Infectious diseases are caused by bacteria, viruses, protozoa, fungi, and rickettsia that invade the body of a susceptible host and cause an illness. They are transmitted from one animal to another by contact with infected urine, feces, and other bodily secretions, or by inhaling pathogen-laden droplets. They may also be acquired by contact with spores in the soil that enter the body through the respiratory tract or a break in the skin. A few are sexually transmitted.
Although pathogens exist everywhere in the environment, only a few cause infection. Fewer still are contagious. Many infectious diseases are species-specific. For example, a dog cannot catch a disease that is specific to a horse, and vice versa. Other infectious diseases are not species-specific, and are capable of causing disease in many animals, including humans. In instances where a disease is zoonotic, public health considerations are discussed.
Many infectious agents are able to survive for long periods outside the host animal. This knowledge is important in determining how to contain the spread of infection.
For many diseases, the best way to prevent them is by vaccination. Immunity and vaccinations are discussed at the end of this chapter.
Bacteria are single-celled microorganisms that are noted for their ability to cause disease.
Brucellosis in Dogs
This disease is caused by the bacteria Brucella canis. It is a major cause of sterility and spontaneous abortion in dogs. Puppies infected in utero are typically aborted at 45 to 59 days after conception. Suspect this disease in any bitch who aborts two weeks before she is due to deliver and whenever a bitch delivers stillborn puppies or puppies who sicken and die.
Dogs with acute infection have enlarged lymph nodes in the groin and/or beneath the jaw. Fever is rare. The testicles of the male may swell in the initial stages, and then become smaller and atrophic as the sperm-producing cells are destroyed. Note, however, that this disease can infect a dog or bitch with- out producing any signs of illness.
In a dog with an acute infection, bacteria are found in the blood, urine, body secretions, and the products of abortion. In a dog with a chronic or inactive infection, bacteria can be transmitted in vaginal secretions during estrus and in semen.
The most common mode of transmission is by contact with infected vaginal discharges following a spontaneous abortion, and by contact with the urine of infected dogs. The disease can spread rapidly throughout a kennel in this manner. Males can acquire the disease through oral and nasal contact with the vaginal secretions of estrus females. Females can acquire the disease through breeding with an infected male. This is of particular concern to breeders, because males can harbor the bacteria for life.
A positive blood culture obtained during an acute infection is the most conclusive diagnostic test. Bacteria can also be cultured from aborted tissue. Blood serum tests will determine if a dog has ever been infected.
Treatment: Brucellosis is difficult to eradicate. A course of intramuscular and oral antibiotics given for a minimum of three weeks will eliminate the disease in 80 percent of dogs. To be considered cured, a dog must be free of the bacteria for at least three months. Since it is difficult to achieve a cure, it is recommended to spay or neuter all infected animals to prevent the transmission of disease to other dogs.
Prevention: All animals should be tested before they are used in a breeding program. Brood bitches should be retested one month before each breeding and, ideally, stud dogs should be retested before every breeding.
Public health considerations: Rare instances of human infection have followed exposure to canine brucellosis. It is important to wear rubber gloves and take proper hygienic precautions when handling all aborted products of conception.