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Pythons and parasites

Using reticulated pythons (Python reticulatus) as part of a rodent control project may sound extreme, but it's an approach on the verge of commercialisation in Thailand. Not that farmers are planning to release thousands of snakes into their fields. But large numbers of farm-bred pythons are being used as living incubators for a rat-killing parasite. Sarcocystis singaporensis, the parasite in question, is a protozoan whose life-cycle depends on the predator-prey relationship of pythons and rats. It reproduces in the intestines of the python, and its spores are excreted in the faeces. Rats become infected by ingesting the spores - most commonly by drinking from water sources that the snakes have defecated in, or by eating invertebrates that have themselves fed on the snake dung. Under normal circumstances this is not a huge problem for the rats. The protozoan multiplies in the blood vessels and forms muscle cysts, tending to make the rats slightly easier to catch, and also causing a small reduction in fertility. But if large numbers of spores are consumed, the damage is much more serious, leading to pneumonia and death in up to 90% of cases.

Bulking up production

Reticulated python
credit: Thomas Jaekel, GTZ

For the last eight years a team of scientists from Thailand's Department of Agriculture have been working with colleagues from German Technical Co-operation and the Bayer Corporation, to research and develop the use of the parasite as a form of biological rodent control. Obtaining sufficient quantities of parasite was the first challenge; attempts to multiply it in cell culture proved unsuccessful, hence the use of pythons. Interestingly, pythons neither suffer from infection, nor develop immunity against re-infection, so can be used for many years of parasite production. Moreover, a single infection, achieved by feeding a snake with an infected rodent, produces an enormous quantity of spores - enough to kill as many as 200,000 rats. The team harvest spores from the faeces of specially farmed snakes. The spores are then purified and concentrated and used to inoculate palatable pellets made from wheat flour, broken maize and other attractants like coconut and fish extracts. Each pellet weighs about a gram, and contains enough concentrated spores to result in rodent death and, more importantly, because the parasite is host specific, the pellets are of no danger to other animals, or to people, if ingested.

The efficacy of the pellets has now been tested in a number of different environments, and with different target rodents, including brown rats in a Thai poultry unit, Malaysian wood rats in an oil palm plantation, and most extensively, ricefield rats (Rattus argentiventer) and Bandicoot rats (Bandicota indica), in Lop Buri district north of Bangkok. Fifty eight hectares of rice-field were treated with pellets over a four month period, and pre-harvest losses to rodents was assessed. The loss figures were compared with two other areas, one where conventional rat control methods were applied, in this case use of zinc phosphide poison and electric fences, and one where no control was attempted. The results indicated that crop losses in the parasite-baited fields were only 0.4%, in comparison to 1.4% in the conventionally protected fields and 5.5% in the unprotected. In all the trials there was a delay of approximately two weeks between setting the baits, and observing rodent fatalities, and during this period there was no observable loss of appetite among the rats. Local farmers asked to comment on the trials cited this as their main reservation about the new control method, but were otherwise very impressed by the potency of the baits. They were also reassured to see that cats, dogs and chickens that came in contact with either the infected rats or the pellets, suffered no ill effects.

Going commercial

The extensive trial in Lop Buri has allowed the parasite pellets to be registered in Thailand as a rodenticide, and already one company, Uniseeds Ltd, is in the process of setting up commercial production. At current estimates, the cost-benefit ratio for using the pellets is approximately 1:3, where damage to rice is in the region of 5%. If production costs can be reduced in the future, the pellets should become a viable alternative to chemical rodenticides.

One difficulty when using micro-organisms as pathogens, is that the target species becomes resistant to the parasite, and in time the control loses its effectiveness. Under natural conditions when a host begins to develop resistance, parasites adapt in order to maintain their status. However, assuming the pellets are being used in areas where the parasite naturally occurs, this process of adaptation will continue. As a result it should always be possible to obtain the most advanced version of the parasite spores for pellet manufacture. This does of course limit the area where the pellets should be used. Sarcocystis singaporensis is widely prevalent in South East Asia, including Myanmar, Malaysia, and much of Indonesia and the Philippines. But for python free - and hence parasite free - areas, other sarcocystis species that live in different host animals might prove useful. One such example, still to be fully researched, is S.dispersa, a parasite that infects barn owls and some species of mice.

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