Estimated reading time: 5 minutes
- Ticks are the cause of significant annual economic losses, primarily because of the diseases they transmit.
- The blue tick is usually the biggest challenge in South Africa because it mainly transmits redwater.
- Cattle that were never previously exposed to Asiatic redwater are now contracting the disease, showing far more severe symptoms because they have no immunity.
- Resistance to chemical agents varies significantly from farm to farm.
- Producers must establish the exact resistance status on their own farm in order to design an effective tick control strategy.
Ticks are the cause of significant annual economic losses, primarily because of the diseases they transmit. In South Africa, the blue tick species is the main culprit.
Dr Luther van der Mescht, a parasitologist at the University of the Free State, says research points to a troubling rise in resistance in blue ticks to commonly used chemical control agents. He stresses the importance of individual farm testing – not only to identify the species present but also to determine which chemicals these ticks have developed resistance to.
Tick species are generally classified as either single-host or multi-host ticks. Blue ticks fall into the single-host category, while species such as the bont-legged and red-legged ticks are the multi-host type.
“The blue tick is usually the biggest challenge in South Africa because it mainly transmits redwater,” Dr Van der Mescht explains. “The issue with the two blue tick species – the African blue tick (Rhipicephalus microplus) and Asian blue tick (Rhipicephalus decoloratus) – is that they are single-host ticks. This means they complete their entire life cycle on a single host, resulting in a shorter generational interval compared to two- or three-host ticks. Since the tick’s entire life cycle is exposed to the same chemical treatment when animals are dipped, resistance can develop much faster.”
His research further shows that African blue ticks are more common in commercial herds, whereas Asian blue ticks dominate in communal herds. However, in the absence of effective biosecurity measures, the Asian blue tick can spread as quickly as and outcompete its African counterpart when animals are moved between areas.
The redwater threat
Blue ticks are the primary vector for the spread of redwater. What is particularly concerning, Dr Van der Mescht explains, is that Asiatic redwater is spreading steadily westward. In the past, Asiatic redwater was largely confined to the warmer, high-rainfall regions of KwaZulu-Natal and the Eastern Cape. Today, however, increasing numbers of cases are being reported in the Free State, with the disease moving deeper into the central and western parts of the province.
“We are currently conducting a study on 20 farms across the Free State, collecting and identifying ticks. The results clearly show that the Asian blue tick is spreading westward, which will significantly increase the incidence of redwater. Cattle that were never previously exposed to Asiatic redwater are now contracting the disease, showing far more severe symptoms because they have no immunity.”
Unlike the African blue tick, which transmits only African redwater, the Asian blue tick can transmit both African and Asiatic redwater. This poses an additional challenge, as the two species are very difficult to distinguish without a microscope. For example, the female African blue tick has three rows of teeth in her mouthparts, while her Asian counterpart has four.
Countering resistance
“Resistance to chemical agents varies significantly from farm to farm. For this reason, producers must establish the exact resistance status on their own farm in order to design an effective tick control strategy. It is not enough to identify which tick species are present; the resistance profile of those populations must also be determined. This can only be done in the laboratory, where female ticks are allowed to lay eggs, and the resulting larvae are tested for resistance.”
The Asian blue tick was originally introduced into the country with a specific resistance profile, but exposure to various chemical classes of control agents has likely altered that profile over time. The three main chemical classes used for tick control are synthetic pyrethroids, formamidines, and organophosphates. Dr Van der Mescht’s research indicates that fewer than 30% of ticks tested remain susceptible to the synthetic pyrethroid, cypermethrin.
“Although the Food and Agriculture Organization (FAO) of the United Nations’ guidelines recommend testing resistance by chemical class, the fact is that variation also exists within classes. For example, both cypermethrin and flumethrin are in the same class, yet flumethrin remains effective in many cases where cypermethrin no longer works. This difference is likely because cypermethrin is widely used as a pesticide and thus more frequently encountered in the environment, while flumethrin is applied specifically for tick control and provides longer-lasting protection.”
Cattle that were never previously exposed to Asiatic redwater are now contracting the disease, showing far more severe symptoms because they have no immunity.
Resistance to formamidines and organophosphates, on the other hand, appears to be much less widespread than in the case of the pyrethroids. This underscores the importance of establishing the resistance profile of each farm individually before implementing a control strategy.
Winter ticks
A third single-host tick species, known as the winter tick, is found mainly on horses during the colder months. It should not be confused with the two blue tick species. Very little research has been conducted on this species, and the extent of its resistance development remains unclear. Fortunately, the winter tick is not associated with any specific diseases. – Izak Hofmeyr, Plaas Media
For more information, email Dr Luther van der Mescht at VanDerMeschtL@ufs.ac.za
