The incubation period, or the time between the bite by an infected mosquito and the onset of clinical signs, ranges from 3 to 14 days. Most people who are infected with West Nile virus either have no symptoms or experience mild illness with flu-like symptoms. In some individuals, particularly the elderly, West Nile virus can cause serious disease that affects brain tissue. At its most serious, it can cause permanent neurological damage and can be fatal.
The majority of people infected with West Nile virus will show no symptoms. An estimated 20% of people infected will develop mild flu-like symptoms which generally last a few days. These symptoms include fever, fatigue, headache, body aches, swollen lymph nodes, and/or a body rash. An estimated 1 in 150 people infected with WNV will progress to a more severe infection called WNV encephalitis (inflammation of the brain), WNV meningitis (inflammation of the membrane around the brain and spinal cord) or WNV meningeoencephalitis (inflammation of the brain and surrounding membrane). Symptoms of this severe disease can last several weeks, and the neurological effects may be permanent. The symptoms include headache, high fever, stiff neck, disorientation, stupor, tremors, seizures or convulsions, paralysis, muscle weakness, loss of consciousness (coma) and/or death.
A milder illness caused by WNV, called West Nile Fever (WNF) is currently reportable by local and state health agencies on a voluntary basis only. This includes those individuals with flu-like illness and no central nervous system signs (such as stiff neck, abnormal cerebral spinal fluid, mental status changes, paresis or paralysis) that have laboratory evidence of WNV infection. In an effort to document where transmission of this new pathogen is occurring, the MDCH encourages the reporting of these individuals to local and state health agencies.
There is no specific treatment for West Nile viral illnesses. Because West Nile infections are caused by a virus, antibiotics are not an effective treatment and no antiviral drugs have been successfully used. Therefore, treatment is supportive. It is aimed at improving the symptoms of an infected person but does not shorten the course of illness. Main treatment concerns involve lowering fever, avoiding dehydration, decreasing brain swelling or dealing with a loss of automatic breathing activity. Individuals with severe infection may require hospitalization. Mild symptoms will usually resolve in a few days.
Horses have proven uniquely susceptible to WNV infection, comprising more than 99 percent of veterinary (i.e. non-human) mammalian cases. Clinical signs of horses which do become ill range from very mild signs to fatal illness. Typical signs include weakness, ataxia (incoordination, stumbling, limb weakness) that either appears suddenly or appears gradually and worsens, somnolence (sleepiness), dullness, muscle twitching (especially of the face and muzzle), listlessness, facial paralysis (droopy eyelids, lower lip), and inability to rise. Some horses may develop mild fevers, blindness, muscle trembling, seizures, and many other signs. The course of illness varies from a few days to a few weeks, and may be rapidly or slowly progressive. Horses that become recumbent have a poor prognosis for recovery, and most horses that reach this stage are euthanized or die. The reported mortality rate for symptomatic WNV-infected horses is 30-40 percent. Most horses, then, recover from the infection, and in most cases, recovery seems to be complete.
As with most viral infections, there is currently no specific treatment for disease caused by WNV. Treatment is essentially supportive, and may include anti-inflammatory medications, intravenous fluid administration, parenteral nutrition, and physical support if the horse is extremely weak or prone to recumbency.
Rabies, Eastern Equine Encephalitis (EEE), Western Equine Encephalitis (WEE), Equine Protozoal Encephalitis (EPM), and other neurologic diseases should be considered in a differential diagnosis for suspect horses.
To the best of our understanding, dogs and cats are very resistant to the West Nile Virus. While a very small number of dog illnesses and possible deaths have been reported across the country, in at least one case the dog in question was unhealthy prior to becoming ill with WNV. This very likely made the dog more susceptible to the virus and also probably contributed to the severity of its illness. Most dogs or cats infected with WNV will show no symptoms of the disease. The only WNV positive dog to date in Michigan developed intermittent seizures and rear limb weakness early in the course of the infection, but has since recovered completely. The CDC has only documented one dog and one cat death in the US since 1999 attributed to WNV infection.
Birds and Mammals
Most corvids infected with WNV die within 3 weeks of infection. Clinical signs prior to death may include incoordinated flying or walking, weakness, lethargy, tremors, and abnormal head posture. Although crows and blue jays account for between 50 and 90 percent of reported avian cases, WNV infection has been identified in over 170 other avian species during the North American outbreak.
Clinical signs and pathologic findings of WNV infection have not been thoroughly described in most mammalian species. In cases where description is given, both clinical illness and pathology are referrable to the nervous system. Therefore, it seems that WNV-associated disease in both equidae and other mammals is primarily a neurologic disease.