The physiology of infective processes of nematode parasites; the stimulus from the animal host

Abstract

The processes by which larvae of Trichostrongylus axei, T. colubriformis and Haemonchus contortus, and eggs of Ascaris lumbricoides (pig strain), Ascaridia galli, and Toxocara mystax infect the host have been studied. A stimulus from the host, depending for its activity on the concentration of undissociated carbonic acid, and on the Eh and pH, was required to start development of the parasite. The stimulus caused larvae to produce 'exsheathing fluid' which completed the second moult, or it caused the production of a 'hatching fluid' so that the eggs hatched. In bicarbonate-carbon dioxide buffers containing reducing agents under mixtures of nitrogen and carbon dioxide the exsheathment of larvae of Trichostrongylus axei and Haemonchus contortus increased as the concentration of the undissociated carbonic acid was increased. A concentration of 0$\cdot $5 $\times $ 10$^{-3}$ M caused 70% of the larvae of Trichostrongylus axei to exsheath at pH 7$\cdot $3 in 0$\cdot $02 M-sodium dithionite in 3 h at 37 degrees C. More than 1$\cdot $5 $\times $ 10$^{-3}$ M was necessary to get similar results with Haemonchus contortus. These species differed from all the others because their exsheathment was not inhibited by high concentrations of undissociated carbonic acid. At pH 6$\cdot $0 the relative activity of reducing agents was sodium dithionite > cysteine > ascorbic acid. At pH 7$\cdot $3 the activity of cysteine and ascorbic acid relative to sodium dithionite was increased. Larvae of Trichostrongylus colubriformis exsheathed readily in hydrochloric acid between pH 1$\cdot $5 and 2$\cdot $5 when the concentration of undissociated carbonic acid was 5 $\times $ 10$^{-3}$ M. Cysteine did not increase the exsheathment. The eggs of Ascaris lumbricoides, Ascaridia galli and Toxocara mystax hatched in bicarbonate-carbon dioxide buffers containing reducing agents under nitrogen-carbon dioxide if the concentration of undissociated carbonic acid was about 0$\cdot $25 $\times $ 10$^{-3}$ M at pH 7$\cdot $3. For the hatching of eggs of Ascaris lumbricoides at pH 7$\cdot $3 the activity of the reducing agents was sodium dithionite > cysteine > ascorbic acid. In all the species which were examined except Trichostrongylus colubriformis the undissociated carbonic acid was more effective at a higher pH. And, as a rule, reducing agents were relatively less effective at higher concentrations of undissociated carbonic acid. The addition of sodium chloride up to 0$\cdot $1 M and sodium taurocholate up to 0$\cdot $05 M usually increased activity. The stimulus for exsheathment was effective within 15 min for larvae of T. axei and 30 min for Haemonchus contortus. Though at these times only a few larvae had exsheathed, during subsequent incubation in water exsheathment often rose to 80% in 3 h. Longer periods were necessary for the stimulus to act on eggs of Ascaris lumbricoides and hatching seldom rose by more than 30% after the stimulus was removed. The process whereby the host provides a stimulus for the resumption of development of the parasite is related to specificity and the site of infection. It is suggested that suspension of development in the infective stage and the dependence upon the host for restarting development may be an adaptation to parasitism.