Trichinella/Trichinellosis

General Characteristics

Basic features of Trichinella

• Intestinal and tissue nematode Nematode A phylum of unsegmented helminths with fundamental bilateral symmetry and secondary triradiate symmetry of the oral and esophageal structures. Many species are parasites. Toxocariasis (roundworm)
• Intracellular parasite
• Associated disease: trichinellosis

Clinically relevant species

• Most notable and worldwide: Trichinella spiralis
• Causes of trichinellosis:
  • Encapsulating/encysting (within host muscles):
    • Trichinella britovi (Europe, Asia ASIA Spinal Cord Injuries, and northern and western Africa)
    • Trichinella murelli (North America)
    • Trichinella nativa (Arctics)
    • Trichinella nelsoni (Eastern Africa)
  • Nonencapsulating:
    • Trichinella papuae (Papua New Guinea)
    • Trichinella pseudospiralis (worldwide)
    • Trichinella zimbabwensis (Tanzania)

Epidemiology

• Worldwide, an estimated 10,000 cases of trichinellosis occur every year.
• In the United States: 
  • 90 cases from 2008 to 2012
  • Decline in cases due to improved swine production
• High prevalence Prevalence The total number of cases of a given disease in a specified population at a designated time. It is differentiated from incidence, which refers to the number of new cases in the population at a given time. Measures of Disease Frequency in:
  • China
  • Former Soviet Union
  • Romania and other parts of Central Europe
  • Thailand
  • Mexico
  • Argentina
  • Bolivia

Pathophysiology

Reservoirs and transmission

• Reservoir Reservoir Animate or inanimate sources which normally harbor disease-causing organisms and thus serve as potential sources of disease outbreaks. Reservoirs are distinguished from vectors (disease vectors) and carriers, which are agents of disease transmission rather than continuing sources of potential disease outbreaks. Humans may serve both as disease reservoirs and carriers. Escherichia coli:
  • Trichinella spiralis: swine/pigs
  • Other animals Animals Unicellular or multicellular, heterotrophic organisms, that have sensation and the power of voluntary movement. Under the older five kingdom paradigm, animalia was one of the kingdoms. Under the modern three domain model, animalia represents one of the many groups in the domain eukaryota. Cell Types: Eukaryotic versus Prokaryotic: wild boars, bear, rodents, horses, moose
• Transmission: raw or undercooked meat (especially pork and bear) containing larvae Larvae Wormlike or grublike stage, following the egg in the life cycle of insects, worms, and other metamorphosing animals. Ascaris/Ascariasis

Pathogenesis

• Encysted larvae Larvae Wormlike or grublike stage, following the egg in the life cycle of insects, worms, and other metamorphosing animals. Ascaris/Ascariasis live within the striated muscles of the reservoir Reservoir Animate or inanimate sources which normally harbor disease-causing organisms and thus serve as potential sources of disease outbreaks. Reservoirs are distinguished from vectors (disease vectors) and carriers, which are agents of disease transmission rather than continuing sources of potential disease outbreaks. Humans may serve both as disease reservoirs and carriers. Escherichia coli animal (some species are not encysted).
• Larvae Larvae Wormlike or grublike stage, following the egg in the life cycle of insects, worms, and other metamorphosing animals. Ascaris/Ascariasis are ingested when undercooked meat is consumed.
• GI phase:
  • After exposure to gastric acid Gastric acid Hydrochloric acid present in gastric juice. Gastroesophageal Reflux Disease (GERD) and pepsin Pepsin Pepsin breaks down proteins into proteoses, peptones, and large polypeptides. Proteins and Peptides, larvae Larvae Wormlike or grublike stage, following the egg in the life cycle of insects, worms, and other metamorphosing animals. Ascaris/Ascariasis are released from the cysts Cysts Any fluid-filled closed cavity or sac that is lined by an epithelium. Cysts can be of normal, abnormal, non-neoplastic, or neoplastic tissues. Fibrocystic Change.
  • Larvae Larvae Wormlike or grublike stage, following the egg in the life cycle of insects, worms, and other metamorphosing animals. Ascaris/Ascariasis invade small-bowel mucosa and then become adult worms.
  • Mating occurs; life span in the small bowel Small bowel The small intestine is the longest part of the GI tract, extending from the pyloric orifice of the stomach to the ileocecal junction. The small intestine is the major organ responsible for chemical digestion and absorption of nutrients. It is divided into 3 segments: the duodenum, the jejunum, and the ileum. Small Intestine: Anatomy lasts approximately 4 weeks.
• Systemic phase:
  • Females release Release Release of a virus from the host cell following virus assembly and maturation. Egress can occur by host cell lysis, exocytosis, or budding through the plasma membrane. Virology larvae Larvae Wormlike or grublike stage, following the egg in the life cycle of insects, worms, and other metamorphosing animals. Ascaris/Ascariasis that enter circulation Circulation The movement of the blood as it is pumped through the cardiovascular system. ABCDE Assessment.
  • Larvae Larvae Wormlike or grublike stage, following the egg in the life cycle of insects, worms, and other metamorphosing animals. Ascaris/Ascariasis migrate to striated muscles.
  • Tissue migration can last up to 1 month, causing symptoms.
  • Highly active muscles affected:
    • Diaphragm Diaphragm The diaphragm is a large, dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. The diaphragm consists of muscle fibers and a large central tendon, which is divided into right and left parts. As the primary muscle of inspiration, the diaphragm contributes 75% of the total inspiratory muscle force. Diaphragm: Anatomy
    • Tongue Tongue The tongue, on the other hand, is a complex muscular structure that permits tasting and facilitates the process of mastication and communication. The blood supply of the tongue originates from the external carotid artery, and the innervation is through cranial nerves. Lips and Tongue: Anatomy
    • Masseter Masseter A masticatory muscle whose action is closing the jaws. Jaw and Temporomandibular Joint: Anatomy
    • Intercostals
    • Extraocular muscles
  • Can reach myocardium Myocardium The muscle tissue of the heart. It is composed of striated, involuntary muscle cells connected to form the contractile pump to generate blood flow. Heart: Anatomy and the brain Brain The part of central nervous system that is contained within the skull (cranium). Arising from the neural tube, the embryonic brain is comprised of three major parts including prosencephalon (the forebrain); mesencephalon (the midbrain); and rhombencephalon (the hindbrain). The developed brain consists of cerebrum; cerebellum; and other structures in the brain stem. Nervous System: Anatomy, Structure, and Classification and lead to cardiac and cerebral inflammatory reaction.
  • As larvae Larvae Wormlike or grublike stage, following the egg in the life cycle of insects, worms, and other metamorphosing animals. Ascaris/Ascariasis encyst, tissue infiltration (by PMNs and eosinophils Eosinophils Granular leukocytes with a nucleus that usually has two lobes connected by a slender thread of chromatin, and cytoplasm containing coarse, round granules that are uniform in size and stainable by eosin. Innate Immunity: Phagocytes and Antigen Presentation) and edema Edema Edema is a condition in which excess serous fluid accumulates in the body cavity or interstitial space of connective tissues. Edema is a symptom observed in several medical conditions. It can be categorized into 2 types, namely, peripheral (in the extremities) and internal (in an organ or body cavity). Edema occur.
  • Calcification follows after a period of months, and after that, they remain in that state for years, but symptoms decrease after months.

Clinical Presentation

• General principles:
  • Incubation Incubation The amount time between exposure to an infectious agent and becoming symptomatic. Rabies Virus period: 1–6 weeks
  • Severity of symptoms depends on amount of larvae Larvae Wormlike or grublike stage, following the egg in the life cycle of insects, worms, and other metamorphosing animals. Ascaris/Ascariasis ( infectious dose Infectious dose Enteric Fever (Typhoid Fever)) consumed.
  • Two phases: intestinal and systemic
• Intestinal phase Intestinal phase Gastrointestinal Secretions:
  •  2–7 days after exposure
  • Can be asymptomatic
  • Abdominal pain Abdominal Pain Acute Abdomen (midabdomen)
  • Diarrhea Diarrhea Diarrhea is defined as ≥ 3 watery or loose stools in a 24-hour period. There are a multitude of etiologies, which can be classified based on the underlying mechanism of disease. The duration of symptoms (acute or chronic) and characteristics of the stools (e.g., watery, bloody, steatorrheic, mucoid) can help guide further diagnostic evaluation. Diarrhea
  • Nausea Nausea An unpleasant sensation in the stomach usually accompanied by the urge to vomit. Common causes are early pregnancy, sea and motion sickness, emotional stress, intense pain, food poisoning, and various enteroviruses. Antiemetics
  • Vomiting Vomiting The forcible expulsion of the contents of the stomach through the mouth. Hypokalemia
• Systemic phase:
  •  1–2 weeks after ingestion
  • Can last up to 8 weeks
  • Fever Fever Fever is defined as a measured body temperature of at least 38°C (100.4°F). Fever is caused by circulating endogenous and/or exogenous pyrogens that increase levels of prostaglandin E2 in the hypothalamus. Fever is commonly associated with chills, rigors, sweating, and flushing of the skin. Fever and chills Chills The sudden sensation of being cold. It may be accompanied by shivering. Fever
  • Periorbital Periorbital Orbital and Preseptal Cellulitis edema Edema Edema is a condition in which excess serous fluid accumulates in the body cavity or interstitial space of connective tissues. Edema is a symptom observed in several medical conditions. It can be categorized into 2 types, namely, peripheral (in the extremities) and internal (in an organ or body cavity). Edema/palpebral edema Edema Edema is a condition in which excess serous fluid accumulates in the body cavity or interstitial space of connective tissues. Edema is a symptom observed in several medical conditions. It can be categorized into 2 types, namely, peripheral (in the extremities) and internal (in an organ or body cavity). Edema: 
    • Proptosis Proptosis Retinoblastoma
    • Chemosis Chemosis Conjunctivitis
    • Conjunctivitis Conjunctivitis Conjunctivitis is a common inflammation of the bulbar and/or palpebral conjunctiva. It can be classified into infectious (mostly viral) and noninfectious conjunctivitis, which includes allergic causes. Patients commonly present with red eyes, increased tearing, burning, foreign body sensation, and photophobia. Conjunctivitis
  • Myalgia Myalgia Painful sensation in the muscles. Ion Channel Myopathy:
    • Face
    • Chest
  • Weakness
  • Dry cough Dry Cough Strongyloidiasis
  • Splinter hemorrhage and/or retinal hemorrhage
  • Petechial rash Rash Rocky Mountain Spotted Fever/ urticaria Urticaria Urticaria is raised, well-circumscribed areas (wheals) of edema (swelling) and erythema (redness) involving the dermis and epidermis with associated pruritus (itch). Urticaria is not a single disease but rather is a reaction pattern representing cutaneous mast cell degranulation. Urticaria (Hives)
  • Headache Headache The symptom of pain in the cranial region. It may be an isolated benign occurrence or manifestation of a wide variety of headache disorders. Brain Abscess
  • Hepatomegaly

Complications

• Myocarditis Myocarditis Myocarditis is an inflammatory disease of the myocardium, which may occur alone or in association with a systemic process. There are numerous etiologies of myocarditis, but all lead to inflammation and myocyte injury, most often leading to signs and symptoms of heart failure. Myocarditis:
  • Transitory passage leads to infiltration of inflammatory cells.
  • Life-threatening arrhythmias are the most common cause of death.
• Meningitis Meningitis Meningitis is inflammation of the meninges, the protective membranes of the brain, and spinal cord. The causes of meningitis are varied, with the most common being bacterial or viral infection. The classic presentation of meningitis is a triad of fever, altered mental status, and nuchal rigidity. Meningitis/ encephalitis Encephalitis Encephalitis is inflammation of the brain parenchyma caused by an infection, usually viral. Encephalitis may present with mild symptoms such as headache, fever, fatigue, and muscle and joint pain or with severe symptoms such as seizures, altered consciousness, and paralysis. Encephalitis
• Pulmonary:
  • Secondary bacterial pneumonia Secondary bacterial pneumonia Influenza Viruses/Influenza
  • Respiratory myositis (involving the diaphragm Diaphragm The diaphragm is a large, dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. The diaphragm consists of muscle fibers and a large central tendon, which is divided into right and left parts. As the primary muscle of inspiration, the diaphragm contributes 75% of the total inspiratory muscle force. Diaphragm: Anatomy)
• Renal failure Renal failure Conditions in which the kidneys perform below the normal level in the ability to remove wastes, concentrate urine, and maintain electrolyte balance; blood pressure; and calcium metabolism. Renal insufficiency can be classified by the degree of kidney damage (as measured by the level of proteinuria) and reduction in glomerular filtration rate. Crush Syndrome
• Thromboembolic disease

Diagnosis

Diagnostic tests Diagnostic tests Diagnostic tests are important aspects in making a diagnosis. Some of the most important epidemiological values of diagnostic tests include sensitivity and specificity, false positives and false negatives, positive and negative predictive values, likelihood ratios, and pre-test and post-test probabilities. Epidemiological Values of Diagnostic Tests

• Serologic evidence:
  • Only detectable after 3 weeks of incubation Incubation The amount time between exposure to an infectious agent and becoming symptomatic. Rabies Virus
  • Methods used:
    • Western blotting Western Blotting Identification of proteins or peptides that have been electrophoretically separated by blot transferring from the electrophoresis gel to strips of nitrocellulose paper, followed by labeling with antibody probes. Blotting Techniques
    • ELISA ELISA An immunoassay utilizing an antibody labeled with an enzyme marker such as horseradish peroxidase. While either the enzyme or the antibody is bound to an immunosorbent substrate, they both retain their biologic activity; the change in enzyme activity as a result of the enzyme-antibody-antigen reaction is proportional to the concentration of the antigen and can be measured spectrophotometrically or with the naked eye. Many variations of the method have been developed. St. Louis Encephalitis Virus
    • Indirect immunofluorescence
    • Latex agglutination test
• Muscle biopsy Biopsy Removal and pathologic examination of specimens from the living body. Ewing Sarcoma:
  • Leads to a definitive diagnosis, but done only if initial tests are inconclusive
  • Symptomatic muscle tissue is biopsied.
  • Examination would show Trichinella spiralis cyst embedded in muscle tissue.

Additional tests

• Laboratory tests:
  • CBC:
    • Eosinophilia Eosinophilia Abnormal increase of eosinophils in the blood, tissues or organs. Autosomal Dominant Hyperimmunoglobulin E Syndrome
    • Leukocytosis Leukocytosis A transient increase in the number of leukocytes in a body fluid. West Nile Virus
  • Metabolic panel and muscle tests:
    • Hypokalemia Hypokalemia Hypokalemia is defined as plasma potassium (K+) concentration < 3.5 mEq/L. Homeostatic mechanisms maintain plasma concentration between 3.5-5.2 mEq/L despite marked variation in dietary intake. Hypokalemia can be due to renal losses, GI losses, transcellular shifts, or poor dietary intake. Hypokalemia
    • ↑ Creatine Creatine An amino acid that occurs in vertebrate tissues and in urine. In muscle tissue, creatine generally occurs as phosphocreatine. Creatine is excreted as creatinine in the urine. Acute Kidney Injury kinase
    • ↑ Aminotransferases
    • ↑ Lactate dehydrogenase Lactate Dehydrogenase Osteosarcoma
    • ↑ Aldolase Aldolase Becker Muscular Dystrophy
• Plain X-ray X-ray Penetrating electromagnetic radiation emitted when the inner orbital electrons of an atom are excited and release radiant energy. X-ray wavelengths range from 1 pm to 10 nm. Hard x-rays are the higher energy, shorter wavelength x-rays. Soft x-rays or grenz rays are less energetic and longer in wavelength. The short wavelength end of the x-ray spectrum overlaps the gamma rays wavelength range. The distinction between gamma rays and x-rays is based on their radiation source. Pulmonary Function Tests: intramuscular calcification

Management

Treatment

• Mild infection: Most infections Infections Invasion of the host organism by microorganisms or their toxins or by parasites that can cause pathological conditions or diseases. Chronic Granulomatous Disease have no complications and are self-limited.
• For infection with systemic symptoms, treatment consists of: 
  • Antiparasitic drugs:
    • Albendazole Albendazole A benzimidazole broad-spectrum anthelmintic structurally related to mebendazole that is effective against many diseases. Anthelmintic Drugs
    • Mebendazole Mebendazole A benzimidazole that acts by interfering with carbohydrate metabolism and inhibiting polymerization of microtubules. Anthelmintic Drugs
  • Corticosteroids Corticosteroids Chorioretinitis (especially for severe complications)
• Prevention:
  • Cook meat up to 77°C
  • Freeze meat at –15°C (not applicable to Arctic species)
  • Proper meat handling
  • Postexposure prophylaxis Prophylaxis Cephalosporins with mebendazole Mebendazole A benzimidazole that acts by interfering with carbohydrate metabolism and inhibiting polymerization of microtubules. Anthelmintic Drugs may be effective if given within 6 days after exposure.

Prognosis Prognosis A prediction of the probable outcome of a disease based on a individual’s condition and the usual course of the disease as seen in similar situations. Non-Hodgkin Lymphomas

• Self-limited
• Complete recovery within 2–6 months
• Increased morbidity Morbidity The proportion of patients with a particular disease during a given year per given unit of population. Measures of Health Status with cardiac or CNS involvement

Differential Diagnosis

• Gastroenteritis Gastroenteritis Gastroenteritis is inflammation of the stomach and intestines, commonly caused by infections from bacteria, viruses, or parasites. Transmission may be foodborne, fecal-oral, or through animal contact. Common clinical features include abdominal pain, diarrhea, vomiting, fever, and dehydration. Gastroenteritis: intestinal phase Intestinal phase Gastrointestinal Secretions of trichinellosis can be mistaken for food poisoning Food poisoning Acute illnesses, usually affecting the gastrointestinal tract, brought on by consuming contaminated food or beverages. Most of these diseases are infectious, caused by a variety of bacteria, viruses, or parasites that can be foodborne. Sometimes the diseases are caused by harmful toxins from the microbes or other chemicals present in the food. Especially in the latter case, the condition is often called food poisoning. Clostridia owing to the constitutional GI symptoms: Viral and bacterial causes of gastroenteritis Gastroenteritis Gastroenteritis is inflammation of the stomach and intestines, commonly caused by infections from bacteria, viruses, or parasites. Transmission may be foodborne, fecal-oral, or through animal contact. Common clinical features include abdominal pain, diarrhea, vomiting, fever, and dehydration. Gastroenteritis must be considered and ruled out using history, physical exam, and stool analysis.
• Myopathies: Autoimmune disorders such as polymyositis Polymyositis Polymyositis (PM) is an autoimmune inflammatory myopathy caused by T cell-mediated muscle injury. The etiology of PM is unclear, but there are several genetic and environmental associations. Polymyositis is most common in middle-aged women and rarely affects children. Polymyositis or dermatomyositis Dermatomyositis A subacute or chronic inflammatory disease of muscle and skin, marked by proximal muscle weakness and a characteristic skin rash. The illness occurs with approximately equal frequency in children and adults. The skin lesions usually take the form of a purplish rash (or less often an exfoliative dermatitis) involving the nose, cheeks, forehead, upper trunk, and arms. The disease is associated with a complement mediated intramuscular microangiopathy, leading to loss of capillaries, muscle ischemia, muscle-fiber necrosis, and perifascicular atrophy. The childhood form of this disease tends to evolve into a systemic vasculitis. Dermatomyositis may occur in association with malignant neoplasms. Paraneoplastic Syndromes present with myalgia Myalgia Painful sensation in the muscles. Ion Channel Myopathy and vasculitis-like symptoms and signs, similar to the systemic signs of trichinellosis. These cases usually involve the proximal muscle groups, unlike in trichinellosis. Investigations for specific autoantibodies Autoantibodies Antibodies that react with self-antigens (autoantigens) of the organism that produced them. Blotting Techniques such as anti–signal recognition protein (anti-SRP) antibodies Antibodies Immunoglobulins (Igs), also known as antibodies, are glycoprotein molecules produced by plasma cells that act in immune responses by recognizing and binding particular antigens. The various Ig classes are IgG (the most abundant), IgM, IgE, IgD, and IgA, which differ in their biologic features, structure, target specificity, and distribution. Immunoglobulins: Types and Functions and anti-Mi2 aid in diagnosis. Management is usually with immunosuppressants Immunosuppressants Immunosuppressants are a class of drugs widely used in the management of autoimmune conditions and organ transplant rejection. The general effect is dampening of the immune response. Immunosuppressants and glucocorticoids Glucocorticoids Glucocorticoids are a class within the corticosteroid family. Glucocorticoids are chemically and functionally similar to endogenous cortisol. There are a wide array of indications, which primarily benefit from the antiinflammatory and immunosuppressive effects of this class of drugs. Glucocorticoids.
• Ascariasis Ascariasis Ascariasis is most often caused by A. lumbricoides. If symptomatic, characteristics typically follow 2 phases, which correlate with the migration of the parasite through the body. The early phase may include cough, dyspnea, and wheezing. The late phase typically includes abdominal discomfort, bloating, nausea, and intermittent diarrhea. Ascaris/Ascariasis: infection caused by the parasitic roundworm Ascaris Ascaris Ascaris is a genus of parasitic nematodes. The infection, ascariasis, is most often caused by A. lumbricoides. Transmission occurs primarily via ingestion of water or food contaminated with Ascaris eggs. Most patients with ascariasis are asymptomatic. Ascaris/Ascariasis lumbricoides: Transmission occurs via ingestion of contaminated water or food. Most patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship are asymptomatic. If symptoms do occur, they can be mild, with only abdominal discomfort, or severe, causing intestinal obstruction Intestinal obstruction Any impairment, arrest, or reversal of the normal flow of intestinal contents toward the anal canal. Ascaris/Ascariasis. Other symptoms, such as cough, are due to migration of the worms through the body.
• Hookworm infection: caused by the helminths Helminths Commonly known as parasitic worms, this group includes the acanthocephala; nematoda; and platyhelminths. Some authors consider certain species of leeches that can become temporarily parasitic as helminths. Anthelmintic Drugs Necator americanus Necator americanus A common parasite of humans in the moist tropics and subtropics. These organisms attach to villi in the small intestine and suck blood causing diarrhea, anorexia, and anemia. Hookworm Infections and Ancylostoma duodenale Ancylostoma Duodenale Hookworm Infections: Patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship will present with iron Iron A metallic element with atomic symbol fe, atomic number 26, and atomic weight 55. 85. It is an essential constituent of hemoglobins; cytochromes; and iron-binding proteins. It plays a role in cellular redox reactions and in the transport of oxygen. Trace Elements deficiency anemia Anemia Anemia is a condition in which individuals have low Hb levels, which can arise from various causes. Anemia is accompanied by a reduced number of RBCs and may manifest with fatigue, shortness of breath, pallor, and weakness. Subtypes are classified by the size of RBCs, chronicity, and etiology. Anemia: Overview and Types and failure to thrive Failure to Thrive Failure to thrive (FTT), or faltering growth, describes suboptimal weight gain and growth in children. The majority of cases are due to inadequate caloric intake; however, genetic, infectious, and oncological etiologies are also common. Failure to Thrive. Diagnosis involves inspection Inspection Dermatologic Examination of human feces for larvae Larvae Wormlike or grublike stage, following the egg in the life cycle of insects, worms, and other metamorphosing animals. Ascaris/Ascariasis and eggs.
• Strongyloidiasis Strongyloidiasis Strongyloidiasis is a common parasitic disease caused by infection with the roundworm Strongyloides stercoralis. Transmission occurs through skin penetration. Strongyloides has a unique life cycle that can be entirely completed in the human host, migrating from the skin to the pulmonary system and then to the GI system. Strongyloidiasis: a disease caused by the roundworm ( nematode Nematode A phylum of unsegmented helminths with fundamental bilateral symmetry and secondary triradiate symmetry of the oral and esophageal structures. Many species are parasites. Toxocariasis) Strongyloides. Strongyloidiasis Strongyloidiasis Strongyloidiasis is a common parasitic disease caused by infection with the roundworm Strongyloides stercoralis. Transmission occurs through skin penetration. Strongyloides has a unique life cycle that can be entirely completed in the human host, migrating from the skin to the pulmonary system and then to the GI system. Strongyloidiasis has various clinical manifestations, including GI symptoms and eosinophilia Eosinophilia Abnormal increase of eosinophils in the blood, tissues or organs. Autosomal Dominant Hyperimmunoglobulin E Syndrome. Diagnosis is via serology Serology The study of serum, especially of antigen-antibody reactions in vitro. Yellow Fever Virus.