Parasitology, ch 5

Congenital transmission =

transmission that occurs through infection of the developing egg or embryo while it is still within the mother's body or at the time of birth/egg laying.

Congenital transmission among invertebrates

transovarial transmission is relatively common. The result is that a parasite can travel rapidly through a population (ex. Babsia bovis in ticks). A young tick can transmit the parasite to a mammal (cattle, human, mouse) with its first blood meal.

Autoinfection

1. A good parasite strategy b/c it avoids the challenges (biological and environmental) of transmission. 2. But: a.) The initial infection by the parasite's parent(s) often sets up a host immune system response that inhibits reinfection. b.) Ultimately, transmission to another host is unavoidable if the parasite or its progeny are to succeed the infected host.

Two fundamental problems with Parasitism:

1. A new host must be found to ensure future generations of the parasite. 2. Identifying a suitable new host. This is especially critical for endoparasites.

Most herbivores will not eat food contaminated with feces if they can avoid it.

1. A proglotid of Taenia sanginata will crawl away from the feces it is voided with, shedding eggs. 2. Dictyocaulis viviparous: Infective larvae of this nematode (cattle lungworm) ride up to 2 meters from a cow par on an exploding sporangiophore of the fungus Philobolus.

Transmission of Cyclospora spp. Transmission Associated With Reproduction

1. Animal sexual intercourse is dangerous: a.) Participants are vulnerable because their focus is on each other and they cannot quickly move or defend themselves. b.) The close contact of two hosts presents parasite with an opportunmity for transmission. c.) Intromittent sex and exchange of body fluids obviates the need for special transmission life stages for the parasite. 2. Transmission via sexual intercourse poses challenges for a parasite: a.) If the host breeding season is short and the animals are not promiscuous, there are few transmission opportunities for the parasite. b.) The sexual act is usually brief, so not much time is available to the parasite for transmission. c.) Most animals have well - developed immune mechanisms to combat pathogens that attempt to establish themselves via the reproductive organs. 3. The importance of transmission via host sexual activity is variable among parasites. For some, it is crucial. For others, this mode of transmission is only one of several modes of transmission.

Enterobius vermicularis a.k.a. pinworm

1. At night the female emerges from the host's body to lay eggs on the perianal region. In a few hours, infective larvae have developed in the eggs. 2. The person awakens, feels itchy, and scratches his/her bottom, lodging the eggs under the fingernails. 3. Result: Reinfection is hands get put into the mouth without washing. Diagnosis: Attach a piece of cellophane tape to a tongue depressor, sticky side up. At dawn, press the infected person (usually a child) around the perianal area. Examine the tape for the thick - walled eggs, which reflect light due totheir lipid content.

Environmental factors on Parasitism

1. Can affect the survival of a parasite transmission stage (cysts, eggs, free - living larvae). 2. The use of inadequate drainage systems. 3. The use of open sewers.

The influence of war on parasite transmission

1. Collapse of the health infrastructrure. 2. Widespread migration, refugee camps. 3. Widespread pollution can weaken the immune system or increase the host's exposure to the infective stage of parasites. There is little information on pollution caused by warfare. War has triggered leishmaniasis in Sudan, Afganistan, and Pakastan. a.) There is an influx of susceptible people to areas that have leishmania. b.) There is a lack of control for sandfly numbers. c.) The health infrastructure is not in good shape. European red fox populations have gone up due to: a.) Anti - rabies measures. b.) Foxes moving into urban areas. Echinococcus multilocularis infections of red foxes have increased. There is concern that foxes will contaminate the environment with Echinococcus multilocularis eggs, leading to human infections.

Pheromermis vesparum Intermediate hosts

1. House the parasite during which some or all of the non-sexual life stages develop. It is essential to the parasite life cycle. 2. Parasite infection itself increases the chance of an intermediate host being eaten because the parasite may damage the intermediate host's health. 3. Some parasites induce specific changes in the morphology, physiology, or behavior of the intermediate host to help ensure its being eaten by the next host.

Humans = a "dead end host". Why:

1. Humans are seldom preyed upon. 2. Most faiths require burying or burning the dead. Some faiths stipulate that the dead be consumed by vultures. These are not considered to be a significant in transmission of disease. A few faiths stipulate that the dead be left out to be eaten by hyenas or dogs. This promotes transmission of Echinococcus granulosis.

Global warming and Parasitism

1. Most scientists agree that it is a genuine phenomenon. a.) Floods from tropical storms can create pools suitable for mosquitoes and snails that serve as vectors/intermediate hosts. b.) Serious storms can destroy infrastructure, disrupt healthcare services, damage homes and sewage systems, and create homeless people or refugees. 2. In the Canadian Arctic, climate change is facilitating the transmission of nematode parasites of musk oxen (Obibos moschatus) and this could have deleterious effects on their health. 3. Many factors affect the transmission of malaria: (from the CDC): Poor rural populations in malaria-endemic areas often cannot afford the housing and bed nets that would protect them from exposure to mosquitoes. These persons often lack the knowledge to recognize malaria and to treat it promptly and correctly. Often, cultural beliefs result in use of traditional, ineffective methods of treatment. Travelers from non-endemic areas may choose not to use insect repellent or medicines to prevent malaria. Reasons may include cost, inconvenience, or a lack of knowledge. Human activities can create breeding sites for larvae (standing water in irrigation ditches, burrow pits) Agricultural work such as harvesting (also influenced by climate) may force increased nighttime exposure to mosquito bites Raising domestic animals near the household may provide alternate sources of blood meals for Anopheles mosquitoes and thus decrease human exposure War, migrations (voluntary or forced) and tourism may expose non-immune individuals to an environment with high malaria transmission. This makes it difficult to access the effect of climate change.

The influence of parasites on host behavior

1. Nonadaptive: Changes to the definitive host. a.) Impairment of host vision, hearing, or other sensory systems. b.) Physical damage to the host central nervous system and/or altering levels of neurotransmitters. c.) Physical damage to the endocrine system and/or altering levels of hormones. 2. Adaptive: Changes to the indeterminate host.

Transmission within the gametes

1. Only intracellular parasites can be transmitted within gametes. Why: Gametes are unicellular structures. 2. Most male gametes, particularly mammalian gametes, are small and contain little cytoplasm, allowing little room for parasites. It is suggested that the evolution of small gamete size may be driven by the host's attempt to control the transmission of parasites within them. One exception: Males of some species of the fruit fly Drosophila produce sperm that are ten times their body length. 3. Transmission via male gametes is rare because; a.) Most male gametes are small. b.) Sperm competition is intense. Anything that compromises their movement would reduce the chance of fertilizing the egg and passing the disease on to the offspring. 4. Female gametes are often large and a number of pathogenic and non-pathogenic symboiants are transmitted within them. Examples: a.) The rickettsiel bacterium Wolbachia found in many invertebrates is typically transmitted via the cytoplasm of the egg. b.) Transmission within gametes is a form of vertical transmission. The reproductive success of the parasite and its host are closely linked. Rickettsiel bacteria include typhus, cat scratch fever, Rocky Mountain spotted fever, etc.

The influence of host behavior on parasite transmission

1. Parasitized prey is often easier to catch and subdue than healthy prey. Becoming infected may not unduly harm the definitive host. ex. Toxoplasma gondi inhibits the intermediate rodent's ability to avoid capture; in fact infected mice become attracted to cat feces. An infected cat suffers little damage. 2. The nematode Anisakis spp. can be spread by eating sushi and sashimi, which are becoming increasingly popular worldwide. 3. Trichinella spiralis, tapeworms: Can be transmitted by eating raw beef or pork, popular in many continental European countries. 4. Human societies vary w.r.t. attitudes towards defecation and disposal of feces. 5. Some animal species (cows) defecate indiscriminately. This is unimportant in migratory animals or those with a large range. It becomes important is situations of limited space (farms, zoos). 6. Some animals defecate in a restricted area or construct latrines or dung piles: a.) Horses often defecate in one corner of a field. b.) Black rhinos (Diceros bicornis) and white rhinos (Ceratotherium simum) defecate in a particular spot, building a dung pile that may reach 1 meter in height. Scattered individual droppings: a.) Fecal contamination is more likely. b.) Droppings may dry out more quickly, killing infective parasites via desiccation or exposure to UV light. Grouped droppings: a.) Fecal contamination is less likely. b.) Droppings do not dry out quickly; the survival of parasite transmission stages may be enhanced.

Hosts and Vectors

1. Paratenic host; a.) Is not essential for parasite transmission. b.) Does improve the chance of transmission occurring. Examples: Ancyclostoma tubaeformae, Ancyclostoma braziliense Third stage larva can invade insect → insect is eaten by rodent → rodent is eaten by definitive host (dog, cat) Mermithid nematode: Definitive host is an invertebrate.

The bed bug Cimex lectularis

1. The male injects his spermatozoa into the female's abdomen ("traumatic insemination"). These migrate to her oviducts where fertilization occurs. Other bugs belonging to the family Cimicadae reproduce in the same way and they are capable of transmitting a number of bacterial and protozoan diseases including Trypanosoma cruzi.

Contaminative transmission

1. The parasite survives until it is ingested by a suitable host. a.) This strategy is energy - demanding and wasteful (vast numbers of eggs or cysts must be produced) b.) It is effective as seen by how many protists and helminthes use it. 2. Many parasites do have mechanisms to increase the chances of finding a host. Active searching for a host Modifying host behavior Use of an intermediate host, paratenic host, or vector Some parasites can be transmitted in a variety of ways.

Marine mammals as "sentinel species":

1. They tend to be long - lived. 2. They feed at high trophic levels. 3. They have a high public profile; dead and dying animals are likely to be reported. 4. Many species have high fat levels in which lipophilic (hydrophobic) chemicals such as DDT become sequestered. 5. Human activities are thought to have contributed to the spread of disease in aquatic animals. Toxoplasma gondii infection in sea otters off California is high. They may be ingesting oocysts in seawater or from filter feeders. T. gondii oocysts have not been identified in any seawater or marine invertebrates or fish in the wild (unlike the cysts of Giardia and Cryptosporidium).

Co - transmission and interactions between infectious agents Burkitt's Lymphoma:

A facial tumor that usually occurs in children living in areas holoendemic for malaria.

Active parasite transmission:

A free - living stage actively searches for and invades its host. Example: Schistosomes; a.) The miracidium actively seeks a snail host, attracted by snail mucosa. b.) The cercaria actively searches its appropriate (bird, mammal) host, attracted by the thermal gradient the host produces.

proto - oncogene:

A gene that is normally present and is important in regulation of cell growth. myc is a proto - oncogene. It encodes a transcription factor that is believed to help regulate 15% of all the genes in the human genome. These include genes involved in cell proliferation, cell growth, apoptosis, and stem cell self - renewal. Myc is very often found to be upregulated in many types of cancers.

oncogene:

A mutated proto - oncogene that can promote transformation of a normal cell into a tumor cell c - myc is an oncogene

Co - transmission and interactions between infectious agents Epstein - Barr Virus (EBV):

A type of herpes virus. Viral DNA becomes integrated into a chromosomal location of the host. Infected lymphocytes experience a chromosomal translocation that moves the c - myc gene in chromosome 8 to be under the control of genes encoding for antibody heavy chain (chromosome 14) or light chains in chromosome 2 (kappa or κ chain)or chromosome 22 (lambda or λ chain). This promotes their transformation into a tumor cell.

Nosocomial transmission:

Acquired in a hospital or as the result of a medical procedure. Gastrointestinal parasites, ectoparasites (lice, mites).

Transmission of Cyclospora spp. Routes of sexual transmission;

Active movement between the host bodies (ex. lice, mites) Transmission within the reproductive fluids (ex. Trichomonas) Transmission through tissue damage caused during copulation. (blood and tissue-dwelling parasites)

Co - transmission and interactions between infectious agents Example # 1:

Anopheles mosquitoes can transmit both malaria parasites and filarial worms. There is some evidence that if an individual mosquito is infected with one of these, it becomes more susceptible to being infected with the other. This concomitant infection may, however, affect the fitness of the mosquito, rendering it shorter lived or not able to fly as far.

Feces:

Are quickly broken down; thus parasite eggs, cysts find themselves washed into streams, rivers, lakes, ponds.

"swimmer's itch':

Cercaria that normally invade aquatic bird invade a human swimming in the water. The cercaria soon dies, but an immune reaction sets in, manifest as a rash.

Lack of personal hygiene (transmission):

Children, senile or otherwise mentally impaired individuals.

the lazy isopod

Chuck the Lazy isopod (the one that eats the fish's tongue and then becomes the tongue of that fish)

Sandfly saliva:

Contains a substance known as Leishmania - enhancing factor: 1. Normally macrophages present pieces of an invader (such as Leishmania) to stimulate the production of parasite - specific T cells. 2. The Leishmania - specific T cells secrete gamma globulin (IFN - γ) when the parasite is present. 3. IFN - γ causes the macrophages to produce superoxide (O2- ), nitric oxide (NO) and other toxic metabolites that kill Leishmania. Sandfly saliva: Also contains anticoagulants and vasodialators. These may attract neutrophils and macrophages, which are take up promastigotes via receptor - mediated endocytosis. A phagolysosome forms, promastigotes develop into amastigotes, and these then focus on the macrophages for invasion and replication. A third member of the innate immune system, the dendritic cell, may also be invaded.

Vector - transmitted parasites:

Delegate host selection to the vector.

Mechanical vectors (transmission):

Dung - frequenting flies (Mus domestica) and other insects (cockroaches)

anthrophilic vector:

Feeds on humans.

zoophilic vector:

Feeds on other animals.

Strongloides stercoralis:

Free- living and parasitic cycles of reproduction. Autoinfection can occur because parasitic female may produce eggs that hatch before being passed. Juveniles penetrate the gut, migrate, and establish themselves in the gut.

Cheap air travel, the car tire business:

Has facilitated the spread of many parasites and their invertebrate vectors.

Geohelminth:

Helminth parasite whose infective stage is found in the soil. Examples: Trichuris trichuria, Ascaris lumbricoides, Ancyclostoma duodenale

Co - transmission and interactions between infectious agents Egypt 1950's:

Intravenous treatment of schistosomiasis spread Hepatitis C virus in parts of the country because of reuse of contaminated needles.

Another example of vertical transmission.

Invertebrates: trans-ovarial transmission. Mammals: a.) trans-placental transmission: The parasite is acquired across the placenta. b.) perinatal transmission: The parasite is acquired through passage through the birth canal or contact with the mother around the time of birth. Most vertically transmitted parasites are avirulent. An exception is Neospora caninum which can cause abortion epidemics in cattle.

Pheromermis vesparum

Larva stages develop within a social wasp → cut their way out of the host, falling into water → mature, mate, female lays eggs → caddis fly or stonefly eats egg → adult, air breathing flying adult is captured by a wasp → egg hatches, slowly develops in wasp → some species alter the behavior of the host, causing it to be attracted to and fall into water.

Co - transmission and interactions between infectious agents Pigs:

May be coinfected with Toxoplasma gondi and porcine circovirus - 2 (PCV - 2). T. gondi is usually not serious in pigs but PCV - 2 is currently a major cause of disease in pigs, causing potentially fatal pneumonia, enteritis, and abortion.

Somehow Plasmodium infection promotes tumor formation.

Maybe: Increased B cell activity as a result of Plasmodium infection increases translocation occurrence. Malaria also suppresses immune mechanisms whereby B cells infected with EBV would normally be eliminated via apoptosis.

Vectors

Mechanical vector: The parasite does not undergo development within the vector and does not invade its tissues (unlike a paratenic host). The parasite is carried on the feet of the vector, its mouthparts, or passes through the mechanical vector's gut. examples: The housefly Mus domestica, the blowfly Calliphora vomitoria, and the cockroach Periplaneta americana are important mechanical vectors for the contaminative transmission of Entaemoba hystolytica. Chickens, dogs, pigs: Will forage in or eat human feces, serving as contaminative vectors for a variety of nematodes, including Ascaris lumbriocoides, Ascaris suum, and Oseophagostomum dentatum. Most blood- dwelling parasites: Use a blood - sucking arthropod (mosquito, sand fly, reduviid bug) or leeches as a vector. In many cases the parasite passes through one of more life cycle in its vector/intermediate host. The vector - parasite relationship may be very specific.

Geophagy (pica):

Most common in children and women, especially during the later stages of pregnancy. a.) Consumption of: Earth, chalk, charcoal, cooking starch, ice... b.) Geophagy is common in primates, other mammals, birds. c.) There is no clear link between geophagy and iron status. d.) Those who consume earth do not do so indiscriminately.

"sporatic Burkett's Lymphoma":

Not associated with either Plasmodium infection nor EBV infection. Tumors are more common on the abdomen than the face. It is associated with the immuno - suppression caused by AIDS. Tumor cells do have the c - myc oncogene.

Raccoons (Procyon lotor)

Raccoons (Procyon lotor) are often infected with the nematode Baylisascaris procyonis. The raccoon constructs latrines on logs or at the base of trees. Small mammals and birds may encounter the nematode at these sites and serve as intermediate hosts. They experience larva migrans and become easy prey for the raccoons.

How religion can influence parasite transmission

Religious practices can affect: 1. What people eat and drink. 2. Their interactions with animals. 3. How they conduct sexual relationships. 4. Where and how they dispose of their dead. Pork consumption: Banned by the Jewish religion, Islam, and Ethopian Orthodox Christianity. This prevents transmission of Trichinella spp. and Taenia solium. Leviticus: hares, swans, owls among others are ranked with hogs as being "unclean". The Hindu religion: Prohibition of beef consumption protects against infection by Taenia saginata. Muslim Feast of Sacrifice (Ἷal - Adha) 1. Those who can afford to do so are to sacrifice a sheep, goat, camel, cow or other bovid to commemorate the prophet Abraham's willingness to sacrifice his son Isma'il. In many countries with a sizeable Muslim population, the number of animals required exceeds local supply. Results: a.) In parts of North Africa and south -western Asia farmers are encouraged to overstock their grazing land. b.) The Islamic calendar does not always match the date of this occasion to the cycles of plant growth and animal reproduction. c.) Large numbers of sheep are shipped from Australia to the Middle East and other Islamic countries, raising concerns about animal welfare. d.) Many animals are killed in backyards or at the side of the road. More are killed than can be consumed. Blood and offal are not safely disposed of and feed rats and domestic and feral dogs. e.) One would expect enhanced transmission of Chrysomya bezziana (an obligate parasite that eats the wounds of a living animal) and Echinococcus granulosis (the hydad - cyst forming cestode). There is little published information on this. f.) There are concerns about anthrax (Bacillus anthracis) and Rift Valley fever transmission.

The influence of parasites on host behavior Example #1:

The cestode Schistocephalus solidus. Intermediate host = stickleback fish Definitive host = a fish eating bird. The pleurocercoid larval stage lives in the peritoneal cavity of the stickleback Gasterosteus aculeatus. Infected fish are rendered more easily caught by a fish - eating bird such as the Arctic tern Sterna paradisaea. How: a.) The pleurocercoid grows so large it compromises the movement of the fish. b.) Oxygen demand increases, causing the fish to swim close to the surface where [O2] is higher. c.) Infected fish exhibit a reduced fright response when attacked.

"night soil" (transmission):

The use of human feces as fertilizer,

Requirement for active search strategy:

There must be plenty of uninfected hosts available. Why: Searching is more energetically expensive than "sit and wait" and in the transmission stage the parasite does not typically eat. Both miracidia and cercariae will attempt to invade unsuitable hosts. There is some interest in using "decoy" snail hosts to control schistosomes.

Congenital infection in humans:

Toxoplasma gondi: The most commonly congenitally transmitted parasite. Leishmania donovani, Trypanosoma cruzi; Congenital transmission occurs, but is thought to be rare (transmission of T. cruzi in this way may be underestimated.) Infection with Toxoplasma gondi may cause a human woman to give birth to more sons than uninfected women. A sex ratio influence is common among invertebrate parasites and their hosts.

The influence of parasites on host behavior Example #2:

Toxoplasma gondii Intermediate host: A rodent. Definative host: cats. a.) The asexual stages encyst in the nervous tissue of the rodent. Results: Neophobia (fear of new objects) is reduced. Activity is increased. The rodent becomes attracted to the aroma of cats. Southern sea otters (Enhydra lutris nereis) suffering from toxoplasmic encephalitis are more easily eaten by sharks, which are infected with the parasite. Filter feeding fish such as anchovies can act as transport hosts if they are eaten by marine mammals while the cysts are in the fish's gut. The effect of Toxoplasma gondii on human behavior Infected men: Become jealous and disregard rules. Have higher testosterone levels. Infected women: Are trusting and conscientious. Have lower testosterone levels. Both men and women: 1. Are more likely to be involved in serious road accidents. 2. Are suspected to develop schizophrenia more often. Schizophrenia: A neuropsychiatric condition characterized by: Emotional and memory problems Difficulties in expressing oneself Controlling one's thoughts Some patients become delusional and hear voices. This may have been adaptive for the parasite when humans were more often prey for large felines, including saber-toothed tigers. Some of the drugs used to alleviate the symptoms of schizophrenia (e.g. haloperidol and valproic acid) also prevent the replication of Toxoplasma gondii.

Contaminatively transmitted gastrointestinal parasites:

Will excyst (protozoans) or hatch (helminths) depending on dues in the digestive tract, including: Temperature, [O2], [CO2], bile composition, digestive enzymes, gut bacteria. Some eggs will hatch in an unsuitable host (ex. Toxocaris canis in human body). These will be unable to complete their development.

endophilic vector

Willing to enter buildings.

Eggs and cysts: adapted to endure a lot of waiting:

a. Thick walls to resist desiccation, attack from bacteria, fungi. b. Plenty of metabolic reserves. c. A low metabolic rate. In some nematodes, it is the 3rd instar that is infective. These are adapted to survive environmental extremes; one example is the 3rd instar of Dictyocaulis viviparous - it wears the cuticle of the 2nd instar as protection.

Hookworm infective larvae:

a.) Are attracted by temperature, CO2, humidity, skin odors. b.) Invasion is via enzymatic secretions, chemical secretions, and physical action. c.) Invsion by a non-human adapted species (ex. Ancyclostoma canium) can give rise to cutaneous larva migrans.

Pollution's effects on parasitism

a.) Can occur naturally (volcanic eruptions) but is more often human - caused. It has been suggested that parasite species diversity be used as a pollution indicator. It has been suggested that parasites be used as indicators of the risk of bioaccumulation (of heavy metals, for example) within the local environment.

Transmission of Cyclospora spp. Modes of transmission:

a.) Consumption of raw fruits/vegetables fertilized with night soil. b.) The use of contaminated water to wash hands and sprinkle produce at market. c.) Flies and other invertebrates transferring the unsporulated oocyst life cycle stage to food. d.) The rise of air travel has probably helped spread the disease to non - endemic regions.

Pheromermis vesparum The paratenic host:

a.) Effects the transmission of the nematode egg to the wasp host. b.) Aids in the dispersal of the nematode as the egg rides the flying caddisfly or stonefly.

Wurchereria bancrofti

a.) Enter the peripheral circulation of the human host only between the hours of 10:00 PM and 2:00 AM. This is probably to coordinate with the hours of peak activity for its mosquito vectors. b.) In some areas the microfilariae do not exhibit periodicity or peak during the daytime. This is probably related to the biting behavior of the local mosquito species or strains (some mosquitoes are day - active).

Leishmania ssp. promastigotes:

a.) Move to the head and mouthparts of infected sandflies. Result: the fly must bite the mammalian host more frequently than uninfected sandflies.

Some animals (hogs) are coprophiles.

a.) Proglottids of Taenia solium (pig tapeworm) remain immobile in the feces they are shed in.

Many nematode parasites do not practice autoinfection. Why:

a.) Some species pass eggs with the feces that do not become immediately infective. b.) Some species require larval development in a vector/intermediate host or as free-living soil forms (rhabdidiforms).

Most sexually transmitted parasites are protozoa. Few helminths are transmitted this way.

a.) The nematode Mehdinema alii is an exception to this generalization. It lives in the hind gut of the "decorated" house cricket Gryllodes sigillatus. Infective daur larvae migrate to the male's genital chamber. The larvae are passed to the female and she can pass these to other males. Most larvae that reach maturity do so in male crickets.

Contaminative transmission:

common among gastrointestinal parasites . a.) Consumption of food or water contaminated with the infective stage. b.) Inhale or accidentally ingest infective stage. Feces, food, fingers, flies

Entoamoeba hystolytica transmission:

contaminated food or water, sexual intercourse, mechanical vector, medical procedures

Some vectors or strains of vectors have specific host choices.

example: The bed bug Cimex lectularis a.) Some strains prefer humans, others rabbits.

A starving vector will switch to humans if needed.

example: The flea Xenopsylla cheopis: a.) Transmits bubonic plague (Pasteurella pestis) b.) Normally feeds on rats. But: Plague is fatal to rats and a flea population will switch to humans as the rat population declines, triggering a human plague outbreak.

Female mice infected with T. gondi:

i. Early phase of latent infection: More males are made. These tend to roam farther than females and so may be better at dispersing the parasite and are more likely to be caught and eaten by cats. ii. Later phases of latent infection: Females are in poorer health. These produce more females than uninfected controls, perhaps because a female offspring, even if small and weak, is likely to reproduce, unlike small, weak male offspring. b.) T. gondi has been found in the semen of a number of mammals using PCR. It is possible that sexual transmission may be more common than previously thought.

Leishmania - enhancing factor:

i. Inhibits the development of macrophages to present fragments of Leishmania and generate Leishmania - specific T cells. ii. Inhibits the ability of IFN - γ to stimulate production of superoxide and other toxins by macrophages. The Leishmania - enhancing factor appears to inhibit the mammalian immune system just long enough for the parasite to become established. It has not been found in any other blood - feeding insect.

Lion crashes in 1994, 2001

i. Unusually long droughts kill vegetation. 2. When rains returned conditions were good for ticks to feed on the weakened herbivores. Ticks transmit Babesia species. 3. Some ticks, such as Rhipicephalus appendiculatus, feed on buffalo and lions and can transmit lion babsias such as Babesia leo and Babesia felis. Canine distemper virus (acquired from domestic or feral dogs) + weakened lion due to lack of prey = the usually nonserious lion babsiosis becomes life threatening. Toxoplasma gondii may also be involved.

Taenia multiceps

metacestode = a coenurus: An enlarged version of a cysticercus in which tens or hundreds of protoscolices develop from the germinal membrane through asexual reproduction. gid: An infected sheep: a.) Will walk around in circles with a high - stepping gait. b.) Results from the large, fluid - filled coenurus: i. Pressure atrophy, localized blockage of the blood supply kills brain cells. c.) The infected sheep is easy prey for the definitive host, a dog or other canid.