About the Parasite
Hydatids is caused by the tapeworm Echinoccoccus granulosus which lives in the gut of dogs. Its life-cycle also involves an intermediate host, which in the case of New Zealand is mainly sheep and, to a lesser extent, cattle. Pigs and deer are rarely involved. Humans are also a host.
Dogs become infected by eating fertile hydatid cysts in raw offal of sheep (liver or lungs). Sheep become infected by grazing on pasture contaminated with tapeworm eggs passed in dog faeces. Neither dogs, nor intermediate hosts, develop clinical signs following infection, so it's difficult to determine whether or not live animals are infected.
Humans can also become infected by contact with infected dogs, and control programmes all over the world are implemented on human health grounds. Dogs are most likely to infect their owner or owner's children.
What effect does hydatids have on humans?
The effect on people can vary from no symptoms to severe illness and death, depending on the number of cysts formed and their site and size.
Generally speaking, the formation of cysts in the body is dangerous and their surgical removal is never straightforward. A major concern during surgery to remove cysts is that the cyst may rupture and spread the disease throughout the body.
Detection of Hydatids in Animals
The only way to detect infection in sheep and cattle is by examination of the offal of slaughtered animals. This occurs in all abattoirs and export meat works in New Zealand, and any sign of hydatids cysts results in laboratory tests being carried out. When infected animals are detected, they are traced back to the property of origin, allowing investigations to be carried out and, if necessary, animal movement controls to be put in place. Investigations can be difficult because the slow growth of cysts in sheep and cattle (as with humans) means that cysts detected at slaughter may have been the result of infections that occurred many years earlier. In the intervening time the animals may have changed owners several times.
History of Hydatids in New Zealand
The parasite was probably introduced to New Zealand with sheep imported from the United Kingdom in the 19th century. It was first identified in humans in New Zealand in 1862. By the first quarter of the 20th century it had been recognised as a significant human health problem.
The first attempts at control were made in 1938, but efforts lapsed during the war years. In the 1950s farmers established local, voluntary, hydatids control committees. The passing of the Hydatids Act in 1959 and the setting up of the National Hydatids Council marked the start of the national control programme that continues today. Funding was largely through a tax on dogs that was added to the dog registration fee and most of the field work was carried out at the district council level. The control programme continued under the Dog control and Hydatids Act 1982 until the Biosecurity Act took over in 1996.
At the beginning of the programme, up to 80 percent of adult sheep carried hydatid cysts and approximately 10 percent of dogs were infected with the tapeworm. Initial control efforts were inspection at slaughter, the regular testing of dogs at dog dosing strips and educating dog owners about the hydatids life cycle and the risks to humans. The mass dog dosing with anthelmintics that started in the 1970s caused a very dramatic decline in incidence. By the mid-1990s the finding of any cysts, fertile or not, was considered an unusual event that warranted property investigation.
Since fertile cysts were found in 1995/96 from three sheep off Arapawa Island, no cysts have been detected in any sheep at post-mortem. From 1996 to 1998, individual cysts in cull cattle were found from one property per year, indicating infection a long time previously. In 1999 and 2000 cysts were detected on one property in cull cows that had been imported from Australia as two year olds in 1994.
However, from 2001 onward no cysts have been found in any animals at slaughter, which each year comprises about 26 million lambs, almost six million adult sheep, 3.5 million cattle and about half a million deer.
For further technical information, see the following articles written by Howard Pharo, National Adviser, Risk Analysis, MAF Biosecurity Authority: 'New Zealand declares 'provisional' freedom from hydatids', Surveillance magazine, Volume 29, No. 3, issue date 20 September, 2002; and 'The long road to Hydatids freedom in New Zealand, Vetscript, 1 October 2002.