Comparison of Early Litomosoides carinii Infections in Cotton Rats and Gerbils

Abstract

Cotton rats (Sigmodon hispidus) and Mongolian gerbils (Meriones unguiculatus) were exposed under similar conditions to the bites of tropical rat mites (Bdellonyssus bacoti) infected with the filariid worm Litomosoides carinii in order to compare early infections in the two rodent hosts. The combined data from four experiments provided a picture of remarkable similarity with regard to number, sex ratio, and length of adult worms and number of microfilariae in the early infections. However, histological examination of lungs showed that tissue reactions were generally more severe in gerbils than in cotton rats. Cotton rats (Sigmodon hispidus), as natural hosts of Litomosoides carinii, have been used in most research with this filariid parasite. Ideally, filaria-free animals raised under known conditions should be used, but the demand for them has not been sufficient to encourage the commercial production of cotton rats. In addition, cotton rats are difficult to handle. These problems have led to our use of the Mongolian gerbil (AMeriones unguiculatus) which is commercially available and easy to handle. The susceptibility of the gerbil to L. carinii infection has been reported following the surgical implantation or the injection of infective larvae (Scott, 1960; Zein-Eldin and Scott, 1961; Zein-Eldin, 1965). We have used young male gerbils mainly in drug evaluation studies with all observations being restricted to mite-induced early infections, i.e., prophylactic tests or therapeutic trials during the 1st month of patency. Therapeutic trials regularly included systematic counting of microfilariae and examinations of adult worms at autopsy. A review of our published (Dickerson and Thompson, 1966; Thompson et al., 1968) and unpublished data confirmed the high susceptibility of gerbils to L. carinii. But the combined data showed that 35 of 132 (26.5%) of the untreated controls had at least a few dead worms at autopsy approximately 3 months after exposure. Usually, dead worms constituted only a low percentage of the recognizable worms. Our experience with cotton rats has been too limited for a comparison of early death of adult worms in the two hosts. However, Wharton (1947) has reported that Received for publication 17 June 1968. occasional cotton rats have dead adult worms but he did not specify the age of infection when this was observed. The convenience of using gerbils and indications that they may be suitable for drug evaluation, encourage systematic comparison of the course of L. carinii infections in the two hosts. This paper deals with early infections following exposure of comparable groups of animals of each species to infected mites under similar conditions. The parameters studied included (1) species and sex of host, (2) early death of adult worms, (3) numbers, length, and sex of adult worms, (4) pulmonary reactions to adult worms, and (5) the microfilaremia. MATERIALS AND METHODS Sources of parasites and vectors The strain of L. carinii used in these experiments was supplied by Dr. E. A. Zein-Eldin, University of Texas, Galveston, and has been maintained cyclically in this laboratory since 1963. Tropical rat mites, Bdellonyssus bacoti, were obtained initially from Dr. Zein-Eldin and later supplemented by a supply from Dr. Frank Hawking, National Institute for Medical Research, Mill Hill, England. Infection procedure Four experiments were carried out using a modification of the tank exposure method of Hawking and Sewell (1948). Metal tanks with dimensions of 23 by 17 by 12 inches, equipped with electrically heated rims (at approximately 40 C) to prevent mites from escaping were used. Tanks were placed on carts equipped with oilfilled moats around the legs as a safeguard against escape of mites in the event of a power failure. Each tank was provided with a wooden frame lid lined with wide-mesh screening, to contain the rodent hosts, and 12XX bolting silk (for milling purposes) to prevent entry of contaminating mites. The design of the experiments is shown in Table I. Four infected gerbils were exposed to large