Sep 19, 2013
Not only is Newcastle Disease (NCD) one of the most important diseases in commercial poultry, it is also a notifiable disease in South Africa and therefore, the state has to be informed of any infection or even suspected infection. This disease, which is caused by a virus, was first identified in 1929 in the city of Newcastle, United Kingdom, hence the name. It is thought, however, to have originated in Indonesia.
The challenge when it comes to identifying and treating this disease lies in the fact that there are numerous strains or types of NCD. These different strains target specific organs in the body and cause damage to certain areas of the body. Some strains of NCD target the respiratory tract, whilst other the intestinal tract and some attack the central nervous system.
Depending on the level of damage caused by NCD, the strains are called lentogenic, mesogenic and velogenic. Lentogenic being the mildest of the strains. In some cases, the mild strains are used in order to manufacture vaccines against the more damaging strains.
How is the disease spread?
The virus is released from the respiratory and intestinal tracts of infected birds. The virus then spreads to adjacent birds. The disease is spread in a number of ways. It could be spread by birds coming into contact with the eyes of an infected bird or by being breathed in. It can also spread through contaminated feed and water. Once this transference has taken place, the disease then multiplies in the new host. The time is takes for the bird to get ill and start spreading the disease is 4 days from exposure.
Depending on the strain, the bird will start to show signs of infection. If the respiratory tract is infected, the birds will show signs of conjunctivitis and or a snick. If the intestinal tract is targeted the bird will have diarrhoea. If the bird has a bobbing tail or a twisted neck, the nervous system has been challenged.
Depending on the strength of the virus, the percentage of mortalities can vary from marginal to over 90%.
In South Africa, the current strain of NCD is velogenic and it targets the respiratory system. Mortalities are high in flocks that have either not been immunised or have been poorly immunised. In some cases, even flocks which have been vaccinated will show mortality. This is mainly caused by a secondary infection of E. coli bacteria targeting the damaged respiratory tract caused by the Newcastle virus.
Diagnosing Newcastle Disease (NCD)
In the old days, samples were taken for virology and attempts were made to grow the virus in eggs. This process took days and thus its effectiveness in the field was of limited value.
Blood samples taken from live birds (all from the same house) can determine antibody levels against NCD. The best way is to take a second set of blood samples 2 weeks later (called paired samples). Again due to the length of time involved this is an impractical.
Presently, the most effective way to diagnose NCD is to take a sample from the trachea or intestinal tract for a test called PCR (Polymerase Chain Reaction). The sample is taken using a cotton swab which is then either sent to the laboratory as it is or it is smeared onto a specific paper called FTA paper. The process involves identifying bits of DNA from the virus involved. In this case NCD viral DNA.
This test is also effective as it allows one to identify and differentiate between a vaccine virus and a field challenge virus. If the presence of a field virus is confirmed, the laboratory will have to inform the farm’s veterinarian, the State Service as well as the State Veterinarian. The farm’s veterinarian together with the State Vetreinaian will then be able to work out a plan going forward.
Preventing Newcastle Disease
Two words come to mind when one looks at the prevention of Newcastle Disease; immunisation and biosecurity.
Proper vaccination and immunisation is critical. Correct vaccine handling, reconstitution and administration will provide immunity to the flocks. There are number of pharmaceutical companies all offering various NCD vaccines. They will all work providing they are administered correctly. Eye drop vaccination is the most effective route followed by sprays and then water application.
In terms of biosecurity, one must ensure that preventative measures are taken to prevent diseases from getting onto farms.