Scientific Papers

Questionnaire study suggests grave consequences of infectious laryngotracheitis, infectious coryza and mycoplasmosis in small chicken flocks | Acta Veterinaria Scandinavica

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This questionnaire-based study investigated the outcome and management of respiratory infections associated with ILTV, A. paragallinarum and M. gallisepticum alone or in combination in chickens from small non-commercial flocks with a laboratory confirmed post-mortem diagnosis. The outbreaks were often associated with severe clinical signs and high mortality and some owners euthanised a subset or all of their chickens. Information on the occurrence of respiratory pathogens in this poultry category in Sweden and elsewhere is scarce. At the time of this study, SCF were not registered in Sweden, and therefore the representativity of the results is unknown. However, the locations of the participating flocks in this study (Fig. 1) suggest that these infections are geographically widespread in Sweden. Several earlier studies from other countries suggest that respiratory infections are common in SCF both in Europe [2, 23] and North America [3, 4, 12]. In Finland, a neighbouring country to Sweden, 12% of 51 backyard chicken flocks were seropositive for ILTV [24]. Moreover, coinfections with several respiratory pathogens were common among the flocks in this study. This was also found in Canada and USA in SCF [3, 4]. Together, these results indicate that respiratory infection is an important issue in small non-commercial chicken flocks in several countries.

Response rates to questionnaires and interview surveys among small poultry flock owners varied widely (4–72%) in previously published studies [2, 11, 25, 26]. Other surveys have been advertised on internet platforms [1, 6, 8] where response rates could not be determined. Differences in response rates may be attributed to a variety of factors such as methodology, target population and incentives to respond. The response rate of this study (61.0%) was higher than in most previous reports. It is plausible that this was related to the combination of a postal informational letter, a known internet access (email addresses supplied at referral of necropsy cases) and an email reminder. Additionally, the questionnaire topic may have been of particular interest to the owners. When interpreting the results of this study, it must be remembered that it is likely to involve some degree of selection bias as the study specifically targeted owners who had submitted chickens for diagnostics and who could choose whether to participate. The severity of clinical signs and the mortality experienced by the responding owners may have been strong motives behind their decision to submit chickens for diagnostic necropsy and to participate in the questionnaire study. In an earlier study, 39% of small flock owners had chickens affected by respiratory disease within 1 to 6 months prior to the study, but only two reported mortalities [2]. This clearly shows that respiratory infections in SCF may not always be associated with the severe signs and mortality reported in our study.

In agreement with several earlier reports [1, 3, 6, 26], a mixed-species flock structure was a common trait in this study. This type of flock structure was associated with detection of M. gallisepticum by PCR, but not with A. paragallinarum or ILTV. Mycoplasma gallisepticum is known to infect a wide variety of poultry including gallinaceous and anseriform species [27], which could explain this association. In contrast, ILTV and A. paragallinarum both have a limited range of primary host species, i.e. mainly chickens [15, 17]. Further identified risks for M. gallisepticum detection in this study were age (young chickens having a higher risk than chicks and adults, P = 0.02) and larger flock size. The latter could represent an indirect effect because larger flock size was also associated with a mixed-species flock structure, but a larger flock could speculatively also consist of chickens from more numerous sources compared to small flocks.

Most of the respondents had added new birds to their flock within a year before the outbreak (82%, Table 3) of which many had introduced live birds rather than hatching eggs. Bird-to-bird transmission from diseased birds or clinically healthy carriers was therefore assumed to be the main source of infection, but transmission of M. gallisepticum through hatching eggs was also a possibility [27]. Owners of SCF may be unaware of the risks of acquiring birds of unknown health status from other farms, live poultry markets or poultry shows and introducing them into their own flock. Moreover, quarantining new chickens for a short period of time may not be sufficient due to the silent carrier state and the fact that respiratory signs may be intermittent. In case of ILTV, virus excretion may be reactivated as a result of mixing birds of different origins or start of lay [28] which means that transmission may not happen soon due to a latency stage. Moreover, more than 30% of the 23 respondents who had owned their flock more than three years before respiratory signs appeared had not acquired new birds during the preceding 12 months of the disease event. Indirect transmission, a long-term carrier state within the flock or in case of M. gallisepticum, a wildlife source could not be completely ruled out.

Another interesting finding in this study was that despite the serious clinical signs reported, less than ten percent of the flocks had received therapy to alleviate the respiratory infection (Table 4). Over-the-counter products were mostly used to alleviate clinical signs. Further, and in accordance with national recommendations to limit the use of antimicrobials (Swedish Medical Products Agency, 2019), none of the respondents reported having used antimicrobial drugs. Mild respiratory infections in SCF will usually not require antimicrobial treatment, unless complicated by secondary bacterial involvement. Further, no drugs are available that will alleviate clinical signs in birds affected by ILTV alone [28]. In this study, full recovery was reported by less than 40% of the flock owners, which could be explained by the severity of the outbreaks, the high occurrence of coinfections, and the long-term clinical signs M. gallisepticum and A. paragallinarum sometimes cause. Similarly, none of the respondents in this study reported having used vaccines to prevent respiratory infections or other poultry diseases. This agrees with earlier findings from Finland and Sweden in which less than two percent of SCF owners had used vaccines [9, 11]. Vaccination of chickens in small flocks, mostly against Marek’s disease, appear to be somewhat more common in some other countries such as the United Kingdom, Canada and USA [1, 25, 29].

In accordance with a previous study [1] the respondents in this study often gained information and advice on animal health from other owners and non-professionals (39.9%, Table 4). Notably, less than 35% or the respondents had been in contact with a veterinarian who was not affiliated to the diagnostic laboratory. Contacts with veterinarians at the diagnostic laboratory (SVA poultry veterinarians) could be assumed to mainly involve questions concerning diagnostics, but they may also have included general information on preventive measures and outbreak management. It was however not clear whether the other veterinary contacts involved practitioners and if they had been consulted prior to after having obtained a laboratory-confirmed diagnosis. A previous report from USA reported that only 42.7% of small-scale and backyard poultry and livestock owners sought veterinary help despite experiencing an animal health concern [5]. The reasons for this may be many and are poorly understood which warrants further study.

Even though cleaning and disinfection routines following the outbreaks (Table 4) were not associated with clinical recovery, the results showed that there is a potential for improvement. Earlier studies have reported similar findings and also suggested that the biosecurity routines on small non-commercial chicken farms may be inadequate [8, 9]. This is an area where veterinary practitioners potentially could offer more help. From this study it appears that there is an urgent need for improved communication between veterinarians and owners of small poultry flocks. Veterinary services should focus on recommendations regarding disease control and prevention, including biosecurity measures and vaccination, in addition to veterinary care when outbreaks occur. Moreover, commercial poultry farmers should limit direct and indirect contacts when selling chickens to small hobby flocks to avoid any exchange of respiratory pathogens.

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