A miniscule tick larva sat motionless on the arched frond of a large tree fern. The young juvenile had recently emerged from a cluster of eggs laid in leaf litter on the forest floor. Drven by the basic instinct to obtain a blood meal, the hatchlings had scattered in various directions. They all eventually ascended an assortment of plants to reach a height suitable for contacting a passing vertebrate. Some had chosen horsetail stems, while others had selected tree trucks. The journey for a few ended when they became entrapped in beads of kauri resin. The tick on the fern leaf waited patiently, since each larva could survive for weeks without food, and aimless wandering would only use up energy. Luckily a group of pachycephalosaurs soon began browsing on the fern foliage. Only glimpses of their bodies appeared among the shaking plants, although now and then a head would suddenly emerge when one reached upward to tear open a trunk and access the tasty fiber within. The vigilant tick was alerted by all the commotion, and sensing the presence of potential hosts, raised short front legs in anticipation and waited for that critical moment when one of the quadrupeds brushed against the frond.
When that occurred, there could be no hesitation, and the larva quickly dropped onto the thick, pebbly skin. For a first meal, the tiny arachnid chose an area of soft tissue around the ear opening. Using deft palpal claws as attachment devices, the tick inserted a barbed tube into the dinosaur's skin. Viscous secretions in the saliva cemented the tick's mouthparts in place and the blood meal began. When satiated, the tick waited until dusk, then dropped to the ground and crawled under some dried leaves at the base of the tree fern. Soon the larva would molt Into a nymphal stage, scale another plant, and await another passing dinosaur. Molting after the second meal into an adult female meant the subsequent feeding would be a longer one, furnishing protein needed for egg production. The large volume of blood consumed at that meal would swell her body up to ten times the original size. During a lifetime, she would deposit several thousand eggs, ensuring survival of the species.
While ticks and mites are not insects but arachnids related to spiders, both probably played important roles in the transmission of pathogens in the Cretaceous world. There are hard and soft varieties of ticks, and in spite of the difference in body shape, texture, and feeding habits, both sexes of all species require vertebrate blood for survival. After inserting a hollow tube, or hy-postome, into the integument, blood is pumped directly into their stomachs. Ticks are very patient and will remain on vegetation for days, weeks, or even months until a host passes close enough for attachment and feeding. A few have elected to live in or near the domiciles of potential victims, where a blood meal is readily available.
Mid-Cretaceous records of both hard and soft ticks45226 show these arachnids were a vital part of that ancient world. While the Cretaceous soft tick appears to have utilized birds, the hard tick had features of extant reptile-feeders and probably consumed dinosaur blood (color plate 11E). The claws on its palps, a unique feature unknown in today's ticks, presumably were used for grasping tuberculate scales.
Presently, ticks parasitize birds, mammals, reptiles, and amphibians. Dining on reptiles is widespread, and seven genera have representatives attacking tortoises, snakes, and lizards.227 Some of the most bizarre associations involve Amblyomma ticks that engorge on pelagic sea snakes and marine iguanas.228 Primitive Aponomma ticks live almost exclusively on reptiles, and one species positions itself on the forelegs, between the toes, in the olfactory sacs, around the cloacal opening, and in the groin of monitor lizards. Another aponommid feasts between the scales, in the eye sockets, and on the head and neck of pythons.229
The saliva of some ticks is poisonous and can cause inflammation, a fatal paralysis, or even anaphylactic shock.230 These neurotoxins are released from the salivary glands and enter the blood of the victim while the tick is feeding. In humans, the paralysis starts in the lower extremities and moves upward to the head region. Walking becomes difficult and the victim becomes easily fatigued as the first signs of paralysis occur. The numbing effect continues and affects the tongue and facial muscles, eventually causing total body paralysis. Without removing the tick, convulsions and death from the shutdown of diaphragm muscles controlling respiration may follow. Similar reactions in dinosaurs, especially among the very young, would have left the animals completely helpless and easy prey.
While feeding on pachycephalosaurs or other dinosaurs, a tick larva may have ingested some bacterial spirochetes that were circulating in their blood. If so, these microorganisms could have multiplied and accumulated in the tick's salivary glands. When taking a second meal, some of these bacteria would be transferred into the new victim and carried by the blood to various body organs. For a brief time, there would be no effect from the multiplying bacteria. However, the animal soon would become sluggish, with stiffening legs and lethargic movements and unable to keep up with the rest of the group during their daily treks to the fern grottos, eventually falling victim to a flesh eater.
Ticks have a bad rap, and rightfully so since they carry some very dangerous pathogens. With an extensive host range, these arachnids are considered the second-most important vectors of human disease after mosquitoes,231232 and have been transmitting pathogens to humans and domesticated animals for thousands of years. Ticks became widely known to the public when they were recognized as vectors of Lyme disease, induced by a spirochete acquired from feeding on diseased rodents or deer in North America and sick birds and rabbits in Europe. Because of severely debilitating effects on humans and the sudden occurrence throughout the United States, Lyme disease has been intensively studied. The first symptoms are often a circular expanding rash beginning around the tick bite. This is accompanied by fever and joint and muscle pains, similar to those felt during the onset of flu. Once the bacteria have become established, nerve inflammation in the joints can lead to difficulty in walking. Diseased animals or those carrying contaminated ticks can spread the infection over considerable distances.230 It is easy to visualize such tick-borne diseases conceivably spreading through dinosaur populations in a similar manner. The intense fatigue associated with these diseases would put infected animals at risk.
Reptiles and birds have their own varieties of Lyme disease carried by ticks, but some lizards are protected from the infection by a substance in their blood that destroys the spirochetes.233 Such immune responses could have evolved in Cretaceous reptiles challenged by infections of ancient spirochetes.
Other bacterial pathogens transmitted by these arachnids cause illnesses known as ehrlichiosis in livestock and dogs, and tularemia in mammals, birds, and amphibians.230 Ehrlichiosis is often fatal to dogs, especially susceptible breeds like German shepherds. Additional tick-borne pathogens include protozoans that cause malarial-like ailments in birds and mammals. These parasites develop in circulating red blood cells and cause fever, sweating, headaches, and chills, and infections often have fatal results. Cattle acquire the disease from ticks that gorged on infected deer (reservoir hosts), and while deer do not appear to suffer, cows can enter lethal comas. Other malarial-like organisms vectored by ticks are responsible for African East Coast fever that is lethal to cattle and buffalo.230
Ticks also spread a number of rickettsia, such as Rocky Mountain disease that causes death in humans.230 Still another tickborne rickettsia that survives in soil and water for months causes Q fever in birds, mammals, and reptiles, including lizards, snakes, and tortoises.234235 People acquire the infection by han dling diseased animals or items contaminated with tick feces and may suffer fever, chills, and even fatalities.230
Viruses carried by ticks cause encephalitis and hemorrhagic fevers, leading to death in a wide range of mammals, including man. Reptiles may play a major role in the disease cycle by being significant reservoir hosts for tick-borne viruses.230 And finally, some African and South American ticks spread filarial parasites to mammals.173
The number of deadly and debilitating pathogens that ticks transmit is astonishing, and there is no reason why dinosaurs would not have been targeted. These aggressive biters are at home on warm- or cold-blooded animals, and no amount of scales, feathers, or hair can impede them. They undoubtedly were quite annoying when large numbers clustered around ear openings, settled under the eyelids, or entered the nostrils. They occur throughout the world in a variety of climates and habitats from tropical forests to deserts, which include conditions found throughout the Cretaceous.
On the basis of our present knowledge, perhaps just as many mites preyed on dinosaurs, since many of the types found today on vertebrates and invertebrates lived in the Cretaceous (color plates 11F, 13B, 14C). In various parts of the world, mites cause a condition known as mange in mammals and birds. Mange mites are adapted to living on the legs of birds in spite of the scaly coverings, and "scaly-leg mites" even cause lesions on the keratinized leg portions of game birds.236 Could the ancestors of these have victimized dinosaurs too?
Of those that attack reptiles, one type exploits geckos, alligators, and rock lizards, while another type prefers skinks and terrestrial lizards.237 Certainly, early members of these mite families would have accepted dinosaurs as hosts.
All stages of vertebrate-parasitic pterygosomid mites take lizard blood,238 and they remain on the host during their complete developmental cycle from a 6-legged larva to an 8-legged adult. Only the females leave the host to deposit their eggs in the surrounding area, and upon hatching, the tiny active larvae secure themselves under scales on the legs, neck, or back.239 Female mites of this group attacking iguanas in Brazil are flattened so they fit snugly beneath the lizard's scales.240 Even after the reptile dies, the parasites will remain attached for weeks, feasting on the carcass until it is completely mummified.
Chiggers (trombiculids) are parasitic only as larvae, then become predators on small invertebrates as nymphs and adults.241 These mites transmit malarial parasites to rodents and lizards,236 as well as filarial nematodes to mammals.173 They certainly existed during the Cretaceous, and the dinosaurs couldn't have escaped their attacks.
Another group that existed back then was the beetle mites, or oribatids. Widespread today on plants, they serve as intermediate hosts of tapeworms, which sometimes cause convulsions and death in humans with heavy infections. It is possible that herbivorous dinosaurs were infected with mite-borne tapeworms, but whether they experienced such severe reactions will remain unknown.
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