Quantitative
enzyme-linked
immunosorbent assays detecting
IgM to the soluble Mycobacterium leprae crude sonicate (CD75) and the synthetic
disaccharide antigen coupled to
bovine serum albumin (
ND-BSA) were assessed for their ability to determine early
infection in families/household contacts of
leprosy patients and employees of a
leprosy center working in close contact with
leprosy patients. Although
IgM to both
antigens (CD75 and
ND-BSA) correlated with the bacterial index (BI) assessed histologically on skin-biopsy samples, the level of
IgM antibodies to
ND-BSA was a much more sensitive
indicator of low bacterial loads. A 4.4-fold difference in antibody levels was observed between the mean group levels of endemic controls (N = 116) and
tuberculoid leprosy patients with a BI of 0 (N = 88), increasing to sevenfold in
tuberculoid leprosy patients with a BI of 1 (N = 20). Using a statistical cut off with endemic controls (mean + 2 S.D.), household/family contacts showed 30% seropositivity (N = 180) as compared to staff contacts who showed 17% seropositivity (N = 55). Percent seropositivity in family contacts was not related to the type of
leprosy of the index case (lepromatous vs. tuberculoid) or the
duration of treatment of the index case. Age of the individual in the family contact group had a significant influence on seropositivity. These results support the hypothesis that, in this community, factors other than the viable bacterial load of the index case, such as
genetic susceptibility, may be influencing the high rate of seropositivity in family contacts.
IgM ND-BSA antibodies seem to provide a good
indicator of low antigenic loads and could prove to be useful in detecting
subclinical infection before the onset of disease. Follow-up studies of these seropositive individuals are in progress to understand the relationship between seropositivity and the progress of clinical disease.