Rabies

Background
Rabies is a viral disease that affects the CNS. The genus Lyssavirus contains more than 80 viruses. Classic rabies, the focus of this article, is the prototypical human Lyssavirus pathogen. Ten viruses are in the rabies serogroup, most of which only rarely cause human disease. The genus Lyssavirus, rabies serogroup, includes the classic rabies virus, Mokola virus, Duvenhage virus, Obodhiang virus, Kotonkan virus, Rochambeau virus, European bat Lyssavirus types 1 and 2, and Australian bat Lyssavirus. In 1997, an unusual bat Lyssavirus caused a brief outbreak of a rabieslike illness in Australia.

The fatal madness of rabies has been described throughout recorded history, and its association with rabid canines is well known. For centuries, dog bites were treated prophylactically with cautery, unfortunately, with predictable results. In the 19th century, Pasteur developed a vaccine that successfully prevented rabies after inoculation and launched a new era of hope in the management of this uniformly fatal disease. Rabies is recognized as a zoonosis worldwide. Animal-control and vaccination strategies currently supersede postexposure prophylaxis in preventing the spread of rabies. Through such programs, rabies has been eliminated in several nations and some areas in the US territories.

Human rabies reflects the prevalence of animal infection and the extent of contact this population has with humans. Less than 5% of cases in developed nations occur in domesticated dogs; however, unvaccinated dogs serve as the main reservoir worldwide. Undomesticated canines, such as coyotes, wolves, jackals, and foxes, are most prone to rabies and serve as reservoirs. These reservoirs allow rabies to remain an indefinite public health concern, and ongoing public health measures are critical to its control. Raccoons, skunks, and insect-eating bats remain the prime vectors in the United States, followed by cats and cattle. Increasingly in the United States, the source of exposures cannot be identified, but the risk of death from rabies is exceedingly low, with fewer than 5 cases documented per year. Opossums are rarely infected and are not considered a likely risk for exposure. Read more »

Brain Abscess

Background

Intracranial abscesses are uncommon, serious, life-threatening infections. They include brain abscess and subdural or extradural empyema and are classified according to the anatomical location or the etiologic agent. The term brain abscess is used in this article to represent all types of intracranial abscesses.

Intracranial abscesses can originate from infection of contiguous structures (eg, otitis media, dental infection, mastoiditis, sinusitis) secondary to hematogenous spread from a remote site (especially in patients with cyanotic congenital heart disease), after skull trauma or surgery, and, rarely, following meningitis. In at least 15% of cases, no source can be identified.

In recent years, the complex array of etiologic agents that cause brain abscess has become better understood.

Pathophysiology

Brain abscess is caused by intracranial inflammation with subsequent abscess formation. In at least 15% of cases, the source of the infection is unknown (cryptogenic). Infection may enter the intracranial compartment directly or indirectly via 3 routes.

Contiguous suppurative focus (45-50% of cases)

Direct extension may occur through necrotic areas of osteomyelitis in the posterior wall of the frontal sinus, as well as through the sphenoid and ethmoid sinuses.This direct route of intracranial extension is more commonly associated with chronic otitic infection and mastoiditis than with sinusitis. Odontogenic infections can spread to the intracranial space via direct extension or a hematogenous route. Contiguous spread could extend to various sites in the central nervous system, causing cavernous sinus thrombosis; retrograde meningitis; and epidural, subdural, and brain abscess. Read more »

Tetanus

Background

The word tetanus comes from the Greek tetanos, which is derived from the term teinein, meaning to stretch. Tetanus appears in military medical documents throughout the ages. Slapping infected dung on the umbilical cords of newborns (ie, as part of ritualistic ceremonies) caused rampant tetanus neonatorum or trismus nascentium in the West Indies and in Africa. Osler’s textbook describes the “eight days sickness” caused by umbilical sepsis, which killed 84 of 125 children within a fortnight of birth in St. Kilda, Scotland. During World War I, tetanus occurred in 1.47 per 1000 British wounded and in 12.5 per 1000 persons involved in the Peninsular campaign. Nicolaier discovered the anaerobic bacillus Clostridium tetani in 1885. In 1889, Koch’s pupil, Kitasato, obtained the bacillus of tetanus in pure culture and associated the disease to animals.

Although rare, the disease has not been eradicated, and early diagnosis and intervention are life saving. Prevention is the ultimate management strategy for tetanus. The 4 clinical types of tetanus are generalized, local, cephalic, and neonatal.

Neonatal tetanus is a major cause of infant mortality in underdeveloped countries, but this form is rare in the United States. Infection results from cord contamination during unsanitary delivery conditions, coupled with a lack of maternal immunization. At the end of the first week of life, infected infants become irritable, feed poorly, and develop rigidity with spasms. This form of tetanus has a very poor prognosis for survival.

Cephalic tetanus is uncommon and usually occurs following head trauma or otitis media. Patients with this form present with cranial nerve palsies. The infection may be localized or may become generalized.

Patients with local tetanus present with persistent rigidity in the muscle group close to the injury site. The muscular rigidity is caused by a dysfunction in the interneurons that inhibit the alpha motor neurons of the affected muscles. No further CNS involvement occurs, and this form has very low mortality rates. Read more »

Typhus

Background

Typhus refers to a group of infectious diseases that are caused by rickettsial organisms and that result in an acute febrile illness. Arthropod vectors transmit the etiologic agents to humans. The principle diseases of this group are epidemic or louse-borne typhus and its recrudescent form known as Brill-Zinsser disease, murine typhus, and scrub typhus. (For more information on pediatric scrub typhus, see the eMedicine article Scrub Typhus in the Pediatric: General Medicine volume.)

Pathophysiology

Epidemic typhus is the prototypical infection of the typhus group of diseases, and the pathophysiology of this illness is representative of the entire category. The arthropod vector of epidemic typhus is the body louse (Pediculus corporis). This is the only vector of the typhus group in which humans are the usual host. Rickettsia prowazekii, which is the etiologic agent of typhus, lives in the alimentary tract of the louse. A Rickettsia- harboring louse bites a human to engage in a blood meal and causes a pruritic reaction on the host’s skin. The louse defecates as it eats; when the host scratches the site, the lice are crushed, and the Rickettsia- laden excrement is inoculated into the bite wound. The Rickettsia travel to the bloodstream and rickettsemia develops.

Rickettsia parasitize the endothelial cells of the small venous, arterial, and capillary vessels. The organisms proliferate and cause endothelial cellular enlargement with resultant multiorgan vasculitis. This process may cause thrombosis, and the deposition of leukocytes, macrophages, and platelets may result in small nodules. Thrombosis of supplying blood vessels may cause gangrene of the distal portions of the extremities, nose, ear lobes, and genitalia. This vasculitic process may also result in loss of intravascular colloid with subsequent hypovolemia and decreased tissue perfusion and, possibly, organ failure. Loss of electrolytes is common.

Some people with a history of typhus may develop a recrudescent type of typhus known as Brill-Zinsser disease. After a patient with typhus is treated with antibiotics and the disease appears to be cured, Rickettsia may linger in the body tissues. Months, years, or even decades after treatment, organisms may reemerge and cause a recurrence of typhus. How the Rickettsia organisms linger silently in a person and by what mechanism recrudescence is mediated are unknown. The presentation of Brill-Zinsser disease is less severe than epidemic typhus, and the associated mortality rate is much lower. Risk factors that may predispose to recrudescent typhus include improper or incomplete antibiotic therapy and malnutrition.

Murine typhus and scrub typhus share the same pathophysiology as epidemic typhus, although they are somewhat milder. The incubation period is approximately 12 days for the typhus group. Prior infection with Rickettsia typhi provides subsequent and long-lasting immunity to reinfection. Read more »