Arthropod Parasites

Arthropod parasites, such as flies, mosquitoes, and midges, assume economic significance by impairing productivity or carrying pathogens. Their greater importance is because they transmit infectious diseases.

Flies

Biting and bloodsucking flies affect cattle by sucking blood and irritating the stock. Because the larvae and pupae of these flies need water for their metamorphic life, they are more plentiful during the rainy season, as com pared to the dry season (Johnson, 1987). Horse flies, belonging to the family Tabanidae, are restless feeders and may bite a number of animals in a short space of time. Thus, they are efficient mechanical transmitters of a variety of viral, bacterial, and protozoan diseases. The bloodsucking stable fly, Stomoxys calcitrans, is very common around milking sheds and on farms where horses are kept (Smeal, 1995). It transmits disease mechanically. The common housefly, Musca domestica, transmits a variety of bacterial and viral infections as a result of its feeding on feces and on the food of humans and other animals.

Blowfly strike (Lucilia cuprina) in sheep is primarily governed by climatic conditions. Flywaves that cause colossal mortalities of sheep occur mainly in spring and autumn when prolonged rainy spells keep the sheep wet to the skin.

The tsetse fly is notoriously important because of its role in the transmission of trypanosomiasis, or "nagana," of cattle and other animals in Africa. It also causes sleeping sickness in humans. Vast tracts of tropical Africa are infested with tsetse flies and are literally closed to successful animal husbandry because of trypanosomiasis.

There are many different species of tsetse fly, and various groups have particular biological requirements. In general, riverine species require the cool, moist environment associated with tree-lined riverbanks. On the other hand, savanna species inhabit dry, open parkland. The distribution of tsetse, as revealed by various surveys, is strongly influenced by environmental conditions such as climate and vegetation cover (Robinson, Rogers, and Williams, 1997a,b).

Studies of the behavior of Glossina pallidipes and G. morsitans have been conducted in Zimbabwe (Torr and Hargrove, 1999). Attributes of samples of tsetse from refuges, odor-baited traps, targets, and mobile baits were compared. The results suggested that during the hot season, refuges were significantly cooler than the surrounding riverine woodland during the day, and tsetse experienced temperatures 2°C cooler than the daily mean in a Stevenson's screen located in woodland. Compared to the catches from traps, refuges had higher proportions of tsetse because temperatures in the refuges do not exceed the lethal level of 40°C. Tsetse populations declined by 90 percent during the hot season. This decline in numbers is not due to direct mortality effects of temperature on adults but may be due, in part, to a doubling in the rates of reproductive abnormality during the hot season and an increase in adult mortality related to a temperature-dependent decrease in the pupal period.

Mosquitoes

Mosquitoes are common and widespread throughout the world. They are important as vectors of Rift Valley fever (hepatitis enzootica) of sheep and other animals, African horse sickness, malaria, lymphatic filariasis, yellow fever, western equine encephalitis, and Dirofilaria immitis (a dog heart-worm) (Speight, Hunter, and Watt, 1999). In semiarid tropical and subtropical regions, severe plagues usually occur after spring and summer rains. A close relationship exists between El Niño and seasonal moquito populations and mosquito-associated diseases (Bouma and van der Kaay, 1996).

Midges

Midges are tiny flies of the family Ceratopogonidae, the most important genera being Culicoides spp. They suck blood and, apart from causing annoyance and worry, can act as vectors for a number of arboviruses, including those that cause ephemeral fever, bluetongue, Akabane disease of cattle, African horse sickness, and epizootic hemorrhagic disease of deer.

Midges pass through the egg, larval, and pupal stages, and the immature stages are aquatic. The number of generations per year depends on temperature. In temperate regions there is usually a single generation, while in the tropics three or four may be completed. These flies are plentiful in the warmer months and are most active at dusk and in the early morning. In climates such as that of northern Queensland, the cycle continues in all the seasons and adult midges are active throughtout the year (Smeal, 1995). Because of their small size they are capable of being carried long distances by wind (Arundel and Sutherland, 1988).

Midge-Related Diseases

Bluetongue. Ecosystems in which bluetongue virus circulates between equine or ruminant hosts and Culicoides have been described in many parts of the world. Orbiviruses transmitted by Culicoides midges are found in areas with varying climates, the main factor being warmth for all or part of the year. Moisture is provided through rain or, in semiarid areas, through irrigation. Spread of the virus is through movement or migration of hosts and through unaided flight or carriage on the wind of infected Culicoides midges (Sellers and Walton, 1992).

The incidence, seasonality, and geographical distribution of bluetongue virus infection in cattle herds in Queensland, Australia, has been exhaustively investigated (Ward, 1996a,b; Ward and Johnson, 1996). Cases of seroconversion, which mostly occurs in autumn and winter, were associated with summer and autumn temperature and rainfall. In the far north, most cases were associated with temperature and rainfall in the summer months. Elsewhere, most cases were associated with autumn temperature and rainfall. It was suggested (Ward, 1996b) that two ecological cycles of infection of cattle exist, supporting a hypothesis of differential transmission by vector species.

In Australia, subclinical infection of cattle with bluetongue occurs mostly in autumn, and the median month of seroconversion is May. The prevalence of infection is suppressed approximately fourfold in a series of dry, cool, autumn seasons, as compared to other combinations of seasons. Occurrence of dry, cool, autumn seasons at least once every four years or less keeps a good check on bluetongue virus infection. The association between the El Niño/ Southern Oscillation Index and cases of seroconversion to bluetongue viruses also indicates that more cases occur during months in which the SOI is positive, compared to months in which it is negative. Studies suggest drought conditions in Australia may affect the endemic stability of blue-tongue virus infection. Instability in the system could lead to cyclical epidemics of infection (Ward and Johnson, 1996).

Horse sickness. The incidence of African horse sickness with climatic conditions has been studied by Baylis, Mellor, and Meiswinkel (1999) and Baylis and colleagues (1998) in Morocco and South Africa. There is evidence of an association between African horse sickness and El Niño/Southern Oscillation (ENSO). Baylis, Mellor, and Meiswinkel (1999) suggested that the association is mediated by the combination of rainfall and drought brought to South Africa by ENSO. The combination of heavy rain followed by drought is thought to affect disease transmission, with breeding sites of the insect vector Culicoides imicola being altered, or, during drought, the animal reservoir for the virus (zebra [Equus burchellii]) may congregate near the few remaining sources of water where they are in contact with and infect more vector midges. High temperatures during droughts increase vector population growth rates and favor disease transmission.

Lice

Lice are small obligate parasites that are highly host specific. They are dorsoventrally flattened, wingless, and have tarsi adapted for clinging to hair or feathers. Lice infestations are common in cattle, sheep, goats, and horses in late winter and early spring.

According to Arundel and Sutherland (1988), Bovicola ovis is the most common and important louse on sheep, causing considerable irritation that results in poor wool growth and damaged wool. Infested sheep rub against objects such as fences and fence posts and bite and chew their fleeces. The fleeces therefore appear deranged and have a pulled and ragged appearance, particularly the areas on the sides behind the shoulder, which sheep can reach with their mouths.

Solar radiation, temperature, and rainfall all have profound effects on lice numbers. Lice is an important parasite of sheep and cannot survive off sheep for more than a few hours under extremes of temperature and light. Laboratory studies have, however, shown that lice held away from sheep could survive for 11 days at 25°C. In shearing sheds in winter and early spring, lice could survive for up to 14 and 16 days, respectively (Crawford, James, and Maddocks, 2001). Shearing has the most dramatic effect in that it physically removes the lice with wool and exposes the remaining lice to extreme weather conditions. Lice numbers drop to the lowest after one to two months of shearing. Saturation of the fleece by heavy and prolonged rain also kills many lice. When temperatures fall from late autumn to early winter, there is an increase in lice numbers, and they decline over summer. However, this pattern is influenced strongly by the timing of shearing.

Ticks

Ticks occur on a global scale but they transmit many more serious viral and protozoan diseases in tropical and subtropical areas than in other regions. Climate and vegetation are the major factors affecting tick distribution (Estrada and Genchi, 2001). When weather conditions are favorable, ticks lie in wait (on grass or rocks) or move in active search of a host. When conditions are unfavorable the ticks return to shelter (under a stone, in litter, under vegetation). Depending on the climatic characteristics of a season and region, ticks may be active during the day (morning or evening). Ticks are active at night in dry plains where strong sunlight prevents any diurnal movement on the ground.

Each species of tick has a particular threshold temperature below which a diapause occurs in all instars. Various climatic factors such as sunlight, temperature, rainfall, and wind patterns condition the presence or absence of a tick species (Shah and Ralph, 1989).

Development and survival of free-living stages of tick are related to temperature, while the duration of survival is influenced mainly by rainfall and consequent relative humidity (Pegram and Banda, 1990). Appropriate relative humidity, rather than wet conditions, is essential for the development and survival of eggs and pupae and the survival of unfed hatched ticks. Each species of tick is adapted to a particular relative humidity range which varies with the instar and its size. The requirements range from extremely humid to very low relative humidity. Immature stages have very specific requirements, while adults can protect themselves better against evaporation because of their larger size and thicker tegument. Immatures adjust their humidity requirements by locating in holes in the ground, cracks in rocks, under litter, at the base of the vegetation layer, and in other sheltered places. Some adults are remarkably drought resistant and can survive for several months or years in a semidesert environment.

In temperate climates, the temperature determines the distribution of ticks. Abrupt or slow changes of temperature can modify the life cycle within a few days or weeks. In northern Eurasia, central Eurasia, and adjoining Mediterranean regions, the greater frequency of activity is mostly in summer.

In tropical climates, the dominant factor is rainfall (Pegram et al., 1989). The start and end of the rainy season influence the different phases of the life cycle. Parasitism is reduced during the dry months (March to June north of the equator) and increases sharply within days following the first major winter rainfall. The population remains stable for a few weeks, then slowly diminishes. At the end of the rainy season there is a marked decrease, with a progressive fall to almost zero in the dry season. In such regions the more rapid tick development pattern is determined by the dry season, and the life cycle takes one year.

Climatic uniformity and the absence of an unfavorable season in equatorial regions allow tick development throughout the year (Smeal, 1995). There is no annual cycle determined by a diapause. Generations overlap or follow one another in a pattern depending on the species.

Every year during the spring and summer, Ixodes holocyclus, a paralysis tick also known as Australian paralysis tick or dog tick, is at its most dangerous along the eastern seaboard of Australia, from around the lakes district in Victoria to the northernmost tip of the country. The female tick produces neurotoxins in its saliva which is injected into the host animal while feeding. These toxins affect nervous tissue causing paralysis in cattle, sheep, and goats (Smeal, 1995).

Mites

Mite-Related Diseases

Sarcoptic mange (barn itch). Sarcoptic mange occurs in all species of animals, causing a severe itching dermatitis. The causative mite, Sarcoptes scabiei, is usually considered to have a number of subspecies, each specific to a particular host. Animals in poor condition or underfed appear to be most susceptible. The disease is most active in cold, wet weather and spreads slowly during the summer months. The females form shallow burrows in the horny layer of the skin in which to lay their eggs. The larval and nymphal stages may remain in the tunnels or emerge onto the skin. The normal exfoliation of the skin eventually exposes the tunnels, and any of the life cycle stages may transmit by contact to other animals (Blood and Radostits, 1989).

Among domestic species, pigs are most commonly affected, but it is an important disease of cattle and camels and also occurs in sheep. It is a notifiable disease in most countries and is important because of its severity.

Sheep itch mite. The itch mite (Psorergates ovis) has been recorded as a parasite of sheep in Australia, New Zealand, South Africa, and the United States. The life cycle, comprising eggs, larvae, three nymphal stages, and adults, takes four to five weeks and is completed entirely on the sheep. Only the adults are mobile, and they effect the spread of the disease by direct contact between recently shorn sheep, when contact is close and prolonged. All stages occur in the superficial layers of the skin and cause skin irritation, leading to rubbing and biting of the affected areas and raggedness of the fleece. Merino sheep are most commonly affected. The highest incidence is observed in this breed, particularly in areas where the winter is cold and wet. There is a marked seasonal fluctuation in the numbers of mites. The numbers are very low in summer, rise in the autumn, and peak in the spring (Blood and Radostits, 1989).

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