A dog taking a walk in Stellenbosch (Photo: Dian Spear)
Dian Spear delves into what could be behind the increase of ticks in the Western Cape.
In the last few weeks, I have seen a tick walking across my computer screen while I was working and another one walking over my dashboard while I was driving.
Why are there so many ticks at the moment?
In the last few months, since we have been able to walk our dogs regularly again, I have noticed more ticks walking around on my dogs after a walk than I can remember seeing before.
My first thought was that it is something to do with all the rain we are having after a drought period, but we also had rain last year.
The dynamics of tick populations involve complicated feedback mechanisms between weather, vegetation and hosts.
Ticks require conditions of high humidity so that they do not desiccate, and dense vegetation close to the ground helps maintain a suitable microclimate for them.
Impact of lockdown?
Interestingly, their need for humid conditions is because, apart from getting hydration from blood, which is their energy source for growth, most ticks don’t “drink” water – they absorb it through their outer cuticle. So, could it be that there are so many ticks because of all the rain? Or, were the ticks just hungry after not having any dogs to feed on during lockdown?
The answer isn’t that simple.
According to research by Prof. Sonja Matthee and Prof. Ivan Horak, the most common tick on dogs in Stellenbosch is the southern African yellow dog tick.
These ticks follow a three-host life cycle, which is common to many ticks and can take years.
The first stage is the larvae, which hatch from eggs and feed on the first host animal.
After feeding, the larvae takes cover and moult into nymphs. The nymphs then feed on an animal, drop off, hide and moult.
The third stage is the adult tick, which finds an animal to feed on by using the sense organ in its front legs, which can sense shadows, body heat, odour and vibrations – many tick species don’t have eyes.
After the female has fed, she lays her eggs – for the yellow dog tick, there are about 5 000 of them. The eggs will hatch when the larvae are developed and it is humid enough.
The southern African yellow dog tick is also found on lions, leopards, cheetahs and wild dogs in Kruger National Park.
Blame it on the rodents
The tick expert Prof. Horak and some of his colleagues conducted long-term research on ticks in Kruger National Park, and they found that the first two stages of the dog tick life cycle, the larvae and nymphs, mostly feed on rodents.
It turns out that rodents play a very important role in this story – they fuel tick growth.
There are more ticks in the vegetation in Kruger National Park in years subsequent to years with good rainy seasons and years when there are explosions of rodents.
Good rains lead to more grass and dense vegetation, which provides cover and food for rodents – rodents eat seeds.
To put it simply: the rain grows the grass, which feeds the
mice, which feed
the immature ticks, which turn into adult ticks. And the timing of this
sequence and its intensity depends on the environmental conditions at play – the rainy season last year and the previous year can influence the number of ticks we find on our dogs today.
To put it simply: the rain grows the grass, which feeds the mice, which feeds the immature ticks, which turn into adult ticks. And the timing of this sequence and its intensity depends on the environmental conditions at play.
It is possible that some adult ticks were waiting for the dogs to come back out for a walk after lockdown.
However, it is more likely that there have been lots of rodents around. I’ve definitely noticed lots of grass, weed and other vegetation growth lately. Perhaps the next tick season is also going to be bad.
– Dr Dian Spear is a Science Communicator with a PhD in Zoology from Stellenbosch.