The helminth parasites of sheep and cattle are classified into nematodes, trematodes, and cestodes. In everyday language these are roundworms, flukes, and tapeworms.
Climate is the major factor determining the development of free-living parasites, and it affects different species of parasites differently. It also affects the survival of the parasites; for example, as temperature varies from warm to cold, and levels of soil and plant moisture vary from moist to dry, populations of the free-living stages of parasites fluctuate. The differential effects of climate for different species of nematodes explains why haemon-chosis is the common parasitic disease in summer-rainfall areas and oster-tagiosis is the common one in winter-rainfall areas (Cole, 1986).
Most helminth parasites undergo a period of development outside the host before becoming infective for another host. During this time in the outside world, the rate of their development and chances of survival are influenced by climatic and other environmental factors.
The rate of development of the extra-host stages tends to rise with increasing temperature. Moisture is required for survival, and extreme desiccation is usually lethal. In the wetter parts of the tropics, extra-host stages are seldom exposed for long to the destructive effects of desiccation, and temperatures are commonly optimal throughout the year. Under these conditions, the survival rate of parasitic forms outside the host is high; they develop rapidly to the infective stage and large populations are established. Even where prolonged dry seasons alternate with very short wet seasons, the extra-host stages of many endoparasites can take full advantage of the warm, wet conditions associated with the latter, while the adult stages which subsequently develop within the host are well protected and safely carried over to the ensuing dry season.
Roundworms or nematodes generally cause major trouble to livestock in tropical areas. The level of parasitism in grazing animals depends to a large extent on the numbers of free-living stages on the pasture. Climate and pasture management affect the survival of these free-living stages of gastrointestinal nematodes. Roundworms or nematodes require oxygen, warmth, and moisture for their development and completion of their full life cycles. They are susceptible to desiccation. In areas where the dry season is prolonged and severe, pastures are often free of infection long before the beginning of the rains. In such areas worm infection is carried over from one wet season to the next by infected carrier animals.
In Australia, the most important parasite in the summer-rainfall region is Haemonchus contortus, while in the winter-rainfall region, the most important are Ostertagia spp., Trichostrongylus spp., and Chabertia ovina (Cole, 1986).
The relationship between climate and gastrointestinal parasites is well established. Temperature is negatively correlated with the level of pasture infectivity (except for Trichostrongylus), and rainfall is positively correlated with pasture infectivity. The number of larvae of gastrointestinal nematodes recovered per kilogram of grass in the grazing lands of Mexico in the dry season was significantly lower than during the rainy season (Hernandez, Prats, and Ruiz, 1992).
Examination of the pattern of pasture contamination and the influence of some climatic factors on the development of ovine trichostrongyles in dry pastures of central Spain revealed two peaks of pasture contamination, from midwinter to early spring, and from midautumn to early winter (Romero, Valcarcel, and Vazquez, 1997).
In the Western Australian Mediterranean climate of hot, dry summers and cool, wet winters, the relative prevalence of Trichostrongylus vitrinus, T. colubriformis, and T. rugatus in sheep are closely correlated with the weather conditions of the region. The prevalence of T. colubriformis was positively correlated with the mean autumn, winter, and spring temperatures. There were suggestions of an association between the amount of rainfall of a locality and prevalence of T. colubriformis. The prevalence of T. rugatus was not correlated with the temperature of any season but was negatively correlated with the mean annual rainfall and length of growing season of a locality (De Chaneet and Dunsmore, 1988).
Fluke or trematode infections are found in all species of domestic animals, including poultry, but cattle and sheep are the principal victims. Flukes are prevalent in most animal and sheep grazing areas of the world.
Fluke commonly use a water snail as an intermediate host. Infection with this species is associated with stock grazing in land that may be infested by migrating snails. The level of infestation with liver fluke is often heaviest following rain, flooding, and irrigation, though in areas of limited infestation, such as banks of streams or dams, it is heaviest in dry times when animals graze these areas more closely.
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