In my experience in immature animals there is a correlation between body weight and number of ectoparasites: the lesser the body weight the larger the number of ectoparasites. In adult animals this is not so obvious: body weight can be more or less normal (although never fat) even with large number of parasites but productivity is reduced. In the case you mention I think is a bit difficult to prove because tick harboring pathogens may well be already detached when the animal exhibit clinical symptoms. For what is worthy is well known that where grazing buffalo (Syncerus caffer) are present cattle are at high risk of East Coast Fever and just a few ticks are enough!   

I believe the main issue is the level of infectivity. Too many ectoparasites may be present but not carrying the causative agent( Protozoa) while just one infected tick is enough to transmit a tick borne disrespectdisrespect

I agree with Saul Lozano, depends of the vector, the pathogen and the host. It is involved the virulence and the amount of the pathogen; the immunity of the host and the abundance and level infection of the vector. For example in Babesia bovis and ticks, it is enough one infected tick larvae to transmit B. bovis to one bovine. But, in field conditions high risk of babesiosis depends of the presence of high charge of infected ticks as well as a few infected ones. It is called enzootic stability. I think each disease has a particular characteristics.

I agree with Saul and Ruben plus, as I mentioned, there is an added complication: "attachment transience" of the infected vector. For various reasons an infected tick may detach  rapidly but in that brief time it may have still been able to infect the host. So consequently if the animal will be clinically sick the tick numbers that animal harbor may be completely unrelated to the pathology. You risk to be presented with two opposite scenarios: a sick animal with a lot of ticks and a sick animal with very few ticks! Alternatively you may find that the sick animal has the same number of ticks as other healthy animals in the herd. 

Also, we need to consider that the pathogen is also a disease factor for the vector. In Babesia bigemina and B. bovis a high infection in the host can kill the tick or reduce considerably the reproductive index, more evident if the ticks feeds on high parasitaemia in the last 24 hrs of engorgement.

Virtually I think there is positive correlation between prevalence of ectoparasites proportional to the risk of pathogen infection.

In nature there are special situations in which the proposed assumption may be different. For example, in immunocompromised human host, Sarcoptes scabiei variety hominis produces the clinical picture called Norwegian scabies. In this case the parasite reproduces much. The stratum corneum becomes a intectante source with high parasite load. The latter contituye a high risk to susceptible hosts nearby (contacts). In this situation the prevalence is not the key factor, but the immunity of the host.

What kind of host animals are you working on?

I think the evidence you are looking for will be hard to find because there are so many other factors obscuring this relationship on different levels like age, sex, behaviour, immune status, population density, community diversity (dilution hosts) etc. of the host. What I've found so far is some literature about the relationship between the prevalence of infestation (for ticks and fleas) and mean abundance, just in case you might be intersted in this rather basic relationship as well...(http://www.ncbi.nlm.nih.gov/pubmed/17032480 and http://www.ncbi.nlm.nih.gov/pubmed/16005879).

All the best.

At first, must be obradititi attention to the type of parasite and host. This correlation is important in the outbreak pathogen. Quantity - number hematofagih species is certainly much more pathogenic than malofaga which are primarily stresogen pathogens.

The fitness of the animals, age and housing conditions are also important when identification of parasites. The cage systems in laying hens infection malofagma receive a drastic decline in production as well as infections with poultry red mite. Normally, the pathological effect of mites is far more important. Cat flea (Ctenocephalides felis felis) in chickens and goats to two weeks, causing significant mortality and the like.

Parasitocenosis ratio is the ratio of the complex ecological, biological and physiological correlations hosts and parasites at which abiotic factors play a major role and therefore often determine the movement of an infection from an extremely benign infection drastic mortality.

Vector role of arthropods that are hematofage are a special problem because of diet and vector role in the spread of infection other etiology. 

My experience in these 28 years of ectoparasites tests indicate that they have a lot of great breadth in the choice of the host but also a higher or lower pathogenicity in relation to primary and secondary hosts. For example, the reproduction of ticks, which was observed in dogs and then found in the lizard was not the same (it was much smaller). Cat fleas was less pathogens in squirrels than in dogs and etc.

Number of ectoparasites is certainly very important, but it must be seen in correlation with the owner - whether it is on the primary or secondary to the cause. The quantity and quality of these infections are often not in the expected correlation.

yes , prevalence of ectoparasite is mostly increase the risk of viral and bacterial diseases , because it serves as biological vector for many pathogens .

yes of course, actually most of the ectoparasite are risk factor of another secondary pathogens.