Overview of Tick Pyemia
Tick pyemia affects lambs 2–12 wk old and is characterized by debility, crippling lameness, and paralysis. Pyemic abscesses are common in joints but may be found in virtually any organ. The disease causes significant economic loss through debilitation and death. The disease is enzootic in many regions of the UK and Ireland where the tick Ixodes ricinus is common, and it is likely to be present in other parts of Europe where the same tick is found.
Staphylococcus aureus is regarded as the main cause of the pyemic abscesses, because it has been isolated consistently from superficial and deep-seated lesions and it is rare to find other bacteria. The bacteria are believed to gain entry into the bloodstream either by direct inoculation during tick feeding, from local superficial wounds, or through the infected umbilicus. However, there is clinical and experimental evidence that I ricinus does not simply act as a vector directly injecting staphylococci into the bloodstream. The main role of I ricinus is as a vector of the rickettsial agent Anaplasma phagocytophilum, which causes tickborne fever (see Tickborne Fever), which in turn creates factors favorable to development of pyemia. 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 were more susceptible to experimental infections with S aureus during the period of neutropenia and that as many as 30% of lambs with tickborne fever may develop staphylococcal infections.
The epidemiology of the disease 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 climatically favoring high tick population and activity.
Abscesses form in various parts of the body, mainly in the 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 dull and lame and often suffer from loss of body condition. Internal abscesses without joint lesions may result in no clinical signs other than the loss of condition, but when lesions are present in the CNS, there may be ataxia, paraplegia, or other nervous signs. The crippling disease lasts for days or weeks, but the disease may also appear as an acute septicemia. On occasion, there may be sudden deaths resulting from multiple internal abscesses without other visible signs. As many as 50% of affected lambs may die, and the survivors recover slowly.
Apart from the joints and other superficial structures, abscesses are commonly found in the liver, lungs, and kidneys. They may 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.
History and clinical signs are valuable indicators. The restriction of the disease to tick-infested areas, its occurrence during seasons of tick activity, and demonstration of A phagocytophilum or specific DNA by PCR in the blood of affected lambs or other sheep in the flock are diagnostic features. Isolation of S aureus from lesions and the absence of other bacteria will help to confirm tick pyemia. The loss of condition and ill-thrift without lameness may 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 by other bacteria such as streptococci and Trueperella pyogenes.
Treatment of clinical cases of tick pyemia with penicillin or tetracycline can be effective, provided the lesions are not too advanced.
Control of tick infestation is the most effective prevention. This can be achieved either by restricting lambs and ewes to low-ground, tick-free pastures for the first few weeks of life or by dipping ewes before lambing and administering acaricides as dips or smears on lambs. In young lambs, pour-on preparations of cypermethrin or smears applied before lambs are moved from lambing fields to hill pastures reportedly control ticks effectively.
Administration of long-acting oxytetracycline at the time of risk can help prevent both tickborne fever and tick pyemia during the first weeks of life. A single injection at double the standard dose given at 3 wk of age can significantly reduce mortality and morbidity in young hill lambs on tick-infested pasture and improve weight gains and condition in the remainder. Prophylactic treatment with a long-acting antibiotic may prevent development of tickborne fever for as long as 3 wk, without pyrexia and immunosuppression, so that the incidence of tick pyemia and other infections such as pasteurellosis and colibacillosis are reduced. Although treatment with oxytetracycline may inhibit the development of immunity, if the lambs eventually develop tickborne fever, they are several weeks older and apparently less susceptible to tick pyemia. Deliberate exposure of lambs by injections, followed by treatment with oxytetracycline, could provide some immunity before the lambs enter tick-infested areas; however, strains specific to the area must be used because some strains of A phagocytophilum have no cross-immunity.