Bloodstream: Enteric fever, AIDS Flashcards
Clinical classification of salmonella
- Typhoidal salmonella
- Non-typhoidal salmonella:
They primarily infect other animals though they can cause food borne gastroenteritis and septicaemia
Kauffman White scene or antigenic classification of salmonella
Based on O antigen, salmonella are classified into serogroups: 1,2,3,…,67
Each serogroup is further differentiated into serotypes
Non motile species of salmonella
S. gallinarum pullorum
Capsulated species of salmonella
S. typhi
S. paratyphi C
S. dublin
Species of salmonella
- S. enterica
6 subspecies (enterica, indica, arizonae)
>3000 serotypes (according to Kaufmann and White scheme using the O,H and Vi antigens) - S. bongori
Serotype of Salmonella typhi
S. enterica enterica Typhi
9,12 (Vi) : d
9,12 are O antigens
Flagellar antigen is of d type
Serotype of salmonella paratyphi A
S. enterica enterica Para A
1, 2, 12 : a
Serotype of salmonella paratyphi B
Salmonella enterica enterica Para B
1, 4, 5, 12 : b; 1, 2
Is flagellar antigen shows phase variations or antigenic variations
Biochemical features of salmonella
All are aerogenic except S. typhi All are non lactose fermenting All are H2S positive except: • Para A • Typhisuis • Cholesuis IMViC -+-+ (except S typhi -+- -) Urease -ve
Conventional biochemical tests for salmonella
- Catalase positive and oxidase negative
- Indole, citrate and urease test negative
- TSI shows:
• gas present except for S. typhi
• abundant H2S present except for:
S. paratyphi A - not produced
S. typhi - speck of H2S present
Cultivation of salmonella
On MacConkey medium: NLF colonies Transport medium: Cary Blair Enrichment media: 1. Selenite F broth 2. Gram negative broth 3. Tatrathionate broth
Salmonella on blood culture
Colony appearance
Blood agar: non haemolytic moist colonies
MacConkey agar: round, translucent pale colonies and non-lactose fermenting
Selective media for salmonella
Low selective media: MacConkey agar Highly selective media: 1. DCA: Non lactose fermenting pale colonies with black centre 2. XLD agar: Red colonies with black centre 3. Wilson Blair’s medium: Isolation of S. typhi from highly contaminated specimens
Features of H antigen of salmonella
Flagella antigen Protein, so highly antigenic Heat and alcohol labile Shows variations H antibodies: Appear earliest after infection and persist for several months
Features of O antigen of salmonella
Somatic antigen Polysaccharide, so lower antigenicity Heat and alcohol stable O antibodies: Follow appearance of H antibodies and disappears in a few weeks
Antibodies in case of remote, recent and active infection of salmonella
In cases of recent or remote infections:
H antibodies- significant titre, but not
O antibodies
In cases of recent infection or active disease:
Both H and O antibodies are significant
Features of Vi antigen of salmonella
Capsular antigen
Polysaccharide so least antigenic
Heat labile but alcohol stable
Present only in casulated species and some E. coli and some citrobacter
Covers O antigen and prevents agglutination with O antibody (so boil the suspension to selectively destroy the Vi antigen)
Features of Vi Antibody against salmonella
Spare for a short time during convalescence
Absence indicates poor prognosis
Persistence indicates carrier state
Infections caused by salmonella
1. Enteric fever/ typhoid: •Typhi - M/C in India •Para A - 2nd M/C in India • “ B • “ C 2. Invasive enteritis out for poisoning: •Zoonosis •All other serotypes - non typhoidal salmonella (GI commensal/ pathogen of reptile, birds, mammals other than man)
Typhoid or enteric fever
Strictly human disease •Incubation P: 1-2 weeks •Infective dose ID50: 10^3 - 10^6 bacilli •Progresses over 3-4 weeks •Resolves for many, if untreated
Risk factors for typhoidal salmonella transmission
1. Stomach acidity: • <1 year of age • antacid consumption • previous Helicobactor pylori infection 2. Intestinal integrity affected: • inflammatory bowel disease • prior GIT surgery
Pathogenesis of enteric fever
- Entry through epithelial cells, M cells:
Trigger formation of membrane ruffles.
BME- bacteria mediated endocytosis (via specialised type III secretion system and ruffles) into vacuoles - Entry into macrophages
- Survival inside macrophages
- Primary bacteremia:
Macrophages reach lymphatics - Spread to RES, gall bladder, kidneys,…
- Secondary bacteremia
Clinical manifestations of enteric fever
- Step ladder pyrexia
- Malaise and anorexia
- Vomiting 🤮
- Faget sign: fever with bradycardia
- Rose spots
- Hepatosplenomegaly
Complications of enteric fever
- Intestinal haemorrhage- M/C
- Intestinal perforation- 2nd M/C
The above 2 occurs mostly in 3rd week - Meningitis, deafness, psychosis
- Arthritis, periostitis
- Nephritis, cholecystitis
- Visceral abscesses
Diagnosis of enteric fever
First week of illness: • blood culture • bone marrow • duodenal aspirate culture Second/third week of illness: Serum specimen for serology like Widal test Third/fourth week of illness: Urine and stool culture
Blood specimen for salmonella diagnosis
- 5-10 ml sterile blood in 50-100 ml of bile broth
- Subculture on MacConkey and blood agar for 10 days
Preferred is 5-10 ml sterile blood
Biphasic/Castaneda medium (BHI infusion with broth)
Rate of positivity for each week: - > 90%
- 75%
- 60%
- 25%
Stool and urine samples for salmonella infection
Positivity rates
Stool:
• selective medium used
• becomes positive at end of 2nd week in 40-50% of cases
Urine:
• becomes positive at end of 3rd week in 30-50% of cases
• salmonella are shed in urine infrequently
Bone marrow aspirate as sample for salmonella
Similar to blood culture, recommended for first week
Advantages:
1. Most sensitive at all stages
2. Remains positive for around 5 days after the start of treatment
Tests for Salmonella
- Widal test
- Typhidot
- Diazo test
- Antigen detection:
(ELISA/ Latex agglutination/ co-agglutination) - Probes/ nucleic acid amplification tests NAATs:
(most specific but limited availability)
NOTE: serology is considered neither sensitive nor specific
Widal test
Tube agglutination test measuring titre of H and O antibodies of salmonella H ab-ag reaction: Fluffy or woolly agglutination Done in Dreyer’s tube O ab-ag reaction: Granular/chalky agglutination Felix tubes
Inference of Widal test
Significant titre of:
O antibodies: >100
H antibodies: >200
Significant titre of O antibodies indicate recent infection
Significant titre of H antibodies indicates the serotype
False positive for Widal test
- Anamnestic response: due to unrelated infections in persons who had prior enteric fever
- If bacterial antigen suspensions are not free from fimbriae
- Persons with inapparent infection
- Persons with prior vaccinations (TAB vaccine)
False negative for Widal test
- Early stage - first week
- Later stage - after fourth week
- Carriers
- Patients on antibiotics
- Prozone phenomenon: antibody excess
Avoided by serial dilution
Diazo test
Biochemical test for salmonella
Defects a phenolic compound in urine
Good sensitivity and specificity only in 1st week
Typhidot test
ELISA based method
Immuno-Chromatographic test for salmonella
IgM and IgG antibodies detected separately
But no quantification
Treatment of salmonella
Empirical: • Ceftriaxone • Azithromycin Fully sensitive strains: 1. Ciprofloxacin 2. Amoxicillin 3. Azithromycin 4. Cotrimoxazole 5. Chloramphenicol
Treatment of MDR strains of salmonella
MDR strains: resistant to Amoxicillin, Cotrimoxazole and Chloramphenicol Using: 1. Ciprofloxacin 2. Azithromycin 3. Ceftriaxone (severe cases)
Treatment of NARST
Nalidixic acid resistant S. typhi Also called fluoroquinolone resistant strains Treatment: 1. Azithromycin 2. Ceftriaxone
Types of carriers of salmonella based on duration
- Convalescent: upto 3 months
- Temporary: 3 months -1 year
- Chronic: >1 year
Carriers of salmonella based on persistence
1. Fecal carriers: • more common • site: gall bladder • seen in patients with biliary tract abnormalities 2. Urinary carriers: • site: kidney • seen in: Urinary tract abnormalities Schistosoma haematobium infections
Is MALDI-TOF useful in identifying Salmonella
No
It can identify salmonella upto genus level
It poorly differentiates between serotypes as they share the same ribosomal proteins
Vaccines for typhoid fever
1. Vi antigen capsular polysaccharide Vi-CPS vaccine: Protection for 2 years Poorly immunogenic in children- T independent 2. Typhoral- oral liver attenuated Lasts for 4 years 3. Parenteral TAB vaccine: Heart killed whole cell vaccine Not used- significant side effects
Drug resistance in Salmonella
- MDR S. typhi:
Resistant to Chloramphenicol, ampicillin and cotrimoxazole - Fluoroquinolone FQ resistance
- Resistance to ceftriaxone
Diagnosis of carriers of salmonella
Treatment
Screening:
Vi agglutination test to detect Vi antibody
Confirmation: culture
specimens - stool or duodenal aspirate
Treatment: ampicillin/amoxicillin + probenecid
Non-typhoidal salmonella NTS
Usually cause gastrointestinal manifestations
In a small proportion, may develop into bacteremia ➡️:
1. Endovascular infection
2. Seedling to various organs leading to metastatic symptoms
Risk factors for bacteremia by non typhoidal salmonella are
1. NTS serotype: S. choleraesuis of pig S. dublin of cattle 2. Age: infants and elderly 3. Immunity: Immunocompromised patients 4. People with pre existing heart conditions
Genera in retroviridae that are pathogenic to humans
- Genus Lentivirus:
Contains HIV-1 and 2 - Genus Deltaretrovirus
Contains HTLV-1
Parts of HIV
1. Envelop • liquid part- host cell membrane • protein part: gp 120 knob-like spikes gp 41 anchoring transmembrane predicles 2. Nucleocapsid: Capsid icosahedral Core contains 2 copies of ss RNA reverse transcriptase, integrase and protease
Structural genes of HIV
- gag gene: matrix and core antigen
- pol gene: reverse transcriptase, protease and integrase
- env gene: gp 120 binding to CD4 and fusion protein gp 41
Non-structural genes of HIV
Essential regulatory: tat: transcriptional transactivator nef: negative factor rev Accessory regulatory: vif viral infectivity factor vpr vpu
HIV serotyping
Based on differences in env gene
1. HIV-1:
3 distinct groups: M (dominant worldwide), N, O
M comprises 11 subtypes or clades A to K (geographically different)
2. HIV-2:
8 subtypes A (common) to H mostly in Africa
HIV
mode of transmission
1. Sexual mode 75%: • heterosexual M/C • anal has higher risk 2. Blood transfusion 5% but highest risk 3. Percutaneous/ mucosal like needles, razors 4. Perinatal 20-40% can occur any time during pregnancy and breast feeding but maximum during delivery
Viral load of HIV in secretions
Maximum in blood, genital secretions and CSF
Variable in breast milk and saliva
Zero or minimal in other secretions
Host receptors involved in HIV entry
- Main receptor CD4 with gp 120:
T cells, monocytes, macrophages, Langerhans cells, astrocytes,… - Co-receptor binding to gp 120:
Usually chemokine receptors like CXCR4 of T cells, CCR5 of macrophages - DC-SIGN, of dendritic cells of mucosa and skin bind and facilitates transport to lymphoid organs
But no entry into dendritic cells
Stages of HIV replication
- Fusion to host cells by gp 41
- Penetration and uncoating
- Reverse transcription
- Transcription to form viral proteins
- Pre-integration complex
- Integration to form provirus
- Latency: infectious to other cells
Pre-integration complex of HIV
Nucleoprotein complex transported into nucleus before integration containing:
- Linear ds DNA
- gag matrix protein
- Accessory vpr protein
- Viral integrase
Stages of HIV progression
Typical or natural course
- Acute HIV disease/ acute retroviral syndrome:
•Primary viremia with flu-like illness - Clinical latency/ asymptomatic stage:
CMI (CD8 T cells) and humoral immunity
HIV replication in lymph nodes for years - Persistent Generalised Lymphadenopathy
- Symptomatic HIV infection/AIDS related complex
- AIDS
Acute HIV disease/ acute retroviral syndrome
- Multiplication in lymph nodes
- Primary viremia with flu-like illness
- Significant fall of CD4 T cells
Persistent generalised lymphadenopathy
PGL
Enlarged lymph nodes of more than 1cm size in two or more non-contiguous sites that persist for at least 3 months
AIDS related complex or
Symptomatic HIV infection
After clinical latency, CD4 T cell starts falling
- Unexplained diarrhoea for at least 1 month
- Severe weight loss, fatigue, malaise and night sweats
- Mild opportunistic infections like oral thrush
Features of AIDS
- Rapid fall in CD4 T cells
- High viral load
- Lymphoid tissue is totally destroyed and replaced by fibrous tissue
- Opportunistic infections
- Neoplasia development
- Direct HIV induced manifestations like HIV encephalopathy
Classification systems for HIV
1. CDC classification: Nine stages based on associated clinical conditions and CD4 T cells 2. WHO clinical staging for adults: Based on clinical conditions only Into 4 stages
High risk groups for HIV
- Extremely high risk:
Female sex workers, men who have sex with men, transgenders and IV drug abusers - Moderately high risk:
• HCW
• haemophiliacs and other blood product recipients
• people with other STIs
Opportunistic infections of HIV infected people
M/C TB in world and India
Fungal: candidiasis, Pneumocytis jirovecii
Viral: Herpes simplex, CMV
Parasitic: Cryptosporidium, Toxoplasma, Strongloides
When is p24 antigen test used for HIV
Is it the best confirmatory test
12-26 days of infection
The IgG antibodies remain for long
IgA is used for serum, mucus secretions and newborn
The best confirmatory method is HIV RNA detection
Specific tests for HIV infection
1. Screening tests: Ab detection • ELISA • Rapid/simple tests 2. Supplementary tests: Ab detection • Western blot assay • Line Immuno-Assay LIA 3. Confirmatory tests: • p24 Ag detection • viral culture • HIV RNA • HIV DNA
NACO strategic algorithms
- Strategy I: transfusion and transplantation safety
Just one test - Strategy IIa: Unlinked anonymous testing
2 tests - Strategy IIa: symptomatic patients
3 tests, first confirmatory test positive ➡️ positive - Strategy III:
Asymptomatic HIV patients, antenatal , pre-op screening
All screening test positive confirmed by 2 tests
Difference between strategy IIb and III of NACO
Strategy IIb: When first two positive ➡️ ➕ve When 2nd negative ➡️ indeterminate or negative Strategy III: When 3 positive ➡️ ➕ve When any confirmatory is negative ➡️ indeterminate When both confirmatory negative ➡️: • high risk - indeterminate • low risk - negative
Prognosis of HIV
- CD4 T cell count- M/C
- HIV RNA load: best
- p24 Ag detection
- Neopterin and β2 macroglobulin level
Diagnosis of paediatric HIV infection
• Baby’s IgG cannot be differentiated from maternal IgG which disappears by 18 months. Screening tests can be done after that. • HIV DNA 🧬 PCR: most recommended after 6 weeks confirmed by same test again • HIV RNA detection • p24 antigen detection
Prophylaxis required for opportunistic infections before starting ART
1. For pneumocystis pneumonia: 2 types 1° and 2° Cotrimoxazole 2. For Crytococcal meningitis Fluconazole 3. Isoniazid preventive therapy for TB
Anti retroviral drugs
- NRTI
- NNRTI
- PI
- NtRTI
- Others like fusion inhibitor
First line treatment for HIV-1 infection for adults with normal serum creatinine levels
TLE
Tenofovir + Lamivudine + Efavirenz
Post exposure prophylaxis for HIV
Occupational exposure within 2 hours
TL + LR regimen
Tenofovir-Lamivudine + Lopinavir-Ritonavir
also used for:
•HIV-2 or HIV-1 and 2 co-infection
• women received with single dose nevirapine in past pregnancy
