Endemic Stability and the SA Tick Picture

#The cattle that die of redwater are often the best-looked-after cattle in the district

Picture a neighbour who runs a tight operation. Spray race every fortnight, records kept, not a tick to be seen. He buys ten in-calf heifers from a Free State farm that dips just as hard. Four months later two are down with redwater and one is dead.

Nothing went wrong with his dipping. His dipping is the reason.

In South African tick country, the animals that die of redwater, gallsickness and heartwater are usually the ones that have never met an infected tick. Mortality of 5 to 10% is not uncommon when susceptible cattle — European breeds especially — are put into an area where the disease is established (Anipedia, University of Pretoria). That figure is the single reason buying-in is a health decision before it is a genetics decision or a price decision.

The concept that explains it is called endemic stability, and it is the most important animal-health idea in this course. It is also the one most often taught backwards.

#The threshold, and the two age windows people blur

Endemic stability for babesiosis (redwater) is generally taken to exist when more than 75% of the population is infected or seropositive before nine months of age (enzootic stability indicator study). Below that, you have an endemically unstable herd — enough challenge to kill animals, not enough to immunise them. The same model gets extended to gallsickness (anaplasmosis), and as you will see later in this lesson, that extension is exactly where the argument starts.

The ARC-Onderstepoort tick-borne disease FAQ gives two related sets of ages, and farmers routinely mash them into one number (ARC-OVI):

Natural non-specific resistance — the biological free ride:

Calves, up to about nine months, for redwater and gallsickness.
Calves younger than six weeks, and lambs and kids in their first week of life, for heartwater.

Recommended vaccination age window — when to use that free ride:

Calves three to nine months for redwater and gallsickness. Note the lower bound. The common shorthand "under nine months" throws it away.
Calves four to six weeks for heartwater; lambs and kids within the first week of life.

The ARC adds that animals can be vaccinated at any age provided reactions are watched and treated — "the latter especially in older animals". That is the whole trade in one sentence. Vaccinating young is what makes reaction management unnecessary. Vaccinating old is what makes it essential.

This course names the window and stops there. Which vaccine, how it is stored, how it is handled and what a reaction needs is a conversation with your veterinarian, against the ARC's own guidance and the pamphlet in the box. There is no dose or schedule anywhere in this module.

#Know which ticks are actually on your animals

A systematic review of ticks on smallholder cattle in South Africa found 26 tick species across seven genera (review, 2024). You do not need all 26. You need to know which ones are on your herd, and whether each one spends its whole life on an animal or drops off between meals — because that decides whether your management lever is the animal or the veld.

Tick	Common name	Life cycle	What it carries
Rhipicephalus (Boophilus) decoloratus	Blue tick (African)	One-host	Babesia bigemina (African redwater), Anaplasma marginale (gallsickness)
R. (B.) microplus	Asiatic blue tick	One-host	B. bigemina and B. bovis (Asiatic redwater), A. marginale
Amblyomma hebraeum	Bont tick	Three-host	Ehrlichia ruminantium (heartwater)
R. appendiculatus	Brown ear tick	Three-host	Theileria parva (Corridor disease, at the buffalo interface)
R. evertsi evertsi	Red-legged tick	Two-host	Paralysis in lambs; also A. marginale and Babesia spp.
Hyalomma truncatum	Bont-legged tick	Two-host	Sweating sickness toxin
Ixodes rubicundus	Karoo paralysis tick	Three-host	Neurotoxin causing Karoo paralysis

Two vector facts do most of the practical work. First, African redwater is carried by both blue tick species, but Asiatic redwater only by R. microplus, which wants higher rainfall — so Asiatic redwater has a much narrower South African range, and the two redwater vaccines are not interchangeable. Second, B. bigemina is broadly present wherever annual rainfall is above about 280 mm, and essentially absent from the northern Western Cape, the Northern Cape, the western Free State, the high Drakensberg and Lesotho (ARC-OVI). If your farm is above 280 mm, redwater is your problem whether or not you have seen it.

A one-host tick like the blue tick lives its whole life on one animal and drops off only to lay. A three-host tick like the bont tick feeds, drops, moults and finds a new host, three times over. That is why a dip interval that flattens blue tick does very little to bont tick, and why the two have different seasons.

#The arithmetic that tells you which month is cheap

One engorged female blue tick lays 2,500 to 3,500 eggs, and ticks can survive six to eight months without a host (Virbac SA). Work that through.

Say you carry 200 engorged females through October because numbers look low and it does not feel worth the trouble. At the bottom of the fecundity range that is 200 × 2,500 = 500,000 eggs, and the same source puts the result at over 400,000 larvae within 30 to 60 days. Those larvae are what you are fighting in February.

Now compare the two decisions. Knock the population down in October and you are killing 200 females. Chase the same population in April and you are trying to kill their offspring's offspring with a chemical that every one of those generations has had a chance to select against.

#Karoo paralysis, including in Gauteng

Ixodes rubicundus does not carry a pathogen. It injects a neurotoxin, and the dose depends on how many ticks are attached. Sheep, goats and young calves go weak in the hindquarters, stagger, and go down. Adult tick activity starts in late summer, peaks in April and May, has a smaller second peak in June, and cases run to about July (MSD Animal Health SA).

The good news is that it reverses: remove the ticks and animals typically recover within 24 to 48 hours. The bad news is where it occurs. Everyone expects it in the Karoo and the southern Free State. Fewer people know there are documented foci near Bronkhorstspruit and Heidelberg in Gauteng, and near Belfast in Mpumalanga (SANBI). Heidelberg is about 30 km from Meyerton. A Highveld smallholder who writes off staggering April lambs as plant poisoning may be looking at a tick his district is not supposed to have.

#The honest complication

Everything above is the standard South African framework, and it is taught with a lot of confidence. The best-known South African field test of it did not produce the clean result advocates quote, and burying that would be dishonest.

Contested — here is both sides

Does relaxing your dipping actually deliver endemic stability?

The case for: Endemic stability is the standard framework in southern African tick-borne disease control. It explains the age–mortality pattern every experienced stockman has seen, and it underpins the ARC's own vaccination and dipping guidance. The alternative — permanent intensive dipping — manufactures a herd whose survival depends on a spray race that never breaks, a drought that never comes and a chemical that never fails.

The case against, and it is real evidence: Rikhotso, Stoltsz, Bryson and Sommerville compared 26 dippings on a 14-day interval against 13 strategic dippings over twelve months in four communally grazed areas at Bushbuckridge, testing 60 cattle per location at the start and the end. In the strategic group, seroprevalence rose for Babesia bovis and B. bigemina but fell for Anaplasma marginale — the opposite pattern to the intensively dipped group. The strategic group also showed higher incidence of clinical tick-borne disease and of abscesses, which the paper records at the start of the survey (Rikhotso et al., JSAVA, 2005).

Separately, the >75%-by-nine-months threshold was developed for babesiosis and has never been definitively validated for Anaplasma marginale (Kansas State VDL).

Where that leaves you: endemic stability is a real phenomenon with softer operational thresholds than the confident literature suggests. The trial does not say the concept is a myth. It says the transition is the dangerous moment, that gallsickness behaves differently from redwater, and that you do not halve your dipping in one season and hope. Lesson 2 is about how that transition is actually run.

Do this on the farm

Before you change a single dipping interval, establish what you are actually dealing with:

Identify the ticks on your animals, not the ticks in the district. Blue tick only, or blue tick plus bont tick? Collect specimens and have them identified.
Find out which pathogens are circulating — serology or blood smears through a DALRRD-approved laboratory, arranged by your veterinarian or animal health technician.
Check your rainfall against the 280 mm line for African redwater.
Write down your herd's exposure history. Bought-in adults from a hard-dipping farm are your highest-risk animals, whatever they cost.
Only then talk to your vet about vaccination and about lengthening intervals. The ARC's position is that the vaccination decision rests on diagnosis, vector prevalence or an intended move — not on habit.

South African specifics

Delta Farms, Meyerton. In the 2006–2017 commercial-farm survey, R. microplus was not recovered from Gauteng at all (van Dalen & Jansen van Rensburg). If that still holds locally, the blue tick here is R. decoloratus, which means African redwater and gallsickness are the diseases in play and Asiatic redwater is not — and the two need different vaccines. Meyerton's roughly 600–700 mm of rain is well above the 280 mm line, so African redwater is present-by-default until proven otherwise.

One thing the research behind this course could not settle: whether the bont tick, and therefore heartwater, occurs at Meyerton. The Highveld sits at or beyond the bont tick's usual margin. That question is load-bearing for a vaccination decision and getting it wrong in either direction is expensive, so it belongs with the Sedibeng state veterinarian, not with a course.

#What to do with this on Monday

Stop thinking of a dip as protection and start thinking of it as an interruption in your animals' education. Some interruptions are worth paying for and some are the reason the herd cannot survive a broken spray race.

Two things change immediately. First, every animal you buy gets assessed on exposure history, not just on price and paperwork — a naive adult heifer walking into your veld is carrying a 5 to 10% chance of not being there in six months. Second, before you touch your dipping interval, you find out which ticks you have and which pathogens are circulating. Everything in the next lesson depends on knowing that.

#Check yourself

4 questions — answers explained as you go

1A farmer's cattle are seropositive for redwater at 40% by nine months of age. What does that tell him?
Why: Endemic stability is generally taken to need more than 75% of the population infected before nine months of age. At 40% he has the worst of both worlds: infected ticks are getting through, but not to enough calves during their window of natural resistance, so a growing share of his adults are susceptible. This is the situation in which vaccination and a diagnosis matter most.
2Why does dipping every 14 days, year after year, raise the risk of animals dying from redwater?
Why: Calves have a natural, non-specific resistance in early life, so an early infection produces immunity with little disease. Remove the exposure and the animal is protected only for as long as the dipping holds. The ARC's own guidance says animals kept under strict intensive tick control have had little or no exposure and are at risk should tick infestation occur.
3A Bushbuckridge trial compared 26 dippings a year against 13. What did the strategic group show?
Why: Rikhotso and colleagues found the strategic group had more clinical disease, and that gallsickness behaved in the opposite direction to redwater. This is the counter-evidence that has to be taught alongside the theory: the concept is sound, but the transition between regimes is the dangerous moment and gallsickness does not follow the babesiosis model.
4A Highveld farmer wants to spend his tick-control money where it does the most good. Which month is the cheapest intervention against blue tick?
Why: One engorged blue tick female lays 2,500 to 3,500 eggs. Killing 200 females in October prevents hundreds of thousands of larvae by December. Chasing the February-to-May peak means treating their descendants, at far greater cost and with far more selection pressure on the chemical. The winter dip is a real intervention too — but it is aimed at multi-host ticks, which peak in July and August.