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High mortality in a sheep flock caused by tick-born diseases – a diagnostic challenge

Dr Roger Daniel BSc BVSc FRCVS

Fellowship Day 2019

Report of presentation

Farm animal pathologist Dr Roger Daniel discussed the value of thoroughly investigating outbreaks of disease where the cause is not immediately obvious. Dr Daniel described a case that encapsulated his work at the Wales Veterinary Science Centre in Aberystwyth which provides postmortem and diagnostic services, undertakes disease surveillance and monitors animal welfare.

In the case discussed, clinical signs began to appear in a flock of 180 hoggs (ewe lambs that had not been bred) and barren ewes about six weeks after they had been put on to hill land above the Elan Valley. The signs included dyspnoea, lethargy and lateral recumbency, and initially six animals were found dead by the farmer.

Postmortem examination of one of the dead animals revealed splenomegaly and lymphadenopathy: “This was an indication of tick-borne disease because we’d seen this sort of thing previously,” Dr Daniel said.

Polymerase Chain Reaction (PCR) testing on the spleen was undertaken by the Moredun Research Institute and confirmed the presence of Anaplasma phagocytophilum, the causative agent of tick-borne fever. “The result was positive, as we expected,” he added.

However, animals continued to be found dead and a second ewe was submitted alive for further investigation. Clinically, lethargy, dyspnoea and lateral recumbency were seen in this animal, and the findings on postmortem examination were similar to those in the first case. “Gross pathology wasn’t giving us any indication of the cause of the high mortality,” he said.

A blood sample had been taken from the animal before euthanasia. This was sent to the Moredun Research Institute and tested positive by PCR for A phagocytophilum. However, histology on samples from the sheep suggested a diagnosis of delayed swayback (copper deficiency), which “threw us a bit”, Dr Daniel admitted. Could swayback be the cause of the increasing mortality, or was there another reason? If so, what other causes should be investigated?

By one week later, 45 animals had died and a third animal was submitted. PCR testing again produced positive results for A phagocytophilum and the postmortem findings were minimal; however, histology in this case indicated a polioencephalomyelitis typical of the viral infection louping ill.

Louping ill is caused by a neurotrophic flavivirus, and results in morbidity and mortality of between 5 and 60 per cent.

Retrospective testing of the samples sent to the Moredun revealed evidence of louping ill virus in the first two animals examined. Immunohistochemistry on samples from the second and third animals, carried out at the Animal and Plant Health Agency (APHA) at Addlestone, detected the presence of louping ill virus antigen in the brain of both.

Dr Daniel explained that A phagocytophilum causes a leukopenia (a reduction in the number of circulating lymphocytes and neutrophils) and, in the case under discussion, this predisposed the animals to infection with louping ill virus. The neurotrophic nature of the virus produced the rapidly fatal polioencephalomyelitis.

“So tick-borne fever and louping ill virus coinfection was the likely cause of the high mortality, and by the end, 53 of this group of animals had died,” he said.

Summing up, he said that the case illustrated the importance of large animal expertise in pathology and specialist laboratory back up in conducting these types of investigations. It also showed the importance of questioning laboratory findings, so that a diagnosis could be reached. To maintain this type of service, government support was needed.

Following Dr Daniel’s presentation, a member of the audience commented on the importance of the sort of surveillance he had described: “Unless we go looking for these things…we’re not going to find these things, and in the changing world in which we are, I think we’re missing more things, and I think the things we’re missing are more significant.”

This was a risk, Dr Daniel agreed, noting that, for example, if it hadn’t been for specialist histopathologists, BSE would not have been spotted at the time it was.