Brugia pahangi: quantitative analysis of infection in several inbred rat strains

We report a comprehensive study of the infectivity of Brugia pahangi in male and female rats of eight different inbred strains. A single infection of any inbred rat strain will produce rats that become microfilaremic, have occult infection, or clear the primary infection. The proportion belonging to any category is determined by the basic susceptibility level of that strain. Patency rates (blood microfilaria+) ranged from 24% (AO rats) to 73% (WKA rats). The period for which microfilaria were in the circulation was directly related to microfilarial burden, with rats carrying less than 50 mf/ml of blood patent for 11.8 weeks +/- 12.2; for 50-499 mf/ml it was 37.6 +/- 14.8 and for 500+ mf/ml it was 63.3 +/- 34.2 weeks. Suckling rats were resistant to infection (0 patent) and weanlings were intermediate in resistance between suckling and adult rats. Female rats were highly resistant to infection. Approximately half of amicrofilaremic rats have occult infections. A high proportion of patent infections involve the testes or testicular lymphatics. In the most susceptible rat strains, more than 95% of the administered L3 or developing L4 parasites were killed within 28 days. During the course of the first 6 months, the ratio of males to females fell significantly, suggesting a shorter life span in male worms. The features of the infectivity/patency patterns in rats are compared with recognized patterns obtaining in human populations. We conclude that rats provide a valuable and underutilized model for the experimental analysis of filarial infections.