Emerging and Evolving Diseases

Most of the important infectious diseases of poultry have been known
for many years and are controlled variously by medication, vaccination,
eradication and biosecurity.

These present fresh challenges for diagnosis, control and understanding

New bugs Emerging viral infections in both humans and animals have been reported with increased frequency in recent years. Advertisement

Most of the important infectious diseases of poultry have been known for many years and are controlled variously by medication, vaccination, eradication and biosecurity. Examples of such diseases that were known before the intensive commercial poultry industry developed are infectious bronchitis (IB), Newcastle disease (ND), lymphoid leucosis. From time to time, new diseases emerge to provide fresh challenges for diagnosis, control and understanding of their epidemiology.

In addition, other well-established diseases evolve in different ways and can also present problems for diagnosis and control. In the wider sphere, it is recognized that emerging viral infections in both humans and animals have been reported with increased frequency in recent years. Examples are SARS, avian influenza, West Nile virus and the Nipah virus. This paper presents a review confined to some of the more recent diseases of viral origin, others that have re-emerged and some experiences from previous emerging diseases. In addition, IB is discussed as an example of a disease in which the causal virus is showing constant evolution. Finally, the impact of infections caused by the avian metapneumovirus (aMPV) are briefly discussed as an example of a still relatively recently emerged viral agent, with some lessons learned.
 

Definitions
For the purposes of this paper, I have used the World Health Organization (WHO) definition of an emerging disease. This is defined as a disease that has appeared in a population for the first time, or that may have existed previously but is rapidly increasing in incidence or geographic range. Diseases that reappear after a period of absence can be considered as re-emerging. Some of these include runting/stunting syndrome and inclusion body hepatitis. I have used the term evolving, since I consider that it accurately describes the constant emergence of novel genotypes of IB virus and we have monitored this phenomenon in Western Europe in the last few years. Arguably, all viruses should be described as evolving, but there are some viral diseases of poultry such as infectious avian encephalomyelitis and fowlpox which appear to be relatively stable. Of course in some instances, virus evolution is a way of producing an emerging disease, so the definitions can overlap.

How do emerging viral diseases originate?
New viral diseases can appear in domestic poultry in a number of ways. The causative virus may originate from another avian species, such as the Egg Drop Syndrome virus, which was found to be a duck virus. Another example may be avian metapneumovirus, where evidence of infection has been shown in ostriches in Africa.

Others have emerged following mutations of familiar viruses, such as the highly virulent strains of Marek’s disease. Various possibilities that can occur through viral recombinations have been discussed by researchers.1  It has been described that recombinations can occur between Marek’s disease virus (an oncogenic herpesvirus), fowlpox virus and various oncogenic retroviruses. These researchers point out that it is now recognized that chicken anaemia virus (CAV) was created by several inter-family recombination events occurring between animal and plant viruses. The modern poultry industry with its intensive husbandry and the multiple opportunities for viral replication is eminently conducive to the spread of new diseases resulting from mutations and recombinations. Some of these mechanisms may have zoonotic implications.

As new viruses emerge it will be necessary to use all available diagnostic methods to determine the viral cause.

Diagnostic and control problems
The emergence of a new disease may present considerable problems of diagnosis, surveillance and control. When an emerging disease is considered to be of viral origin, the traditional method of agent identification had been to attempt virus culture in cell monolayers, fertile eggs or organ cultures. However, the shortcomings of our existing culture systems are self-evident.

Cultivation was easily achievable with EDS adenoviruses, and rotaviruses were first cultured with the help of trypsin and centrifugation. However, CAV has not been cultivable in conventional cell types, but needs transformed cells in suspension. Turkey coronaviruses, considered to be an important player in poult enteritis mortality syndrome (PEMS), can so far only be cultivated in the intestine of turkey embryos and hepatitis E virus has so far resisted attempts to grow it.

It seems likely that intestinal viruses are involved in the multifactorial syndromes like PEMS and runting/stunting and are proving especially difficult to cultivate. Perhaps intestinal organ cultures, though notoriously self-destructive, should be reconsidered. The inability to cultivate the causal virus can also delay the development of serological tests for surveillance or studying immune responses. In some instances this has been obviated using particular recombinant proteins but only where the important viral protein genes can be identified and expressed.

Where cultivation has proven difficult, and before the advent of molecular methods, immunofluorescence staining of tissue sections using convalescent serum has helped to show the likely tissue and cellular targets of viruses such as aMPV. Electron microscopy of intestinal and tissue suspensions or embedded sections has indicated morphology and size of viruses present.

More recently, molecular methods and in particular, the polymerase chain reaction has been used to identify characteristic virus sequences in clinical material. This technology is rapid and can, when followed by sequencing or random fragment length polymorphism, give precise identity to a genotype.

It is also possible to trace the geographic movement of viruses by means of molecular epidemiology more conveniently than any cultural system could. However, although much more rapid than virus isolation, where a virus is uncultivable, PCR does not provide live virus for further study, for example for pathogenesis elucidation or vaccine development. In these cases, infectivity has to be determined in live birds using filtered virus-rich tissue or gut material. In future, it seems likely that the development of microarray technology will enable the rapid screening of emerging disease material for a wide range of possible agents.

Emerging and re-emerging diseases
The two most prominent viral emerging diseases to affect poultry in recent years are PEMS and hepatitis E virus infection, associated with hepatospelenomegaly or big liver and spleen disease (BLS).

PEMS is a highly infectious disease of young turkeys defined by spiking mortality, diarrhea, growth depression, stunting and immunosuppression. It was first described in young turkeys in the southwestern U.S., but is now know to be in the U.K. and elsewhere. It appears to be multifactorial in nature and can be transmitted using gut extracts and also with bacteria-free extracts of thymus.2

Turkey coronaviruses (TCV) and turkey astroviruses have been implicated, together with reoviruses and entropathogenic E.coli. However, the role of these viruses is uncertain, since they cannot replicate the disease when given alone but they can be detected in PEMS-free flocks. Because of the uncertainty of the aetiology, and particularly the role of viruses, control is best achieved by basic disease prevention principles such as all-in all-out and thorough cleansing and disinfection.

BLS was first described in Australia in the 1980s and later in the U.K. and U.S. Affecting broiler breeders and layer breeders, it is characterized by reduced egg production, increased mortality and enlarged livers and spleens. The putative causal virus is a 30-35 nm diameter non-enveloped RNA herpesvirus referred to as avian hepatitis virus (HEV) which shares 50-60 per cent nucleotide identity with pig and human types of the virus and 80 per cent identity with the Australian strains of BLS virus.3 Attempts to propagate the virus in cell cultures or eggs were unsuccessful. Gross liver lesions have been produced in specific pathogen-free (SPF) adults using virus in bile extract.4

Although due to the inability to cultivate this virus a traditional vaccine cannot be made, successful immunization of chicks against hepatitis E virus using a recombinant HEV capsid has been reported.5 Diagnosis depends on clinical and post mortem signs and RT-PCR methods, which are now being used for virus identification. The origins of this virus are unknown but it may have represented a mutation of a pig or human type.

Other diseases described as being emerging are infectious laryngotracheitis in Australia, runting/stunting syndrome in the U.S. and inclusion body hepatitis in Canada. All these conditions have been present in the UK and there would be described as reemerging.

Previous experiences of emerging diseases
Experiences in investigating some earlier emerging diseases have been varied. For example, EDS was caused by a virus that could easily be grown in the laboratory and, in addition, it hemagglutinated erythrocytes, immediately providing a convenient serological test. Further work on the virus showed that it was of duck origin. Diarrhea due to rotaviruses could easily be identified by electron microscopy of gut contents but the viruses resisted cultivation unless trypsin was added to the medium and cells were centrifuged.

The runting/stunting syndrome is a complex multi-factorial problem whose aetiology has not been fully resolved. Although some reoviruses have been implicated and indeed some vaccine manufacturers include anti-stunting reovirus components in their products, such vaccines would have been expected to solve the problem had reoviruses been that important. One factor, which has caused reoviruses to be implicated as a cause of many conditions is that they are so simple to grow in cell culture. A recent report described the detection of reoviruses in corvids, which were dying in New York State. Whether they were the cause remains to be investigated.

The chicken anaemia virus had been present in poultry populations but proved difficult to work with because it will only grow in the laboratory in transformed cells. aMPV infection was first described in South Africa where it had a devastating effect on the turkey industry. After spreading to Europe, the virus was characterized as the first avian pneumovirus but proved difficult to grow until it was found to cause deciliation in tracheal organ cultures. It could then be adapted to cell cultures. However, both APV and IB virus would be much simpler to work with in the laboratory if primary isolation could be made in a convenient cell culture.

Evolving diseases
A disease in which evolution can be followed by constant surveillance is IB. The coronavirus constantly generates variants because of the instability of a hypervariable region in the S1 spike gene. The S1 spike is biologically very important, with roles in cell attachment, virus immunity and induction of immunity. Variants occurring due to mutations or recombinants can arise against which existing vaccines are poorly protective.

We have conducted an RT-PCR survey of IBV types associated with IB-like conditions in commercial chicken flocks in Western Europe between 2002 and 2006.6 Positive samples were sequences over a 340 nucleotide region of the S1 gene. From more than 500 flocks tested, 60 per cent were positive and most genotypes were, as expected, of the kind employed as vaccines, such as Massachusetts, 793B (major variant), D274 and Arkansas. About 50 per cent of viruses comprised vaccines. However, two major variants were monitored, Italy02 and QX. Italy02, of unknown origin, was the most prevalent wild type, but the use of dual vaccination with different available vaccines can control it and a homologous vaccine does not appear necessary.

QX genotype, first encountered in 2004, is identical in S1 sequence to a virus isolated in China in 1996. This virus appears to have travelled from the Far East to western Europe in only slightly longer than it took HPAI H5N1 to cover the same distance. However, while migratory waterfowl are considered to have played a major part in H5N1 transcontinental spread, there is no evidence that IBVs are transmitted in the same way. The means of long distance spread of IBV remains unclear.

Avian metapneumoviruses were first encountered as a cause of turkey rhinotracheitis in the late 1970s. They are recognized as a major cause of loss to the turkey industry and a player of uncertain importance in chronic respiratory disease in chickens. Two subtypes, A and B, were recognized on the basis of G glycoprotein sequences, in most parts of the world except U.S. and Australia, until subtype C was identified in turkeys in Colorado, then Minnesota in the mid-1990s. Since then, a subtype D in turkeys and a C-like virus in ducks were described in France. Surprisingly, a C-type virus has been reported in pheasants in Korea. Thus the possible evolution and global spread of this virus are puzzling.

American workers have reported evidence of infection with aMPVs in some wild birds species but whether they are instrumental in transmitting disease has still to be fully investigated.

Conclusions
It can be expected that new viral diseases will continue to emerge from time to time in the poultry industry, in the various ways listed above. Indeed it may be that some of these are already present in domestic poultry. Perhaps new diseases will emerge caused by viruses in genera not yet recognized as pathogens in domestic poultry such as pestiviruses, toroviruses or lentiviruses. It will be necessary to use all available diagnostic methods to determine the viral cause. Although frequently overlooked in the rush to use modern methodologies, full characterization of the gross and microscopic pathology of a new disease is imperative, to obviate confusion when similar outbreaks are described in different parts of the world.7

New technologies such as microarrays are likely to be valuable in speedy recognition of likely causes although from experience we know that some conditions are multifactorial. Perhaps a vain hope is that there could be more collaboration between laboratories in investigating such diseases, although commercial considerations are often not conducive to this.

With regard to evolving viruses such as IB virus, continuous monitoring of genotypes is essential for determining what viruses are prevalent and highlighting the emergence of new types which are likely to require modification of vaccine strategies.

References

1. Davidson, I., and de Silva, R.F. (2007) Creation of diversity in the animal virus world by interspecies and intraspecies recombinations: lessons learned from poultry viruses. Virus Genes, 36, 1-9
2. Schultz-Cherry, S., Kapczynski, D.R., Simmons, V.M., Koci, M.D., Brown, C., and Barnes , H.J. (2000) Identifying agent(s) associated with poult enteritis mortality syndrome: importance of the thymus. Avian Diseases. 44, 256-65.
3. Haqshenas, G., Shivaprasad, H.L., Woolcock, P.R., Read, D.H. and Meng, X.J. (2001) Genetic identification and characterization of a novel virus related to human hepatitis E virus from chickens with hepatitis-splenomegaly syndrome in the United States. Journal of General Microbiology. 82, 2449-2464.
4. Billam, P., Huang, F.F., Sun, Z.F., Pierson, F.W., Duncan, R.B., Elvinger, F., Guenette, D.K., Toth, T.E. and Meng, X.J. (2006) Systematic pathogenesis and replication of avian hepatitis E virus in specific pathogenfree adult chickens. Journal of Virology. 2005, 79, 3429 -37.
5. Guo, H., Zhou, E.M., Sun, Z.F. and Meng, X-J. (2007) Protection of chickens against avian hepatitis E virus (avian HEV) infection by immunization with recombinant avian HEV capsid protein. Vaccine. 25, 2892-9.
6. Worthington, K.J., Currie, R.E. and Jones R.C. (2008) An RT-PCR survey of infectious bronchitis virus genotypes in Western Europe from 2002 to 2006. Avian Pathology (in press).
7. McNulty, M.S. (1997) Chicken anaemia virus – a glimpse of the future? British Poultry Science. 38, 7-13.