Plasmodium hylobati: A Malaria Parasite of the Gibbon

Abstract

A natural infection of Plasmodium hylobati in Hylobates moloch, from Sarawak, was studied. When received, the infection was chronic as evidenced by a low level of parasitemia. After splenectomy, the parasitemia reached high levels and infection of 4 different anopheline mosquitoes was obtained. The same gibbon was reexposed to infection by the intravenous inoculation of sporozoites from Anopheles b. balabacensis mosquitoes. The prepatent period was 9 days. Plasmodium hylobati was first described by Rodhain (1941) from a malarial infection in a Hylobates lensciscus (= moloch) gibbon from Java. The parasite has also been seen (Garnham, 1966) in blood films from ailing gibbons from Sarawak. In June 1968 a H. moloch gibbon, suspected of being naturally infected with P. hylobati, was received in Chamblee, Georgia, for detailed studies of the parasite since only the blood forms were known. This animal was captured in January 1966 on the upper stream of the Tinjar river in the Baram District (fourth division), Sarawak. It was taken to Kuching where Dr. Hwang examined its blood and found malaria parasites. Films and eventually the animal itself were sent to one of us in London for further study. On arrival in July 1966 no parasites were seen in the thin film, but a few days later scanty ring forms were detected in thick films. Parasitemia continued to increase, and schizonts but no gametocytes appeared. The numbers of parasites varied from week to week but were usually at a low level. It seemed probable that the best anopheline vector would be Anopheles balabacensis and as no colony of this species was available in England, the animal was sent in June 1968 to the United States for investigations on the sporogonic and exoerythrocytic cycles of the parasite. At the time of its arrival, the parasitemia was approximately 14 per mm3. The infection remained at a low level for the next 9 months, after which it was splenectomized (March 1969). Here, we intend to report on the sporoReceived for publication 29 April 1971. * Present address: Imperial College Field Station, Ashurst Lodge, Ascot, Berks, England. t Present address: Central America Malaria Research Station, USAID, APO New York 09889. gonic aspects as well as some observations on the course of the infection. The results of the exoerythrocytic studies are reported in the following paper. MATERIALS AND METHODS The mosquitoes were all from laboratory-reared colonies. The Anopheles balabacensis balabacensis mosquitoes were originally from Thailand; the colony was established from eggs obtained from the Walter Reed Army Institute of Research, Washington, D. C. The A. stephensi were from Delhi, India, and were originally obtained from the London School of Hygiene and Tropical Medicine. The A. freeborni were from Marysville, California, and have been maintained in the laboratory since 1944. The A. maculatus, from Malaysia, were obtained from the Institute for Medical Research, Kuala Lumpur, Malaysia, in 1964. The A. quadrimaculatus were the Q-1 strain from southeastern United States and were obtained from Technical Development Laboratories, CDC, Savannah, Georgia. Blood smears were stained with Giemsa and the gametocyte counts determined by examination of the thin blood film until 500 WBC had been seen. Total parasite counts were made using the thick film. For the mosquito feeding, the animal was tranquilized and the mosquitoes, caged in small ice cream carton cages, were allowed to engorge by feeding on the animal's belly. After feeding, the mosquitoes were incubated in a constant temperature incubator at 25 C. Cotton pledgets soaked with 10% Karo solution were supplied daily. Beginning at 6 and continuing through 15 days after feeding, representative mosquito samples were removed from the cages, dissected, and microscopically examined for the presence of oocysts on the gut and/or sporozoites in the salivary glands. Beginning on the 6th and continuing through the 12th day after feeding, 100 oocysts on the guts of A. b. balabacensis mosquitoes were measured using an ocular micrometer. Beginning on the 12th day, salivary glands were examined for the presence of sporozoites. Gland infections were graded as reported earlier (Collins et al., 1966). For the reinoculation of the gibbon with sporo-