PRODUCTOS VACA LECHERA: PRODUCTOS PESOS MUERTOS:

Usually Nematodirus spp. are generally parasites of even-toed ungulates (Artiodactyla),

usually in pecoran ruminants although there are a small number of species in rodents (Hoberg et al. 1988; Hoberg 2005). N. spathiger have been recorded from sheep, cattle,

goats, deer, some gazelles, camels and rodents and N. filicollis has been recorded from

sheep, goats, cattle and deer (May 1920; Lichtenfels and Pilitt 1983). Both species have been found in domesticated ruminants and their wild counterparts (van Dijk and Morgan 2009). Several species of Nematodirus have been recorded from the host of

interest, the domesticated sheep, including: N. spathiger, N. filicollis, N. abnormalis,N. helvetianus and N. battus (Brunsdon 1967; McKenna 2009). The latter is not found in

New Zealand sheep and N. helvetianus is more commonly found in cattle (Brunsdon

1967; Pomroy 1997; McKenna 2009).

2.3.1 Host age and immunity

Nematodirus spp. are found in young lambs, being one of the first parasites that infect

the lamb in spring (Brunsdon 1960; Vlassoff and McKenna 1994). Infections of

Nematodirus occur mostly in lambs less than nine months old (Tetley 1935; Brunsdon

1963b) and in particular between two and five months old (Brunsdon 1967). Initially lambs have little capacity to resist infection (Vlassoff et al. 2001). However, resistance

eventuates and results in: increased prepatent period, stunted adult worms, fewer numbers establishing in the host and inhibited larval development and fewer eggs produced (Brunsdon 1962). Resistance develops rapidly, with egg output declining suddenly and in certain cases within two months after the initial infection (Brunsdon 1963). Additionally it is a strong response and in general lasts throughout the animal’s life. Adult ewes rarely have Nematodirus burdens, even during the post-parturient rise Nematodirus are absent, though most other trichostrongyloid genera are found

(Brunsdon and Vlassoff 1971; Vlassoff 1973). It seems both age and acquired resistance contributes to the response to Nematodirus. Age resistance per se was reported in

previously worm-free two-tooth sheep (18 months), with a lower FEC, fewer worms and a longer prepatent period, than in lambs (6 months) (Brunsdon 1962). It also appeared that age-related resistance was stronger against N. filicollis than N. spathiger.

However Tetley (1935) concluded that age-related resistance did not occur, as a 10 month old lamb that was naïve to nematodes gained an infection of Nematodirus.

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Though Tetley’s studies are conducted with few animals they instead suggest acquired immunity was most important as he had already shown infections occur more predominately in lambs less than 9 months. Brunsdon (1963b) also discovered that the stimulation of resistance to N. filicollis and N. spathiger was due to the pre-patent

development of larvae and not the results of an established patent infection.

2.3.2 Pathology

Only a few studies have given details of the pathology related to N. spathiger and N. filicollis. The clinical signs of disease in lambs with Nematodirus infections are: loss of

appetite, sometimes failure to gain weight or weight loss, profuse diarrhoea and dehydration (Thomas 1959a; Kates and Turner 1953, 1955; Brunsdon 1961). The outbreak of disease commonly occurs suddenly (Brunsdon 1961). Lambs that were given an experimental dose of N. spathiger presented with tissue damage to the

intestinal wall. In histological sections of the jejunum, the mucosa was eroded (Kates and Turner 1955). Damage to the mucosa occurred within five days of infection, with the loss of epithelium from the villi and necrosis of the lamina propria. Damage was so severe in some parts few villi remained. Slightly longer infections resulted in similar damage though some of the anterior ends of the immature worms penetrated the muscularis mucosa (Kates and Turner 1955). The consensus is that tissue damage is caused mostly by the developing larvae and the clinical signs are a result of this damage. The disease is more difficult to diagnose due to these immature worms, as eggs are not being produced the FEC does not reflect the actual parasitic burden (Brunsdon 1961), however it could be argued that FEC never reflects the true worm burden for

Nematodirus as they are poor egg-producers as evidenced by Chalmers (1985) study. 2.4 Pathogenicity

Nematodirus is described as a genus with pathogenic potential (Kates and Turner 1953);

however there appears to be a lack of experimental studies conducted on the individual effects of N. filicollis and N. spathiger in New Zealand lambs. References to their

pathogenicity are related to reports of clinical disease and mortality when acute outbreaks occur. Of such reports, infections of up to 45,000 worms per host have been found with mortality rates of 6-10% (Brunsdon 1961; Anonymous 2010). International studies on pathogenicity of N. spathiger in experimental infections have shown effects

27 uninfected lambs compared to lambs administered a large dose (300,000-900,000) of N. spathiger larvae (Kates and Turner 1953). Similarly a 26.5lb weight difference occurred

after 10 weeks with a mixed Haemonchus contortus and N. spathiger infection

compared to the uninfected control (Kates and Turner 1960). Evidently this is an additive effect of the dual species, as the study showed the weight gain was not as pronounced in a pure H. contortus infection (Kates and Turner 1960). It would be

thought a similar effect would be found to these studies using New Zealand isolates of

N. spathiger, given the similarity in clinical signs and mortality.

Before 1950 Nematodirus was considered non-pathogenic in New Zealand, however

since this time, acute outbreaks occurred, as mentioned above (Brunsdon 1961). The more current thinking is that these two species are classed as of major importance in New Zealand sheep (Pomroy 1997). Brunsdon (1961) indicated Nematodirus generally

only occur in lambs in moderate numbers. It seems this would be true today, possibly as a result of interventions with anthelmintics; and as a UK-based study suggested,

Nematodirus normally contributes to the mixed nematode burden that a lamb

experiences (van Dijk and Morgan 2009).