Lecture 5 -- Epidemiology of Parasitic Gastro-Enteritis (PGE)

What is Parasitic Gastro-Enteritis (PGE)?

A condition causing diarrhea in lambs in August to September

• Primarily influenced by T. circumcincta and Trichostrongylus spp

• Cooperia spp. does not necessarily cause PGE but it often contributes to overall infection with T. circumcincta and Trichostrongylus spp.

What are the factors that influence the severity of PGE?

• Concurrent infection

• Nutritional status of sheep

• Development of immune response

  • Young lambs’s immune response takes several months to mature

What are the typical clinical signs of PGE?

• Diarrhea

• Poor weight gain or overall weight loss in lambs.

What factors might be important in leading to outbreak of PGE?

1. Peri-parturient rise in ewes

   • When ewes graze alongside lambs, they can contribute to pasture contamination

   • Because of reduced immunocompetence of ewe around parturition, egg output particularly increase prior to and for several weeks after parturition from ewe

2. Hypobiosis of larvae

   • Decreased temperatures in autumn trigger arrested development of larvae inside the sheep (Stop in L4 and not going to make it to L5 until following spring) → Stay over winter → Extends PPP to 5-6months

3. Overwintered L3

   • If the pasture was grazed by the lambs previous year, there will be L3 on that pasture and survive over the winter

When does the peri-parturient rise (PPR) occur?

From ~2 weeks before to 6 weeks after lambing

Why can L3 larvae overwinter on pasture?

When larvae moult from L2 to L3, they retain L2 cuticle = Double ensheathed → Protects them from harsh environmental condition

How do overwintered L3 larvae add to infection pressure even when their numbers are too low to cause clinical disease?

• When lambs are typically turned out to pasture in March, they initially pick up low level of overwintered L3 from the environment

• After 3weeks PPP, they begin to shed parasite eggs in their faeces, which further contaminates the pasture

When do overwintered L3 larvae gradually die out, and why?

June

• Because they lose viability and have limited energy reserve

• After the beginning of June, there are few or no overwintered larvae on pasture

What factors does the development of trichostrongyle larvae (from eggs → L1→ L2 → L3) depend on?

• Temperature (Most important)

  • Cooler temperature slow down development

• Humidity

• Rainfall for larval dispersal

When does egg output / pasture infective larvae peak, and why?

~ August due to asynchronous development of egg

• Because temperature-dependents development rates differ, larvae appear on pasture a the same time

  • Eggs passed early in spring develop slowly due to cooler temp. and may take several months to reach L3

  • Eggs passed later develop rapidly in warmer conditions

• Result: L3 larvae from eggs shed at different times appear on pasture simultaneously

Describe a year in the life of T. circumcincta.

March-April:

• Overwintered L3 on grass

• Ewes contaminate pasture from PPR

May-June:

• Lambs start to graze → Shed eggs after 3 weeks PPP

June-July:

• Day-night temp. increases → Development of eggs starts to speed up