Dynamics of Prepatent Infections of Guinea Pigs with the Ruminant Parasite, Trichostrongylus colubriformis (Nematoda)

Abstract

Inoculating guinea pigs with from 625 to 20,000 larvae of Trichostrongylus colubriformis resulted in progressively more worms becoming established in 8or 12-day-old infections; however, the percentage of the inoculum that became established was essentially the same at all dose levels. Inoculating guinea pigs with from 10,000 to 160,000 larvae resulted in progressively earlier onset of clinical disease and death; 20,000 or more larvae constituted the LD1oo. Over half the 20 guinea pigs given 160,000 larvae died within 48 hr. Larvae penetrated the mucosa of the small intestine within 24 hr, and some fifth-stage worms were still embedded 12 days after inoculation. Live and dead worms were continually expelled during the prepatent period of infection. Since the original report (Herlich et al., 1956) of the guinea pig's suitability as an experimental host for the ruminant nematode parasite, Trichostrongylus colubriformis, this host-parasite system has been used in a number of immunological investigations (Gordon et al., 1960; Silverman et al., 1962; Wagland and Dineen, 1965; Dineen and Wagland, 1966; and Herlich, 1966). The studies of Herlich et al. (1956), Sturrock (1963), and Connan (1966) have indicated that expulsion of adult worms occurs with the onset of nematode egg production about 15 days after inoculation (DAI). This finding suggests that the host-parasite system is of optimal experimental value during the prepatent period of infection. A study of the dynamics of this phase of the infection is herein reported. MATERIALS AND METHODS As there is no apparent sex difference in the susceptibility of guinea pigs to infection with T. colubriformis (Herlich, 1958), young (200 to 400 g) of both sexes were used randomly in all experiments. Infective larvae of T. colubriformis were of the RLS isolate (Herlich, 1966) maintained in pure culture by continuous passage through helminthfree ruminants. Larvae used in these experiments were either from fresh cultures or from aqueous suspensions stored in a refrigerator at 4 to 5 C for no longer than 3 months. Such storage has no adverse effects on larval infectivity (Herlich, 1966). Within any single experiment, all guinea pigs were given larvae from the same suspension. Seven experiments were conducted to determine Received for publication 1 October 1968. the percentage of the larval inoculum that would become established as a result of 9 different inoculation levels and the effects of such infections on the guinea pigs. The number of larvae in the inoculum ranged from 625 to 160,000 per guinea pig, with the number being increased by twofold increments, viz. 625, 1,250, 2,500. Specific numbers of larvae administered and the intervals between inoculation and necropsy are shown in Tables I through III. The standard error of each level of inoculum was determined to provide 95% confidence in the uniformity of larval doses; the limits of variability for each dose level are shown in Tables I through III. There were 10 guinea pigs in each dose group in all experiments, with a total of 260. Inoculation and postmortem procedures have been described elsewhere (Herlich, 1966). The data were analyzed by Student's t