Rabies transmission

TEM micrograph with numerous rabies virions (small dark-grey rod-like particles) and Negri bodies (the larger pathognomonic cellular inclusions of rabies infection).

Rabies is mostly transmitted to humans, and between animals, through the saliva of infected animals. Transmission is generally through a bite from any infected animal.^[1]

Transmission between humans is extremely rare, although it can happen through organ transplants, or through bites.^[1]

After a typical human infection by bite, the virus enters the peripheral nervous system. It then travels along the nerves towards the central nervous system. During this phase, the virus cannot be easily detected within the host, and vaccination may still confer cell-mediated immunity to prevent symptomatic rabies. Once the virus reaches the brain, it rapidly causes encephalitis and symptoms appear. This is called the "prodromal" phase and at this time, treatment is usually unsuccessful. Rabies may also inflame the spinal cord producing myelitis.

Animals

Dog with rabies virus

In 1932, Dr. Joseph Lennox Pawan of Trinidad in the West Indies first discovered that infected vampire bats could transmit rabies to humans and other animals.^[2][3] Any mammal may become infected with the rabies virus and develop symptoms, including humans. Most animals can be infected by the virus and can transmit the disease to humans. Infected bats, monkeys, raccoons, foxes, skunks, cattle, wolves, dogs or cats provide the greatest risk to humans. Rabies may also spread through exposure to infected domestic farm animals, groundhogs, weasels, bears and other wild carnivores. Small rodents such as squirrels, hamsters, guinea pigs, gerbils, chipmunks, rats, and mice and lagomorphs like rabbits and hares are almost never found to be infected with rabies and are not known to transmit rabies to humans.^[4]

The virus is usually present in the nerves and saliva of a symptomatic rabid animal.^[5][6] The route of infection is usually, but not necessarily, by a bite. In many cases the infected animal is exceptionally aggressive, may attack without provocation, and exhibits otherwise uncharacteristic behaviour.^[7] (Note that "uncharacteristic behaviour" may include uncharacteristic friendliness as well as the stereotypically violent mode of rabies. Since rabies can be transmitted through contact with saliva, not just through bites, this "tame" mode of rabies is no less dangerous.) Transmission may also occur via an aerosol through mucous membranes; transmission in this form may have happened in people exploring caves populated by rabid bats.^[8]

Transplantation

Rabies is known to have been transmitted between humans by transplant surgery on rare occasions. Infections by corneal transplant have been reported in Thailand (two cases), India (two cases), Iran (two cases),^[9] the United States (one case), and France (also a single case).^[10] Details of two further cases of infection resulting from corneal transplants were described in 1996.

In June 2004, three organ recipients died in the United States from rabies transmitted in the transplanted kidneys and liver of an infected donor from Texarkana.^[11] There were bats near the donor's home, and the donor had told others that he had been bitten.^[12] The donor is reported to have died of a cerebral hemorrhage, the culmination of an unidentified neurological disorder, although recipients are said to have been told the cause of death had been a car crash. Marijuana and cocaine were found in the donor's urine at the time of his death, according to a report in The New England Journal of Medicine.^[13]

[The surgeons] thought he had suffered a fatal crack-cocaine overdose, which can produce symptoms similar to those of rabies. "We had an explanation for his condition," says Dr. Goran Klintmalm, a surgeon who oversees transplantation at Baylor University Medical Center, where the transplants occurred. "He'd recently smoked crack cocaine. He'd hemorrhaged around the brain. He'd died. That was all we needed to know."^[14]

Because of doctor-patient confidentiality rules, doctors involved with this case would not talk about it on the record, but a few did say that had Beed not had cocaine in his blood, the E.R. doctors might have investigated his symptoms more aggressively instead of assuming he had overdosed. (Because no autopsy was done, doctors have not been able to establish whether the rabies or the drugs actually killed him.)^[14]

In February 2005, three German patients in Mainz and Heidelberg were diagnosed with rabies after receiving various organs and cornea transplants from a female donor. Two of the infected people died. Three other patients who received organs from the woman have not yet shown rabies symptoms. The 26-year-old donor had died of heart failure in December 2004 after consuming cocaine and ecstasy. In October 2004, she had visited India, one of the countries worst affected by rabies worldwide. Dozens of medical staff were vaccinated against rabies in the two hospitals as a precautionary measure. Associated Press reports that "Donated organs are never tested for rabies. The strain detected in the victims' bodies is one commonly found in bats, health officials said." According to CNN, "Rabies tests are not routine donor screening tests, Virginia McBride, public health organ donation specialist with the Health Resources and Services Administration, said. The number of tests is limited because doctors have only about six hours from the time a patient is declared brain-dead until the transplantation must begin for the organs to maintain viability."

References