Ticks are among the most significant disease vectors affecting livestock in southern Africa, posing a constant threat to animal health, farm profitability and even human safety. From transmitting deadly diseases such as redwater, heartwater and gall sickness to causing hide damage, paralysis and production losses, ticks have a far-reaching impact across farming systems. In this Q&A, the spotlight falls on the diseases spread by different tick species, how their life cycles influence infestation levels, and why effective control remains a critical challenge for livestock producers.
Compiled by Charl van Rooyen
Question: Which diseases are transmitted by ticks?
Answer: Hard-shield ticks are, economically speaking, the most important external parasites of production animals in the tropical and subtropical parts of the world. Livestock production is the largest farming sector in southern Africa, and cattle farming is the most important of all, in spite of the unfriendly climate with its sparse and unpredictable rainfall. But it is not only the climate that hampers farming; pests such as external parasites do too.
It is estimated that 80% of the world’s 1 214 billion herd of cattle are adversely affected by ticks. They are, therefore, a serious threat to successful animal production. Ticks proffer various threats, such as being vectors of diseases (that sometimes cause death) and toxins (poisons), causing anaemia and damaging hides and skins. They can also transmit diseases to people. Ticks that have long mouthparts, such as the bont tick and the bont-legged tick, cause great economic losses to livestock farmers because they sometimes damage the skins or hides to such an extent that marks or holes occur. As a result, the value of such hides or skins is reduced when they are sold.
In addition, the wounds they cause sometimes become septic and result in the formation of an abscess. And this can also result in screw worm infection and mastitis in cows. Because ticks are blood-suckers, severe infestations can lead to anaemia in cattle, which results in weight and production losses. They are also associated with certain animal diseases, such as redwater (babesiosis), heartwater and gall sickness (anaplasmosis) because ticks are carriers (vectors) of the organisms that cause these diseases. In some species, the toxins they transmit in their saliva cause diseases such as sweating sickness, Karoo paralysis and spring lamb paralysis.
Ticks also carry organisms that cause biliary fever in horses and biliary in dogs. Humans can contract the dreaded Congo fever (a viral disease) from the bite of a tick. In this case, the villain is an infected bont legged tick. Humans can also contract tick bite fever, which is transmitted by several species of ticks. If a dog is bitten by the bont-legged tick, a large wound sometimes develops on its skin at the site of the bite.
Ticks are divided into three categories, according to their life cycles, namely one-, two- or three-host ticks.
One-host ticks
All three life stages of the tick (larva, nymph and adult) occur on the same host. This category is represented by the blue tick, previously thought to be a Boophilus sp., but now considered to be a Rhipicephalus sp. which is a vector of redwater, gall sickness and tick fever (in humans). Its life cycle takes about two months to complete, but development from larva to engorged female takes only three weeks.
When a female is engorged, she falls off the host and lays between 2 500 and 4 500 eggs. The eggs hatch after three to eight weeks, a larva having six legs emerges from the egg and climbs up a grass stalk. While cattle graze, larvae climb onto them and suck blood for seven days. They then moult and a nymph emerges, still on the animal. They suck blood for another seven days, moult and adult females and males emerge. The ticks mate and the female sucks so much blood during the last day or two of the life cycle that she is easily visible, and that’s where the name, blue tick, comes from.
Prof Ivan Horak made the following calculation: If 68 fully engorged females can be counted on an animal with the naked eye, this number represents only a seventh of all the females on the animal; so there are, in fact, 476 female ticks on the animal. The males live on the animal for a longer period (sometimes weeks). There are two males for each female; so on the hypothetical animal above there would be 952. Thus, in total, there are 1 428 adult ticks on the animal, two nymphs for each (2 856 nymphs) and two larvae for each nymph (5 712 larvae). So, for the 68 ticks that can be seen with the naked eye, there will be about 10 000 ticks, in various developmental stages, on the animal.
In the warmer, humid areas they occur throughout the year; mainly on cattle, but at times also on horses, donkeys, sheep and goats. Two kinds of blue tick are carriers of red water; one that transmits African redwater (Rhipicephalus decoloratus) and one that carries African and Asiatic redwater (Rhipi cephalus microplus).
Severe losses occurred, especially during the last part of the nineteenth century. In southern Africa, intensive dipping programmes were enforced by the State up to the 1960s. As a result, blue ticks and also redwater were effectively controlled. After that, there was a slackening in the control measures. At the same time, resistance to acaricides in the dips began to build up in ticks. Since then, losses resulting from redwater have begun to increase again.
Two-host ticks
The two juvenile stages (larvae and nymphs) occur on the same host (for example, cattle, birds, game and rodents) and the adult stage occurs, in particular, on cattle, horses, sheep, goats and the larger game animals.
These are the red-legged and the bont legged ticks. Their life cycle is longer; three months to the one-host tick’s 21 days. The female of the red-legged tick (Rhipicep halus evertsi evertsi), which often attaches in the animal’s ear (juveniles) or under the tail, falls off when she is engorged and lays about 7 000 eggs on the ground. The larvae that hatch climb onto the host (cattle, rodents or birds) where they suck until they are engorged with blood. They then moult to become nymphs which engorge with blood before falling off. They then climb onto the next host, suck blood until engorged and become males and females. They mate, the females suck a lot of blood and then fall off to lay eggs and so continue the cycle.
The red-legged tick is the vector of biliary fever in horses, East Coast fever and red water in cattle, and tick bite fever in people. They prefer to attach to the tail switch, feet and under the tail. The female lays about 5 000 eggs. Bont-legged ticks (Hyalomma spp.) produce a toxin that causes sweating sickness in calves. They have long mouthparts which could damage the animals’ hides. Secondary infection sometimes occurs, causing bacterial abscesses, footrot and teat loss in cows, which could result in mastitis.
Blowfly maggots could appear in the infected areas. Bont-legged ticks have a similar life cycle. Adult ticks are found mainly on the tail switch and under the tail. Those occurring between the claws of sheep and goats can cause severe lameness.
Three-host ticks
All three stages of these ticks occur on different hosts. Among the best known ticks in this group are the bont tick, the brown ear tick and the Karoo paralysis tick. The engorged female falls off its host and lays a clutch of eggs. The larvae that hatch climb onto a host, suck blood until full and fall off. These then moult before climbing onto the next host as nymphs which also suck blood until engorged when they fall off, moult and then climb onto another host as adults.
The life cycle of the bont tick or heartwater tick (Amblyomma hebraeum) takes about five months to complete. A female can lay up to 18 000 eggs.
The life cycle of the Karoo paralysis tick (Ixodes rubicundus) takes 24 to 35 months to complete, and the females lay about 2 000 eggs.
The life cycle of the brown ear tick (Rhipi cephalus appendiculatus) takes eight months to two years to complete and each female lays about 5 000 eggs.
The favourite attachment site of the larvae of Rhipicephalus appendiculatus is the skin of the animal’s head, neck, dewlap and the legs; those of the nymphs are the neck and base of the ears. The adult ticks prefer the belly, skin of the inner ear and the neck. The bont tick is the carrier for the organisms that cause heartwater.
They have well-developed mouthparts that cause painful wounds which can lead to abscesses forming and damage to the hide. The brown-ear tick prefers high-rainfall areas and attaches particularly to the animal’s ear. In severe infestations, they can cause such extreme damage that the ears fall off. They also attach to the skin around the eyes, cheeks, top profile and neck.
This tick carries the organism that causes Corridor (buffalo) disease and East Coast fever. The ticks can cause damage to the ears as well as tick toxicosis.
Information supplied by Drs Tom Strydom and Faffa Malan.
Also read:
Ask the Vet: Ticks outwit chemical remedies
Combat ticks in a natural way
Antiparasitic resistance threatens livestock productivity
