It’s known that a dog can be owned for some lawful purposes. We are, however, not allowed to keep a dog as a “pet”, since it is not a very clean animal.  As for the findings of scientific research relative to keeping dogs, the prominent Muslim scholar, Sheik Yusuf Al-Qaradawi, states in his well-known book, The Lawful and the Prohibited in Islam:

Some lovers of the West in Muslim countries claim to be full of love and compassion for all living creatures and they wonder why Islam warns against this “best friend” of man. For their benefit, we quote a lengthy excerpt from an article by the German scientist, Dr. Gerard Finstimer, in which the author sheds light on the dangers to human health resulting from keeping dogs or coming in contact with them. He says:
The increasing interest shown by many people in recent times in keeping dogs as pets has compelled us to draw public attention to the dangers which result from this, especially because pet dogs are hugged and kissed and permitted to lick the hands of the young and the old, and what is worse, to lick the plates and utensils which are used by human beings for eating and drinking.
Besides being unhygienic and uncouth, this practice is bad and abhorrent to good taste. However, we are not concerned with such matters, leaving them to be addressed by teachers of etiquette and good taste.
Rather this article is intended to present some scientific observations.
From the medical point of view, which is our main concern here, the hazards to human health and life from keeping and playing with dogs are not to be ignored. Many people have paid a high price for their ignorance, as the tapeworm carried by dogs is a cause of chronic disease, sometimes resulting in death.
This worm is found in man, in cattle, and in pigs, but it is found in fully-developed form only in dogs, wolves and rarely in cats. These worms differ from others in that they are minute and invisible; consequently, they were not discovered until very recently. Biologically, the developmental process of this worm has some unique characteristics. In the lesions caused by them, one worm gives rise to many heads which spread and form other and varied kinds of lesions and abscesses. These heads develop into full-grown worms only in dogs’ tonsils. In humans and in other animals they appear as lesions and abscesses completely different from the tapeworm itself. In animals the size of an abscess may reach that of an apple, while the liver of the infected animal may grow from five to ten times its normal size. In human beings the size of the abscess may reach that of a clenched fist or even the head of an infant; it is filled with yellow fluid weighing from ten to twenty pounds. In the infected human it may cause diverse kinds of inflammations in the lungs, muscles, spleen, kidneys, and brain, and appears in such different forms that specialists, until very recently, had difficulty in recognizing it.
In any case, wherever this inflammation is found, it poses great danger to the health and life of the patient. What is worse is that in spite of our knowledge of its life history, origin, and development, we have not been able to devise a cure for it, except that in some instances these parasites die out, possibly because of antibodies produced in the human body. Unfortunately, cases in which such parasites die without causing damage are rare indeed. Moreover, chemotherapy has failed to produce any benefit, and the usual treatment is surgical removal of the abscessed parts of the body. For all the
se reasons we should use all possible resources to fight against this dreadful disease and save man from its dangers.
Professor Noeller, through post-mortem dissection of human bodies in Germany, found that the incidence of infection with dogs’ worms is at least one percent. In some places such as Dalmatia, Iceland, southeastern Australia, and Holland, where dogs are used for pulling sleds, the incidence rate of tapeworm among dogs is 12 percent. In Iceland, the number of people who suffer from the inflammation caused by this worm has reached the rate of 43 percent. If we add to this the human suffering, the loss of meat because of infection of cattle, and the permanent danger to human health because of the presence of tapeworms, we cannot be very complacent toward this problem.
Perhaps the best way to combat the problem is to limit the worms to dogs and not let them spread, since in actuality we need to keep some dogs. We should not neglect to treat dogs when necessary by getting rid of the tapeworms in their tonsils and perhaps repeating this process periodically on shepherd dogs and watchdogs.
Man can protect his life and health by keeping a safe distance from dogs. He should not hug them, play with them, or let them come close to children. Children should be taught not to play with dogs or to fondle them. Dogs should not be permitted to lick children’s hands or come to places where they play. Unfortunately, dogs are allowed to roam about everywhere, especially in places where children play, and their bowls are scattered throughout the house. Dogs must have their own separate bowls, and they must not be allowed to lick bowls and plates used by humans. They should not be allowed inside grocery stores, restaurants, or marketplaces. In general, great care must be taken that they do not come in contact with anything which is used by people for eating and drinking.
We already know that the Prophet (peace and blessings be on him) forbade mixing with dogs, and that he warned against their licking plates and against keeping them without necessity. How is it possible that the teachings of an unlettered Arab, Muhammad, should agree with the latest findings of scientific research? Truly, we cannot say anything except to repeat the words of the Qur’an: (Nor does he speak from (his own) desire. It is nothing other than a Revelation sent down.) (An-Najm 53: 3-4)