Wetenschap - 7 januari 1999

Parasites endanger ecosystems and human welfare

Parasites endanger ecosystems and human welfare

Parasites endanger ecosystems and human welfare

Parasites are the forgotten half of the world's huge biodiversity. At the same time they pose a major threat to human welfare, said Andy Dobson from Princeton University


The ecologist spoke last month to a large audience about his research on parasites infecting animals and humans. Dobson was one of the guest speakers of the annual meeting of the Netherlands/Flanders Ecological Society (NEVECOL) and the Netherlands Society for Aquatic Ecology (NVAE), held at the International Agricultural Centre in Wageningen

Dobson believes that both the danger and presence of parasites are is widely underestimated. Dobson argues that ecologists in particular should start investigating parasites since these pose a gigantic ecological problem. A large number of wildlife, cattle and humans, especially those in the Third World, are infested by parasites. Ecologists can make a valuable contribution towards developing techniques to control parasite infections, he says. Surveys in the US have shown that internal parasite infections are widespread in all types of cattle, exceeding 90% in most cases. The vast majority of cattle infected show no clinical signs whatsoever throughout their productive life: losses from parasitic infections are primarily due to subclinical effects including reduced weight gain and reduced milk production

Worms and other parasites are also abundant in small children in the Third World. Dobson: In Trinidad for example, many children are infected by up to eight different kinds of parasite species. The problem is, according to Dobson, that too few doctors specialize in parasite infections. The fact that humans often show no clinical signs contributes to the widespread neglect of the dangers of parasites. Consequently, no preventive measures are being taken. Dobson: We are seeing more and more evidence that parasite infections affect the growth rate of children but do not kill them. Intellectual development is also hampered.

One of the first ecologists to specialize in parasites, Dobson has been conducting research on red grouse infected by nematodes (parasitic worms) in order to develop techniques to control parasite outbreaks. The birds were divided into groups. When the populations crashed due to infection, one group was treated with medicinal drugs to cure them. The outcome of the experiment was somewhat surprising: the birds which received treatment initially got rid of the worms but as the number of grouse increased, more hosts were provided for the worms. Consequently, the worm population recovered. Dobson concluded that drugs had no real effect on the worm population

In trying to find ways of eradicating parasites or building immunity to their attacks, nature can teach us useful lessons, says Dobson. When by the end of the last century almost all cattle and wildebeest in the African Serengeti Park had been wiped out by Rinderpest, some animals managed to outsmart the deadly parasite. Wild dogs which fed from carcasses of infected cattle and hereby apparently vaccinated themselves against the virus

Whether this is applicable to other parasite species and hosts, is highly uncertain. Dobson believes much more research is needed to fully understand and prevent the dramatic effects of parasites on ecosystems and human welfare. H.B

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