The researchers studied the reaction of carp to the single cell parasite Trypanoplasma borreli. A related parasite causes sleeping sickness in humans. Fish farmers are far from happy about the parasites because they can cause epidemics which result in fish mortality.
Each day the researchers took the same carp out of an aquarium containing parasites. The carp received an injection of a placebo and were then put back in the aquarium. The injection imitated the vaccinations which fish receive in fish farms. The carp were taken out of the water for a total of one minute per day.
None of the stressed carp survived the more than 40 days. Six of the ten carp who were left alone did survive however.
During the stress study the biologists discovered how the stress hormone cortisol brings about a poor defence against the parasite. Stressed carp produce extra cortisol. In carp, cortisol inhibits four crucial proteins which normally ensure the protection of the fish. In addition to this, cortisol ensures that certain protective white blood cells die. Cortisol also reduces the quantity of antioxidant in the cells, which renders the fish more susceptible to their own immune system.
In turn the parasite ensures that the carp immune system is so highly activated that the carp dies as a result of this. The parasite stimulates the carp to produce an excessive amount of nitrogen monoxide. This oxidant is toxic for the majority of intruders, including the parasite. However, the carp produces so much that the cells which must clear up the intruders are weakened. The parasite can then reproduce unhindered.
The vast majority of carp, more than 90% of the world production, are reared in China. That is also where the majority are consumed. The Dutch are less enthusiastic about carp, in part because it has so many bones. In the wild about 80% of carp are infected with the parasite. The majority of wild carp can cope with this.
Further information can be obtained from Dr Jeroen Saeij (Cell Biology and Immunology Group, Wageningen University), tel. 31-31-748-3708 or 31-31-748-2732 (Dr Geert Wiegertjes), fax 31-31-748-3955, e-mail jeroen.saeij@celb.edc.wau.nl. The doctorial thesis was defended on 4 September 2002. Dr Saeij's supervisors was Prof. W.B. van Muiswinkel.
The research was funded by the Netherlands Organisation for Scientific Research (NWO).