Vector-transmitted diseases

Vector-transmitted diseases are spread through the bites of infected arthropods (e.g. mosquitoes and ticks).

The main vector-transmitted diseases in cattle are 3-day sickness (bovine ephemeral fever) and Akabane disease.

Equine infectious anaemia is a vector-disease affecting horses that can significantly increase after flooding.

Many arboviruses are present in Australia, especially in the tropical north. The main ones of concern are:

orbiviruses (bluetongue, epizootic haemorrhagic disease)
flaviviruses (Murray Valley encephalitis, Japanese encephalitis, Kunjin)
alphaviruses (Ross River, Barmah forest)
bunyaviruses (Akabane, Aino)
rhabdoviruses (bovine ephemeral fever).

National Arbovirus Monitoring Program

The Australian National Arbovirus Monitoring Program (NAMP) is responsible for monitoring the distribution of insect-borne viruses.

Monitoring activities

The program currently monitors the distribution of the 3 most economically important insect-borne viruses that affect livestock for trade:

The program's objectives are:

Trade support: Facilitating the export of live sheep, cattle and goats to countries sensitive to bluetongue, Akabane and bovine ephemeral fever (BEF or 3-day sickness) by providing scientific information for the development of export protocols and to meet export certification requirements.
Bluetongue early warning: Detecting incursions of exotic strains of bluetongue virus into North Queensland through surveillance of the northern bluetongue virus endemic area.
Risk management: Detecting seasonal changes in the distribution of the 3 viruses and their vectors. Surveillance carried out under the program provides producers with early warning of movement by these endemic diseases into new areas where cattle that have not previously been exposed are likely to suffer severe symptoms, including abortion, fever, loss of coordination and, in some cases, death.

Data for the program is gathered by monitoring cattle located in sentinel herds – groups of cattle previously unexposed to arboviral infections. They are blood tested at regular intervals to detect the incidence of viral infection. New animals are selected each year to replace those that develop antibodies to the viruses. Insect traps near the animals in these herds show whether insect vectors are present.

Zones and maps

The distribution of arboviruses is determined by their insect vectors, whose ranges vary according to weather conditions. Those carried by Culicoides midges (such as bluetongue and Akabane) are mainly confined to the north and east of Queensland. Those carried by mosquitoes (such as bovine ephemeral fever, Murray Valley encephalitis, Ross River and Kunjin) are more widely distributed.

A bluetongue-free zone has been established from surveillance data obtained by the monitoring program. This has given some Australian producers access to bluetongue-sensitive overseas markets that were not previously available to them.

Vector-transmitted diseases in cattle

The main vector-transmitted diseases in cattle are three-day sickness (bovine ephemeral fever) and akabane disease.

Three-day sickness

Increased mosquito and midge populations after high rainfall can result in outbreaks of three-day sickness. Consider vaccinating before the wet season as you may not be able to muster stock in wet weather. Even if this were possible, the disease peak may be well past before any significant immunity could develop. (Stock would have to be vaccinated twice, 2 weeks apart, and it would take about 4 weeks for full immunity to develop.)

The three-day sickness epidemic may cause:

ongoing neuromuscular impairment (staggering or incoordination) in some animals
some reduced or delayed fertility in females due to abortion or fevers
temporary or permanent infertility in bulls.

Neuromuscular disease requires owners to monitor and treat animals for welfare and management reasons.

Fertility issues in females may require some management intervention such as:

diagnosing pregnancy and culling empties, and replacing with more heifers
reducing culling for non-fertility reasons (age)
extending mating periods
accepting reduced calvings and making up numbers in the next season.

In multiple sire herds, bull fertility may be less of an issue but we highly recommend some bull assessments or introducing new known fertile bulls. In single-sire herds it is especially critical to do bull assessments to determine the effects of disease and post-flood stress on bull fertility.

Learn more about bovine ephemeral fever.

Akabane disease

Akabane disease is spread by biting insects, but disease rarely shows until calving time when a range of presentations from abortions, limb malformations and dumb calves can occur. There is no preventative treatment, but owners need to be aware of the possibility in naive cattle.

Find out more about Akabane disease.

Equine infectious anaemia

Equine infectious anaemia (EIA) is a viral disease of horses transmitted by tabanidae and other biting flies. Central and western Queensland river systems are known to be areas of endemic EIA infection. The increase in biting flies following flooding can result in some increased transmission of EIA.

Symptoms

The disease can present from an inapparent infection through to a severe disease with depression, rapid loss of condition, weakness, fever and death. Anaemia develops as the disease progresses. Nasal and eye discharge, jaundice and small haemorrhages in membranes of the mouth may be seen. Other symptoms can include increased heart rate, a heart murmur, swelling of the lower body and legs, dragging of hind legs, diarrhoea and abortion. Some horses may appear to recover but then relapse, especially if placed under stress.

Prevention

It is important that you take some preventative measures. These could include:

testing horses and removing known infected animals
isolating valuable, non-infected horses from other horse populations
applying frequent insecticidal treatment to horses to minimise their chance of being bitten by insects.

Also consider...

Learn more about equine infectious anaemia.

Mosquito-borne viruses in horses

There are 2 types of mosquito-borne viruses namely arboviruses and flaviviruses.

Alphaviruses

Alphaviruses include Ross River virus (RRV) which can cause disease and death. Clinical signs in horses of Ross River virus include:

fever
reduced appetite
muscular soreness
reluctance to move
sometimes swollen joints and glands.

Flaviviruses

Flaviviruses can infect the brain and nervous system of horses and humans. Most flaviviruses are exotic to Australia, however, Kunjin virus and Murray Valley Encephalitis (MVE) occurs in Australia and has caused deaths of horses and humans. Although most horses infected with flaviviruses remain clinically well, others may develop abnormalities of the brain and spinal cord. Clinical signs can include:

changes in behaviour
facial paralysis
difficulty swallowing
ataxia (wobbliness)
blindness
inability to rise.

Prevention

Mosquito control is extremely important in helping to prevent arboviruses and flaviviruses. You can achieve mosquito control by:

regularly applying insect repellents
rugging horses
stabling horses at dusk and dawn to decrease mosquito bites during the most active periods.

Stable fans will reduce mosquito numbers by circulating the air, as mosquitoes prefer still air. Clear stagnant water from around stables and paddocks. If water sources contain a large number of mosquito larvae, consider stocking with fish that feed on such larvae or applying biological and or chemical control measures.

Also consider...

Read about Kunjin virus in Australia.
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