28 August 1998

Keep up guard against

The doomsday scenario of a

deadly strain of antibiotic-

resistant salmonella now

seems less likely, but

producers cannot afford to

drop their guard.

Peter Grimshaw reports

THIS year could see the recent explosion of Salmonella typhimurium type DT104 declining significantly. This is the strain responsible for most of the well-publicised human infections and for costly outbreaks on many dairy farms.

The good news is that the level of infections in cattle peaked in 1996. It began to decline in 1997, and there appears to be a further reduction so far this year.

Robert Davies, at the Veterinary Laboratories Agency, Weybridge, Surrey, confirms that the number of reported infections in humans has followed a similar pattern.

He says the history of DT 104 is similar to that for other strains of S typhimurium which have occurred among animals in recent years. "The most likely reason for this is that a level of immunity has been reached in the population, particularly in cattle," Dr Davies explains.

Independent veterinary consultant Tony Andrews is not so sure, and he believes that more effort should be applied to discovering the mechanisms behind the characteristic decline in virulence of a new pathogen. "All these organisms appear to go through a natural cycle of rapid increase and then decline," he says. "If we knew why they declined, wed have a better chance of restricting the initial scale of the outbreak."

A relatively recent arrival in the British Isles, probably in the gut of exotic or migratory birds, type DT 104 rapidly took hold in a number of species, and directly or indirectly infected humans. It has caused and still causes quite severe problems for farmers, particularly in cattle, and was fatal to some members of the public who ate contaminated meat.

Farmers rarely overlook Salmonella typhimurium DT 104 because it usually causes quite serious problems when it first turns up in a cattle herd. Characteristically, says Dr Andrews, it usually tends to affect up to 15% of one group of animals, of any age, resulting in acute dysentery and occasionally rapid death, sometimes without any warning signs.

After a short period, scouring and loss of condition gradually ease, although infective organisms may continue to be excreted by individual animals that have recovered. The picture is similar among calves, though possibly with a higher proportion of animals affected, and often with several deaths.

Other animals such as rodents and birds can also be infected, and because they are no respecters of boundaries, they can spread it to other farms. The organism itself is robust and can survive for many months outside a host animal.

Because of the implications for humans, salmonella tends to be better reported than some other conditions.

Any salmonella isolated in a food animal species has to be reported to MAFF under the Zoonosis Order. Health and environmental authorities are also more likely to follow up incidents, and to attempt to trace their sources.

"Luckily, we do seem to have a vaccine that is reasonably effective for this one," says Dr Andrews. "No vaccine is 100% effective, but it seems to work better on the DT 104 strain than it does on some others."

He says a properly conducted vaccination programme can significantly reduce the level of excretion of infective bacteria in a herd. If this is combined with effective cleaning of the farm environment and thorough vermin control, the disease can effectively be defeated.

Most vets have mastered the techniques of control without resorting to blanket antibiotic treatment. This is generally confined to the affected group, with the remainder of the herd being vaccinated.

Once a farm has been apparently clear of the disease for a year or more, farmers may be tempted to think their problems are over, but this could be a mistake.

Dr Davies says continuing the vaccination programme is quite expensive. But he warns that cattle farms that have had an outbreak are very prone to re-infection. "Its quite common for the vaccination to be dropped after a year or 18 months, and for the infection to re-occur after that."

Salmonella typhimurium DT 104 affects all ages of cattle but a higher proportion of calves will be infected, says Tony Andrews.