Small but DEADLY
W hat to know about common poultry diseases
Infections and poultry diseases are caused by several microorganisms, namely bacteria, viruses, protozoa and fungi. If the bird has a good immune system, it can resist low grade invasions of these organisms. If the bird comes under stress which often lowers immunity, the bird becomes susceptible. These organisms then multiply rapidly in the bird and sickness follows.
Disease-causing bacteria are found in large numbers in the poultry house. Once they invade the bird, they multiply rapidly through cell division. One cell divides into 2, 2 cells divide into 4, 4 divide into 8 and so on. Bacteria can survive and multiply outside of the bird providing there is food and the conditions are right. They multiply under laboratory conditions forming colonies. It’s the patterns of the colonies
they develop which helps identify the bacteria.
Some bacteria produce the condition of the disease in the bird. Other bacteria just lower the bird’s resistance, so allowing other bacteria to invade the bird and produce disease. Bacteria produce toxins which are antagonistic to the bird.
The greater the number of bacteria, the larger the amount of toxin produced until the bird dies. These toxins are of various sorts and affect the bird in different ways. The virulence (pathogenicity) of these organisms is a measure of how fast it produces a disease. The more virulent, the faster the disease develops. This virulence may vary within a species of bacteria. The bacteria within the bird may not multiply very rapidly until conditions are right and then it explodes, multiplying rapidly so producing large amounts of toxin. This is especially so on multi-aged farms where the virulence will increase as the disease goes through each batch of birds.
The bird must be in a susceptible state for the disease to become established and the bird has various mechanisms to help prevent invasion. The most important of these is keeping the bird stress free. Good management in ensuring good temperature control, good feed, good housing and correct stocking densities, etc. This ensures that the immune system does not weaken so making bacterial outbreaks less likely. A healthy skin free of cuts and scratches will keep bacteria out. The cilia and the mucous on the membranes in the respiratory tract will also rinse bacteria away. That’s why it’s essential to keep ammonia levels down as ammonia will destroy the cilia.
Climate and seasons also play a role in that some diseases affect birds more in cold weather and some more during hot weather. Some species of birds are more resistant to certain bacteria than others. The bacteria will invade a type of bird and grow and multiply without producing any evidence of the disease but in another type of bird be devastating to its health. The age of the bird can influence its susceptibility to certain bacteria. With this in mind, good management is the most important factor in preventing disease outbreaks.
Viruses are the smallest of the microorganisms that cause disease and can only be seen with an electron microscope. Most can only survive for a few hours outside
of the body. They are killed by direct sunlight, heat and some disinfectants. They cannot multiply outside of living tissue and can only multiply within a host cell. They cannot be treated with antibiotics. Vaccination is the only way to prevent viral diseases. Each vaccine is specific for each virus i.e. vaccination for Newcastle disease will only be effective against Newcastle disease.
These are single celled animals which are parasitic in poultry by living on the contents of the cell. They have a complex life cycle. An example of this is Coccidiosis.
Fungi are a group of organisms which include moulds and yeasts. They grow outside of the bird and can be ingested or inhaled by the bird. They produce toxins in the bird.
Viral Diseases Newcastle Disease (NCD)
This is caused by a virus of which there are four forms: Lentogenic (mild pathogenicity), Mesogenic (intermediate pathogenicity), Velogenic or Neurotopic (high pathogenicity), Viscerotopic Velogenic (very high pathogenicity). The mild form is used as a vaccine, the moderate is often seen in birds that have not been properly vaccinated, and the very virulent form can cause extremely high mortalities in unprotected flocks.
The virus can be transmitted through the air over reasonably short distances. Coughing dislodges the virus from the respiratory tract and this is then carried in the air from bird to bird. The virus can also be carried by mechanical means, such as on clothing, equipment, trucks, etc .This is probably the most common form of transmission and can be prevented by strict biosecurity. Short rest periods and multi-age sites often result in the older birds re-infecting the younger birds. Viruses can→
also be spread by wild birds, neighbouring poultry, or even vermin like rats and mice.
The incubation period is 4 – 6 days and the spread of the disease is very rapid. Symptoms are respiratory difficulty, twisted necks and diarrhea. The tail feathers by the Cloaca are often stained green from Bile as the birds stop eating. Egg production drops and eggshell quality is reduced. Upon postmortem, hemorrhaging of the proventriculus is seen and lesions are sometimes found in the respiratory tract. In the picture the hemorrhaging of the proventriculus can be clearly seen.
There is no treatment for this disease. Birds can only be protected by vaccination. It is best to consult with a veterinarian to design a vaccination programme for your area and circumstances. Vaccination techniques need to be good to achieve titer levels that will resist a field challenge. Titer is a measure of antibody production or immunity.
Gumboro Disease/infectious Bursal Disease (IBD)
This is a viral disease which attacks the immune system of the chicken. In young birds the Bursa of Fabricius is mainly responsible for producing antibodies which kill off disease causing pathogens. This organ can be found next to the Cloaca and when cut open has folds in it. This disease attacks this organ thus severely affecting the bird’s immune system. Incubation period is about 2 days. In a very virulent form, it can cause high mortality but normally Coli septicaemia follows on from the disease. There is no cure, prevention is by vaccination, but antibiotics are often used to prevent secondary bacterial infections.
The disease is confined to younger birds. Birds become morbid, stop eating, have ruffled feathers and can have whitish diarrhoea. Birds can often be seen picking at their vents.
On post mortem when the skin is peeled back some haemorrhaging can be seen on the thigh muscles and sometimes the breast. The Bursa is swollen and when cut open haemorrhaging can be seen. It is best to send birds to the laboratory for a positive diagnosis. Note the swollen Bursa of Fabricius.
Control should rather be by prevention so biosecurity is very important. The virus is capable of remaining viable outside of the chicken for several months so proper washing and disinfection of houses becomes very important. It is mainly spread by people and on equipment.
There is no treatment for this disease as it is a viral disease, but antibiotics are often given for secondary infections. Parents are→