Treatment of Canine Filariasis with Tetracycline
A. Fleming, a bacteriologist in Scotland, accidentally discovered the world’s first antibiotic substance penicillin in 1928. It became clear that penicillin was useful as a treatment for bacterial infections, and after that, as you know, various antibiotics were developed and frequently utilized as essential treatment for bacterial infections. Let me introduce a case in which an antibiotic can be used for treatment of a parasitic disease.
Canine filariasis, a serious parasitic disease for dogs, is caused by a blood-feeding mosquito parasite (Dirofilaria immitis) which enter a dog’s pulmonary artery and heart, causing chronic cardiovascular and respiratory problems. The mosquitoes mainly live in southern Honshu where they cause problems for dogs and their owners. Treatment was either killing parasites by a poisonous arsenic compound or surgical removal of parasites, both of which cause a big burden on dogs and their owners. Later, the discovery of ivermectin which is a macrolide antibiotic by the Novel Prize winner Satoshi Omura, made the establishment of prevention method for this disease possible. However, if we look overseas, this disease and zoonosis are still serious parasitic diseases. Apart from ivermectin, now let me explain the application of tetracycline for treatment of canine filariasis.
Tetracycline, which is an antibiotic produced by microorganisms, has effect for treatment by inhibiting growth of microorganisms such as bacteria, rickettsiae, and chlamidia. Doxycycline and minocycline in addition to tetracycline are also known are in the same group of antibiotics. Then, why is tetracycline effective for canine filariasis? The answer is related to the symbiotic relationship between a parasite and a bacterium.
It is known that Wolbachia pipientis, which is rickettsiae, frequently live symbiotically with canine filarial nematodes. Wolbachia has the ability of changing the host’s reproductive system in favor of itself, for example, Wolbachia kills male symbiotic arthropods whereas females survive and contribute to the reproduction. To change the reproductive system of filarial nematodes to parthenogesis, a male is not necessary for reproductive, and Wolbachia gives oneself an advantage. If Wolbachia are eliminated from filarial nematodes, they could not survive or reproduce. Thus, the elimination of Wolbacia expels filarial nematodes. Therefore, the use of tetracycline which is essential for rickettsiae is available for the elimination of Wolbachia.
It has been gradually clarified that antibiotics which have been frequently used against bacterial infections are also available for treatment or prevention of parasite diseases. Considering that antibiotics are also utilized for treatment of cancer as an antineoplastic drug, I consider antibiotics to be one of the greatest discoveries of the 20th century.