Medical Entomology Past Paper

## DANIEL ROSASI - Comprehensive Study Guide --- ## SECTION A: LONG ANSWER QUESTIONS (20 Marks Each) ### Question 1: Life Cycle of a Three-Host Tick (20 Marks) **With an illustration, outline the life cycle of a three-host tick** **Answer:** **Life Cycle Stages:** - **Egg Stage (Fall)**Adult females drop off the third host after feeding, usually in the fall - Females lay eggs in the environment - Eggs hatch into six-legged larvae - Larvae overwinter in this stage - **Larval Stage (Spring - First Host)**In spring, larvae seek out and attach to the first host, usually a small rodent - Larvae feed on the first host - Later in summer, engorged larvae leave the first host - Larvae molt into nymphs, usually in fall - Ticks overwinter in the nymphal stage - **Nymphal Stage (Following Spring - Second Host)**During the following spring, nymphs seek out and attach to the second host - Second host is usually another rodent or lagomorph - Nymphs feed on the second host and drop off later in summer - Nymphs molt into adults off the host in late summer or fall - Adults overwinter in this stage - **Adult Stage (Next Spring - Third Host)**The next spring, adults seek out and attach to a third host - Third host is usually a larger herbivore (cervids, bovids), carnivore, or human - Adults feed and mate on the third host during summer - Females drop off the host in fall to continue the cycle - Females may reattach and feed multiple times **Important Notes:** - The three hosts do not necessarily have to be different species or different individuals - Humans may serve as first, second, or third hosts --- ### Question 2: Myiasis (20 Marks) **Discuss myiasis under the following sub-headings** #### A. AETIOLOGY (3 Marks) - Myiasis is the invasion of organs and tissues of humans or other vertebrates by fly larvae - Fly larvae feed on living or dead tissues or, in intestinal myiasis, on the host's ingested food - Flies in several genera may cause myiasis in humans **Important Species:** - **Dermatobia hominis** - Primary human bot fly - **Cochliomyia hominivorax** - Primary screwworm fly (New World) - **Chrysomya bezziana** - Old World screwworm - **Cordylobia anthropophaga** - Tumbu fly - **Cuterebra, Oestrus, Wohlfahrtia** - Animal parasites that occasionally infect humans #### B. PATHOGENESIS (5 Marks) **Types of Myiasis:** **1. Accidental Myiasis** - Usually involves eating food contaminated by eggs or larvae of non-parasitic flies (e.g., house flies) - Larvae may survive for some time in the intestine - No flies are specially adapted to cause intestinal myiasis in humans - **Symptoms:** Abdominal pain, diarrhea, blood discharge, vomiting - Living larvae may be passed in excreta or vomit **2. Obligatory Myiasis** - Occurs in other mammals, not typically humans - Essential for fly maggots to live on a live host for at least part of their life - **Examples:** Larvae of *Cordylobia anthropophaga*, *Cochliomyia hominivorax*, *Chrysomya bezziana*, *Dermatobia hominis*, *Wohlfahrtia magnifica* - All obligatory parasites of humans and other vertebrates **3. Facultative Myiasis** - Larvae are normally free-living, often attacking carcasses - Under certain conditions may infect living hosts - **Urogenital myiasis:** Occasionally occurs in humans, usually involving larvae of *Musca* or *Fannia* species - Ovipositing flies attracted to unhygienic discharges lay eggs near genital orifices - Hatching larvae enter genital orifice and pass up urogenital tract - Considerable pain may be caused during urination - Larvae may obstruct passages, causing discharge of mucus, blood, and eventually larvae **Different Terms for Myiasis by Body Part:** - **Dermal myiasis** - Affects different parts of the body - **Cutaneous myiasis** - Skin infection - **Intestinal/enteric/gastrointestinal myiasis** - Digestive system - **Ophthalmic myiasis** - Eyes - **Creeping myiasis** - When larvae burrow under skin surface producing creeping eruption - **Traumatic myiasis** - When wounds become infested #### C. DIAGNOSIS (3 Marks) - Diagnosis made by finding fly larvae in tissue - Identification to genus/species level involves comparing morphological structures:Anterior and posterior spiracles - Mouthparts - Cephalopharyngeal skeleton - Cuticular spines - Travel history can be helpful for genus/species identification #### D. TREATMENT (4 Marks) - **Simple removal:** When larvae occur in wounds, sores, dermal or subdermal tissues, removal under aseptic conditions is usually relatively simple - **Complex removal:** When larvae are deeply embedded in underlying tissues or have penetrated mucous membranes, eyes, frontal sinuses, or cavities, removal is more difficult - Surgery may be needed for deeply embedded larvae #### E. CONTROL (6 Marks) - Ash/salt on tables/latrines to discourage landing of flies - Curtains to entrances of eateries to prevent fly entry - Screens on windows to prevent fly entry - Use of insecticides to k