Longer survival time means lice must be hit hard
By Hannah Velten
CHEWING lice can live off their hosts for 15-16 days, rather than the assumed few hours, and lice may inadvertently help spread ringworm spores.
Both theories highlight the need to treat sheep and cattle against lice before housing, says Peter Bates, senior entomologist at the Vet Lab Agency (VLA), Weybridge.
The survival of chewing lice, reported in the Australian Veterinary Journal, has major implications for bio-security, says Dr Bates. "Where lousy sheep/cattle have been housed or transported in lorries, clean stock following them could easily become infested. Lice are the last thing producers need in housing."
David Harwood of VLA Winchester shares similar concerns. "Bringing animals into a herd could also introduce lice, ringworm or mange. To be safe, vets should clinically inspect new arrivals, take fleece/hair samples and decide on a treatment/preventative regime."
New stock must be kept in a separate airspace and in buildings 3m (9ft) away from stock on farm, he adds.
Lice infestation causes extreme irritation to stock and usually occurs in animals that are already in poor condition. "Lice take advantage of debilitated groups. Producers must search for the underlying problem, such as poor nutrition or underlying disease such as pneumonia or coccidiosis," says Mr Harwood.
Cattle are at risk from blood-sucking, as well as chewing lice, and infestation can kill calves and cause invisible damage to leather, reducing its quality, adds Dr Bates. "If producers got returns for leather quality, treatment against ectoparasites would undoubtedly increase."
Although louse populations peak between January and March, the parasites start to multiply during early winter. "The chances of eradicating lice are greatest when treatments are used on fewer lice at the beginning of housing," advises Dr Bates.
Sheep usually attract chewing lice, which can be killed off by either a syntheic pyrethroid or organophosphate dip. "Dipping should ideally be completed two weeks before housing. Treatment just before housing increases the risk of pneumonia, as hot, wet sheep create damp air.
"When sheep are shorn or clipped prior to housing, an SP pour-on treatment is sufficient, as shearing removes a large proportion of lice," he adds.
Mr Harwood says chewing lice will survive injectables targeting sheep scab. "When treated sheep are still itching, chewing lice could be the cause. Take a sample of fleece and skin debris to be sent to the lab for identification," he advises.
For cattle, an SP pour-on will kill both types of lice, but when blood-suckers are identified a systemic injectable can be used, advises Dr Bates.
"When possible, cattle should be treated before housing to allow lice to die."
Young cattle bought in from grass which are usually wormed can be treated with an avermectin pour-on, which controls both worms and sucking/chewing lice, says Mr Harwood.
Dr Bates and his French counterparts believe lice also help ringworm spores to find a host and possibly act as carriers of ringworm between cattle. "Scratching against walls and timber to relieve the irritation caused by lice increases contact with ringworm spores, which can survive for years in housing environments. When lice are absent, cattle are less likely to rub."
As with lice, ringworm in a group of calves will indicate animals are debilitated, which must be investigated, warns Mr Harwood.
"Many farms live with a low level of ringworm infection which does not cause animals any undue harm. Ringworm infection will naturally regress over time as the animals immune system takes over, not because holly is hung in the buildings.
"But when ringworm becomes more widespread or causes animals to become poorly and loose condition, call the vet in," he advises.
A wash or spray can be used to treat infected stock, but tough crusts may prevent the preparation getting to the root of infection. "Do not handle infected animals without gloves on," warns Dr Bates.
Mr Harwood says producers have an obligation to treat or prevent ringworm under Health and Safety legislation because it is a zoonotic disease. "Ringworm infections can be serious, particularly in young children handling calves, when spores contact faces and hands." Ringworm in humans causes itching and pain when it penetrates muscles, adds Dr Bates.
Vaccination against ringworm is the best way to prevent outbreaks on farms known to have a problem, advises Mr Harwood. "Programmes should be complete before calves are exposed to the risk of ringworm, such as at housing or when calves are mixed after weaning." *
Lice infestation is increasing and lice may be spreading ringworm spores, says Peter Bates.
Louse picture reproduced by kind permission of Pfizer.
• Treat before housing/mixing.
• Bio-security important.
• Humans at risk from ringworm.