AGRICULTURE NEWS - Fowl typhoid is a disease of chickens caused by a host-specific serovar of Salmonella – Salmonella Gallinarum (SG).
Outbreaks of SG can be very severe, in the worst cases almost the entire flock can die over a period of a few months.
Unfortunately, outbreak control is a difficult and poor long-term response to antibiotics is typical. For this very reason, prevention through good biosecurity measures, including vaccination, is crucial.
Causative Organism
SG is one of the few host-specific Salmonellae. This simply means that it currently only infects avian species and poses minimal risk to human health. SG is adapted to a non-intestinal environment and does not have flagella, making it a non-motile organism.
Diagnosis
A suspected diagnosis of SG can be made based on post-mortem lesions – these are seldom conclusive, however. The liver may show a bronze discolouration as well as some mottling due to pin-point bleeding. The spleen is typically enlarged, this may be severe – this normally marble-sized organ can reach the size of a golf ball! White mottling of the spleen is a common finding.
A definitive diagnosis requires a culture of the organism, this is usually best done using cotton swabs from the liver. Serological tests are not very useful in SG diagnosis.
Responsible Control Measures
SG is a notifiable disease in terms of the Animal Diseases Act and an outbreak must be reported to the responsible state veterinarian.
In most cases, state veterinary services will not cull the affected flocks and will leave most of the work controlling the disease to the affected farmer.
The ideal approach to SG outbreaks would be to cull affected flocks and to dispose of them appropriately where the carcasses cannot be accessed by people or scavenging animals.
The carcasses do not pose a direct health threat to people or mammals but there is a risk that the disease can be spread from carcasses that are moved from burial sites to wherever they are consumed.
Destruction of large numbers of birds is of course very expensive and in the absence of compensation, other approaches have to be followed.
Responsible disposal of mortalities during an outbreak remains crucial to preventing the spread of the disease.
Treatment
Most strains of SG are usually sensitive to a range of antibiotics – this can be confirmed by antibiogram testing.
Antibiotic treatment, however, is typically only moderately effective. Mortality is usually suppressed while the birds are on treatment but returns to the rate before treatment quite quickly once treatment is withdrawn.
Vaccination
Both modified live, and inactivated vaccines are available against SG.
To be honest, all have their limitations in terms of efficacy.
Live Vaccines
Two live vaccines are registered specifically for use against SG in South Africa – both contain the SG9R strain of SG which is genetically modified to reduce its ability to cause disease symptoms in chickens.
The greatest difficulty with both these vaccines is that the protection in vaccinated birds lasts only about 12 weeks and where challenge loads are high it becomes necessary to revaccinate birds in lay (by injection) every 2-3 months to avoid further outbreaks.
Because SG belongs to the same group of Salmonella (Group D) as Salmonella Enteritidis (SE), use of SE vaccines will also give some protection to birds against SG challenge. There are live and inactivated SE vaccines available.
Inactivated Vaccines
Presently only one inactivated vaccine including SG is registered for use in South Africa – Ventri Salmonella Polyvalent Vaccine (Afrivet) – which also contains SE and ST (Salmonella Typhimurium) antigens.
Autogenous SG vaccines are available and are used by some producers.
Inactivated vaccines will generally offer longer-lasting immunity (up to 30 weeks duration is claimed) than the live vaccines.
They will also give better humoral immunity than live modified vaccines and thereby possibly better protection against systemic infection than modified live SG vaccines. They are unable to colonize the GIT and offer more limited local immunity than the live vaccines for that reason.
Transmission
SG is present in large quantities in the manure of infected flocks. This means that farm hygiene is critical for the success of SG eradication programmes.
SG outbreaks in Hi-rise commercial laying farms where manure is allowed to accumulate beneath sheds for long periods of time are particularly difficult to manage as birds keep getting re-exposed to the disease from the manure pile.
Rats, wild birds and even red mites also spread the disease between infected sheds.
The movement of infected hens (such as cull hens) is a serious risk. Because diarrhoea is a common clinical sign of SG infection, the number of dirty eggs on a farm will typically increase, dirty eggs are also a potential source of infection for pack stations.
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