[Frontiers in Bioscience 2, d471-481, September 15, 1997]|
THE COLOSTRUM-DEPRIVED, ARTIFICIALLY-REARED, NEONATAL PIG AS A MODEL ANIMAL FOR STUDYING ROTAVIRUS GASTROENTERITIS
Department of Animal Science, North Carolina State University, Raleigh, NC 27695-7626 and Center for Gastrointestinal Biology and Diseases, Chapel Hill and Raleigh, NC
Received 9/5/97 Accepted 9/10/97
7. CLINICAL MANIFESTATIONS OF ROTAVIRUS INFECTION
A detailed overview of the rotavirus replication cycle as studied in continuous cell cultures from monkey kidneys has been described by Estes (65) and it appears that a similar replication process may happen in the enterocytes of the small intestine. The pathogenesis of diarrhea caused by rotavirus infection as studied in miniature swine piglet has been described by Graham et al. (66).
Despite the variability of experimental conditions described above, induced rotavirus gastroenteritis in neonatal pigs, particularly when porcine rotavirus isolates were used (12, 14, 16, 17, 20, 22), has been characterized clinically by anorexia, diarrhea, ocassional vomiting, loss of weight and high titers of rotavirus shedding in feces.
The onset of diarrhea in practically all studies occurred within 2 to 3 days after rotavirus inoculation. The duration of diarrhea, however, was quite variable. Middleton et al. (8), using conventional pigs which were removed from the sow at 2 to 3 days of age and inoculated per os at 6 days of age with human rotavirus, reported that diarrhea began 32 hours after virus ingestion and lasted for only 24 hours. In another study (7), none of the gnotobiotic pigs intranasally inoculated with human rotavirus developed any clinical signs of infection during the following 3 to 4 weeks; however, pigs infected with porcine rotavirus had profuse diarrhea as early as 18 hours after viral infection. Others reported diarrhea duration varying from 3 to 6 days (20) to 10 to 18 days (11, 17) after rotavirus inoculation. In all cases, severity of diarrhea diminished with time after rotavirus inoculation.
Large amounts of virus in feces were found shortly before or at the onset of diarrhea (7, 8, 10, 11, 14, 21, 22); the amount of virus shed in infected pigs feces exceeded the quantity of virus ingested (8). Rotavirus was detected in infected pigs feces regardless of whether or not they had diarrhea (15).
Refined experimental procedures used in our rotaviral gastroenteritis research during the last 2 years have provided additional detailed information, particularly on the extent of anorexia, severity of diarrhea and magnitude of fecal virus shedding, as well as on the effect of rotaviral infection on growth and mortality rate during the post-inoculation period. The detailed experimental protocol including a description of the scales used for feces scores and agglutination scores used to assess fecal rotavirus shedding has been previously reported (22). Under our experimental conditions, the onset of diarrhea occurred within 48 hours after virus ingestion, profuse diarrhea lasted for at least 5 days (from 2 to 7 days post-inoculation) and diarrhea progressively ceased thereafter (Figure 1A). Furthermore, high levels of rotavirus shedding were found in feces of infected pigs from 2 until 7 days after virus ingestion and declined thereafter (Figure 1B). In this particular study, infected pigs were inoculated with 7.8 x 104 rotavirus particles per pig at 6 days of age (Gomez et al., unpublished results).
Figure 1. Effect of rotavirus infection on feces consistency (A) and fecal rotavirus shedding (B). Each value is the mean ± SEM of 10, except for day 19 when n = 9, pigs per group.
The inoculation of rotavirus produced vomiting in ~40% of pigs within 24 to 48 hours after rotavirus ingestion and before the onset of diarrhea. Because of the vomiting and the relatively long period, practically one week after virus ingestion, of anorexia (Figure 2B) along with the diarrhea produced by rotavirus infection, infected pigs lost or gained little body weight during the week following rotavirus inoculation (period from 6 to 13 days of age, Figure 2A). Thereafter, infected pigs resumed growth to the extent that their average daily gain during the period from 13 to 19 days of age (Figure 2A) was similar (p > .05) to that of non-infected pigs (228 ± 12 vs. 232 ± 17 g per day, respectively). Values are means ± standard error. However, the average body weight of infected pigs at the end of the trial (19 days of age) was significantly lower (p < .01) than that of non-infected pigs (3.34 ± .26 vs. 4.61 ± .29 kg). Except for the rotavirus inoculation, non-infected pigs were otherwise simultaneously but separately reared under similar conditions (Gomez et al., unpublished results).
Figure 2. Effect of rotavirus infection on body growth (A) and diet intake (B).
In a few studies (10, 14), 25% to 30% of gnotobiotic rotavirus infected pigs died by 4 to 5 days after virus ingestion. However, in most of the reports, the effects of rotavirus infection on mortality of pigs have been seldom studied mainly because of the short duration of the experimental periods required to ascertain intestinal damage caused by rotavirus infection (7 - 10, 12, 13, 15) rather than the long term effect on pigs' survival. The dose and infectivity of rotavirus inocula, the age and condition of pigs at time of inoculation and the duration of the experimental periods appear to be the main factors responsible for the variable cumulative mortality caused by rotaviral enteritis. In several experiments carried out in our laboratory, colostrum-deprived pigs reared under similar conditions and infected by 5 to 6 days of age with the same source of rotavirus at doses between 106 and 107 rotavirus particles have shown mortality rates between 40% and 50% by the end of the experimental periods (10 to 12 days post-inoculation)(21; Gomez, unpublished results); lowering the rotavirus dose to 105 or 104 has reduced the mortality to 10% and 20% (Gomez, unpublished results). Increasing the dose of rotavirus to 8.4 x 108 resulted in a cumulative mortality of 70%, with most of the mortality (45% from 15% to 60%) occurring between 6 and 8 days after virus ingestion (22).