Weather and the ecology of bursate nematodes

Abstract

ABSTRACT. --The fr6e-living stages of bursate nematodes are affected in different ways by meteorological factors. The eggs and 1st and 2nd stage larvae live in the feces, and these factors act on them through the feces. The embryo in the egg is protected by the eggshell. The 1st and 2rid stage larvae feed on micro-organisms, and are affected by the species of microorganisms and their metabolic products. The 3rd stage, infective larvae do not feed but leave the feces and migrate onto the vegetation, where they are more directly affected by meteorological factors than the earlier stages. They are partially protected by their sheath. The degree day concept can be used to predict the length of time required for development to the infective stage. Under constant conditions at 100% relative humidity and 20-35 ° C, Trichos}rongylus colubriformis requires 90-115 degree days (temperature times number of days) to reach the infective stage, and Haemonchus contortus requires 95-144 degree days. The great majority of eggs dies within the 1st day after reaching pasture, altho most of them would develop to the infective stage in the laboratory. Only 0.03% of H. contortusand T. colubriformis eggs placed on pasture were recovered from the vegetation as infective larvae. When larvae of 1t. contortus were placed on pasture, 1.6% (about 50 times as many but still only aout 1/30 of their potential) were later recovered. The cause of this die-off is unknown. Two types of potential worm burden prediction must be made-general and specific. The former can be made on the basis of temperature and moisture, but the latter requires information on many modifying factors.

Caecal threadworms Trichostrongylus tenuis in red grouse Lagopus lagopus scoticus: effects of weather and host density upon estimated worm burdens

“…The host has no effective acquired immunity to reinfection (Shaw & Moss, 1989 a) and parasite burdens are closely related to faecal egg counts (Moss et al 1990a). Such simplicity offers the opportunity to examine ecological principles of nematode/host interactions without some of the complexities that make it hard to predict parasite burdens in many other host species (Levine, 1980). The present study addressed two main questions: (1) Were estimated burdens of caecal threadworms in red grouse most strongly associated with host density or with weather conditions?…”

“…This distinction, between the likely demographic consequences of relatively constant and widely varying transmission rates, is similar to the dichotomy in ecological theory between density-dependent and density-independent factors in limiting animal populations. Thus, Anderson (1991) recommended density-dependent regulation to epidemiologists, whereas Levine (1980) emphasized the density-independent variables of temperature and moisture for predicting nematode population size (intensity) in the host.…”