The disease of turkey rhinotracheitis (mercifully abbreviated to TRT) first appeared in the UK in 1985. As its name suggests, it first affected turkeys. Fattening turkeys showed severe respiratory disease, mainly starting in the upper respiratory tract (rhino = nose, trachea = windpipe and -itis = inflamed). This often led to pneumonia and E.coli infection. The virus was properly identified in 1988 as an avian pneumovirus (a group of viruses which also includes respiratory viruses capable of causing disease in human babies, calves and mice). Already, by that time, veterinarians, producers and research workers were suspicious that the same virus was responsible for some problems in chickens which had emerged over the same time period. This was called a more general Avian rhinotracheitis (ART) as turkeys were not involved! Work now suggests that the turkey is the natural host of the virus but that chickens and even guinea fowl and pheasants can become infected. There is also some evidence that wild birds and some sea birds can act as a reservoir of the virus.
Swollen head syndrome (SHS)
The most obvious manifestation in chickens has been in broilers and broiler breeders which have shown infections of the nose and head leading to the aptly titled condition of swollen head syndrome (SHS) where the birds seemed to be suffering from an obvious head cold with swelling of the head. Breeders have also shown nervous signs and drops in egg production.
Antibody levels and diagnosis
The clinical picture in commercial layers and layer breeders is less clear cut. This has made diagnosis of the condition in layers quite difficult. If the flock has been monitored for antibodies in the blood regularly, the appearance of TRT antibodies at a time when egg production or egg quality problems are being experienced may be a pretty good pointer that TRT was the culprit. However, some flocks seem to produce antibodies early in life without showing any problems so a positive blood test in itself is not always diagnostic.
So how do I spot infection in my flock?
At least three syndromes have been described in layers.
a) The nervous form.
This has been seen in free range and cage flocks with severe twisting of the neck, with the head and neck arched forwards, either between the legs or to one side. The cause of these nervous signs is severe local infection of the spongy bones of the head. Birds also showed green diarrhoea due to not finding food. This frequently has an adverse effect on egg production. Clearly, the condition could be confused with fowl pest (Newcastle disease).
b) Peritonitis.
A persistent chronic mortality as peritonitis has been seen, sometimes starting as an IB-like mild upper respiratory disease which then seems to precipitate the peritonitis. There is a variable response to infeed antibiotics. Egg production can show typical IB like dips but may take longer to recover and leave the flock with an ongoing seconds problem of pale shells, thin shells and possibly shell-less eggs.
c) The infamous white egg problem.
We are clearly a long way from solving this problem but avian pneumovirus infection seems one of a number of likely candidates. Possibly this again starts with mild upper respiratory tract infection and a slight mortality blip but generally hens are unaffected.
Diagnosis
Serology (blood tests) can be useful as a retrospective indication of infection but you always need a baseline system to assess when the birds go positive due to the fact that many flocks will seroconvert early (possibly in rear) without subsequent problems in lay, even though they will then have positive antibodies. This is, yet again, a good advert for routine blood sample monitoring of layer flocks rather than waiting for a disease problem and then trying to find which agent to blame!
What can I do to prevent infection?
Good disease biosecurity is the key to prevention of the introduction of most infections. However, a respiratory virus which may be carried by wild birds suggests that infection can be airborne, especially in areas with a concentrated turkey population. Vaccines are available which are effective at controlling this infection, currently being available as a live primer and a dead injection. TRT virus is likely to become a common component of the triple injection vaccinations which will help reduce costs and the number of injections needed.
Should I vaccinate?
The decision to vaccinate for TRT really depends on the expected risk or previous experience of laying flocks in a particular geographical area. There are some areas of the country where infection of layers is common whereas others remain free. Your veterinary surgeon will be able to alert you to the possible risks in your area. If vaccination is considered worthwhile, then this may be as two doses of live vaccine in rear or one dose of live and the follow up injection prior to lay. The latter programme is likely to give the best level of protection.
Conclusions
The evidence of avian pneumovirus infection as a cause of mortality and egg problems in commercial layers is increasing. Full vaccination programmes may be appropriate in some areas but this should be supported by good diagnostic information and knowledge of the disease situation in your area.
