Infectious bronchitis (IB) of chickens has been a disease of major economic importance for many years and continues to cause losses due to reduced growth in broilers and reduced egg production and quality in layers and breeders. IB is usually considered to be a respiratory infection, but an aspect which has received little attention, is enterotropism amongst certain strains of the virus. We have described a strain ('Moroccan IBVG') which replicated in the gut (on the tips of the villi) and persists at that site for longer than in the respiratory tract. Furthermore, following early infection of chickens, this virus is re-excreted from both tracts when the birds reach sexual maturity. We have recently examined several other British strains of IBV and found that the enterotropism such as is shown by IBVG is not uncommon. Thus some strains of this virus exhibit pathogenesis of infection which resembles that of enteric coronaviruses of other species such as the cat, dog or pig. This has led to collaboration with the feline group in their studies of canine and feline coronavirus infections.
The implications of IBV strains surviving in chicken populations through enteric replication are: (i) they may avoid the humoral response induced by conventional IB vaccines given by spray or eyedrop, (ii) faecal-oral infection may be an important means of spread, (iii) re-excretion at sexual maturity could act as an unexpected source of infection and (iv) long-term persistence in a single chick or closed flock might predispose to mutation of the original virus to a new antigenic typeTurkey rhinotracheitis (TRT) has been and continues to be the most important disease in the turkey industry in the last ten years. The causal agent, a pneumovirus, causes loss of production and sometimes deaths in growing turkeys and loss of egg production in breeders. A manifestation in some broiler or broiler-breeder chicken flocks is 'swollen head syndrome'.
Since the mid 1980s, this laboratory has been in the forefront of investigations on identification of the agent and studies of its pathogenesis for the turkey and chicken (resulting in some 10 papers published). Since 1987, we have received funding from a commercial company to investigate production of an attenuated vaccine. In practice, this has proved more difficult than was first envisaged, the main problem being striking the right balance between attenuation, immunogenicity and failure to revert to virulence. However, much valuable ancillary basic research has been done on the immune response of the host to this virus and molecular changes to viruses with different degrees of attenuation.
The pathogenesis, latency and epidemiology of another respiratory pathogen of chickens, infectious laryngotracheitis virus, has been studies for several years with a succession of AFRC/BBSRC grants in collaboration with the small animal infectious diseases group. We have defined the carrier state and shown that carriers are an important source of virus. Furthermore, it appears that many of the strains circulating and causing disease in the field may be derived from the present conventional vaccine. Present studies are concerned with molecular characterisation of the virus with a view to improving diagnostic techniques and developing genetically engineered vaccines.
Avian reoviruses are ubiquitious and can be found in most healthy poultry flocks. In chickens, they have been isolated from several disease conditions but only in the case of tenosynovitis, an important infectious cause of lameness in broiler chicken breeds, is there a definite association between the virus and the pathological condition. From the mid 1970s to the present, almost 30 papers from this department have been published on reovirus infections in the chicken, including early events in pathogenesis, effects of age at infection, breed of chicken mixed infections and effects of immunosuppression.
Until recently, avian reoviruses have been considered to be fundamentally enteric pathogens and while egg transmission does occur at a low rate, faecal-oral spread is the main means of transmission. It has been generally held that all strains are trypsin insensitive and therefore well equipped to survive the harsh conditions in the gut. However, we recently showed that some avian reoviruses are sensitive to trypsin and that enteric survival is very short. Continued work in this area has shown that trypsin sensitivity is not uncommon. Indeed seven of 21 strains, including common vaccine strains were found to be sensitive. Other points to emerge were that (i) trypsin sensitivity is linked to sensitivity to amylase also, (ii) egg-transmission of a sensitive strain occurred at a much lower rate than that of a resistant strain and (iii) following oral inoculation of specific-pathogen- free chicks with high doses of sensitive virus, trypsin-resistant virus is being excreted by 7 days p.i. The phenomenon of trypsin sensitivity, which implies different pathogenesis of infection and perhaps alternative means of spread for the sensitive strains is the subject of continued study.
Reoviruses have frequently been implicated in the infectious stunting syndrome of chickens, although it is generally accepted that they do not play a primary role. A paradox of avian reovirus infections is that following oral infection of chicks, very high titres of virus are found in the intestine at four days p.i. but there is no enteritis and no visible microscopic changes in the gut epithelium. Recent work using intestinal organ cultures has shown that some reoviruses can suppress certain brush border enzymes and this may contribute to intestinal dysfunction.
Commercial vaccines are available world-wide for the control of avian reovirus infections and especially tenosynovitis. However, it is generally acknowledged that avian reoviruses are poorly immunogenic and there is a need for improved vaccines. We are currently receiving funding from an American vaccine company for the development of a new reovirus vaccine based on one of our UK strains rather than the American S1133 which is the basis of most commercial products.
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