The clinical, parasitological and haematological parameters
of tick-borne fever (TBF) in sheep were assayed. Infections were
characterised by incubation periods of one to four days followed
by fevers which lasted from four to 12 days. The prepatent periods
ranged from two to seven days and. the visible parasitaemias per¬
sisted for six to ten days. Other clinical signs were minimal.
Haematological changes, however, were dramatic; an initial leucocytosis
was followed by a profound leucopaenia attributable first
to a fall in lymphocyte and eosinophil numbers and later to a neutropaenia.
The nature of the TBF-induced lymphocytopaenia was investi¬
gated and found to be associated with a significant decrease in the
number of peripheral B-lymphocytes and with a small reduction in
the number of T-lymphocytes.
Evidence of immunosuppression was sought and found by measuring
the primary and secondary antibody responses of TBF-infected sheep
to a commercial clostridial vaccine. Injections of the vaccine
elicited primary and secondary serum antibody responses in both TBFinfected
and normal sheep but the antibody titres in the infected
sheep were observed to be significantly lower than the titres in
the normal sheep. In contrast to the suppressed humoral immune
responses the cell-mediated response as measured by a delayed skin
hypersensitivity reaction to dinitrochlorobenzene was unimpaired in
the TBF-infected sheep.
The effect of TBF on phagocytosis by neutrophils was examined.
The phagocytosis of staphylococci was considerably reduced during
the TBF parasitaemia; this reduction was attributed to the limited
phagocytic activity of the parasitised neutrophils
Tick-borne fever affected the nasal carriage of Pasteurella
haemolytica in sheep; the rate of isolation of the organism from
swabs of the nasal secretions increased during the reaction to
Cytoecetes phagocytophila infection. The attempt to produce
pyaemia in lambs by superimposing TBF on an already existing
naturally acquired staphylococcal infection was unsuccessful. On
the other hand, concurrent infections of sheep with C_. phagocyto¬
phila and parainfluenza-3 virus produced respiratory distress and
deaths. The exacerbation of the respiratory disease in the animals
with dual infections was associated with a suppressed antiviral
It is postulated that the observed potentiation of concurrent
infections stems from the combined effect of the neutrophilic
malfunction and immunosuppression induced by C. phagocytophila