Case Study 2
Studying QX disease in oysters and its impact on the Australian eastern seaboard.
ROBERT ADLARD, QueenslandUniversity: The disease I’m looking at in rock oysters is called ‘QX disease’. It was first described in Queensland, hence the ‘Q’. It’s a protozoan parasite, which ends up targeting the digestive gland of the oyster—which is the bit inside the yummy bit you eat—and there it packs out, it proliferates and it starts to sporulatein that particular organ and effectively stops the oyster from absorbing any nutrients, so they starve to death.
When we started the current project, about two years ago, one of the basic things we didn’t know really was where the disease occurred. If you’re managing a disease like this, you want to prevent potentially infected stock moving from one estuary to an estuary where the disease hasn’t been reported. Basic to that, you need to know where the disease is and where it isn’t. That then brings you to, well, how are you going to diagnose it? Ok, we can find it very easily when it’s sporulating and in disease outbreak status. In an oyster it’s not very hard to diagnose at all. But is that the situation you find it in all the time? What periods of the year is it in that kind of development? So these were all questions we were starting to look at when we started the current project. We looked at 22 different estuaries on the east coast of Australia, three in Queensland and the rest in New South Wales. With the help of NSW fisheries, who’s a partner in this project, we ended up getting thousands of oysters into the museum. Our initial diagnosis was done using a very simple and inexpensive test where we had a look at the digestive gland and touched that on a slide, called a tissue imprint method. It’s very cheap, you stain it and you look at it very quickly. However, the sensitivity was an issue, so we developed DNA PCR-type diagnostic assays.
The George’s River is a good example of the kind of impact that this disease can have if it outbreaks in a river. In 1979, the George’s was producing something like $17 million worth of stock. That was at its peak. When I started working the George’s, around the early 90s, it wasn’t as high. It’s a river in the middle of Sydney basically, so there’s a lot of pressure on it and a lot of multiple uses going on there. But it was a very healthy industry in the 90s. In 1994, when I diagnosed the QX disease, there were twenty or so farmers working in the river. Today, you’ll find two brothers who are still working in the George’s and that’s all. Their main source of income is actually removing the oyster infrastructure, all the posts and rails in the river, because effectively the farming in the river is no longer viable.
I don’t think there’s any doubt that if you stress any aquatic animal, then it’s going to be more susceptible to a range of things. We have to look after the estuary, certainly. If you’re growing a product which is often eaten raw, there is some concern in just what it’s growing in.
It’s a very seasonal disease, too. In mid summer if you’re starting to see oysters that are sick or gaping or dead oysters on trays, often dead is probably too late to do any particular tests on. Rather than take samples, I think it should probably be reported. I think it’s probably better, once that report’s made, for appropriate people to go and take samples themselves.
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