Ticks are developing resistance to pesticides, may become uncontrollable

Ticks that suck blood from cattle are developing resistance to the pesticides that kill them and have become increasingly difficult to control. If not contained, the spread and resistance to pesticides of the small parasites will affect farmers’ incomes and could reduce the country’s meat and milk production.

South Africa has approximately 12.2-million cattle. The ticks that live on cattle carry pathogens that can cause deadly diseases such as anaplasmosis, which causes fever and anaemia, and babesiosis, which causes fever and progressive anaemia.

The potential impact of ticks on agriculture is magnified by the sector’s peculiar circumstances. South Africa’s farming community consists of two agricultures: a large number of subsistence farmers with limited resources and a smaller number of commercial farmers. The country has many tick species and a diverse habitat and climate in which ticks can breed and cattle can be farmed. Warm, humid conditions usually lead to faster tick development which could result in quicker resistance development.

As a result of these factors, the impact of ticks and tick-borne diseases can be more severe than in developed countries.

I research parasites including ticks and set out to investigate acaricide resistance in the invasive Asian blue tick, which is considered one of the most successful cattle ticks in the world, yet its resistance status was uncertain in South Africa. The goal was to determine the resistance status and link this with current and past acaricide use.

The study found many tick populations have become resistant to at least two of the three main acaricide types (chemical classes) used in South Africa.

The reasons for the growing resistance may include poor farm management practices such as underdosing, overdosing and dosing too frequently with acaricides. Underdosing leaves more resistant survivors. Overdosing or dosing too frequently kills most ticks but the few that survive will become “super-resistant” and transfer their genes to the next generation.

We found acaricide resistance was highly variable across South Africa, probably because of different farm management practices. This needs constant monitoring at a local level for us to understand the scale of the problem and prevent mass tick resistance.

If ticks continue to build resistance to acaricides, this will affect farmers’ incomes and could lead to lower meat and milk production.

How ticks breed

The pesticides that kill ticks are known as acaricides. There are only three main acaricides used in South Africa.

When a cow is infested with ticks, most of the pests will be killed when the cow is dipped in acaricide. However, some ticks survive the dip, reproduce and pass their acaricide-resistant genes to their offspring. This will eventually result in generations of ticks that are completely resistant.

Male ticks typically mate with a few female ticks on a cow. The female ticks drop off, lay thousands of eggs and die. The eggs hatch within three to six weeks and a new generation (larvae) is born. Ticks that are fully resistant to acaricides can emerge within as little as two years after the introduction of a new acaricide.

The additional problem is the long time it takes to develop new acaricides. It takes approximately eight to 10 years and millions of dollars to clinically test and register new products.

Why ticks are becoming resistant

Underdosing, overdosing and dosing too often with acaricides drive the development of resistance. Underdosing leaves more survivors with resistant genes. Overdosing or dosing too frequently kills most ticks but the survivors will transfer their genes to the next generation.

Cattle in communal systems that move around freely and cross borders transport resistant ticks to other populations where they mate and further spread acaricide-resistant genes.

Making the situation worse are ticks that breed faster, such as the invasive Asian blue tick, and develop resistance much faster than, for example, the endemic brown ear tick.

South Africa has cattle breeds that are naturally less resistant to ticks. They can sustain larger tick populations and therefore contribute more to the problem.

A further problem is anti-tick vaccines are not yet commercially available in Africa. This is mainly due to limited research and development, specially on the kind of vaccines needed for African tick species.

How to solve the problem

Livestock inevitably develops resistance to acaricides. But this usually happens over a much longer period if pesticides are used correctly.

There are actions that can be taken to slow resistance. 

Livestock should be quarantined when moved to a new farm to prevent the spread of resistant ticks.

The number of acaricide treatments for each animal must be kept to a minimum so  ticks do not have as many opportunities to develop resistance.

Tick resistance to acaricides must be monitored. Identifying ticks and characterising their resistance profiles in specialised laboratories could be used to make informed decisions regarding acaricide use which can mitigate resistance. Once this baseline is established, resistance should be monitored continuously.

Government veterinary services should create awareness of acaricide resistance and support under-resourced producers.

Acaricide resistance testing laboratories should be established by government to monitor and advise farmers. No support is being provided to farmers by the authorities.

Acaricides that have different ways of killing ticks must be rotated so ticks don’t build resistance.

Ticks can also be killed by plant extracts. These are popular in developing countries because they are cheaper than synthetic acaricides. However, they are not as effective. Combining botanical extracts and synthetic acaricides could be a solution, but this has not been fully explored.

• Luther van der Mescht is a senior lecturer in the department of zoology and entomology, University of the Free State

This article was first published in The Conversation