Tick pyemia is mainly a staphylococcal superinfection of lambs already infected with the bacterium Anaplasma phagocytophilum, the causative agent of tickborne fever. It was initially assumed that ticks were injecting Staphylococcus aureus during feeding. However, further studies confirmed that tick pyemia results from a decrease in the phagocytic ability of neutrophils infected with A phagocytophilum. Crippling abscesses then form in joints and other parts of the body. Diagnosis is based on clinical signs, isolation of bacteria from lesions, and demonstration of A phagocytophilum in granulocytes in blood smears and by PCR assay. Penicillin or tetracycline can be used for treatment.
Tick pyemia mainly affects lambs 2–12 weeks old, but it can occur in ruminants of all ages. It is characterized by debility, crippling lameness, and paralysis. Pyemic abscesses are common in joints but can be found in virtually any organ. The disease causes considerable economic loss through debilitation and death.
Tick pyemia is enzootic in many regions of the UK and Ireland where the tick Ixodes ricinus is common, and it is likely present in other parts of Europe where the same tick is found. Tick pyemia in lambs in the UK was also reported to be associated with infestation with the tick Haemaphysalis punctata harboring Babesia motasi and Theileria luwenshuni.
Etiology of Tick Pyemia in Lambs
Staphylococcus aureus is regarded as the main cause of the abscesses in tick pyemia because it has been isolated consistently from superficial and deep-seated lesions, and it is rare to find other bacteria in these lesions. The name tick pyemia reflects that this condition occurs in sheep grazing pastures infested with ticks. It was initially assumed that the ticks were injecting S aureus during feeding. However, further studies confirmed thatthe condition is due to a decrease in neutrophils' phagocytic ability once they are infected with the bacterium Anaplasma phagocytophilum (1). Therefore, the key role of I ricinus in tick pyemia is not as a vector of S aureus but rather as a vector of A phagocytophilum, which in turn predisposes lambs, adult sheep, and other ruminants to secondary infection with S aureus and other pathogens.
When ruminants are infested by I ricinus ticks harboring a ruminant-specific variant of A phagocytophilum, they may develop the febrile disease tickborne fever and recover after a week or two. However, because the bacterium infects neutrophils and other granulocytes and causes immunosuppression in other ways, infected animals are prone to superinfection with other bacteria, viruses, and fungi. Therefore, sheep, cattle, and other ruminants recovering from tickborne fever are also prone to tick pyemia, abortion, pneumonia, louping ill, pneumonic pasteurellosis, and other secondary infections.
Lambs affected with tickborne fever have severe leukopenia, and their peripheral blood neutrophils are less capable of phagocytizing and killing S aureus. Experimental studies have shown that lambs with tickborne fever are more susceptible to experimental infections with S aureus while neutropenic and that as many as 30% of lambs with tickborne fever may develop staphylococcal infections.
The epidemiology of tickborne fever is closely related to the biology of I ricinus. The disease is limited to areas populated by I ricinus and to seasons of the year favoring high tick populations and activity.
Clinical Findings of Tick Pyemia in Lambs
In tick pyemia, abscesses form in various parts of the body, mainly joints, tendon sheaths, and muscles, resulting in lameness—hence the common use of the term “crippled lambs.” In some outbreaks, > 30% of lambs may be affected; they are usually listless and lame and often suffer from loss of body condition.
Internal abscesses without joint lesions may not result in clinical signs other than loss of condition; however, when CNS lesions are present, ataxia, paraplegia, or other neurological signs may occur.
The crippling disease lasts for days or weeks, but tick pyemia may also appear as acute septicemia. On occasion, sudden deaths may occur from development of multiple internal abscesses without visible clinical signs. Up to 50% of affected lambs may die, and survivors recover slowly.
Lesions
Other than in joints and other superficial structures, abscesses in tick pyemia are commonly found in the liver, lungs, and kidneys. They can also be present in the meninges of the spinal cord and in the pericardium and myocardium. The diaphragm, thymus, and adrenal glands are less commonly affected. Ticks are often found attached to an inflamed area.
Diagnosis of Tick Pyemia in Lambs
Clinical signs
Isolation of S aureus from lesions
Demonstration of A phagocytophilum by PCR assay
History and clinical signs are valuable indicators of tick pyemia. Diagnosis is based on the restriction of the disease to lambs introduced to tick-infested areas during seasons of tick activity and on the demonstration, by PCR assay, of DNA specific to A phagocytophilum in blood samples or of typical cytoplasmic inclusions in neutrophils in peripheral blood. Isolation of S aureus from lesions and the absence of other bacteria helps confirm tick pyemia.
The loss of condition and ill thrift, without lameness, can be difficult to recognize as tick pyemia, and the acute condition can be confused with other septicemic diseases. Tick pyemia may also resemble other suppurative infections of the newborn, including navel ill and joint ill, due to infections with other bacteria such as streptococci and Trueperella pyogenes.
Treatment and Control of Tick Pyemia in Lambs
Penicillin or tetracycline
Tick control
Treatment of clinical cases of tick pyemia with penicillin or tetracycline can be effective, provided lesions are not too advanced.
Administration of long-acting oxytetracycline (20 mg/kg, IM, once) to newborn lambs at risk during the first weeks of life can help prevent both tickborne fever and tick pyemia. A single injection at double the standard dose of oxytetracycline (40 mg/kg, IM) administered at 3 weeks old can substantially decrease mortality and morbidity rates in young hill lambs on tick-infested pasture and can improve weight gain and condition in the remainder of the flock.
Prophylactic treatment with long-acting oxytetracycline can prevent development of tickborne fever and accompanying pyrexia and immunosuppression for up to 3 weeks; consequently, the incidence of tick pyemia and other secondary infections, such as pasteurellosis and colibacillosis, will be decreased (2, 3, 4).
Although treatment with oxytetracycline may inhibit the development of immunity to tickborne fever, if treated lambs eventually develop this infection, they will be several weeks older and less susceptible to tick pyemia. Deliberate exposure of lambs by injecting them with A phagocytophilum and then treating them with oxytetracycline could provide some immunity before they enter tick-infested areas; however, lambs must be injected with region-specific bacterial strains because some strains of A phagocytophilum have no cross-immunity. In addition, use of prophylactic antimicrobial agents in food-producing animals may be prohibited in some countries.
Control of tick infestation is the most effective prevention of tick pyemia. This can be achieved either by restricting ewes and lambs for the first few weeks of life to low-ground, tick-free pastures or by dipping ewes before lambing and administering acaricides as dips or smears on lambs. Pour-on preparations of cypermethrin or smears applied before to young lambs before they are moved from lambing fields to hill pastures reportedly also control ticks effectively.
Key Points
Tick pyemia is a secondary staphylococcal infection of lambs concurrently infected with the bacterium Anaplasma phagocytophilum.
The primary clinical sign is lameness caused by abscesses in joints or muscles.
Penicillin or tetracycline can be used for treatment.
Reducing tick infestation is the most effective form of control.
For More Information
Macrelli M, Phipps P, McGinley L, Medlock J, Johnson N. First report of fatal tick pyaemia caused by heavy infestation with the red sheep tick, Haemaphysalis punctata and co-infection with Babesia and Theileria species. Vet Rec Case Rep. 2020;8:e001267.
References
Woldehiwet Z. The effects of tick-borne fever on some functions of polymorphonuclear cells of sheep. J Comp Pathol. 1987;97(4):481-485. doi:10.1016/0021-9975(87)90026-0
Brodie TA, Holmes PH, Urquhart GM. Some aspects of tick-borne diseases of British sheep. Vet Rec. 1986;118(15):415-418. https://europepmc.org/article/med/3754669
Brodie TA, Holmes PH, Urquhart GM. Prophylactic use of long-acting tetracycline against tick-borne fever (Cytoecetes phagocytophila) in sheep. Vet Rec. 1988;122(2):43-44. https://pubmed.ncbi.nlm.nih.gov/3363826/
Tuomi J. Experimental studies on bovine tick-borne fever: 5. Sensitivity of the causative agent to some antibiotics and to sulphamezathine. Acta Pathol Microbiol Immunol Scand. 1967;71:109-113. doi:10.1111/j.1699-0463.1967.tb05148.x
