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Endoparasites - Seasonal Variation in Worm Burdens

The number of parasites infecting animals ("parasite load") varies with the seasons. This burden is generally heavier in warm months declining in winter. This is largely due to the requirement of parasites for warmth and moisture in their development outside the host. Eggs hatch more readily and immature parasite forms develop faster with high humidity and warm temperatures. In summer months, the worm's life cycle is shorter but a higher proportion of larvae may be killed if conditions are very dry. Over-wintered infective third stage larvae can contribute to parasite burdens in the spring. Therefore, worm burdens in sheep increase tremendously during the warmer seasons.

Seasonal variations affect the activity of parasites within animals, although under NZ conditions reasonable burdens of nematode worms infecting sheep are found to a greater or lesser extent throughout the year. Development generally slows down in the late autumn-early winter. Infective larvae that enter hosts in autumn may stop developing and remain in larval stages for some time. This inhibited or arrested development is called hypobiosis. A similar phenomenon occurs with parasites in sheep living in areas that become very dry for some period of the year. Arrested development is advantageous to the parasite as it is a means of delaying egg production until the external environment permits larval development.

In the hypobiotic state, parasites save energy by not producing eggs that would die if excreted from the sheep. Furthermore, hypobiotic parasites are more resistant to drugs intended for their destruction. In spring, or when moist conditions return, arrested larvae resume their development and adult parasites begin to lay increased numbers of eggs. The newly mature parasites produce more eggs which yield more infective larvae. This effect of increased egg production is known as the spring rise in egg counts. Spring lambing occurs concurrently with the spring rise (periparturient rise). Therefore, the increase in the population of infective larvae coincides with the increased availability of new susceptible hosts. The result is a heavy increase in the number of adult parasites infecting a flock. Spring and periods of heavy rainfall are; therefore, important times to treat sheep with antiparasitic drugs.

The interactions of seasonal variation in worm burdens, hypobiosis, and spring rise have been examined to give detailed reasons for the seasonality of endoparasite infections. Studies of the trichostrongyle family (Ostertagia, Haemonchus, Trichostrongylus, Cooperia, Nematodirus and Mecistocirrus) have revealed that egg output from sheep endoparasites peaks at certain times during the warm months. The first rise is in late spring or early summer and is due to increased shedding of eggs from parasites in ewes one or two months following lambing. This is a manifestation of the spring rise. Lambs kept with ewes become infected by larvae developing from these eggs. As these worms become mature, more eggs are shed onto pasture. This establishes a second rise in eggs and larvae in late summer or early fall.

Increased numbers of larvae on pasture produce higher rates of infection. Clinical signs of disease may become more frequent following these peaks.

Summer rainfall may influence the ability of some parasite larvae to develop. Haemonchus and Trichostrongylus larvae, for example, thrive in the warm moisture associated with summer rains, whereas development of Ostertagia larvae is curtailed by heat, especially in conditions of low humidity. Ostertagia larvae progress rapidly to the infective stage in autumn. Rainfall has been associated with disease inflicted by Fasciola. Heavy rains in early summer favour development of the snail intermediate hosts, which causes a greater prevalence and severity of Fasciola infections in the following winter.

Hypobiosis – Nematode Numbers, Various Stages – Cold Climate


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