Development of Brugia pahangi in the Jird, Meriones unguiculatus, with Notes on Infections in Other Rodents

Abstract

Infective larvae of Brugia pahangi dissected from Armigeres subalbatus that had fed 8 to 9 days previously on an infected dog were inoculated subcutaneously into the testicular area of 42 jirds, Meriones unguiculatus. All animals became infected. Early developmental stages were recovered from the pelt, carcass, lymph nodes, fat, testes, heart, and lungs. The third molt occurred from 6 to 9 days and the fourth molt from 18 to 24 days postinfection. Males started the last two molts from 1 to 2 days earlier than females. Twenty-five of 33 jirds developed patent infections with microfilaremias persisting for at least 13 weeks. Microfilarial densities varied considerably and appeared to be somewhat dependent on the size of the larval inoculum used. Jirds infected with 10 to 50 larvae usually developed low microfilaremias; high microfilarial densities were frequently see following inoculation of 70 to 100 larvae. The mean prepatent period for all infections was 67 days; the shortest time was 57 days, the longest 84 days. Adult worms were usually found either in the heart and lungs or in the testes, but the highest and most stable microfilaremias occurred in animals with a majority of worms in the testes. Other potential rodent hosts investigated included a second species of jird, M. libycus, a wood rat, Neotoma lepida, a kangaroo rat, Dipodomys merriami, and golden hamsters. Developing worms were found in all but the kangaroo rats, but microfilariae were produced only in M. libycus. Worms were found almost exclusively in the heart and lungs of these other rodents. Attempts to infect small rodents with lymphatic-dwelling filarial worms of man and other vertebrates have generally been unsuccessful. Probably the most concerted effort was that of Ramachandran and Pacheco (1965) who achieved limited success with Brugia pahangi in cotton rats. Other workers, Laing et al., 1961; Edeson et al., 1962; Zaini et al., 1962; Ahmed, 1967a, b; and Chong and Wong, 1967, tried to establish either B. pahangi or subperiodic B. mialayi in mice, rats, hamsters, and other rodents with little or no success. The need for a susceptible small animal host for studies on the biology, host-parasite relationships, and chemotherapy of human filariae has been stressed repeatedly (WHO Tech. Rep. No. 359, 1967). The ability of jirds to serve as suitable hosts for a variety of helminth infections suggested their possible usefulness as hosts for filariae of the genus Brugia. This study reports on the experimental transmission of B. pahangi to the jird, Meriones unguiculatus. Received for publication 13 February 1970. * This investigation was supported by the United State-Japan Cooperative Medical Science Program administered by the NIAID of the NIH, Department of Health, Education, and Welfare, U. S. Public Health Service Research Grant AI-07770. MATERIALS AND METHODS A dog infected with B. pahangi was obtained in 1968 from Parke, Davis and Company; this strain originated in Malaysia and was brought to the United States in 1958 where it was maintained in cats and dogs (Schacher, 1962). A Singapore strain of Armigeres subalbatus, maintained in the insectary of the Division of Infectious and Tropical Diseases at UCLA, was used as the vector mosquito (Barr, 1964). Mosquitoes were fed on an anesthetized dog that had a microfilarial density of 200 to 400 per 20 mm3 of blood at the time of exposure. Infected mosquitoes were then maintained at a temperature of 24 to 26 C and a relative humidity of 70 to 80% for 8 to 9 days prior to their dissection. Mosquitoes were stunned in test tubes immersed in an ice bucket and were then divided, using fine forceps, into head, thorax, and abdomen. These parts were immediately placed in tissue culture fluid (Medium NCTC 135, available from North American Biologicals, Inc., 16500 N.W. 7th Avenue, Miami, Florida), at a pH of 6.7 to 6.9, in a Baermann funnel. Larvae rapidly migrated out of fragmented mosquitoes and settled to the bottom of the funnel. They were then drawn off and counted. Larvae handled in this manner did not adhere to the glassware, and remained active for several hours, provided the fluid did not become alkaline. This was superior to dissecting mosquitoes individually in terms of time and labor involved and the numbers and vigor of larvae obtained. The rodent hosts used were: Meriones unguiculatus, the dark-clawed mongolian jird; a second jird species, M. libycus; two native California rodents, Neotoma lepida, a wood rat, and Dipodomys merriami, a kangaroo rat, and commercially obtained