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Internal Parasites of Sheep and Goats

Description
Common Internal Parasites of Sheep and Goats



Common internal parasites of sheep and goats include the following:

  • Haemonchus contortus { Barberpole Worm }

  • Trichostrongylus spp { Hair Worms }

  • Fasciola hepatica { (Liver fluke)

  • Moniezia expansa { Tapeworm }

  • Strongyloides papillosus { worms }

  • lungworms

  • coccidia



    See belowfor pictures


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    The commonest g.i. nematodes are Haemonchus contortus, Ostertagia circumcincta, and Trichostrongylus axei and colubriformis.

    Haemonchus is however the most important of all because of its capacity for sucking blood. The others are of little importance from the point of view of prevalence or pathogenic effects since they are rarely present in sufficiently high numbers to cause clinical disease, although they contribute in no small way to economic losses due to sublinical problems. However, because the epidemiology and control of these g.i. worms is similar, devising a control program for the most pathogenic - H. contortus - will effectively control them all.

    Strongyloides papillosus is fairly common in suckling non-immune lambs.

    The cestode, Moniezia expansa is prevalent in lambs during summer but is of little consequence.

    The major nematode pathogens of sheep are also the major pathogens of goats. However, there are some important differences. Browsing goats and goats reared in confinement are not exposed to the same nematodes as grazing sheep. In addition, dosages of anthelmintics for goats should be similar to those of cattle rather than sheep because cattle and goats metablize anthelmintics more rapidly than sheep.



    The Periparturient Rise (PPR)


    The major g.i worms have a direct life cycle. Eggs passed in the feces of infected hosts hatch and develop through three larval stages to L3, the infective stage. Under optimal conditions of moisture and temperature, this can happen in as little as a week. Parasitic development from L3 to egg-laying adults takes approximately 2-4 weeks.

    These nematodes have developed several adaptations to ensure their survival and transmission from one host generation to the next - arrested development, PPR, prolonged survival on pasture and a long adult life -span. These g.i. nematodes are one of the most important impediments to growth of intensively grazed lambs.

    L3's of Ostertagia, Trichostrongylus and Nematodirus survive well on pasture overwinter. Therefore sheep turned out to graze in spring are first exposed to infections with these speces. Haemonchus contortus more readily survives winter as arrested L4's. Since Haemonchus preparasitic stages require generally higher temperature for maximum development, L3's of H.C tend to be found later in the grazing season. In addition Trichostrongylus spp comprise the bulk of the adult worm population in sheep in fall and winter since thay have a longer adult life span than the others, such as Haemonchus.

    As previously mentioned, because of the dominant features of temperature and moisture on development, survival and transmission of larval populations, there are marked seasonal variations of g.i. nematodes both on pasture and in host animals.

    In a flock of sheep raised on pasture during spring, summer and fall and housed in winter, the pattern is as follows.

    About 4 weeks after the start of lambing, the fecal egg counts of these ewes begins to rise and peaks about 8 weeks after lambing. Since this was first noticed 50 years ago in spring lambing ewes, it was called the Spring Rise.

    Thus the term Spring Rise refers to a regular occurrence of a seasonal increase in the fecal nematode egg counts of sheep. It ocurs in all classes of sheep ,rams, non-pregnant females, pregnant and lactating ewes.

    In the latter, it is very pronounced and is called the PPR. since a similar event occurs in ewes lambing and lactating at other times of the year apart from spring. So that in spring-lambing ewes, the PPR is an amplifcation of an already occuring Spring Rise.

    Two factors associated with the PPR are : the immune status of the host and arrested development.

    Immunity against the g.i. nematodes develops slowly in sheep and lambs are not fully immune until they are about 6-9 months old. Adult sheep are immune and the expression of this immunity is the immune expulsion of an existing worm population - called Self Cure - triggered by the intake of infective larvae.

    However, in pregnant and lactating sheep, there is a decline in the immune status of the host - called the post/peri parturient relaxation of resistance believed to be caused by the hormones of pregnancy and lactation, particularly prolactin. The consequences of this relaxation of resistance is the impairment of immune expulsion i.e. no self-cure. and the establishment of new infections since these animals are now susceptible to infection.

    Many of the g.i. nematodes, particularly Haemonchus survive the winter as arrested early L4's. Resumption of development occurs in late winter-early spring at a time when most female sheep are in late pregnancy or early lactation.



    These various components interrelate as follows:


    The PPR in fecal nematode egg counts comes from three sources

    - Arrested larvae resuming their development.
    - Ingestion of larvae that have overwintered on pasture
    - Increased egg output from an existing worm population.

    The Post parturient relaxation of resistance means that these worm burdens will be maintained rather than expelled.

    After weaning this immuno-suppressive effect is removed, sheep regain their immune competence, resistance is restored and these adult worms are expelled by the process of self-cure.

    The major consequence of the PPR is that eggs passed by ewes are the major source of infective larvae for lambs during the grazing season.

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    Sequence of events


  • Lambing

  • PPR

  • Self-cure

  • Development of L3's on pasture

  • Infection of grazing sheep & lambs

  • Re-contamination of pasture with eggs and development to L3's

  • Arrested development in fall-grazing sheep

  • Decline overwinter of pre-parastic stages on pasture.

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    Disease considerations


  • Disease in ewes from the PPR - clinical and subclinical - milk production.

  • Disease in ewes, lambs and other grazing sheep due to the summer rise in L3's on pasture.

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    Parasite control in sheep


  • Preventing the PPR is the key to controlling g.i. nematodes in sheep. This must be done with an anthelmintic effective against arrested larvae - Levamisole or Ivermectin. The objective is to prevent disease in lactating ewes and to prevent ewes from contaminating spring pastures.

  • Treat lambs at weaning and move to a safe pasture - WHY?

  • Prevent by treating in spring. Four treatments at 3, 6, 9, and 12 weeks after turnout onto pasture in spring will effectively prevent the development of the second, disease-producing generation of worms.

  • Ivermectin and Levamisole are the drugs of choice because they are effective against arrested and benzimidazole resistant worms - resistance being a major problem in sheep g.i. nematodes.
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