Page 172 - The Veterinary Laboratory and Field Manual 3rd Edition

Parasitology 141

to be the result of poor nutrition and/or poor
husbandry (for example, high stocking rate, lack
of grazing rotation and so on) which limits the
immune capability of the host animal and also
increases the degree of pasture contamination.
Good animal husbandry is a vital component of
parasite control.

PatHoLoGIcaL FIndInGS
Infected animals may develop watery or slightly
liquid diarrhoea, often with mucus (mucoid
enteritis) and sometimes with blood (haemor-
rhagic enteritis), but these signs may also be
seen in other diseases. At necropsy there is pres-
ence of nematode parasites in the digestive tract
(some may not be easily seen by the naked eye
Figure 3.23 Life cycle of a typical ruminant round- so a magnifying glass is useful). The intestinal
worm (for example, Haemonchus contortus). (A) wall and abomasal mucosa may be thickened
Roundworm eggs are shed in the faeces at the and swollen (oedematous). Sometimes there is
morula stage of development (B), (B)–(C) The L1 nodule development in the mucosa of the intes-
stage develops and the eggs hatch in the faeces in tine and enlargement (hypertrophy) of the
a day or two (if the conditions are favourable). The mesenteric lymph nodes.
second moult (L2) starts and continues complete,
in the environment. (D) The infective L3 remains dIaGnoSIS
within the L2 cuticle until it is ingested by the host • Clinical signs.
(sheep). (E) The L2 sheath is cast off in the aboma- • Microscopic examination of the faeces to find
sum and the L3 parasitic stage undergoes a moult the parasite stages (eggs, larvae). If larvae are
to the L4. The L4 moults to the L5 within the host present in faeces it is an indication that the
and develops into the sexually mature adult. (A) faecal sample is not fresh (unless lungworm
Eggs are passed in the faeces and the cycle starts is suspected, see page XX).
again. The time scale for the life cycle depends on • Faecal culture may be necessary to allow dif-
the environmental conditions but is commonly 2–3 ferentiation of strongyle eggs. This is done
weeks. The number of eggs produced by the para- by hatching the eggs and examining the
site will depend on the immune status of the host morphology of the larvae. Details of larval
and the fertility of the parasites present. identification can be found in Taylor et al.
(2007).
are only indicated by lower than expected weight • Post-mortem and total worm count with
gain or increased susceptibility to other diseases. identification of adult parasites.
Mild infection is often termed ‘sub clinical’. It • Note that regular examination of faecal sam-
should be noted that animals which develop high ples for the presence of parasite eggs is very
faecal egg counts may do so because they have important for good livestock husbandry. This
limited immunity to the parasites; this may be is because even low parasite burdens can
a result of a lack of previous exposure in young limit production and apparently healthy ani-
livestock, but in adult animals it is more likely mals may contaminate pasture with a large

Vet Lab.indb 141 26/03/2019 10:25