🪱Parasitology Unit 5 – Arthropod Parasites and Vectors

Key Arthropod Groups

• Arachnida includes ticks, mites, and spiders which can transmit diseases or cause direct harm through bites or infestations
• Crustacea encompasses crabs, shrimp, and copepods some of which serve as intermediate hosts for parasitic worms (Diphyllobothrium)
• Insecta is the largest class containing mosquitoes, fleas, lice, and triatomine bugs that spread various pathogens (malaria, plague, typhus, Chagas disease)
• Myriapoda consists of centipedes and millipedes which can inflict painful bites or secrete irritating substances but rarely transmit diseases
• Pentastomida contains tongue worms that parasitize the respiratory tract of vertebrates and can cause zoonotic infections in humans (Linguatula serrata)

Arthropod Anatomy and Physiology

• Exoskeleton provides protection, support, and prevents water loss but limits growth requiring periodic molting (ecdysis)
• Segmented body plan allows for specialization of different regions (head, thorax, abdomen) and appendages (mouthparts, legs, wings)
  • Mouthparts adapted for piercing, sucking, or cutting to enable blood-feeding or tissue damage (mosquito proboscis, tick hypostome)
• Open circulatory system with hemolymph bathing tissues and organs transporting nutrients, waste, and hormones
• Respiratory system varies among groups including tracheae, book lungs, or cutaneous respiration
• Excretory system consists of Malpighian tubules that remove nitrogenous waste and regulate hemolymph composition
• Nervous system includes a ventral nerve cord with ganglia and sensory organs (compound eyes, antennae, sensilla) for detecting hosts and environmental cues

Life Cycles and Reproduction

• Holometabolous development involves distinct egg, larva, pupa, and adult stages with complete metamorphosis (mosquitoes, fleas)
  • Larvae often occupy different habitats and have different feeding strategies than adults
• Hemimetabolous development has egg, nymph, and adult stages with gradual metamorphosis (ticks, lice, triatomine bugs)
  • Nymphs resemble miniature adults and typically have similar feeding habits
• Parthenogenesis allows for asexual reproduction in some species (mites, aphids) leading to rapid population growth
• Polyembryony occurs in some parasitoid wasps where a single egg divides to produce multiple genetically identical offspring
• Reproductive strategies like high fecundity, short generation times, and host-seeking behaviors contribute to their success as parasites and vectors

Vector-Borne Diseases

• Malaria is caused by Plasmodium protozoa transmitted by Anopheles mosquitoes leading to recurring fevers, anemia, and potentially fatal complications
• Dengue virus is spread by Aedes mosquitoes causing fever, headache, and joint pain with severe cases resulting in hemorrhagic fever or shock syndrome
• Lyme disease is a bacterial infection transmitted by Ixodes ticks characterized by a distinctive rash (erythema migrans), flu-like symptoms, and potential long-term complications
• Chagas disease is caused by the parasite Trypanosoma cruzi spread by triatomine bugs leading to heart and digestive system damage
• Zika virus is transmitted by Aedes mosquitoes and can cause microcephaly in infants born to infected mothers and Guillain-Barré syndrome in adults

Transmission Mechanisms

• Biological transmission involves the development or multiplication of the pathogen within the vector before being transmitted to the host
  • Plasmodium sporozoites develop in mosquito salivary glands and are injected during blood feeding
• Mechanical transmission occurs when the vector physically transfers the pathogen without it undergoing development
  • Horse flies can spread equine infectious anemia virus between horses via contaminated mouthparts
• Vertical transmission is the spread of pathogens from an infected female vector to her offspring (transovarial) or from an infected host mother to her young (perinatal)
  • Zika virus can be passed from a pregnant woman to her fetus resulting in congenital abnormalities
• Horizontal transmission refers to the spread of pathogens between individuals of the same generation through direct contact, contaminated surfaces, or vectors
• Sylvatic transmission cycles involve the circulation of pathogens between wild animals and vectors while urban cycles occur between humans and vectors in populated areas

Impact on Human and Animal Health

• Morbidity refers to the incidence and severity of disease in a population which can lead to decreased quality of life, disability, and economic burden
• Mortality is the number of deaths caused by a disease and can be particularly high for some vector-borne infections in endemic areas (malaria)
• Zoonotic diseases are those that can be transmitted from animals to humans often with vectors serving as a bridge (Lyme disease, Zika virus)
  • Emergence of zoonotic diseases is facilitated by factors like habitat encroachment, global travel, and climate change
• Veterinary impact includes diseases that affect livestock (bluetongue virus in sheep) and companion animals (heartworm in dogs) leading to animal suffering and economic losses
• Social and psychological effects of vector-borne diseases can include stigma, anxiety, depression, and post-traumatic stress disorder (PTSD) in affected individuals and communities

Control and Prevention Strategies

• Insecticide-treated bed nets (ITNs) protect individuals from mosquito bites during sleep and have significantly reduced malaria transmission in endemic areas
• Indoor residual spraying (IRS) involves applying long-lasting insecticides to the walls and surfaces of homes to kill resting mosquitoes
• Larval source management targets the aquatic breeding sites of mosquitoes through habitat modification, biological control (Bacillus thuringiensis israelensis), or chemical treatment
• Topical repellents containing DEET, picaridin, or permethrin can be applied to skin or clothing to prevent arthropod bites
• Chemoprophylaxis is the use of preventive medications to protect against infection such as antimalarial drugs for travelers to endemic areas
• Vaccination is available for some vector-borne diseases (yellow fever, Japanese encephalitis) but is still in development for others (dengue, Zika)
• Integrated vector management (IVM) employs a combination of control methods tailored to local conditions and includes community participation, public education, and monitoring and evaluation

Current Research and Future Directions

• Genetic modification of vectors aims to reduce their ability to transmit pathogens through techniques like CRISPR-Cas9 gene editing and the release of insects with a dominant lethal gene (RIDL)
• Wolbachia is a bacterial symbiont that can be introduced into mosquito populations to reduce their susceptibility to viruses like dengue and Zika
• Vaccine development continues for complex vector-borne diseases with a focus on novel approaches like recombinant viral vectors, DNA vaccines, and structure-based antigen design
• Improved diagnostics are being developed for rapid, point-of-care testing and enhanced surveillance of vector-borne diseases in resource-limited settings
• Climate change modeling is being used to predict the potential spread of vectors and diseases into new geographic areas and inform preparedness and adaptation strategies
• One Health approach recognizes the interconnectedness of human, animal, and environmental health and promotes cross-sectoral collaboration in the control and prevention of vector-borne diseases
• Insecticide resistance management is crucial for preserving the effectiveness of current control tools and involves strategies like rotation, mosaics, and mixtures of different insecticide classes
• Community engagement and social mobilization are essential for the success and sustainability of vector control programs and involve educating and empowering communities to take ownership of prevention efforts