Arthropod-borne summer livestock diseases

As the weather grows hotter and wetter across the country, the incidence of livestock diseases transmitted by arthropods increases.

Damp, warm and rainy summer months present ideal breeding conditions for blood-sucking arthropods such as mosquitoes, midges, sandflies and ticks, increasing the risk of livestock contracting arthropod-borne diseases. At this time of year, a farmer must stay on top of vaccination and dosing programmes and take steps to minimise disease problems.

Mosquitoes transmit many important arboviruses in the rainy season, including West Nile virus (WNV), Middelburg virus, Shuni virus, Wesselsbron virus and Rift Valley fever (RVF). Biting midges transmit blue tongue and African horse sickness virus (AHSV). Some vectors transmit diseases to humans and disturb livestock to the point that animals do not feed properly. Consequently, beef cattle, sheep and poultry may suffer weight loss, and dairy cows may produce less milk. The risk of serious illness and death increases.

“Horses that develop neurological signs of WNV have over a 30% chance of dying or being disabled, while unvaccinated horses that become infected with AHSV have a 50% to 90% risk of dying,” says Prof Marietjie Venter, professor at the Zoonoses Research Unit at the University of Pretoria, and director of the One Health and International Emerging Infections programme for the Global Disease Detection of the US-Centers of Disease Control and Prevention in South Africa.

“WNV is best-known for severe neurological signs such as stumbling, hind- and forelimb paralysis, difficulty in rising, and death in horses,” says Venter. “However, it can also infect other species, including humans, during the rainy season, causing fever, rash, headaches, joint and muscle pain. In severe cases, it leads to paralysis, brain infections and even death in human patients.”

Middelburg virus and Shuni virus can infect many species, including horses and wildlife. All develop neurological signs in severe cases.“Usually, animals infected with WNV, and in all likelihood the Middelburg or Shuni virus, cannot transmit the disease directly to humans or back to mosquitoes, although caution should be taken when handling the brain or spinal cord of animals that die,” she says.

Wesselsbron virus and RVF may cause abortion storms (large-scale abortion outbreaks at a rate above 10%) and death in sheep and cattle. “Farmers can become infected when handling the aborted foetuses of these animals. Animals that die of any of these arbovirus diseases should not be used for human or animal consumption. Cases of WNV and Shuni virus have been reported in dogs and even crocodiles, so carnivores are also at risk.”

Keeping mosquito populations under control helps prevent these diseases from becoming established. Control methods include draining standing water where mosquitos breed, such as excess irrigation water, flower pots and containers, cleaning water tanks and buckets daily, and fitting mosquito screens and fans to stables and sheds.

“Other preventative measures include spraying valuable animals such as horses with insect repellents containing DEET or alpha cyano pyrethroids, such as Cypermethrin, to help protect against biting insects. Vaccines exist for AHSV, WNV, RVF and Wesselsbron virus,” Venter says. “AHSV is the most important horse disease, and horses should be vaccinated annually, as it is required by law.”

Tick-borne diseases can cause heavy stock losses and prevent livestock in disease-free areas from being introduced onto farms with a high tick load. According to Dr Rhoda Leask, University of Pretoria senior lecturer: Small Stock Health and Production, heartwater is a very important disease spread by the Amblyomma tick and affects cattle, sheep and goats. Gallsickness is spread by ticks and affects mostly cattle, while redwater is spread by infected blue ticks.

Following summer rains, says Leask, worms are a problem, particularly in sheep. Other diseases to be wary are three-day stiffness sickness and lumpy skin disease. A farmer should work closely with a vet in protecting livestock against diseases, through drugs and other control measures.