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Immunosuppressive viruses in poultry

Course: Tackling common poultry diseases | Last Updates: 7th October 2015

Stephen Lister
Partner Crowshall Veterinary Services and member of the Veterinary Residues Committee
Biography >>

Some viruses cause diseases that result in distinct symptoms. But there are those which can lie undetected while suppressing the bird’s immune system.

These so-called immunosuppressive viruses prevent the immune system from fighting off disease or responding to vaccination.

Effect of immunosuppressive viruses

To fully understand the impact of these viruses, it is useful to consider the basis of the bird’s immune system.

A healthy immune system operates by identifying infectious agents that are “foreign” and then killing or removing them. Many of the mechanisms are complex, but the starting point is an intact physical barrier against the outside world.

The first layer of defence is skin, which covers the body preventing entry to sensitive tissues and organs beneath. However, to breathe and digest food, the body must have a more intimate contact with its environment via the respiratory and intestinal tracts.

The need to absorb oxygen, excrete carbon dioxide and digest and absorb all the essential nutrients from feed eaten requires there to be complex linings to the nose, windpipe and intestines.

Therefore, there is a need for allowing selective entry to essential elements, but excluding a range of harmful infectious agents. This is achieved by a sticky mucus lining giving physical protection.

If viruses and other pathogens do manage to breach these defences and gain access to the bloodstream or deeper tissues then the complex internal immune system takes over in a healthy bird.

The role of vaccines

Scientists make use of these immune mechanisms to produce a range of vaccines designed to stimulate the immune system, fooling it into thinking that there is a disease challenge that needs defending against. This primes the immune system such that when and if, it meets the real thing later it is already prepared to produce the right defensive response, and in double quick time.

This manipulation of the immune system is an essential part of any disease control strategy for modern poultry production. However, this response and activity comes at a cost. The bird has to expend a lot of energy every time it responds to a disease challenge, or indeed vaccination.

The immune system competes with other tissues for energy and building blocks for growth.

For example, during disease challenges affecting the bird’s gut, there may be a 25% increase in use of nutrients from feed and a 5% increase in amino acid requirement.

Hen-in-cageAs a result of these costs, it is clearly advantageous to reduce the challenges on the immune system to a minimum in the first place, to avoid this waste of energy, nutrients and effort by the bird. This highlights the importance of other aspects of an overall biosecurity programme, not merely relying on the bird’s sophisticated immune system or a vaccination programme alone.

The secret of success for the immune system in fighting off infection is that it should be functioning optimally. Unfortunately, in poultry production there are a number of viruses, as well as non-specific stressors and nutritional inadequacies, which can damage various parts of this complex system.

Viruses and their effects

On the viral side, it has been known for many years that Gumboro disease virus, Chick Anaemia virus and Marek’s disease virus can have a direct effect on the major tissues of the bird’s immune defence system. Indeed there is also evidence that combinations of two or more of these viruses can amplify these adverse effects.

Other viruses, such as Reoviruses, can have similar, but less marked effects. In turkeys, Haemorrhagic Enteritis virus and the viruses associated with the Poult Enteritis Mortality Syndrome play similar roles in damaging the immune system.

With Gumboro disease, infections occurring within the first two weeks of life cause little or no mortality, but can damage certain organs and hence impair antibody production.

This leaves birds susceptible to a range of infections which they would normally have resisted. Even superficial skin infections can become well established, Clostridial and Staphylococcal infections from the environment leading to severe lesions, notably Gangrenous Dermatitis.

Other flocks show more chronic respiratory disease due to combined bacterial and viral assaults on the sensitive lining, resulting in mortality or increased downgrading at processing. Even if the early damage to the bursa (an organ involved with the immune system) is mild, or partial, the reduction in antibody production might not show any clinical effects, but lead to flocks which just show poorer weight gain or unevenness due to insidious, low grade bacterial challenges.

Infectious Process and CAV

It is also thought that this sort of “silent” infection can lead to a variety of other processing issues. One of the largest causes of condemnation in broilers in the USA is Infectious Process, a form of avian cellulitis. The subcutaneous bacterial infection appears to gain access through skin wounds and it is considered that birds with a poorly functioning immune system are much more prone to this condition.

In the USA alone, this condition is estimated to cause losses of $50m (£31.4m) a year.

Chick Anaemia virus (CAV) infection can have dramatic effects on antibody production, but also impairs production of red blood cells.

It has been associated with severe clinical disease, often associated with secondary bacterial or fungal lesions, the most notable of which is Blue Wing disease, a severe clostridial infection, predominantly affecting the skin on the wings.

Research at the Veterinary Research Laboratory, Stormont, Belfast clearly demonstrated the cost of this. Broiler flocks affected by clinical CAV infection from a vertically transmitting parent flock had a net income per 1000 birds 17-19% lower than that recorded in unaffected flocks. Furthermore, the flock had a 3.3% lower average slaughter weight and mortality was 2% higher. Other work suggests that the depression of average weight can be as much as 12%.

Even more significant is the sub-clinical effects of CAV infection. They found in a large scale commercial trial with data from more than 1m broilers that horizontally acquired CAV infection (persistent virus on the broiler farm) was capable of causing significant economic loss resulting from sub-clinical infection. Even in flocks not reporting any obvious clinical disease signs, the infection was responsible for a 13% drop in net income per 1000 birds, a 2% penalty in food conversion and 2.5% lower average liveweight at slaughter.

The researchers concluded that these results showed that “sub-clinical CAV infection has a substantial effect on commercial broiler performance and profitability”.

To conclude, there is a variety of viruses which have a significant, irreversible and damaging effect on the bird’s immune system, and as a consequence, on bird performance.

Effective cleansing of buildings is a key factor in reducing viral load.

Avoiding the effects of immunosuppressive viruses

The best approach to minimising disease is to use the best vaccines available, properly apply them and ensure an appropriate level of nutrition to enable the best possible response to these vaccines.

However, reducing the pressure on vaccines by reducing the viral load and their damage on the immune tissues is the way to maximise that response.

The key tips are:

  • Reduce vehicle and visitor movements
  • Cleanse and disinfect all vehicles and equipment entering site
  • Use protective clothing, hand sanitisers, boots and footdips, at least between sites, but where possible between houses and flocks within a premises
  • Exclude wild birds from houses
  • On free-range sites try to limitwild bird contact by avoiding feeding on range, clearing up any feed spillages and siting flocks away from wild bird haunts
  • Have an effective vermin control plan in place
  • Effective terminal cleansing and disinfection with a proven broad-spectrum disinfectant, such as DuPont Virkon. This reduces the number of pathogens and the weight of disease challenge and, hence, greatly enhances the biosecurity programmes. It can only be achieved with sufficient turnaround/down time to allow removal of all litter and the required contact times for the disinfection products used before restocking.
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