The silence of the lambs – aggressive anaplasmosis a growing concern in sheep on pasture

Background

For the past 10 years, the incidence and severity of anaplasmosis (also known as tick-borne fever) in sheep have been increasing in southern Sweden and on Gotland. The disease has also developed a more aggressive course than previously; animals that would normally not require treatment, such as older ewes, are becoming ill, sometimes resulting in death. Animals that have been treated preventively with tick control products also become ill, and animals die despite treatment being initiated. One recent example is a sheep flock in Skåne that, in the spring of 2024, experienced what is known as an “abortion storm” among its ewes. One fifth of the flock’s ewes aborted their fetuses and three ewes died within four days. Test results showed that the bacterium Anaplasma phagocytophilum, which causes anaplasmosis, was responsible for the outbreak. During 2023–2024, Anaplasma infections also caused several deaths among sheep and cattle on Gotland. That tick-borne fever is associated with deaths in cattle is very unusual, and clinical disease normally subsides within a week.

Anaplasmosis is one of the most common tick-borne infections of clinical relevance in animals in Sweden, primarily affecting sheep and cattle at pasture, but it can also cause infection in, among others, goats, camelids, horses, dogs, cats, wild ungulates and humans. Humans can develop so-called human granulocytic anaplasmosis (HGA, previously known as ehrlichiosis), which in some cases can cause prolonged unexplained fever; in rare cases, kidney damage and other organ damage may also occur. Severe infections are treated with antibiotics, usually tetracyclines, which are broad-spectrum antibiotics. In infected animals, the Anaplasma bacterium (which is an intracellular organism) can be detected in the blood.

Our changing climate, with increasing temperatures during the summer months, a longer growing season and milder winters, has led to an increased population and wider distribution of our common tick (Ixodes ricinus), as well as new tick species being found in Sweden that may potentially become established in the country. In addition to common ticks, Anaplasma bacteria can also be transmitted by several tick species. Some of these are already established in Sweden, e.g. the red sheep tick (Haemaphysalis punctata), which is common on Gotland and in the Kalmar region, the taiga tick (Ixodes persulcatus) and the rodent tick (Ixodes trianguliceps), while others are not established but have been detected in Sweden more frequently in recent years, e.g. the ornate dog tick (Dermacentor reticulatus). In Sweden, Anaplasma species other than A. phagocytophilum have been detected in sheep. The first Swedish findings of A. bovis and A. capra were diagnosed in 2013 in a sheep flock on Gotland. It is unclear why new species occur specifically on Gotland, but it is known that the ornate sheep tick is a vector for A. bovis.

Anaplasmosis/tick-borne fever leads to increased animal suffering and animal losses due to deaths and abortions, while weakened animals, particularly young lambs at pasture, may become easy prey for predators such as eagles and foxes, as well as experiencing reduced growth among surviving animals. Treatment of sick animals also results in increased antibiotic use and higher production costs. In addition to production losses, more cases of disease lead to an increased workload for animal owners and staff. Flocks in particularly affected areas of eastern Gotland have lost approximately 20% of their lambs, corresponding to up to 80–90 lambs on some farms, and have experienced production losses of more than SEK 100,000 per season. Additional financial losses may arise when pastures have to be avoided due to the tick burden and the disease that occurs as a result, which may mean that the animal owner is unable to make use of grazing subsidies.

Purpose

The project aims to reduce the impact of the tick-borne infection anaplasmosis in grazing sheep, a disease that causes significant animal suffering and production losses.

Implementation

The occurrence of Anaplasma bacteria in ticks collected from pasture and from lambs and sheep with disease on Gotland and in southern Sweden will be investigated. Blood and organ samples from lambs and sheep will also be analysed.

Sheep farmers’ preventive routines against tick infestations and treatment routines in cases of disease will be mapped, and risk factors will be analysed.​

Surveys

The surveys are aimed at Swedish sheep owners and vets. The questions concern management and grazing practices that may influence the incidence of anaplasmosis/grazing fever, as well as experiences of the disease.

It takes just 10 minutes to complete the survey.

The survey consists of three sections: i) general, ii) anaplasmosis, and iii) treatment and economics.

Even if you have no experience of anaplasmosis/grazing fever, we would appreciate it if you could answer the questions on management and grazing practices.

You can find the questionnaires here:

Sheep owners: https://netigate.se/ra/s.aspx?s=1308289X567643781X58279

Veterinarians: https://netigate.se/ra/s.aspx?s=1308290X567591888X52298

Results

The new knowledge about which Anaplasma species the ticks carry, any co-infections, as well as the risk factors associated with the occurrence and course of disease, will form the basis for updated and improved advice on the disease and how it can be prevented. Overall, the new knowledge will create better conditions for implementing preventive measures in time to minimise animal suffering, antibiotic use and production losses, and to strengthen the competitiveness of Swedish sheep production.

Link to the referral document can be found here: https://www.sva.se/media/sdggu3tc/remiss-projekt-aggressiv-anaplasma-ett-hot-mot-vaara-betande-noet-och-faar.pdf