New insights in avian influenza

Avian influenza is worldwide in distribution. Avian influenza viruses vary in their pathogenicity and their effects range from a mild respiratory or asymptomatic disease, low pathogenic avian influenza (LPAI) (subtype H1,3,5,6,9), to catastrophic losses associated with viscerotropic infections, highly pathogenic avian influenza (HPAI). HPAI are always subtype H5 or H7.

Outbreaks of HPAI result in severe losses in production, disruption in operations and high costs for control and prevention. The influenza virus easily enters the body through the mucosae and can replicate in all internal tissues and is excreted through feces and other secretions. Prevention is better than cure! 

Transmission of the disease

Wild birds serve as reservoirs (H1-H16) and transmit infection to commercial flocks.  

Rapid multiplication of AI virus occurs in commercial flocks, ultimately affecting all poultry operations in a region, unless appropriate controls are implemented. The virus is relatively resistant to environmental exposure and can infect birds placed in decontaminated units housing a previously infected flock. Indirect infection can occur by moving flocks, equipment and personnel, and by wind dispersal of virus-laden dust and feathers. Direct infection occurs following contact between infected carriers and susceptible flocks. 

Clinical Signs 

Highly pathogenic avian influenza (HPAI) results in an acute and precipitous decline in performance with rapidly ascending mortality characterized by both respiratory and nervous signs. Flock morbidity is apparent following exposure to AI virus, but mortality is variable depending on the pathogenicity of the strain.

Mild strains (LPAI) result in low morbidity and mortality, however a decline in performance (decreased laying rate) is often noticed. In contrary, severe symptoms have been reported in Belgium, induced by a LPAI (H3N1) such as discoloring of the eggs, increased mortality (40-60%), severe drop in egg production and feed consumption, increase in secondary infections. 

When infected with AI, Birds are more susceptible to secondary infections such as E. coli and other respiratory pathogens (MG/MS, ND, ILT, IB).

Pathology 

Highly pathogenic avian influenza is characterized by subcutaneous hemorrhages and edema of the head. Vesicles may be present on the comb and wattles. Hemorrhages are observed in the serosa of all viscera and in the mucosa and lymphoid structures of the intestinal and respiratory tracts. Mild influenza results in tracheitis, pulmonary edema and if secondary bacterial infection occurs, airsacculitis is observed. However, in latest cases of LPAI in Belgium, also liver, kidney and spleen congestion, coelomitis and salpingitis is observed. 

Diagnosis 

Diagnosis can by done by RT-PCR assay of organs or swabs.

Treatment and control 

Avian influenza is a notifiable disease listed by the OIE. There is no treatment available. Therefore, implementation of biosecurity measures, in line with OIE international Standards, is the key to control the disease. Putting in place accurate warning systems as well as prevention measures is essential as part of an effective strategy to prevent and control avian influenza.  

Because of the stability of the virus in the environment and highly contagious nature, strict biosecurity measures and good hygiene are essential in protecting against disease outbreaks. Take following points into account:

• Keep poultry away from wild birds

• Remove all elements that could attract wild birds, including poultry feed placed outside the building

• Maintain strict control over access to flocks by vehicles, people and equipment

• Ensure the disinfection and cleaning of property, poultry houses and equipment

• Avoid the introduction of birds of unknown disease status into the flock

• Report any bird illnesses and deaths

• Ensure appropriate disposal of manure, litter and dead animals

• Vaccinate animals if allowed by regional regulations

In some regions, such as in European countries, vaccination against AI is forbidden by law. Contaminated flocks are culled. In other countries, such as Egypt, commercial AI vaccines are available. Poultry flocks are vaccinated against H9 and H5. Those vaccinations do not always induce the required serological response and protection. However, some unpublished research shows the serological response to AI vaccination can be enhanced by in feed supplementation of beta-(1.3)-glucans. In addition, birds receiving beta-glucan did not suffer from performance effects of a LPAI infection.