June Kivi, of Wellington, asks :-

A TV expose showed video footage from a pig farm where rats were living with the pigs, and carcasses of dead pigs were left with the living pigs. Apart from the animal welfare issues, this kind of farming practice raises also human health issues. Could you please explain what is the chance of getting Trichinosis and/or Creutzfeld-Jacob disease ("mad cow disease") when consuming meat from the pigs living in these kind of conditions?

Our concern is raised by the following facts: 1. Trichinosis: A typical life cycle for T spiralis involves humans, pigs and rodents. 2. Cannibalism among farm animals may be related to Creutzfeldt-Jacob disease (mad cow disease), and in pigs Trichinosis.

Callum Irvine and Bruce Welch, veterinarians with the New Zealand Veterinary Association, responded.

CJD is not believed to be present in New Zealand and even in countries where it exists it has not been associated with pigs.

Trichinosis has been diagnosed in New Zealand but there have been very few cases in recent decades. Recent surveys done in both the North and South Island in commercial pig farms producing exported pork did not find the parasite to be present. In NZ it is thought to be associated with some wildlife species and possibly in smaller "para-commercial" pig farms. Commercial farms don’t provide an ideal mechanism for the parasite to establish and perpetuate itself.

Rats could infect pigs if the pigs ate infected rats (unusual - pigs fed ad-lib have a much feed as they require) or if they ate rat faeces, although the latter route of transmission is not as likely as rat faeces are only infective for a short time after they get infected with Trichinella. Also the rats themselves would have to become infected with Trichinella which can't happen on a negative(not infected) farm and doesn’t happen easily where pigs are positive (infected). All farms have rodent populations varying from very small to large, but despite that Trichinella still hasn’t been found in commercial pig farms in recent times. Thus while we couldn’t say there is no risk we would say the risk was small, and if pork from an infected farm was cooked adequately the risk to consumers would be removed.

For a person to become infected with Trichinella the following would have to occur :-

1. The parasite enter the commercial farm (hasn’t happened yet to our knowledge).

2. Rats become infected with Trichinella from eating dead pigs (would have to entail poor housekeeping on the farm) which then in turn infect the pigs, or else pigs pass it between each other which doesn’t happen easily.

3. The pork from infected pigs is eaten without being cooked properly.

New Zealand's health system is good and would be likely to diagnose human cases of Trichinella if they occurred here. We can’t say that there's no risk but the risk is certainly small. That doesn’t decrease the importance of good rodent control on all farms