American Cotton Rat (Sigmodon hispidus) as an Experimental Host for Brugia pahangi

Abstract

Infective larvae of Brugia pahangi, dissected from Aedes aegypti (selected Liverpool strain) 9 to 11 days after feeding on an infected cat, were injected subcutaneously into the right and left groin of young cotton rats of both sexes. Although the size attained by the worm in the cotton rat was exceeded by that earlier described in the cat, the third molt was observed 2 days earlier than it had been in the cat; the fourth molt occurred later. In three of 18 cotton rats that were maintained for 55 days or longer after inoculation, microfilaremia was observed on the 83rd, 94th, and 95th days, respectively. In two animals, observed for 25 weeks, microfilaremia increased gradually to approximately 150 per 20 mm3 blood and exhibited continuous and considerable fluctuations during a 24-hr period. In over 90% of mosquitoes which fed on an infected cotton rat, the infective larvae, 1 to 19 in number, developed at a normal rate and produced patent infection in 10 weeks when inoculated into a kitten. In contrast to findings in the cat, no worms in the cotton rat were found in the lymphatics; subadults were recovered from subcutaneous tissues and the carcass, and mature worms usually were found in the right heart and lungs. The successful transmission of Brugia malayi and Brugia pahangi to domestic cats and other animals (Edeson and Wharton, 1958; Edeson et al., 1960) opened up new lines in filariasis research. The domestic cat was shown to be a good experimental host for B. malayi and B. pahangi infections, and the domestic dog a good host for B. pahangi. Besides the hazards of epidemics of distemper wiping out months of research work, in general it has been the experience of many workers that it is laborious and uneconomical to maintain cats and dogs in large numbers in the laboratory. In an attempt to find other suitable laboratory animals, Laing et al. (1961) inoculated six hamsters, six white rats, six white mice, and two dogs with subperiodic B. malayi and obtained patent infection in two hamsters, two dogs, and one white rat. Preliminary studies by Edeson et al. (1962) showed that four out of seven cotton rats inoculated with B. malayi developed patent infections. The purpose of the present study was to ascertain the suitability of the cotton rat (Sigmodon hispidus) as a convenient laboratory host for B. pahangi. MATERIALS AND METHODS Eggs of a selected strain of Aedes aegypti highly susceptible to B. malayi, B. pahangi, Wuchereria Received for publication 12 April 1965. *Present address: Department of Parasitology, Faculty of Medicine, University of Malaya, Kuala Lumpur, Malaysia. bancrofti, and Dirofilaria immitis infections were obtained from the Liverpool School of Tropical Medicine (Macdonald, 1962) and cultured in the insectary. Infective larvae for inoculation were obtained by feeding the mosquitoes on cats infected with B. pahangi and the mosquitoes were dissected 9 to 11 days later. Infective larvae obtained from dissected mosquitoes were placed in a few drops of normal saline and injected subcutaneously in the right and left groin region of the cotton rat as shown quantitativ ly in Table IV. The total inoculum was given on the same day. The strain of B. pahangi used was that which was isolated in Malaya, brought to New Orleans in 1958 (Schacher, 1962), and maintained in cats a d dogs. Cotton rats, obtained from animal dealers (Tumblebrook Farms, Brant Lake, New York), were 4 to 8 weeks of age when inoculated. Both male and female rats were used and were maintained in individual cages. Starting at 55 days after inoculation, samples of 20 mm3 of peripheral blood from the tail or leg veins were examined routinely on rats maintained for observations on patent infections. In rats which did not show patent infections, Knott's (1939) concentration technique was used on larger samples of blood obtained by draining the orbital sinus with a fine heparinized capillary pipette. On other occasions, up to 1 ml of blood was taken by heart puncture with a syringe. Blood films were stained with Giemsa's (35 to 40 drops in 100 ml of phosphate buffer, pH 7 to 7.2) and microfilariae were counted. All rats that were sacrificed at periodic intervals as well as those which died were examined for worms. The animal was skinned completely, and the skin was soaked in warm saline and allowed to stand 1 to 12 hr. Lymphatic nodes and glands such as popliteal, inguinal, sacral, mesenteric, axillary, cervical, thoracic, abdominal, and others were