Report – ZL2102 – Implications of Parasites

ZL2102, 2000-10-11

Implications of Transmission Thresholds of Parasites


For any invading species it is useful to distinguish between three distinct phases of the invasion process: getting there, becoming established and spreading. However, for parasites’ establishment and maintenance thresholds are fairly clear and sometimes measurable in practice (Dobson & May, 1986).

In spite of the advances in the prevention and treatment of parasitic infections, diseases caused by parasites even today are more common than any other kind of disease, particularly in subtropical and tropical areas and countries (Rohde, 1993).

This paper will discuss transmission thresholds for invading species to new ecosystems.

Direct vs. Indirect Lifecycles

The parasites can have direct or indirect lifecycles. In direct life cycles there is no intermediate host(s) and the parasites occur on or in the definitive host from the beginning, where sexual reproduction occurs. These parasites usually have a greater reproduction potential than the host does. In parasites with indirect life cycles there can be up to six vectors. The vectors are the sites for asexual reproduction of the parasite (Dr. D. Barton, School of Tropical Biology, J.C.U. 2000 pers. comm.).

Micro vs. Macro Parasites

Malaria is a micro parasite with an indirect lifecycle. Because of the indirect lifecycle the parasite tends to have a higher threshold due to the dependence on its vector/s. If the conditions are not suitable for the vector/s it will become much harder for the parasite to establish a sustainable population.

In contrast, parasites like hookworms, that are macro parasites and have direct lifecycles, do not have any vectors to depend on so it is therefore much easier for them to establish a viable population

For any invading species it is useful to distinguish between three distinct phases of the invasion process: getting there, becoming established and spreading. It is very hard to determine the exact factors for any animal or plant species’ minimal critical size (referred to as a threshold) for the population to become established and to maintain itself. However, for parasites’ establishment and maintenance thresholds are fairly clear and sometimes measurable in practice (Dobson & May, 1986).

Thresholds host densities are given with a formula that differs slightly depending on the circumstances, the simplest formula is given here as Caughley and Sinclair (1994).

R0 = (a + b + n ) / Xb )

R0 = net reproductive rate of parasite or pathogen.

X = host density.

a = disease induced mortality rate.

b = mortality rate from other causes.

n = recovery rate.

b = measure of intensity of transmission.

If the transmission threshold is equal to 1, then the parasite population is maintaining itself and the epidemic. If the transmission threshold is less than 1, the disease will decrease and the parasites will die out and lastly if the transmission threshold is more than 1, the parasite population and epidemic will flourish and spread (Caughley and Sinclair, 1994).

Mechanisms that influence the maintenance and spread of the disease are due to traits of both the parasite and the host, particularly by the mortality rate (a ) of the host caused by the disease and also the reproductive rate of parasite or pathogen (R0). (Dobson & May, 1986; Caughley and Sinclair, 1994).

Parasites with a direct lifecycle have lower transmission thresholds than do parasites with an indirect lifecycle. This is due to the obvious fact that parasites with direct lifecycles do not have any vectors and therefore they live anywhere where suitable hosts live.

On the other hand, parasites with an indirect lifecycle tend to have higher transmission thresholds, as they are dependent on one or more vectors to complete their lifecycle and it is therefore harder for them to establish a sustainable population in a new ecosystem. If, however, the parasites do establish within a new ecosystem a long lived vector can retain eggs in their tissues and act as some kind of buffer (protection against extinction) for the parasite to wait for better circumstances to be able to spread (Caughley and Sinclair, 1994).

A good example of this is schistosomes where the parasites penetrate and develop within a snail and emerge out into a freshwater habitat where they penetrate human skin and are carried finally to the bladder or gut (Caughley and Sinclair, 1994).

The parasites can remain endemic to the area, despite wide human extermination attempts on the snails causing fluctuations in the snail population, due to the longevity of humans and the snails enormous reproductive capacity, schistosomes can stay inside human tissues until enough snails are present (Caughley and Sinclair, 1994).

Social unrest, e.g. wars and crises.

20-40 Million people are estimated to be refugees or displaced in other ways. Over 80% of these live in environments where, vector-borne diseases are endemic and health services are disrupted and any health input is usually provided by international emergency resources.

The main threats to refugees or displaced people are lack of immunity, movement of migrating/fleeing populations through areas with endemic vector-borne diseases, settlement in such areas, loss of cattle and other livestock (which can cause the vector to change the behavior and start feeding on humans), stress and malnutrition – which affect the immune system negatively (Molyneux, 1997)

The socioeconomic standard

Social unrest usually causes instability resulting in disruptions of all governmental sectors including health, reduced support from Non Governmental Organizations (NGO) and also causes the private sector an inability to support (Molyneux, 1997).

Most common infectious vector-borne diseases in unstable and in countries with low socioeconomic standard are malaria, typhus and relapsing fever (Molyneux, 1997).


Recent studies suggest that the effects of the climate fluctuations caused by El Niño Southern Oscillation (ENSO) is related to the periodicity of malaria. Correlation has also been found between the anomalies in sea surface temperature and malaria in east pacific. (Molyneux, 1997). The impact of the now so famous topic, global warming and global environmental change, would have a positive effect on the increase of malaria. Areas to be affected worst will be areas that fringe to the infection, such as the highlands of Ethiopia, Madagascar and Kenya (Lindsay and Birley, 1996).

Increased temperature and rainfall are major factors for a disease to spread, one examples of this is malaria in Papua New Guinea, where after an extra wet wet-season the mosquitoes breed and cause a major epidemic from the normal endemic malaria population. Another example occurred during 1995-1996 when the Townsville area also had an extra wet wet-season and the Ross River Fever spread and caused an epidemic in the Townsville area where the infection normally is endemic (Dr. D. Barton, School of Tropical Biology, J.C.U. 2000 pers. comm.).


The effects of deforestation on intermediate host- borne diseases have had lot of attention from researchers. What has been found is that all major vector insect groups are affected by deforestation. In South America considerable impact of deforestation has been shown to cause epidemics of malaria, the leishmaniases, shistosomiasis, loaiasis and Chagas disease (Molyneux, 1997).

Water Resources

Major problems in poorer countries are damming up rivers to be able to extract drinking water and to collect fresh water for irrigation, which have increased and intensified the outbreaks of shistosomiasis. This happened in the Senegal River Basin in West Africa after the completion of the Diama Dam at St Louis. Other research in Mali has showed, upstream from the dam construction there, that intestinal and urinal shistosomiasis had become more frequent than malaria that used to be endemic in the area (Molyneux, 1997).

Another aspect of water use and transmission is rice cultivations that, throughout the tropical world, usually lead to an increased risk of shistosomiasis. In South-East Asia the risk also exists for Japanese B Encephalitis (JBE); particularly if there is a pig population adjacent to the rice cultivations which acts as a host reservoir of JBE (Molyneux, 1997).


Caughley, G. & Sinclair, A.R.E. (1994) Wildlife Ecology and Management. Blackwell Science, Cambridge, USA.

Dobson, A.P. & May, R.M. (1986) Patterns of Invasions by Pathogens and Parasites.In Mooney, H.A. & Drake, J.A. (Eds) Ecology of Biological Invasions of North America and Hawaii. pp. 58-76, Springer Verlag, New York.

Lindsay, S.W. & Birley, M.H. (1996) Climate change and malaria transmission. Annals of Tropical Medicine & Parasitology. 90, 573-588.

Molyneux, D.H. (1997) Patterns of change in vector-borne diseases. Annals of Tropical Medicine & Parasitology. 91 (7) 827-839.

Rohde, K. (1993) Ecology of Marine Parasites. 2nd Ed. CAB International, Wallingsford.

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