas ( 1 6 , 1 7 , 90) . The extent to which the agent is introduced into susceptible flocks by this route of transmission is difficult to estimate and will probably be highly influenced by the form of production. A recent study demonstrated that more than 8 0 % of the diagnosed cases of P. multocida infections in poultry in Denmark during the years 1995 to 1997 included poultry which had been in contact with wild fauna (17). This included contact with mammals, but the role of these as a reservoir has not yet been thoroughly investigated by more recent molecular typing methods. The virulence properties for poultry of mammalian isolates of P. multocida also remain to be investigated, whereas the clones isolated from wild birds have been found to be identical or closely related to those isolated from domestic poultry. These clones have also been demonstrated to be virulent for poultry in experimental infections (16, 17, 71). However, dogs, cats and pigs, in particular, may act as reservoirs for strains of P. multocida which are virulent for poultry (54, 8 1 , 88 , 95) . Carrier birds are generally believed to play a major role in spreading the disease (11 , 81) . Many studies suggest that survivors wthin a diseased flock may act as reservoirs of infection, but until recently, limited information has been available concerning the possibility of carriers in flocks of poultry with no history of previous outbreaks of fowl cholera. However, an investigation by Muhairwa et al. has indicated that a high carrier rate of P. multocida ssp. multocida and septica may exist in apparently healthy poultry flocks (including chickens and ducks) (64). Surprisingly, many birds carried P. multocida on the cloacal mucosae. The importance of this finding in explaining the spread of the infection is unclear, as excretions from the mouth, nose and conjunctiva of diseased birds are generally believed to be the primary source of contamination of the environment; transmission by aerosol has been reported to be less important (5). Other potential sources of infection are carcasses of birds which have died of the infection, and equipment or insects which have been in contact with infected birds. Transmission of P. multocida through the egg is not believed to represent a risk (81), although contamination of eggshells could theoretically occur during passage through the cloaca. However, given the delicate nature of the micro-organism, contamination of this type is likely to be insignificant.

To minimise the risk of importing the disease through trade of hatching eggs, day-old chicks, point of lay pullets, etc., the guidelines indicated in the International Animal Health Code (68) should be followed. However, it should be noted that to further reduce the risk of introducing the disease through imported stock, all imported birds should originate from establishments with a high level of biosecurity, and which follow all-in/all-out principles. Imported eggs and birds should originate from confined flocks only. Mouse inoculations of pooled swabs from mucosal membranes of imported birds must be negative (sampling level to be defined).

Rev. sci. tech. Off. int. Epiz., 19 (2) 629

Disease The site of infection for P. multocida is generally believed to be the respiratory tract (60, 81) . However, inoculation through oculo-nasal-oral routes may also generate typical lung lesions and a progressive bacteraemia (73) , indicating that other mucosal membranes may serve as portals of entry. The ability of P. multocida to survive passage of the gastro-intestinal tract appears to be limited (81) , but the presence of P. multocida on the cloacal surface of carrier birds indicates that some organisms may survive passage (64) . The observation that some strains of P. multocida can be virulent and immunogenic following oral administration also suggests that intestinal invasion or interaction with the intestinal mucosae occurs to some degree ( 3 2 , 5 5 ) . Localisation of P. multocida in the bursa may occur following a bacteraemia, since P. multocida has been detected in the bursa of intratracheally infected chickens (J.P. Christensen and M. Bisgaard, unpublished data). Pasteurella multocida may also enter the tissues through cutaneous lesions and result in septicaemia (80) or localised cutaneous lesions (28, 33) . Following an upper respiratory tract infection, P. multocida may subsequently spread to the lungs and multiply before entering the bloodstream (61). Once in the bloodstream, P. multocida either multiply rapidly (87) or localise in the liver and spleen where initial multiplication occurs before a massive bacteraemia (70, 93) . Death is presumed to be due to the effects of endotoxin (19, 39 , 76) , as signs of acute fowl cholera have been reproduced by injection of endotoxin from P. multocida (41 , 77) .

A wide range of signs may be observed in infections with P. multocida, depending on the nature of the infection. Few signs may be observed in peracute and acute infections (often referred to as cholera). In these cases, death is often the only sign of disease in the flock. In more protracted cases, mucous discharges from the mouth, nose and ears, cyanosis, general depression, ruffled feathers and diarrhoea may be observed. In chronic infections, signs are principally due to localised infections of leg or wing joints, comb, wattles and subcutaneous tissue of the head (36, 81) , oviduct (7) and the respiratory tract (81). Severe forms of dermal necrosis in turkeys have also been reported (28, 33) .

Lesions In the case of peracute or acute forms of the disease, the post-mortem findings are dominated by general septicaemic lesions including vascular disturbances, as reflected by general passive hyperaemia and congestion thoughout the carcass. Petechial and ecchymotic haemorrhages are often present in the abdominal and coronary fat, and haemorrhages may be observed in the intestinal mucosae and on subserosal surfaces in the thoracic and abdominal cavities. The liver and spleen are often swollen and may contain multiple small focal areas of coagulative necrosis or the organs may undergo more generalised necrosis. The lungs are often involved, especially in turkeys, where the lesions may be very characteristic. In the most acute forms of infection, the lung lesions are dominated

by haemorrhages, but this is soon followed by necrosis and fibrinous pleuro-pneumonia where affected areas are clearly marked from unaffected tissue. A unilateral or bilateral productive inflammation of pleura and lungs with extensive exudation of fibrin is common. Histologically, the lesions are mainly associated with heterophilic infiltrations (80, 81) .

In chronic forms of P. multocida infections, suppurative lesions may be widely distributed, often involving the respiratory tract, the conjunctiva and adjacent tissues of the head (81). Caseous arthritis and productive inflammation of the peritoneal cavity and the oviduct are common in chronic infections. A fibrino-necrotic dermatitis including caudal parts of dorsum, the abdomen and breast, and involving cutis, subcutis and the underlying muscle, has been observed in turkeys and broilers (28, 33) .

Diagnostic methods The history of the disease, clinical signs and gross lesions may be helpful in diagnosis, but are insufficient to allow a definite diagnosis of the disease. The. final diagnosis depends on isolation of the organism.

Primary isolation is usually 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 (67). Pasteurella multocida can be readily isolated from viscera of birds dying from peracute/acute fowl cholera and often from suppurative lesions of chronic cases. In cases of acute fowl cholera, bipolar organisms can be demonstrated in liver imprints using Wright's or Giemsa stain (67, 81) . Immunofluorescent microscopy has been used to identify P. multocida in tissue and exudate (81).

More recently, the polymerase chain reaction technique has been used with success to detect carrier animals within turkey flocks (52). However, the specificity and sensitivity of this test need to be reinvestigated considering the present uncertain taxonomy of the P. multocida complex. Consequently, for surveillance purposes and to investigate for carrier animals, the most sensitive method still appears to be mouse inoculation (52, 64) . However, differences in pathogenicity of different clones of P. multocida for mice remain to be investigated (64). Swabs should be taken from the cloaca and pharynx. Following inoculation of the swabs in broth medium and thorough shaking, 0.2 ml to 0.5 ml of the contents is injected into mice by the intraperitoneal route. If P. multocida is present, the mice usually die within 2 4 h to 48 h and the organisms can be isolated in pure culture from heart, blood, liver and spleen (81). Isolation attempts on selective media or blood agar may represent an alternative (62), but this method appears to be less sensitive than mouse inoculation (64).

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Following isolation, identification is based on the results of biochemical tests. The most valuable characteristics for differentiation of Pasteurella multocida from other relevant organisms are shown in Table I. However, simple diagnostic keys do not allow a firm diagnosis within the family Pasteurellaceae. For this reason, extended characterisation, including the use of reference strains, is recommended (6). For further delineation of P. multocida into subspecies multocida, septica and gallicida, the characteristics to be used are shown in Table II.

Serological tests for the presence of specific antibodies are not used for diagnosis of fowl cholera, but have been used in order to test immunity in vaccinated poultry (67).

Public health implications Disease in humans caused by P. multoäda is not uncommon, and P. multocida may be considered a zoonotic organism (8). This is substantiated by the observation that the disease apparently occurs predominantly among the farming population (8). No reports exist of direct transmission from poultry to man or vice versa, but the possibility for such infections cannot be excluded. The organism is a common cause of infection following animal bites or scratches which are mostly caused by dogs or cats (including large cats) (3). Bite wound infections caused by pigs have also been reported (30, 31) . A severe cellulitis may develop which may progress to osteomyelitis and subsequently to septicaemia (29) .

Pasteurella multocida may also be involved in respiratory tract infections, either as a primary or secondary infectious agent (96). In patients with dysfunction of the liver in particular (74, 96) , P. multocida is known to cause bacteraemia which may localise in joints, respiratory tract or progress and cause sepsis. In addition to these principal types of infections, P. multocida has been isolated from a variety of infections, including peritonitis, puerperal sepsis, neonatal sepsis, brain abscesses and urinary tract infections (8). The significance of the different subspecies of P. multocida in relation to diseases reported has not yet been elucidated.

Methods of prevention and control Control of fowl cholera thoughout the world depends principally, on vaccination. Extensive management systems still dominate in many parts of the world and under these conditions, control of P. multocida infections is almost impossible, because of the wildlife reservoir. Animal welfare concerns have also increased the use of non-confined production farms in the industrialised world, resulting in a significant risk of introducing the infections to commercial flocks.

Many live and inactivated (bacterins) fowl cholera vaccines have been developed and tested in attempts to control the disease (81). As modified live vaccine strains can revert to

Table I Characteristics used to differentiate between Pasteurella multocida and other relevant micro-organisms

Characteristic Pasteurella Pasteurella Pasteurella Pasteurella Haemophilus Pasteurella Ornithobacterium Riemerella Characteristic multocida gallinarum avium volantium paragallinarum langaa rhinotracheale anatipestifer

Nitrate + + + + + + (reduction) Urease - - - - — _ + D Arginine - - — — _ _ (+) dihydrolase

(+)

Ornithine + _ D _ decarboxylase Indole + - - _ _ _ D(+) xylose + D D D D _ _ D ( - ) mannitol + - - + + + _ D ( - ) sorbitol D* - - D + _ D(+) galactose + + + + _ + (+) _ Maltose - + - + D _ (+) _ Trehalose D* + + + — _

(+)

Dextrin - + - + _ _ (+) ct-galactosidase - - - - _ _ + + ct-glucosidase D* + + + D - + +

+ : > 9 0 % of strains positive within one to two days - : > 9 0 % of strains negative (+) : > 9 0 % of strains positive within three to fourteen days D : different * : character used for separation of subspecies of P. multocida

Rev. sci. tech. Off. int. Epiz., 19 (2) 631

Table II Characteristics used for identification of subspecies of Pasteurella multocida

Pasteurella multocida Characteristic ssp. multocida ssp. séptica ssp. gallicida

L(+) arabinose D ( - ) arabinose Dulcitol D ( - | sorbitol L(-) fucose Trehalose cc-glucosidase

+ : > 9 0 % of strains positive within one to two days - : 2 9 0 % of strains negative D : different

their pathogenic phenotypes and tend to cause disease in

immunocompromised birds, most commercial vaccines are of

the bacterin type. The vaccines normally contain P. multocida

of serotypes A:l , A:3 and A:4 which has been grown in vitro,

emulsified in an oil adjuvant or aluminium hydroxide (47).

Bacterins are inexpensive to produce and provide some

degree of protection, consequently limiting the incidence and

severity of clinical disease (83) . The principal disadvantages of

the bacterins are that these vaccines have to be injected, often

resulting in tissue reactions (22) , and only induce immunity

to homologous serotypes (75) . As a result, the development of

safe live vaccines is highly desirable to allow the use of a less

laborious route of administration and to obtain

cross-immunity. The principal live attenuated vaccines

currently used, primarily in North America, are the Clemson

University strain and the M-9 strain, both of which are of

serotype A:3,4. Both strains have been implicated in

outbreaks of fowl cholera (44, 91) , and as a consequence,

several attempts have been made to further modify these

strains. Temperature sensitive mutants of both strains have

been constructed which are not capable of growth at 42°C

(42, 45) . Although some protection has been obtained with

these mutants, some mortality was still observed (46) . More

recently, emphasis has been placed on creating non-reverting

auxotrophic mutants of P. multocida by mutating the aro-A

gene (47, 48 , 83) and on selection of clones with a reduced

growth rate (32). Promising results have been obtained in

preliminary vaccination trials with some of these strains.

Guidelines for the production and use of bacterins are

outlined in the Manual oj Standards for Diagnostic Tests and

Vaccines (67) .

Perceived risks of importing the disease No country can be considered free of fowl cholera, primarily

because the causative agent, P. multocida, has a broad habitat,

including mucosal surfaces of a wide range of domestic and

wild birds and mammals. Consequently, control of the

disease on a national basis should focus on the application of

appropriate biosecurity measures at the site of production,

rather than import restrictions. However, to ensure that

imported stock are free of the infection, the guidelines

mentioned earlier should be followed (see section entitled

'Epidemiology'). Further processed poultry products are not

considered to present a major risk for transmission of the

infection, due to the delicate nature of P. multocida.

Choléra aviaire J.P. Christensen & M. Bisgaard

Résumé Pasteurella multocida, sous-espèce multocida, est l 'agent le plus f réquent du choléra aviaire, bien que les sous-espèces septica et gallicida de P. multocida puissent éga lement provoquer, dans une cer ta ine mesure, des maladies apparentées au choléra aviaire. Cependant, les propr iétés v i ru lentes de ces diverses sous-espèces pour di f férents hôtes n'ont pas été é luc idées. La gravi té et l ' inc idence des in fect ions dues à P. multocida peuvent var ier cons idérab lement en fonct ion de plusieurs fac teurs liés à l 'hôte (dont l 'espèce et l 'âge des volai l les in fectées) , à l 'env i ronnement et aux souches bactér iennes en cause. Aucun fac teur unique n'a été associé à une di f férence de v i ru lence entre souches. Les fac teurs de v i ru lence possibles sont notamment les su i van t s : la capsu le , l 'endotoxine, les protéines de la membrane extér ieure, les systèmes de

632 Rev. sci. tech. Off. int. Epiz, 19 (2)

f ixat ion du fer, les proté ines de choc the rm ique , la product ion de neuramin idase, les enzymes de cl ivage des ant icorps. Les tox ines RTX {repeats in toxin) ne semblent pas produi tes par P. multocida, mais l 'exotoxine de P. multocida (PMT) pourra i t cont r ibuer à la v i ru lence dans cer ta ines infect ions aviaires. L 'épidémiologie du choléra aviaire est complexe. Les systèmes de sérotypage t radi t ionnels ne sont guère uti l isés dans les études épidémio log iques. Ces dern ières années, des méthodes de typage molécula i re ont été appl iquées à des souches aviaires de P. multocida d 'or ig ines diverses. Selon les résul tats obtenus avec ces nouvel les méthodes, les o iseaux sauvages pourra ient être à l 'or igine de l ' infect ion des volai l les d 'é levage. Il n'est pas exclu que les mammifères jouent un rôle s imi la i re, mais cela n'a pas encore été conf i rmé. Les oiseaux porteurs de l 'agent pathogène pourra ient jouer un rôle déterminant dans la t ransmiss ion du choléra. Les surv ivants semblent représenter un r isque, mais des enquêtes plus récentes indiquent que des porteurs de P. multocida peuvent exister sans r isque dans des élevages de vola i l les, en l 'absence de foyers antér ieurs de choléra aviaire. Il faudra at tendre des recherches plus poussées pour mesurer l ' importance de ce phénomène.

L' infect ion due à P. multocida est généra lement local isée dans l 'apparei l respi rato i re. Elle peut évoluer en infect ions sura iguës/a iguës ou en infect ions chron iques. Dans le premier t ype d ' in fect ion, les s ignes cl in iques sont rares et on observe pr inc ipa lement des lésions de sept icémie général isée. Dans les fo rmes chron iques d ' in fect ions dues à P. multocida, des lésions suppurat ives peuvent être largement répandues, souvent local isées dans l 'apparei l respi rato i re, la conjonct ive et les t issus pér i -céphal iques.

Le diagnost ic dépend, dans tous les cas, de l ' isolement du baci l le responsable. Pour déceler les in fect ions in f rac l in iques, il est recommandé d ' inoculer à la sour is des pré lèvements chois is, mais l 'ampl i f icat ion en chaîne par polymerase et l ' isolement sur cer ta ins mi l ieux de cul ture peuvent éga lement const i tuer des al ternat ives. L'élevage en c laustrat ion est probablement le moyen le plus ef f icace de prévenir l ' in t roduct ion de P. multocida. Cependant, les systèmes d'élevage extensi f sont prédominants dans de nombreuses régions du monde et, dans ces systèmes, la mesure prophylact ique recommandée est cel le de la vacc ina t ion . Ma lheureusement , les problèmes liés à la product ion de vacc ins à bactér ies v ivantes qui so ient à la fo is ef f icaces et sans danger ne sont tou jours pas résolus. Par conséquent , la prophylaxie dépend tou jours des bactér ines, qui présentent des inconvénients s igni f icat i fs par rappor t aux vacc ins à germes vivants.

Mots-clés Anatomo-pathologie - Choléra aviaire - Épidémiologie - Identification - Maladies aviaires - Pasteurella multocida - Prévention.

Cólera aviar J.P. Christensen & M. Bisgaard

Resumen Pasteurella multocida subespecie multocida es la causa más común de cólera aviar, aunque P. multocida subespecies septica y gallicida pueden también en cierta medida provocar enfermedades af ines. Comoquiera que sea, hasta el momento no se han di luc idado las propiedades de las dist intas subespec ies por lo que respecta a su v i ru lencia para dist intos huéspedes.

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La gravedad e inc idencia de las in fecc iones causadas por P. multocida pueden var iar notablemente en func ión de diversos fac to res l igados al huésped (especie y edad de las aves afectadas) , al entorno o a la cepa bacter iana de que se t ra te . No habiéndose determinado ningún fac to r único que expl ique las var iac iones de v i ru lencia observadas entre dist intas cepas, cabe suponer que éstas obedecen a un conjunto de fac to res , entre ellos los s iguientes: la cápsu la , la p roducc ión de endotox inas, las proteínas de membrana externa, los s is temas de f i jac ión del h ierro, las proteínas de choque té rm ico , la p roducc ión de neuramin idasa y los enzimas que degradan ant icuerpos. Aunque P. multocida no parece elaborar tox inas de t ipo RTX (repeats in toxin), es posible que su exotoxina (PMT) contr ibuya a su v i ru lencia en algunas in fecc iones aviares. La epidemiología del cólera aviar parece comple ja . Los s is temas t rad ic iona les de caracter izac ión de serot ipos son de poca ut i l idad para los estudios ep idemio lóg icos. En los úl t imos años se han ensayado métodos de t ip i f i cac ión molecu lar en cepas aviares de P. multocida de dist into or igen. Los resul tados obtenidos con estos nuevos métodos l levan a pensar que las aves salvajes pueden const i tu i r un foco de in fecc ión para las explotac iones avícolas industr ia les. Aunque hay menos pruebas al respecto , t ampoco cabe exclu i r la posibi l idad de que los mamíferos desempeñen un papel similar. Las aves por tadoras parecen intervenir dec is ivamente en la t ransmis ión del cólera aviar. En este sent ido, los e jemplares superv iv ientes de bandadas a fec tadas parecen const i tu i r un pel igro, aunque invest igac iones más rec ientes han revelado que pueden exist ir por tadores de P. multocida en bandadas sin n ingún antecedente de brotes de cólera aviar. En cualquier caso, conviene esperar nuevas invest igac iones para va lorar el verdadero a lcance de esta observac ión. La in fecc ión por P. multocida se instala, según opinión general izada, en el t rac to respirator io. El proceso in fecc ioso puede tomar fo rmas diversas, desde la peraguda/aguda hasta la c rón ica . En el pr imer caso se observan pocos signos cl ín icos antes de que sobrevenga la muerte del an imal , y entre las lesiones observadas predominan las propias de una sept icemia general izada. En las formas crón icas puede observarse la presencia general izada de lesiones supurat ivas, que suelen afectar el t rac to respirator io, la conjunt iva y los te j idos encefá l icos adyacentes.

El d iagnóst ico está cond ic ionado s iempre al ais lamiento del microorganismo. Para detectar in fecc iones subcl ín icas se recomienda inocular ratones con las muestras sospechosas, aunque la ampl i f icac ión en cadena por la pol imerasa o la eventual posibi l idad de aislar la bacter ia en medios select ivos const i tuyen posibles métodos al ternat ivos. El medio más ef icaz de impedir la in t roducc ión de P. multocida es seguramente la segregac ión de los e jemplares in fectados. Sin embargo, en muchas partes del mundo predominan los s is temas de producc ión extensivos, en cuyo caso se recomienda la vacunac ión como medida prevent iva. Lamentablemente, y dadas las d i f icu l tades que todavía plantea la e laborac ión de vacunas vivas ef icaces y seguras, la lucha contra la enfermedad sigue dependiendo de vacunas preparadas con bacter inas, que presentan notables desventajas en comparac ión con las vacunas vivas.

Palabras clave Anatomo-patología - Cólera aviar - Enfermedades aviares - Epidemiología -Identificación - Pasteurella multocida - Prevención.

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