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The epidemiology of parasitic diseases

Although the reasons for the occurrence of parasitic diseases are multiple and often interactive, the vast majority occur for one of four basic reasons. These are:

Each of these will be discussed in turn, giving examples.

AN INCREASE IN THE NUMBERS OF INFECTIVE STAGES

This category involves parasitic diseases which occur seasonally, and although more distinct in zones with a wide climatic variation, may also be observed in zones with minor variations in climate such as the humid tropics.

A multiplicity of causes is responsible for the seasonal fluctuations in the numbers, and availability of infective stages, and these may be conveniently grouped as factors affecting contamination of the environment, and those controlling the development and survival of the free-living stages of the parasites and, where applicable, their intermediate hosts.

CONTAMINATION OF THE ENVIRONMENT

The level of contamination is influenced by several factors.

BIOTIC POTENTIAL

This may be defined as the capacity of an organism for biological success as measured by its fecundity. Thus, some nematodes, such as Haemonchus contortus and Ascaris suum, produce many thousands of eggs daily, while others, like Trichostrongylus, produce only a few hundred. Egg production by some external parasites such as the blowfly, Lucilia sericata, or the tick, Ixodes ricinus, is also very high, whereas Glossina spp produce relatively few offspring.

The biotic potential of parasites which multiply either within an intermediate or final host is also considerable. For example, the infection of Galba (Lymnaea) with one miracidium of the trematode Fasciola hepatica can give rise to several hundred cercariae. Within the final host, protozoal parasites such as Eimeria, because of merogony and gametogony, also give rise to a rapid increase in the contamination of the environment.

STOCK MANAGEMENT

The density of stocking can influence the level of contamination and is particularly important in nematode and cestode infections in which no multiplication of the parasite takes place outside the final host. It has the greatest influence when climatic conditions are optimal for development of the contaminating eggs or larvae, such as in spring and summer in the northern hemisphere.

A high stocking density will also favour the spread of ectoparasitic conditions such as pediculosis and sarcoptic mange, where close contact between animals facilitates the spread of infection. This may occur under crowded conditions in cattle yards, or from mother to offspring where, for example, sows and their litters are in close contact.

In coccidiosis, where large numbers of oocysts are disseminated, management procedures which encourage the congregation of stock, such as the gathering of lambs around feeding troughs, may lead rapidly to heavy contamination.

In temperate countries, where livestock are stabled during the winter, the date of turning out to graze in spring will influence contamination of pasture with helminth eggs. Since many helminth infective stages, which have survived the winter, succumb during late spring, the withholding of stock until this time will minimise subsequent infection.

IMMUNE STATUS OF THE HOST

Clearly, the influence of stocking density will be greatest if all the stock are fully susceptible, or if the ratio of susceptible to immune stock is high, as in sheep flocks with a large percentage of twins or in multiple suckled beef herds.

However, even where the ratio of adults to juveniles is low it must be remembered that ewes, sows, female goats, and to a lesser extent cows, become more susceptible to many helminths during late pregnancy and early lactation due to the periparturient relaxation in immunity. In most areas of the world, parturition in grazing animals, synchronised to occur with the climate most favourable to pasture growth, is also the time most suitable for development of the free-living stages of most helminths. Thus, the epidemiological significance of the periparturient relaxation of immunity is that it ensures increased contamination of the environment when the number of susceptible animals is increasing.

There is some evidence that resistance to intestinal protozoal infections such as coccidiosis and toxoplasmosis is also lowered during pregnancy and lactation, and so enhances spread of these important infections.

On the credit side, host immunity will limit the level of contamination by modifying the development of new infections either by their destruction or arrest at the larval stages, while existing adult worm burdens are either expelled or their egg production severely curtailed.

Although immunity to ectoparasites is less well defined, in cattle it develops against most species of ticks, although in a herd this expression of resistance often inadvertently results in an overdispersed population of ticks with the susceptible young animals carrying most of the ticks.

In protozoal diseases, such as babesiosis or theile- riosis, the presence of immune adults also limits the likelihood of ticks becoming infected; however, this effect is not absolute since such animals are often silent carriers of these protozoal infections.

HYPOBIOSIS/DIAPAUSE

These terms are used to describe an interruption in development of a parasite at a specific stage and for periods which may extend to several months.

Hypobiosis refers to the arrested development of nematode larvae within the host and occurs seasonally, usually at a time when conditions are adverse to the development and survival of the free-living stages. The epidemiological importance of hypobiosis is that the resumption of development of hypobiotic larvae usually occurs when conditions are optimal for free-living development and so results in an increased contamination of the environment. There are many examples of seasonal hypobiosis in nematodes including Ostertagia/Teladorsagia infections in ruminants, Hyostrongylus rubidus in pigs and Trichonema spp in horses.

Diapause in arthropods, like hypobiosis in nematodes, is also considered to be an adaptation phenomenon whereby ectoparasites survive adverse conditions by a cessation of growth and metabolism at a particular stage. It is most common in temporary arthropod parasites in temperate climates. In these, feeding activity is restricted to the warmer months of the year and winter survival is often accomplished by a period of diapause. Depending on the extremity of the northern or southern latitudes, this may occur after one or several generations. For example, the headfly, Hydrotoea irritans, in northern latititudes, has only one annual cycle and overwinters as a mature larva in diapause. Other insects, such as Stomoxys calcitrans or blowflies in these latitudes, have several generation cycles before entering diapause. Diapause occurs less in parasites which continuously infect the hosts, such as mange mites or lice.

To date, similar phenomena have not been ascribed to protozoa, although there is one report of latent coc- cidiosis occurring in cattle for which a similar hypothesis has been proposed.

DEVELOPMENT AND SURVIVAL OF INFECTIVE STAGES

The factors that affect development and survival are mainly environmental, especially seasonal climatic change and certain management practices. Current changes in the global climate are anticipated to influence the infective stages of many parasites and/or the prevalence of some intermediate hosts. For example, the trend towards warmer wetter seasons has been one factor attributed to the increase in prevalence of Fasciola hepatica infection in ruminants in some temperate regions.

THE MICROHABITAT

Several environmental factors which affect the microhabitats of free-living parasitic stages are vital for development and survival. Thus moderate temperatures and high humidity favour development of most parasites, while cool temperatures prolong survival. The microclimate humidity depends, of course, not only on rainfall and temperature, but on other elements such as soil structure, vegetation type and drainage. Soil type influences the growth and species composition of the herbage and this, in turn, determines the degree to which a layer of ‘mat’ is formed between the soil and the herbage. The mat is abundant in older pastures and holds a permanent store of moisture in which the relative humidity remains high even after weeks of drought. The presence of this moisture and pockets of air trapped in the mat limit the rate of temperature change and these factors favour the development and survival of helminth larvae, ticks, larval stages of insects and coccidial oocysts.

In contrast, the use of rotational cropping of pastures reduces the influence of ‘mat’ and therefore parasite survival. In the arid tropics pasture growth is usually negligible causing a similar effect.

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