15.3 Vector Control & Neglected Tropical Diseases

Vector Control & Neglected Tropical Diseases

Neglected Tropical Diseases (NTDs) are a diverse group of communicable conditions that prevail in tropical and subtropical environments. In Kenya, NTDs predominantly afflict impoverished rural communities and informal urban settlements where access to potable water, adequate sanitation, and quality healthcare is limited. The Kenya National Master Plan for the Elimination of NTDs establishes strategic priorities combining preventive chemotherapy (Mass Drug Administration [MDA]), integrated vector management, water, sanitation, and hygiene (WASH) infrastructure, and clinical morbidity management. For the Clinical Officers Council examinations, clinicians must master the parasite life cycles, vectors, intermediate hosts, diagnostic criteria, and precise pharmacological regimens for these debilitating conditions.

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1. Lymphatic Filariasis (Bancroftian Filariasis / Elephantiasis)

Lymphatic filariasis is a chronic vector-borne parasitic infection caused by filarial nematodes that reside within human lymphatic vessels.

Etiology & Vector Bionomics in Kenya

• Etiological Agent: Wuchereria bancrofti accounts for 100% of lymphatic filariasis cases in Kenya (neither Brugia malayi nor Brugia timori occurs in Africa).
• Primary Vectors:
  • Culex quinquefasciatus: The primary urban and peri-urban vector along the Kenyan Coast. Breeds in stagnant, organically polluted water, wet pit latrines, blocked open drains, and cesspools.
  • Anopheles gambiae and Anopheles funestus: Serve as secondary rural vectors in agricultural coastal zones.
• Endemic Geography: Hyperendemic in six Coastal counties: Mombasa, Kilifi, Kwale, Lamu, Tana River, and Taita Taveta.

Pathogenesis & Nocturnal Periodicity

• Infective third-stage larvae (L3) are inoculated into human skin during a mosquito blood meal -> migrate to afferent lymphatic channels and regional lymph nodes -> mature over 6 to 12 months into adult thread-like worms (females measure 8–10 cm; males 4 cm) -> adults live for 6 to 8 years.
• Adult worms induce intense mechanical obstruction, lymphatic endothelial proliferation, marked vessel dilatation (lymphangiectasia), and valvular incompetence, resulting in chronic retrograde lymphedema.
• Gravid females release millions of sheathed microfilariae into the bloodstream. In East Africa, microfilariae exhibit marked nocturnal periodicity: they circulate in highest numbers in peripheral blood between 10:00 PM and 2:00 AM, coinciding with the nocturnal biting peak of Culex mosquitoes. During the day, they sequester in the deep pulmonary capillaries.

Clinical Manifestations

1. Asymptomatic / Subclinical Infection: Microfilaremic patients with hidden lymphatic dilatation and subclinical renal damage (hematuria, proteinuria).
2. Acute Adenolymphangitis (ADL / Dermato-lymphangio-adenitis): Episodic, agonizing attacks of high-grade fever, chills, painful regional lymphadenitis, and retrograde lymphangitis (erythema tracking centrifugally down the limb). Driven primarily by secondary bacterial superinfections (Group A Streptococcus, Staphylococcus aureus) entering through cracked interdigital skin fissures.
3. Chronic Irreversible Manifestations:
   • Hydrocele: Accumulation of clear fluid within the tunica vaginalis of the scrotum. This is the most common clinical manifestation of lymphatic filariasis in men.
   • Lymphedema & Elephantiasis: Typically begins in the feet and ascends to the lower leg (occasionally arms, breasts, or vulva). Characterized by non-pitting brawny edema, skin thickening, dark pigmentation, deep cutaneous folds, and warty hyperkeratotic papillomatous excrescences (mossy foot).
   • Chyluria: Rupture of dilated retroperitoneal lymphatics into the renal collecting system, causing milky, lipid-rich urine.

Diagnostics

• Nocturnal Thick Blood Film: Giemsa-stained smear collected strictly between 10:00 PM and 2:00 AM. Reveals sheathed microfilariae with a characteristic tail terminal nuclei-free space.
• Circulating Filarial Antigen (CFA): Rapid immunochromatographic Filariasis Test Strip (FTS); detects soluble adult W. bancrofti antigens in fingerprick blood. Highly sensitive; can be performed any time of day or night.
• Scrotal Ultrasonography: Visualizes the pathognomonic "filarial dance sign"—erratic, undulating movements of live adult worms within dilated intrascrotal lymphatic vessels.

Treatment & Elimination Protocols

• Mass Drug Administration (MDA): Kenya implements annual community-wide preventive chemotherapy for at least 5 consecutive years with $\ge 65%$ epidemiological coverage: $\text{Albendazole (400 mg single dose)} + \text{Ivermectin (150--200 }\mu\text{g/kg single dose)}$ (Note: In areas non-endemic for onchocerciasis or loiasis, Diethylcarbamazine [DEC 6 mg/kg] + Albendazole is utilized globally; however, Kenya's national elimination program historically instituted Albendazole + Ivermectin to mitigate severe filarial adverse reactions).
• Morbidity Management and Disability Prevention (MMDP):
  • Meticulous foot and leg hygiene: Washing the limb twice daily with clean water and mild soap, drying carefully between toes, applying antibiotic/antifungal ointments to fissures, elevating the leg at night, and wearing comfortable footwear to prevent secondary bacterial ADL episodes.
  • Surgical hydrocelectomy (complete excision/eversion of the tunica vaginalis) to resolve hydroceles.

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2. Trachoma

Trachoma is the leading infectious cause of irreversible blindness worldwide, classified by the WHO as a target for global elimination.

Etiology & Vector Transmission

• Etiological Agent: Chlamydia trachomatis serovars A, B, Ba, and C (obligate intracellular bacterium).
• Vector & Transmission: Transmitted by Musca sorbens (the eye-seeking bazaar fly that breeds preferentially on exposed human feces deposited on the soil). Transmission also occurs via direct contact with infective ocular and nasal discharges from young children, and shared contaminated fomites (towels, clothing).
• Hyperendemic Arid Counties: Turkana, Narok, Kajiado, Samburu, Baringo, West Pokot, and Marsabit.

Pathophysiology & The Natural History of Blindness

Repeated cycles of infection and re-infection in early childhood produce chronic follicular conjunctival inflammation -> subconjunctival cicatricial scarring -> scar retraction rolls the eyelid margin inward (entropion) -> eyelashes turn inward to rub against the globe (trichiasis) -> continuous mechanical trauma from eyelashes abrades the corneal epithelium -> secondary corneal ulceration, bacterial keratitis, and dense fibrous corneal opacity (CO) producing irreversible blindness.

WHO Simplified Trachoma Grading System

1. TF (Trachomatous inflammation -- Follicular): Presence of 5 or more follicles (each $\ge 0.5\text{ mm}$ in diameter) in the central upper tarsal conjunctiva.
2. TI (Trachomatous inflammation -- Intense): Pronounced inflammatory thickening of the upper tarsal conjunctiva that obscures >50% of the normal underlying deep tarsal vessels.
3. TS (Trachomatous Scarring): Presence of easily visible white fibrous bands, lines, or scars in the tarsal conjunctival stroma.
4. TT (Trachomatous Trichiasis): At least one eyelash rubbing against the eyeball, or evidence of recent epilation (plucking) of inward-turned eyelashes.
5. CO (Corneal Opacity): Easily visible corneal clouding overlying the pupil that obscures at least part of the pupil margin, causing severe visual impairment.

The WHO SAFE Strategy for Trachoma Elimination

• S -- Surgery for Trichiasis: Bilamellar Tarsal Rotation (trabut or weiss procedure) performed by trained clinical officers under local anesthesia. Corrects the eyelid deformity, turning lashes outward to preserve vision.
• A -- Antibiotics for Active Infection: Mass Drug Administration (MDA) when TF prevalence among children aged 1 to 9 years is $\ge 5%$ in a sub-county:
  • Oral Azithromycin: Single dose of 20 mg/kg (maximum adult dose: 1,000 mg).
  • Topical 1% Tetracycline Eye Ointment: Applied to both eyes twice daily for 6 consecutive weeks for infants <6 months of age, pregnant women in the first trimester, or individuals with macrolide hypersensitivity.
• F -- Facial Cleanliness: Promoting daily washing of children's faces with clean water to clear crusty ocular and nasal secretions, reducing fly attraction.
• E -- Environmental Improvement: Constructing ventilated improved pit (VIP) latrines to eliminate open defecation (denying Musca sorbens its essential breeding medium); increasing community access to clean water.

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3. Schistosomiasis (Bilharzia)

Schistosomiasis is a chronic trematode (blood fluke) infection driven by freshwater snail vectors.

Comparative Biology & Pathology: S. mansoni vs. S. haematobium

Clinical Management of Schistosomiasis

• First-Line Chemotherapy: Praziquantel (PZQ) administered as a single oral dose of 40 mg/kg (or split into two divided doses 4 hours apart) taken with a meal to improve tolerability. In mass school deworming campaigns, dosing is determined using the standardized WHO dose pole.
• Female Genital Schistosomiasis (FGS): S. haematobium eggs trapped in the vulva, vagina, and cervix produce characteristic sandy mucosal patches, contact bleeding, and dyspareunia, multiplying the risk of HIV transmission threefold to fourfold.

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4. Soil-Transmitted Helminths (STHs)

Soil-transmitted helminth infections are caused by nematode parasites whose eggs undergo embryonic maturation in warm, moist soil contaminated by human feces.

Species & Clinical Syndromes

1. Ascaris lumbricoides (Giant Roundworm):
   • Ingestion of embryonated eggs from contaminated soil or food.
   • Larval pulmonary transit induces Loeffler's syndrome (cough, wheezing, transient pulmonary infiltrates, marked eosinophilia).
   • Adult worms (20–35 cm) inhabit the jejunum, consuming host nutrients. In young children with heavy worm burdens, tangled masses of worms cause mechanical intestinal obstruction at the ileocecal valve, or migrate into the biliary tract causing acute obstructive cholangitis or pancreatitis.
2. Trichuris trichiura (Whipworm):
   • Ingestion of soil-borne eggs; adult worms embed their thread-like anterior ends into cecal and colonic mucosa.
   • Heavy infections produce Trichuris Dysentery Syndrome (chronic mucoid bloody diarrhea, tenesmus, severe microcytic anemia, growth failure) and classic rectal prolapse in young children due to mucosal edema and straining.
3. Hookworms (Necator americanus & Ancylostoma duodenale):
   • Infective filariform larvae penetrate intact skin ("ground itch" papules), migrate to lungs, are swallowed, and attach to duodenal/jejunal mucosa via sharp teeth (Ancylostoma) or cutting plates (Necator).
   • Worms ingest blood and produce mechanical lacerations with anticoagulant secretion. A. duodenale causes blood loss of 0.2 mL/worm/day; N. americanus causes 0.03 mL/worm/day.
   • Heavy chronic infections produce profound microcytic hypochromic iron-deficiency anemia, hypoproteinemia, anasarca, fatigue, cognitive impairment in school children, and maternal anemia in pregnancy.

Public Health Deworming Regimen

• Under Kenya's National School-Based Deworming Programme (NSBDP), school-aged and preschool children receive periodic mass deworming:
  • Albendazole: 400 mg single oral dose (chewable tablet).
  • (Alternative: Mebendazole 500 mg single oral dose).
  • Administered annually in communities where baseline STH prevalence is 20% to 50%, and twice yearly where baseline prevalence exceeds 50%.

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5. Rabies (Hydrophobia): Prevention & Post-Exposure Prophylaxis (PEP)

Rabies is an acute, progressive, incurable viral encephalomyelitis caused by the Rabies virus (Lyssavirus, family Rhabdoviridae), a bullet-shaped, negative-sense, single-stranded RNA virus.

Transmission & Pathogenesis

• Transmission: Inoculation of infectious saliva via bites, scratches, or licks on broken skin/mucous membranes from an infected mammal. In Kenya, domestic dogs (Canis familiaris) cause >95% of all human cases; wild carnivores (jackals, hyenas) serve as secondary reservoirs.
• Centripetal Axonal Migration: Virus replicates locally in striated muscle -> enters unmyelinated nerve endings via nicotinic acetylcholine receptors -> ascends retrogradely along peripheral motor axons at 12 to 100 mm/day toward the spinal cord and dorsal root ganglia -> rapid trans-synaptic ascent to the central nervous system.
• CNS Destruction & Centrifugal Spread: Produces severe, fatal encephalomyelitis characterized by pathognomonic cytoplasmic inclusion bodies (Negri bodies) in hippocampal pyramidal and cerebellar Purkinje neurons. The virus then travels centrifugally down autonomic nerves to the salivary glands, cornea, and skin.
• Incubation Period: Typically 1 to 3 months (ranges from 1 week to >1 year, depending on bite severity, viral load, and anatomical distance from the central nervous system; facial bites carry the shortest incubation).

Clinical Presentation: Furious vs. Paralytic Rabies

• Prodrome (2 to 10 days): Fever, malaise, headache, and cardinal sensory changes: intense tingling, burning paresthesias, or intractable pruritus at the healed bite site.
• Furious (Encephalitic) Rabies (80% of cases): Hyperactivity, fluctuating agitation, autonomic dysfunction (profuse salivation, lacrimation, dilated pupils), aerophobia (violent spasms triggered by air currents), and hydrophobia (involuntary, agonizing diaphragmatic and pharyngeal spasms provoked by the sight, sound, or attempt to swallow water). Case fatality is 100% once clinical neurological signs emerge.
• Paralytic (Dumb) Rabies (20% of cases): Ascending flaccid paralysis mimicking Guillain-Barré syndrome, progressing to coma and respiratory arrest.

Life-Saving Post-Exposure Prophylaxis (PEP) Protocol

POST-EXPOSURE PROPHYLAXIS (PEP) MASTER ALGORITHM:

Step 1: IMMEDIATE WOUND CARE (Reduces viral load by >90%)
  • Immediately flush and scrub the bite wound with running water and soap for a FULL 15 MINUTES.
  • Apply virucidal antiseptic: 10% Povidone-Iodine or 70% Alcohol.
  • ABSOLUTE RULE: DO NOT SUTURE the wound (suturing drives virus into nerve endings).
  • Administer Tetanus Toxoid and broad-spectrum antibiotics (Amoxicillin-Clavulanate).

Step 2: STRATIFY BY WHO EXPOSURE CATEGORY
  • Category I: Touching/feeding animals, licks on intact skin.
    -> ACTION: Wash hands/skin; NO VACCINE OR RIG NEEDED.
  • Category II: Minor scratches/abrasions without bleeding, nibbling of uncovered skin.
    -> ACTION: Immediate 15-minute wound wash + FULL RABIES VACCINE SERIES (Days 0, 3, 7, 14, 28).
  • Category III: Single/multiple transdermal bites or scratches, bleeding, licks on broken skin,
    mucosal contamination, or ANY contact with bats.
    -> ACTION: 15-minute wound wash + RABIES IMMUNOGLOBULIN (RIG) + FULL VACCINE SERIES.

Step 3: INFILTRATE RABIES IMMUNOGLOBULIN (RIG) LOCALLY
  • Human RIG (HRIG): 20 IU/kg body weight; or Equine RIG (ERIG): 40 IU/kg body weight.
  • Technique: Infiltrate the FULL calculated dose directly into and around all wound margins.
  • If anatomical volume allows leftover RIG, inject IM at a distant site (e.g., anterolateral thigh).
  • CRITICAL: NEVER inject RIG into the same syringe or same anatomical limb as the vaccine!

Step 4: ADMINISTER CELL-CULTURE RABIES VACCINE (ARV)
  • Intramuscular Essen Regimen (1.0 mL IM in deltoid muscle; anterolateral thigh for infants):
    -> Doses administered on Day 0, Day 3, Day 7, Day 14, and Day 28.
  • NEVER administer rabies vaccine in the gluteal muscle (fat depot impairs immune response).