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Essentially, virus particles, or "virions," consist of nucleic acid enclosed in a protein coat. They lack metabolic activity and do not possess ribosomes or most enzymes necessary for replication. Both the lipid and the protein coats protect the nucleic acid from enzymatic degradation. The protein coat, or "capsid," consists of repeating, identical subunits called "capsomeres. In the first type, capsomeres are arranged as a regular polyhedron with 20 triangular faces and 12 corners. Other viruses exhibit helical symmetry in which a helix is formed of ribonucleoprotein and nucleic acid. Helical viruses are always enveloped, whereas icosahedral viruses may be enveloped or non-enveloped. The envelope is derived from host cell membranes and modified by insertion of one or more spikelike glycoproteins. These and other proteins on the surface of enveloped or non-enveloped viruses are important for two reasons: they provide specific interaction with receptors on host cells, and they serve as the major antigens of the virus. Figure 373-1 demonstrates schematically the marked variety in size, shape, and structure of human viruses. The genome may be linear or circular and may exist as single or multiple segments. Viruses are classified by the International Committee on Taxonomy of Viruses according to the scheme presented in Table 373-1 (Table Not Available). The following order of virion characteristics is used: nucleic acid type, presence or absence of envelope, genome replication strategy, positive- or negative-sense genome, and genome segmentation. Because virus structure varies and genomes are complex, mechanisms of replication are diverse. Following a random collision between a virus particle and a cell surface, attachment occurs by binding of a surface protein of a virus to a host cell virus receptor. The nature of the viral attachment protein has been identified for a number of viruses. Penetration of the plasma membrane of the cell occurs by endocytosis, a process similar to receptor-mediated endocytosis of non-viral ligands, or by non-endocytic pathways such as direct translocation across the plasma membrane. After acidification of the endosome, the viral membrane fuses with that of the vesicle, releasing the nucleocapsid. After uncoating of the viral nucleic acid, macromolecular synthesis of nucleic acid and protein occurs. Viruses cause cell injury by a number of mechanisms: directly by lysis resulting from viral replication, by lysis induced by antiviral antibody and complement, or by cell-mediated immune mechanisms recognizing infected host cells. As virus infection spreads and sufficient numbers of cells are injured, disease results. Viral infection may be limited to the initial site of infection or may spread through the lymphatic, blood, or nervous system to distal sites. A role for viral toxins has never been established, and such enzymes as are virus coded have a role in viral replication but not directly in cellular injury, and they do not affect host tissues at distant sites. However, products of inflammation released from sites of cell injury and circulating interferon and other lymphokines contribute to the signs and symptoms of viral infection. In addition to lytic effects on cells, viral infection may transform cells so that they proliferate continuously and, in vertebrates, mammals, and humans, may produce tumors, sometimes as a result of the occurrence of viral oncogenes in such viruses. Antiviral antibodies develop in response to viral infection and to immunization with attenuated or inactivated virus or viral components. In the serum, antibodies of all classes and subclasses of immunoglobulins are found; in addition, secretory antibodies consisting predominantly of immunoglobulin A (IgA) molecules develop on mucosal surfaces in response to infection of their surfaces. They are of critical importance in diseases in which the primary site of inoculation is a mucosal surface.

In such cases, circulating volume must be restored to ensure adequate cerebral perfusion; therefore, fluid replacement is required until perfusion normalizes. Hyperthermia can adversely affect outcomes in children with acute brain injuries, so maintain normal body temperatures. Disability­Neurologic Deficit Assess pupillary size and reaction to light and the level of consciousness. Pediatric Glasgow Coma Scale assessments can be done as part of the secondary survey (Table 11­5). Gastric distention from positive-pressure ventilation increases the chance of vomiting and aspiration. The nasogastric route should be avoided in patients with significant midface injuries, as its use increases the risk of intracranial placement. Exposure & Environment Significant injuries can be missed unless the child is completely undressed and examined fully, front and back. Because of their high ratio of surface area to body mass, infants and children cool rapidly. Urinary Catheter An indwelling urinary bladder catheter should be placed to monitor urine output. Contraindications are based on the risk of urethral transection; signs include blood at the meatus or in the scrotum or a displaced prostate detected on rectal examination. After the initial flow of urine with catheter placement, the urine output should exceed 1 mL/kg/h. Skin 5 4 3 2 1 6 5 4 3 2 1 Best upper limb motor response: Child (Infant modification)b Search for lacerations, hematomas, swelling, and abrasions. Cutaneous findings may indicate underlying pathology (eg, a flank hematoma overlying a renal contusion), although surface signs may be absent even with significant internal injury. Head Check for hemotympanum and for clear or bloody cerebrospinal fluid leak from the nares. Battle sign (hematoma over the mastoid) and raccoon eyes are late signs of basilar skull fracture. A score < 8 usually indicates central nervous system depression requiring positive-pressure ventilation. This can be done clinically in children older than 4 or 5 years of age with normal neurologic findings on examination (including voluntary movement of all extremities) who are able to deny midline neck pain or midline tenderness on palpation of the neck and who have no other painful injuries that might obscure the pain of a cervical spine injury. If radiographs are indicated, a cross-table lateral neck view is obtained initially, followed by anteroposterior, odontoid, and in some cases oblique views. Normal studies do not exclude significant injury, either bony or ligamentous, or involving the spinal cord itself. Therefore, an obtunded child should be maintained in cervical spine immobilization until the child has awakened and an appropriate neurologic examination can be performed. The entire thoracolumbar spine must be palpated and areas of pain or tenderness examined by radiography. Genitourinary System If urethral transection is suspected (see earlier discussion), perform a urethrogram before catheter placement. Management of kidney injury is largely nonoperative except for renal pedicle injuries. Delayed diagnosis of fracture may occur when children are comatose; reexamination is necessary to avoid overlooking previously missed fractures. Treatment of open fractures includes antibiotics, tetanus prophylaxis, and orthopedic consultation. Central Nervous System Most deaths in children with multisystem trauma are from head injuries, so optimal neurointensive care is important. Significant injuries include diffuse axonal injury; cerebral edema; subdural, subarachnoid, and epidural hematomas; and parenchymal hemorrhages. Extensor or flexor posturing represents intracranial hypertension, not seizure activity, until proven otherwise. If accompanied by a fixed, dilated pupil, such posturing indicates that a herniation syndrome is present, and mannitol or 3% hypertonic saline should be given if perfusion is normal. Mild prophylactic hyperventilation is no longer recommended, although brief periods of hyperventilation are still indicated in the setting of acute herniation.

Tetracycline, 500 mg four times a day, or doxycycline, 100 mg twice a day, should be continued for at least 1 week after the patient becomes afebrile (usually 2 to 3 days). Those patients who receive no antibiotics also do well, with a recovery rate of more than 99%. Doxycycline and a quinolone have been somewhat effective, but cures have not been achieved even after 2 years of continuous therapy. The location of the organism in an acid environment inside the phagolysosome interferes with the activity of antibiotics. Experimental studies designed to alkalize the fluid helped to eradicate the organisms in phagocytes. The combination of doxycycline and chloroquine in these studies was most effective and may be useful in patients with chronic Q fever. Surgical resection of infected valves is usually required because the large vegetations cause hemodynamic deficiencies in cardiac function. For those persons at high risk, such as researchers working with sheep, veterinarians, or exposed laboratory workers, vaccine can be obtained under an investigational new drug application. Focusing on controlling disease in the workplace is more effective than attempting to control the disease in animals. Three recommended measures include knowing the serologic status of the employees, not permitting pregnant women or persons with valvular heart disease to be in the high-risk jobs, and confining the research on sheep to a building dedicated solely to that purpose. Approximately 200 different infectious agents, many of them rare, cause disease in humans and fulfill the definition of zoonoses. There are more than 30 million cats in the United States and more than 40 infectious diseases have been transmitted by this creature. Almost all arthropod transmitted infectious agents in the United States are due to either ticks or mosquitos, with ticks being the more common villain and Lyme disease being the most common arthropod-transmitted infectious disease in the United States. The risk of developing a zoonosis is increased by direct animal contact, outdoor activities, exposure to and inhalation of infectious air particles, insect bites, contact with previously infected human blood products, and contact with and ingestion of infectious agents transmitted by animal-contaminated water and insufficiently cooked meat, eggs, dairy products, fish, and shellfish. Raw shellfish are the garbage filters of the ocean and can transmit at least 25 different infectious or toxic illnesses to humans. In addition, the farmer, pet owner, hunter, laboratory researcher, and cave explorer, among others, is at higher risk than the general population to develop a zoonosis. Infective agents transmitted by these routes from animal sources essentially include members of all microbial classes: viruses, bacteria, fungi, and parasites. Immunocompromised hosts such as splenectomized patients, transplant patients, patients with the acquired immunodeficiency syndrome, as well as pregnant women and their fetuses are at high risk of developing clinical disease when exposed to these various infectious agents. New emerging infectious diseases seem inevitable because of increased interest in xenotransplantation, global warming trends, human intrusion in previously underexplored or never-explored sites, and an increasing threat of biological terrorism or warfare. Preventive measures to decrease infection in the compromised host include utilizing routine pet care immunization, neutering pets, using caution when handling pet fomites, and avoiding ingestion of undercooked meat, fish, and eggs. Non-animal-associated environmental- or travel-related infectious diseases can be confused with zoonoses. The vast majority of clinical diseases caused by Legionella pneumophila, Plasmodium falciparum, Entamoeba histolytica, Giardia lamblia, Pseudomonas pseudomallei, Chromobacterium violaceum, Aeromonas hydrophila, Francisella philomiragia, and airborne fungi such as Blastomyces dermatitides, Coccidioides immitis, and Histoplasma capsulatum are acquired through environmental exposure and are only rarely related to animal hosts. One exception is Sporotrichosis schenckii, which generally is considered an environment-acquired pathogen from vegetation injuries. This fungus has been associated with the zoonotic transmission from draining cutaneous ulcers of cats to owners and animal handlers. Histoplasmosis has been acquired by explorers (spelunkers) in caves contaminated by bat guano. Unfortunately, some descriptive disease titles can be misleading to clinicians and thus can interfere with correctly considering the possible diagnosis. The transmission of tick-borne Rocky Mountain Spotted Fever actually occurs more commonly in the southeastern United States than in the Rocky Mountains and has even been acquired in the middle of New York City. Vegetarians and other strict non-pork-eating persons have been seriously infected with the pig tapeworm Taenia solium as a result of fecal contamination of food from unsuspected infected human sources. Fleas on dogs Cat scratch or bite Flea bite dermatitis Pulex irritans Cat-scratch disease Bartonella species Note: See Table of Contents and Index for more detailed discussion of each disease. Viral incubation of the three influenza species in the pig with reassortment of antigens and subsequent spread to humans of virulent "new" influenza strains can lead to influenza pandemics that in sheer number (billions) surpass any past epidemics of smallpox or plague. Leprosy, a human-to-human-transmitted illness of biblical notoriety, is endemic in at least three animal species, including thearmadillo.

In general, the frequency of cardiac disease in Central America and Mexico in seropositive persons is low, even though rates of seropositivity may be substantial. Also in these countries heart disease tends to develop later in life than in Brazil, Bolivia, or Argentina. Yet in some areas of the West, bites from aggressive and abundant reduviid bugs can be a source of annoyance to , and allergic reactions in, suburbanites and outdoorspeople. A local inflammatory lesion called a chagoma may develop at the site of entry of the parasite. Histologically, the chagoma shows mononuclear cell infiltration, interstitial edema, and intracellular aggregates of amastigotes in cells of the subcutaneous tissue and muscle. Biopsy specimens from enlarged lymph nodes show hyperplasia, and amastigotes may be present in reticular cells. Skeletal muscle tissue from muscle biopsy specimens has shown organisms and focal inflammation. In acute cases that have a fatal outcome there is invariably myocarditis with an enlarged heart. Microscopically, degeneration of cardiac muscle fibers and prominent but patchy areas of inflammation with nests of amastigotes in the muscles are observed. The heart in those patients with chronic disease who die suddenly, presumably of ventricular arrhythmias or heart block, may be normal in size or only moderately enlarged. Other patients with chronic chagasic cardiomyopathy experience cardiomegaly and die of intractable failure. The hearts are both hypertrophied and dilated, with thinning, especially at the apex to form a characteristic apical aneurysm. Mural thrombi, with subsequent embolization of the lungs and peripheral organs, are frequently seen. Microscopic findings in the heart are not specific, consisting of focal mononuclear cell infiltrates, hypertrophy of cardiac fibers with patchy areas of necrosis, variable fibrosis, and edema. The components of the conduction system of the heart most often involved by inflammatory changes are the sinoatrial and atrioventricular nodes, as well as the right branch and left anterior branches of the bundle of His. The microscopic pathologic changes are disappointingly similar to those in the heart, again with no or very few organisms. This type of parasympathetic denervation may also be found in other hollow viscera, such as duodenum, ureters, or biliary tree. The presence of lesions and organisms in the placenta may be associated with abortion, stillbirth, or acute disease in the fetus. However, pregnancy may result in a normal fetus, even though placental lesions are present. Some of these patient maintain a low-level parasitemia demonstrable only with very sensitive techniques. So, one point of view is that indeterminate cases have a smouldering disease process that will become evident later. However, there are no tests that can predict whether or when evidence of chronic disease will develop. In countries of lower endemnicity the risk of chronic disease is correspondingly less. Key features of the chronic disease that must be explained include the following: (1) a latent period of up to 20 years from presumed initial infection with T. Genetic diversity in parasite strains, including variation in animal virulence, may explain geographic differences in disease. Yet, this does not preclude a concomitant autoimmune reaction to parasite antigens that have been shown to share antigenic epitopes with neural tissues. Also, reports of activation of disease are increasing, especially with brain involvement similar to that produced by Toxoplasma sp. When those initially exposed do have clinical manifestations, the disease is an acute systemic infection. The incubation period under natural conditions cannot be established accurately but is probably at least a week. A local area of erythema and induration (chagoma) may develop in the skin at the site of parasite entry. The chagoma is often accompanied by regional adenopathy and persists for several weeks. Meningoencephalitis is another serious complication, particularly in very young patients.