Longitudinal Study on Seroreactivity of Goats Exposed to Colostrum and Milk of Small Ruminant Lentivirus-infected Dams

Abstract

Introduction: Small ruminant lentivirus (SRLV) causes caprine arthritis-encephalitis in goats and maedi-visna disease in sheep. Transmission is via ingestion of colostrum and milk from infected dams or long-term direct contact between animals. Lifelong seroconversion can occur several weeks after infection via ingestion. However, sub-yearling lambs that ingest contaminated colostrum may be able to clear the infection and become seronegative. Whether a similar phenomenon occurs in goats remains unknown. Therefore, the serological status of goats was studied longitudinally from the moment of natural exposure to colostrum and milk of SRLV-positive dams through the age of 24 months.

Material and methods: Between February 2014 and March 2017 a dairy goat herd was studied which had been infected with SRLV for more than 20 years and carried maedi-visna virus-like genotype A subtype A17. Thirty-one kids born to dams seropositive for SRLV for at least a year beforehand were followed. They ingested colostrum immediately after birth and then remained with their dams for three weeks. The goats were tested serologically every month using two commercial ELISAs. The clinical condition of the goats was also regularly assessed.

Results: Out of 31 goats, 13 (42%) seroconverted at the age ranging from 3 to 22 months with a median of 5 months. Two goats seroconverted in the second year of life. The other eleven did so before the age of one year; two of these reverted to seronegative status. Only 9 out of 31 goats (29%) seroconverted in the first year of life and remained seropositive. They were early and stable seroreactors to which SRLV was transmitted lactogenically. The age at which they seroconverted ranged from 3 to 10 months with a median of 5 months. In 8 of the 18 persistently seronegative goats, a single isolated positive result occurred. No goats showed any clinical signs of arthritis. The level of maternal antibodies at the age of one week did not differ significantly between the stable seroreactors and the remainder.

Conclusion: Seroconversion appears to occur in less than 50% of goats exposed to heterologous SRLV genotype A via ingestion of colostrum and milk from infected dams and is delayed by 3-10 months. The natural lactogenic route of transmission of SRLV genotype A in goats appears to be less effective than this route of genotype B transmission reported in earlier studies.

Keywords: caprine arthritis-encephalitis; colostral antibodies; humoral immunity; maternal antibodies; seroconversion.

Fig. 1

Timeline of the serological status of 13 goats that seroconverted during the study. The orange band is the duration of maternal antibodies, the blue band is the time of seronegativity in both ELISAs, and the red band is the time of seropositivity in at least one ELISA. The direct transition from the period of maternal antibodies to the period of active seroreactivity by goat no. 28 results from the overlapping seropositivities of this goat in the two ELISAs used – see Fig. 3 for details

Fig. 2

Quantitative results of an indirect ELISA based on synthetic viral peptides (sp-iELISA; sample-to-positive control serum ratio, S/P%) and a competitive ELISA based on surface glycoprotein (SU-cELISA; percent inhibition, PI) in intermittent (goat no. 9 and goat no. 18) and late seroreactors (goat no. 13 and goat no. 20). Dashed lines indicate the manufacturer’s cut-off value used to interpret the results of the sp-iELISA (red; S/P% = 50%) and the SU-cELISA (green; PI = 35%). The arrows indicate the first positive result of the sp-iELISA (red) and the SU-cELISA (green) or the times at which an intermittent seroreactor was seropositive

Fig. 3

Quantitative results of an indirect ELISA based on synthetic viral peptides (sp-iELISA; sample-to-positive control serum ratio, S/P%) and a competitive ELISA based on surface glycoprotein (SU-cELISA; percent inhibition, PI) in early and stable seroreactors. Dashed lines indicate the manufacturer’s cut-off values used to interpret the results of the sp-iELISA (red; S/P% = 50%) and the SU-cELISA (green; PI = 35%). The arrows indicate the first positive result of sp-iELISA (red) and SU-cELISA (green)

Fig. 4

Quantitative results of an indirect ELISA coated with the mixture of synthetic viral peptides (sp-iELISA; sample-to-positive control serum ratio, S/P%) of 18 seronegative goats in the first 24 months of their lives. The red dashed line indicates the manufacturer’s cut-off value of S/P% = 50%. The dark solid line under the x axis indicates the time period in which maternal (colostral) antibodies were detected. The asterisk (*) indicates significant difference (α=0.05) compared to the age of 4 months

Fig. 5

Quantitative results of a competitive ELISA coated with viral surface glycoprotein (SU-cELISA; percent inhibition, PI) of 18 seronegative goats in the first 24 months of their lives. The green dashed line indicates the manufacturer’s cut-off value of PI = 35%. The dark solid line under the x axis indicates the time period in which maternal (colostral) antibodies were detected. The asterisk (*) indicates significant difference (α=0.05) compared to the age of 4 months