Rabies is a dangerous viral disease that affects the central nervous system and can be fatal if left untreated. The rabies virus is primarily transmitted through the saliva of infected animals, most commonly through a bite. Dogs are one of the most common carriers of rabies worldwide. A key question regarding rabies transmission is whether dogs can spread the disease without showing any symptoms.
What is Rabies?
Rabies is caused by a virus in the family Rhabdoviridae, genus Lyssavirus. There are multiple strains of the rabies virus that can infect different species, including dogs, bats, raccoons, foxes and skunks. In dogs, the rabies virus attacks the central nervous system, causing inflammation of the brain and spinal cord. Rabies has a virtually 100% fatality rate once clinical signs appear.
The rabies virus is transmitted through infected saliva, usually via a bite from a rabid animal. The virus enters the wound and travels along peripheral nerves to the spinal cord and brain. After an incubation period that can last weeks to months, the virus causes encephalitis and symptoms including:
- Fever
- Headache
- Confusion
- Agitation
- Hyperactivity
- Difficulty swallowing
- Excessive salivation
- Paralysis
Once clinical signs appear, rabies is virtually always fatal. However, vaccination against rabies after exposure but before symptoms arise is highly effective at preventing the disease.
How is Rabies Transmitted Between Dogs?
The rabies virus is primarily passed between dogs through bites and direct contact with infected saliva or nervous tissue. Less common routes of transmission include:
- Getting saliva or nervous tissue from an infected dog in an open wound or mucous membrane (eyes, nose, mouth)
- Ingesting infected material
- Inhalation of aerosolized rabies virus
- Transplacental transmission from mother to fetus
Infected dogs can shed large amounts of rabies virus in their saliva starting up to 10 days before showing clinical signs. The vast majority of rabies transmission between dogs occurs through bite wounds. The virus is highly neurotropic and travels quickly along peripheral nerves to the central nervous system after entering a bite wound.
What is the Incubation Period for Rabies?
The rabies virus incubation period is the time from initial infection to when clinical signs appear. In dogs, the incubation period may last:
- 2 weeks to 6 months
- Typically 1-3 months
- On average 3-8 weeks
There are two distinct phases of rabies virus infection:
- Initial peripheral replication at the site of virus entry
- Centrifugal spread via peripheral nerves to the central nervous system and rapid replication in the brain and spinal cord
The lengthy incubation period is due to the slow replication of the virus locally and its slow spread to the central nervous system along axons. The incubation period tends to be shorter in dogs with bites to highly innervated body regions like the head and face.
Can Dogs Transmit Rabies Before Showing Symptoms?
Yes, dogs can transmit rabies to other animals and people before showing clinical signs of the disease. This occurs because:
- Rabies virus is shed in the saliva of infected dogs starting up to 10 days before symptoms appear.
- Dogs initially shed virus at the peripheral site of infection, allowing transmission via bites at this time.
- The virus spreads centrally along nerves at a slow rate, delaying the onset of symptoms.
- By the time a dog shows clinical signs, it has been capable of shedding virus for a period of days to weeks.
Therefore, dogs in the incubation phase without symptoms are still contagious and able to transmit rabies to other mammals through their saliva.
Shedding of Rabies Virus Before Symptom Onset
Several studies have examined shedding of rabies virus in the saliva of infected dogs prior to symptom development:
- In one study, virus shedding was detected in saliva 3-5 days before symptoms appeared.
- Another study found viral RNA in saliva up to 10 days before symptoms.
- The onset of infectious viral shedding typically precedes clinical signs by 1-7 days.
Infected dogs can clearly shed infectious virus for a period of days before symptom onset, posing a transmission risk during the incubation phase.
Initial Peripheral Viral Replication
After entering through a bite wound, the rabies virus initially replicates at peripheral sites like muscle and epithelial tissue near the inoculation site. Local shedding can transmit the virus at this stage before it spreads to the brain and spinal cord. This early peripheral phase lasts approximately 7-10 days in dogs.
Slow Central Spread Along Nerves
The relatively slow spread of the rabies virus along peripheral nerves to the central nervous system accounts for the typically long incubation period. The virus travels at a rate of 7-13 mm/day along axons. This delay between initial infection and central nervous system invasion allows transmission from dogs that appear normal but are incubating the virus.
What are the Signs of Rabies in Dogs?
The first clinical signs of rabies in dogs include:
- Fever
- Behavioral changes
- Hydrophobia (fear of water)
- Loss of appetite
- Vomiting
- Hyperactivity
As the disease rapidly progresses, dogs develop:
- Incoordination
- Seizures
- Paralysis
- Aggression
- Excessive salivation
- Jaw paralysis
Ultimately most rabid dogs progress to a paralytic or “dumb” form of the disease before death.
Dumb vs Furious Rabies
Rabies in dogs can take two different forms:
- Dumb or paralytic form – This is the most common form, with progressive paralysis leading to death within a few days.
- Furious form – Dogs with this form are restless, excitable, and prone to bite at objects. Death occurs within 3-4 days.
Regardless of the manifestation, any change in a dog’s normal behavior could be an early sign of rabies. Suspect rabies infection in any dog showing unexplained illness, behavioral changes, or neurologic dysfunction.
How Long Can Rabies Incubation Last in Dogs?
The rabies incubation period can vary significantly, lasting anywhere from 2 weeks to 6 months in dogs. However, most dogs develop clinical signs within 1-3 months after an infective bite. According to the 2020 Compendium of Animal Rabies Prevention and Control:
- 50% of rabies cases develop symptoms within 2-9 weeks
- 80% of cases show signs within 3-10 weeks
- Nearly 100% of infections become symptomatic within 4-12 weeks
While prolonged incubation periods up to 6 months are possible, they are exceptionally rare. This table summarizes the typical rabies incubation periods in dogs:
| Incubation Period | Timeframe |
|---|---|
| Minimum | 2 weeks |
| Common | 3-8 weeks |
| Average | 3-12 weeks |
| Maximum | 6 months |
Viral strain differences and bite location influence incubation time. Shorter periods of 1-2 weeks are more likely with bite wounds to highly innervated regions like the face and head.
How Can Human Exposure to Rabies Be Prevented?
Preventing human rabies depends on controlling the disease in domestic and wild animal reservoirs and preventing bites from these species. Recommendations to avoid rabies exposure include:
- Vaccinating all pet dogs and cats against rabies
- Keeping pets confined and not allowing them to roam freely
- Reporting stray animals to animal control
- Staying away from wildlife
- Having any animal that exhibits neurologic signs evaluated for rabies
- Washing any animal bite wound thoroughly with soap and water
- Seeking post-exposure prophylaxis after any high-risk bite
Public health officials also use rabies vaccines strategically in oral baits to immunize wild carnivore reservoirs like foxes, raccoons, skunks and coyotes.
Conclusion
In summary, dogs can transmit rabies to humans and other mammals during the incubation period before showing recognizable symptoms. This is due to early peripheral viral shedding in saliva, a period of several days to weeks between salivary shedding and symptom onset, and the slow progress of the virus from the bite site to the central nervous system along axons.
Any dog exhibiting unexplained behavioral changes, signs of neurologic dysfunction, or otherwise acting abnormally should be evaluated for rabies. Seek immediate medical attention for assessment and potential post-exposure prophylaxis if bitten by a dog with confirmed or suspected rabies infection. With proper prophylaxis delivered promptly after virus exposure, human rabies deaths can be prevented.