Signs, Impact and Biology

Mosquitoes

Many species of mosquitoes have been shown to feed on cattle (Figure 1). When at epidemic levels, such as following recent rains or in very wet environments, attacks by mosquitoes may be enough to cause a marked loss of condition in cattle, reduction in milk yield and in severe instances even death. These effects are likely due to irritation caused by mosquito bites, and cattle will react with vigorous stamping of the feet, switching of the tail, twitching of the skin and tossing of the head. However, it was estimated in one instance that cattle under significant attack lost approximately 166 mL of blood per night, suggesting there may be direct effects as well. Mosquitoes have been observed to bite in most body areas on cattle, but when mosquitoes are in high numbers attacks are usually most intense on the legs and underline. Species of mosquito differ in their preference for feeding time and in some cases, there can also be continuous biting during the day. However, most mosquitoes feed at night with peaks from 6:30 pm to 8.00 pm with another similar peak after sunrise.

Mosquitoes are known to be vectors of a wide range of diseases, but in Australia the main concern for cattle is bovine ephemeral fever (BEF or three-day sickness) that can cause widespread sickness and death in some animals. The virus has been isolated from both mosquitoes and biting midges in Australia, but various factors suggest that mosquitoes are the principal vectors. Most outbreaks of BEF are associated with the rapid growth of mosquito and biting midge populations in wet periods.

Midges

Biting midges of the genus Culicoides are small blood feeding flies less than 3 mm long. In Australia they are commonly called sandflies although this term is more correctly applied to a different group of biting flies.

Cattle usually show no reaction to the bites of Culicoides biting midges. Horses, however, can suffer from Queensland itch which is a reaction to the bite of Culicoides brevitarsis. The main impact of Culicoides on cattle is caused by the diseases they transmit, especially akabane disease and bluetongue virus. Despite some evidence to suggest that biting midges are able to transmit bovine ephemeral fever, mosquitoes are now thought to be the major vector of this virus.

Nineteen species of biting midge feed on cattle in Australia but only 8 have been associated with transmitting bluetongue diseases to cattle, these 8 are called the vector species. The most important vector species is Culicoides brevitarsis which transmits both bluetongue and akabane viruses (Figure 1). There is little local evidence that other species are able to transmit akabane virus, but it is possible that some of them may be capable.

Distribution

All 8 vector species arrived in Australia from Asia or New Guinea, probably on wind currents. New strains of bluetongue virus are occasionally found in northern Australia and these are thought to have been introduced by infected midges arriving on wind currents. The distribution of the vector species is provided in Table 1. As these species are tropical, their distribution in Australia is confined to warmer climates. Three species have limited distributions; two are only present in northern WA and NT and one is only present in northern QLD. The remaining 5 species have dispersed more widely. Six species are present in northern QLD but 3 are confined to the far north. Culicoides brevitarsis is the most widespread species and is the only one regularly found in southern QLD and in NSW. This species is present from the Kimberly in WA, across the northern half of the NT and QLD and the eastern half of QLD into NSW as far south as Sydney. In good seasons this species can move further south and west to cover all but the southwestern corner of QLD. Seasonal changes in the distribution of vectors is monitored by the National Arbovirus Monitoring Program.

Table 1. Distribution of vector species of bluetongue virus in Australia

Breeding site

Three of the vector species, C. brevipalpis, C. brevitarsis and C. wadai are closely associated with cattle and breed in cattle dung pats. Both C. oxystoma and C. peregrinus breed in mud. The breeding site of the other 3 species is unknown.

Life cycle

Only female biting midges feed on blood which they need to produce eggs. They go through an endless cycle of feeding on blood, digesting the blood meal to produce eggs, laying the eggs then searching for another blood meal. Females mate only once in their lifetime and store the spermatids in special organs to be released when they lay eggs. The life span of midges is likely to vary between species but is typically less than 1 month.