|dc.description.abstract||This thesis records a series of experimental observations on Bunostomum
trigonocephalum. The studies were made under the following broad headings:
a) pre-parasitic development
b) comparison of routes of infection and studies of the mechanism
of infection including exsheathment of the infective larvae
c) the pathogenesis and patnogenicity of the parasite for the sheep
The morphology and development of the first 1 second and third larval
stages was examined, The moult of the first to the second stage was observed
and the first stage cuticle appeared to disintegrate rather than be cast as
an intact sheath. This moult occurred after about 5 days incubation at 21°c.
Little larval growth occurred after the first moult and the second moult followed
comparatively quickly, 80% of the larvae were in the infective stage after
7 days incubation.
0 0 Survival of infective larvae was compared at temperatures of 0 C, 10 c,
0 0 0 0
20 C, 27 C and 37 C. Larvae survived longest at 10 C.
Infective larvae were administered to sheep intravenously, subcutaneously,
intraruminally, orally and percutaneously. The largest infections were established
after percutaneous administration.
Infective larvae applied to the skin of sheep exsheathed rapidly and
penetrated the keratinised layers of the skin. They were also found to exsheath
on and penetrate into the skin of calves, gtli.nea pigs and rabbits.
There was evidence that larvae migrated to the lungs in the guinea pigs and
patent infections were established in some calves,
During exsheathment the anterior portion of the sheath appeared to disintegrate
allowing the larvae to emerge, This is unlike the processes which
have been described in other strongylate nematodes.
A variable proportion of the infective larvae were found to exsheath
"spontaneously" in faecal cultures,
Attempts were made to find what stimulates the infective larvae to exsheath,
The results were inconclusive. The highest rate of exsheathment
was obtained by·exposing larvae to buffer of pH 2. The addition of co2 enhanced
the exsheathment rate but only at pH 2. Even so, the results were
variable and difficult to reconcile with conditions on the skin surface.
After infection of sheep the prepatent period was found to range from
54 to 60 days. The 24 hour egg-output per female worm was estimated in 5
infected sheep and was found to range from 2579 to 10480 eggs. Most of the
Bunostomum were found in the intestine between 5 and 35 feet posterior to
the gastric pylorus. Numbers of Bunostomum in excess of 2-300 caused a
significant and progressive decline in haemoglobin, packed cell volume and
total serum protein levels over observation periods of up to 16 weeks after
infection, The serum protein decline was primarily due to a fall in albumin
concentration. The animals so affected also showed evidence of anorexia,
retarded growth rate and loss of weight, Diarrhoea occurred sporadically.||en_US